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This manual documents how to use the GNU compilers, as well as their features and incompatibilities, and how to report bugs. It corresponds to GCC version 3.3. The internals of the GNU compilers, including how to port them to new targets and some information about how to write front ends for new languages, are documented in a separate manual. See section `Introduction' in GNU Compiler Collection (GCC) Internals.
1. Compile C, C++, Objective-C, Ada, Fortran, Java, or treelang You can compile C or C++ programs. 2. Language Standards Supported by GCC Language standards supported by GCC. 3. GCC Command Options Command options supported by `gcc'. 4. C Implementation-defined behavior How GCC implements the ISO C specification. 5. Extensions to the C Language Family GNU extensions to the C language family. 6. Extensions to the C++ Language GNU extensions to the C++ language. 7. GNU Objective-C runtime features 8. Binary Compatibility 9. Known Causes of Trouble with GCC If you have trouble using GCC. 10. Reporting Bugs How, why and where to report bugs. 11. How To Get Help with GCC How to find suppliers of support for GCC. 12. Contributing to GCC Development How to contribute to testing and developing GCC.
Funding Free Software How to help assure funding for free software. The GNU Project and GNU/Linux
GNU GENERAL PUBLIC LICENSE GNU General Public License says how you can copy and share GCC. GNU Free Documentation License How you can copy and share this manual. Contributors to GCC People who have contributed to GCC.
Option Index Index to command line options. Keyword Index Index of concepts and symbol names.
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Several versions of the compiler (C, C++, Objective-C, Ada, Fortran, Java and treelang) are integrated; this is why we use the name "GNU Compiler Collection". GCC can compile programs written in any of these languages. The Ada, Fortran, Java and treelang compilers are described in separate manuals.
"GCC" is a common shorthand term for the GNU Compiler Collection. This is both the most general name for the compiler, and the name used when the emphasis is on compiling C programs (as the abbreviation formerly stood for "GNU C Compiler").
When referring to C++ compilation, it is usual to call the compiler "G++". Since there is only one compiler, it is also accurate to call it "GCC" no matter what the language context; however, the term "G++" is more useful when the emphasis is on compiling C++ programs.
Similarly, when we talk about Ada compilation, we usually call the compiler "GNAT", for the same reasons.
We use the name "GCC" to refer to the compilation system as a whole, and more specifically to the language-independent part of the compiler. For example, we refer to the optimization options as affecting the behavior of "GCC" or sometimes just "the compiler".
Front ends for other languages, such as Mercury and Pascal exist but have not yet been integrated into GCC. These front ends, like that for C++, are built in subdirectories of GCC and link to it. The result is an integrated compiler that can compile programs written in C, C++, Objective-C, or any of the languages for which you have installed front ends.
In this manual, we only discuss the options for the C, Objective-C, and C++ compilers and those of the GCC core. Consult the documentation of the other front ends for the options to use when compiling programs written in other languages.
G++ is a compiler, not merely a preprocessor. G++ builds object code directly from your C++ program source. There is no intermediate C version of the program. (By contrast, for example, some other implementations use a program that generates a C program from your C++ source.) Avoiding an intermediate C representation of the program means that you get better object code, and better debugging information. The GNU debugger, GDB, works with this information in the object code to give you comprehensive C++ source-level editing capabilities (see section `C and C++' in Debugging with GDB).
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For each language compiled by GCC for which there is a standard, GCC attempts to follow one or more versions of that standard, possibly with some exceptions, and possibly with some extensions.
GCC supports three versions of the C standard, although support for the most recent version is not yet complete.
The original ANSI C standard (X3.159-1989) was ratified in 1989 and published in 1990. This standard was ratified as an ISO standard (ISO/IEC 9899:1990) later in 1990. There were no technical differences between these publications, although the sections of the ANSI standard were renumbered and became clauses in the ISO standard. This standard, in both its forms, is commonly known as C89, or occasionally as C90, from the dates of ratification. The ANSI standard, but not the ISO standard, also came with a Rationale document. To select this standard in GCC, use one of the options `-ansi', `-std=c89' or `-std=iso9899:1990'; to obtain all the diagnostics required by the standard, you should also specify `-pedantic' (or `-pedantic-errors' if you want them to be errors rather than warnings). See section Options Controlling C Dialect.
Errors in the 1990 ISO C standard were corrected in two Technical Corrigenda published in 1994 and 1996. GCC does not support the uncorrected version.
An amendment to the 1990 standard was published in 1995. This
amendment added digraphs and __STDC_VERSION__ to the language,
but otherwise concerned the library. This amendment is commonly known
as AMD1; the amended standard is sometimes known as C94 or
C95. To select this standard in GCC, use the option
`-std=iso9899:199409' (with, as for other standard versions,
`-pedantic' to receive all required diagnostics).
A new edition of the ISO C standard was published in 1999 as ISO/IEC 9899:1999, and is commonly known as C99. GCC has incomplete support for this standard version; see http://gcc.gnu.org/gcc-3.3/c99status.html for details. To select this standard, use `-std=c99' or `-std=iso9899:1999'. (While in development, drafts of this standard version were referred to as C9X.)
Errors in the 1999 ISO C standard were corrected in a Technical Corrigendum published in 2001. GCC does not support the uncorrected version.
By default, GCC provides some extensions to the C language that on rare occasions conflict with the C standard. See section Extensions to the C Language Family. Use of the `-std' options listed above will disable these extensions where they conflict with the C standard version selected. You may also select an extended version of the C language explicitly with `-std=gnu89' (for C89 with GNU extensions) or `-std=gnu99' (for C99 with GNU extensions). The default, if no C language dialect options are given, is `-std=gnu89'; this will change to `-std=gnu99' in some future release when the C99 support is complete. Some features that are part of the C99 standard are accepted as extensions in C89 mode.
The ISO C standard defines (in clause 4) two classes of conforming
implementation. A conforming hosted implementation supports the
whole standard including all the library facilities; a conforming
freestanding implementation is only required to provide certain
library facilities: those in <float.h>, <limits.h>,
<stdarg.h>, and <stddef.h>; since AMD1, also those in
<iso646.h>; and in C99, also those in <stdbool.h> and
<stdint.h>. In addition, complex types, added in C99, are not
required for freestanding implementations. The standard also defines
two environments for programs, a freestanding environment,
required of all implementations and which may not have library
facilities beyond those required of freestanding implementations,
where the handling of program startup and termination are
implementation-defined, and a hosted environment, which is not
required, in which all the library facilities are provided and startup
is through a function int main (void) or int main (int,
char *[]). An OS kernel would be a freestanding environment; a
program using the facilities of an operating system would normally be
in a hosted implementation.
GCC aims towards being usable as a conforming freestanding
implementation, or as the compiler for a conforming hosted
implementation. By default, it will act as the compiler for a hosted
implementation, defining __STDC_HOSTED__ as 1 and
presuming that when the names of ISO C functions are used, they have
the semantics defined in the standard. To make it act as a conforming
freestanding implementation for a freestanding environment, use the
option `-ffreestanding'; it will then define
__STDC_HOSTED__ to 0 and not make assumptions about the
meanings of function names from the standard library, with exceptions
noted below. To build an OS kernel, you may well still need to make
your own arrangements for linking and startup.
See section Options Controlling C Dialect.
GCC does not provide the library facilities required only of hosted implementations, nor yet all the facilities required by C99 of freestanding implementations; to use the facilities of a hosted environment, you will need to find them elsewhere (for example, in the GNU C library). See section Standard Libraries.
Most of the compiler support routines used by GCC are present in
`libgcc', but there are a few exceptions. GCC requires the
freestanding environment provide memcpy, memmove,
memset and memcmp. Some older ports of GCC are
configured to use the BSD bcopy, bzero and bcmp
functions instead, but this is deprecated for new ports.
Finally, if __builtin_trap is used, and the target does
not implement the trap pattern, then GCC will emit a call
to abort.
For references to Technical Corrigenda, Rationale documents and information concerning the history of C that is available online, see http://gcc.gnu.org/readings.html
There is no formal written standard for Objective-C. The most authoritative manual is "Object-Oriented Programming and the Objective-C Language", available at a number of web sites
There is no standard for treelang, which is a sample language front end for GCC. Its only purpose is as a sample for people wishing to write a new language for GCC. The language is documented in `gcc/treelang/treelang.texi' which can be turned into info or HTML format.
See section `About This Guide' in GNAT Reference Manual, for information on standard conformance and compatibility of the Ada compiler.
See section `The GNU Fortran Language' in Using and Porting GNU Fortran, for details of the Fortran language supported by GCC.
See section `Compatibility with the Java Platform' in GNU gcj,
for details of compatibility between gcj and the Java Platform.
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When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The "overall options" allow you to stop this process at an intermediate stage. For example, the `-c' option says not to run the linker. Then the output consists of object files output by the assembler.
Other options are passed on to one stage of processing. Some options control the preprocessor and others the compiler itself. Yet other options control the assembler and linker; most of these are not documented here, since you rarely need to use any of them.
Most of the command line options that you can use with GCC are useful for C programs; when an option is only useful with another language (usually C++), the explanation says so explicitly. If the description for a particular option does not mention a source language, you can use that option with all supported languages.
See section Compiling C++ Programs, for a summary of special options for compiling C++ programs.
The gcc program accepts options and file names as operands. Many
options have multi-letter names; therefore multiple single-letter options
may not be grouped: `-dr' is very different from `-d
-r'.
You can mix options and other arguments. For the most part, the order you use doesn't matter. Order does matter when you use several options of the same kind; for example, if you specify `-L' more than once, the directories are searched in the order specified.
Many options have long names starting with `-f' or with `-W'---for example, `-fforce-mem', `-fstrength-reduce', `-Wformat' and so on. Most of these have both positive and negative forms; the negative form of `-ffoo' would be `-fno-foo'. This manual documents only one of these two forms, whichever one is not the default.
See section Option Index, for an index to GCC's options.
3.1 Option Summary Brief list of all options, without explanations. 3.2 Options Controlling the Kind of Output Controlling the kind of output: an executable, object files, assembler files, or preprocessed source. 3.3 Compiling C++ Programs Compiling C++ programs. 3.4 Options Controlling C Dialect Controlling the variant of C language compiled. 3.5 Options Controlling C++ Dialect Variations on C++. 3.6 Options Controlling Objective-C Dialect Variations on Objective-C. 3.7 Options to Control Diagnostic Messages Formatting Controlling how diagnostics should be formatted. 3.8 Options to Request or Suppress Warnings How picky should the compiler be? 3.9 Options for Debugging Your Program or GCC Symbol tables, measurements, and debugging dumps. 3.10 Options That Control Optimization How much optimization? 3.11 Options Controlling the Preprocessor Controlling header files and macro definitions. Also, getting dependency information for Make. 3.12 Passing Options to the Assembler Passing options to the assembler. 3.13 Options for Linking Specifying libraries and so on. 3.14 Options for Directory Search Where to find header files and libraries. Where to find the compiler executable files. 3.15 Specifying subprocesses and the switches to pass to them How to pass switches to sub-processes. 3.16 Specifying Target Machine and Compiler Version Running a cross-compiler, or an old version of GCC. 3.17 Hardware Models and Configurations Specifying minor hardware or convention variations, such as 68010 vs 68020. 3.18 Options for Code Generation Conventions Specifying conventions for function calls, data layout and register usage. 3.19 Environment Variables Affecting GCC Env vars that affect GCC. 3.20 Running Protoize Automatically adding or removing function prototypes.
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Here is a summary of all the options, grouped by type. Explanations are in the following sections.
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3.2 Options Controlling the Kind of Output Controlling the kind of output: an executable, object files, assembler files, or preprocessed source. 3.4 Options Controlling C Dialect Controlling the variant of C language compiled. 3.5 Options Controlling C++ Dialect Variations on C++. 3.6 Options Controlling Objective-C Dialect Variations on Objective-C. 3.7 Options to Control Diagnostic Messages Formatting Controlling how diagnostics should be formatted. 3.8 Options to Request or Suppress Warnings How picky should the compiler be? 3.9 Options for Debugging Your Program or GCC Symbol tables, measurements, and debugging dumps. 3.10 Options That Control Optimization How much optimization? 3.11 Options Controlling the Preprocessor Controlling header files and macro definitions. Also, getting dependency information for Make. 3.12 Passing Options to the Assembler Passing options to the assembler. 3.13 Options for Linking Specifying libraries and so on. 3.14 Options for Directory Search Where to find header files and libraries. Where to find the compiler executable files. 3.15 Specifying subprocesses and the switches to pass to them How to pass switches to sub-processes. 3.16 Specifying Target Machine and Compiler Version Running a cross-compiler, or an old version of GCC.
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Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking, always in that order. The first three stages apply to an individual source file, and end by producing an object file; linking combines all the object files (those newly compiled, and those specified as input) into an executable file.
For any given input file, the file name suffix determines what kind of compilation is done:
file.c
file.i
file.ii
file.m
file.mi
file.h
file.cc
file.cp
file.cxx
file.cpp
file.c++
file.C
file.f
file.for
file.FOR
file.F
file.fpp
file.FPP
file.r
See section `Options Controlling the Kind of Output' in Using and Porting GNU Fortran, for more details of the handling of Fortran input files.
file.ads
file.adb
file.s
file.S
other
You can specify the input language explicitly with the `-x' option:
-x language
c c-header cpp-output c++ c++-cpp-output objective-c objc-cpp-output assembler assembler-with-cpp ada f77 f77-cpp-input ratfor java treelang |
-x none
-pass-exit-codes
gcc program will exit with the code of 1 if any
phase of the compiler returns a non-success return code. If you specify
`-pass-exit-codes', the gcc program will instead return with
numerically highest error produced by any phase that returned an error
indication.
If you only want some of the stages of compilation, you can use
`-x' (or filename suffixes) to tell gcc where to start, and
one of the options `-c', `-S', or `-E' to say where
gcc is to stop. Note that some combinations (for example,
`-x cpp-output -E') instruct gcc to do nothing at all.
-c
By default, the object file name for a source file is made by replacing the suffix `.c', `.i', `.s', etc., with `.o'.
Unrecognized input files, not requiring compilation or assembly, are ignored.
-S
By default, the assembler file name for a source file is made by replacing the suffix `.c', `.i', etc., with `.s'.
Input files that don't require compilation are ignored.
-E
Input files which don't require preprocessing are ignored.
-o file
Since only one output file can be specified, it does not make sense to use `-o' when compiling more than one input file, unless you are producing an executable file as output.
If `-o' is not specified, the default is to put an executable file in `a.out', the object file for `source.suffix' in `source.o', its assembler file in `source.s', and all preprocessed C source on standard output.
-v
-###
-pipe
--help
gcc. If the `-v' option is also specified
then `--help' will also be passed on to the various processes
invoked by gcc, so that they can display the command line options
they accept. If the `-W' option is also specified then command
line options which have no documentation associated with them will also
be displayed.
--target-help
--version
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C++ source files conventionally use one of the suffixes `.C',
`.cc', `.cpp', `.c++', `.cp', or `.cxx';
preprocessed C++ files use the suffix `.ii'. GCC recognizes
files with these names and compiles them as C++ programs even if you
call the compiler the same way as for compiling C programs (usually with
the name gcc).
However, C++ programs often require class libraries as well as a
compiler that understands the C++ language--and under some
circumstances, you might want to compile programs from standard input,
or otherwise without a suffix that flags them as C++ programs.
g++ is a program that calls GCC with the default language
set to C++, and automatically specifies linking against the C++
library. On many systems, g++ is also
installed with the name c++.
When you compile C++ programs, you may specify many of the same command-line options that you use for compiling programs in any language; or command-line options meaningful for C and related languages; or options that are meaningful only for C++ programs. See section Options Controlling C Dialect, for explanations of options for languages related to C. See section Options Controlling C++ Dialect, for explanations of options that are meaningful only for C++ programs.
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The following options control the dialect of C (or languages derived from C, such as C++ and Objective-C) that the compiler accepts:
-ansi
This turns off certain features of GCC that are incompatible with ISO
C90 (when compiling C code), or of standard C++ (when compiling C++ code),
such as the asm and typeof keywords, and
predefined macros such as unix and vax that identify the
type of system you are using. It also enables the undesirable and
rarely used ISO trigraph feature. For the C compiler,
it disables recognition of C++ style `//' comments as well as
the inline keyword.
The alternate keywords __asm__, __extension__,
__inline__ and __typeof__ continue to work despite
`-ansi'. You would not want to use them in an ISO C program, of
course, but it is useful to put them in header files that might be included
in compilations done with `-ansi'. Alternate predefined macros
such as __unix__ and __vax__ are also available, with or
without `-ansi'.
The `-ansi' option does not cause non-ISO programs to be rejected gratuitously. For that, `-pedantic' is required in addition to `-ansi'. See section 3.8 Options to Request or Suppress Warnings.
The macro __STRICT_ANSI__ is predefined when the `-ansi'
option is used. Some header files may notice this macro and refrain
from declaring certain functions or defining certain macros that the
ISO standard doesn't call for; this is to avoid interfering with any
programs that might use these names for other things.
Functions which would normally be built in but do not have semantics
defined by ISO C (such as alloca and ffs) are not built-in
functions with `-ansi' is used. See section Other built-in functions provided by GCC, for details of the functions
affected.
-std=
Even when this option is not specified, you can still use some of the
features of newer standards in so far as they do not conflict with
previous C standards. For example, you may use __restrict__ even
when `-std=c99' is not specified.
The `-std' options specifying some version of ISO C have the same
effects as `-ansi', except that features that were not in ISO C90
but are in the specified version (for example, `//' comments and
the inline keyword in ISO C99) are not disabled.
See section Language Standards Supported by GCC, for details of these standard versions.
-aux-info filename
Besides declarations, the file indicates, in comments, the origin of each declaration (source file and line), whether the declaration was implicit, prototyped or unprototyped (`I', `N' for new or `O' for old, respectively, in the first character after the line number and the colon), and whether it came from a declaration or a definition (`C' or `F', respectively, in the following character). In the case of function definitions, a K&R-style list of arguments followed by their declarations is also provided, inside comments, after the declaration.
-fno-asm
asm, inline or typeof as a
keyword, so that code can use these words as identifiers. You can use
the keywords __asm__, __inline__ and __typeof__
instead. `-ansi' implies `-fno-asm'.
In C++, this switch only affects the typeof keyword, since
asm and inline are standard keywords. You may want to
use the `-fno-gnu-keywords' flag instead, which has the same
effect. In C99 mode (`-std=c99' or `-std=gnu99'), this
switch only affects the asm and typeof keywords, since
inline is a standard keyword in ISO C99.
-fno-builtin
-fno-builtin-function
GCC normally generates special code to handle certain built-in functions
more efficiently; for instance, calls to alloca may become single
instructions that adjust the stack directly, and calls to memcpy
may become inline copy loops. The resulting code is often both smaller
and faster, but since the function calls no longer appear as such, you
cannot set a breakpoint on those calls, nor can you change the behavior
of the functions by linking with a different library.
With the `-fno-builtin-function' option only the built-in function function is disabled. function must not begin with `__builtin_'. If a function is named this is not built-in in this version of GCC, this option is ignored. There is no corresponding `-fbuiltin-function' option; if you wish to enable built-in functions selectively when using `-fno-builtin' or `-ffreestanding', you may define macros such as:
#define abs(n) __builtin_abs ((n)) #define strcpy(d, s) __builtin_strcpy ((d), (s)) |
-fhosted
Assert that compilation takes place in a hosted environment. This implies
`-fbuiltin'. A hosted environment is one in which the
entire standard library is available, and in which main has a return
type of int. Examples are nearly everything except a kernel.
This is equivalent to `-fno-freestanding'.
-ffreestanding
Assert that compilation takes place in a freestanding environment. This
implies `-fno-builtin'. A freestanding environment
is one in which the standard library may not exist, and program startup may
not necessarily be at main. The most obvious example is an OS kernel.
This is equivalent to `-fno-hosted'.
See section Language Standards Supported by GCC, for details of freestanding and hosted environments.
-fms-extensions
-trigraphs
-no-integrated-cpp
The semantics of this option will change if "cc1", "cc1plus", and "cc1obj" are merged.
-traditional
-traditional-cpp
-fcond-mismatch
-funsigned-char
char be unsigned, like unsigned char.
Each kind of machine has a default for what char should
be. It is either like unsigned char by default or like
signed char by default.
Ideally, a portable program should always use signed char or
unsigned char when it depends on the signedness of an object.
But many programs have been written to use plain char and
expect it to be signed, or expect it to be unsigned, depending on the
machines they were written for. This option, and its inverse, let you
make such a program work with the opposite default.
The type char is always a distinct type from each of
signed char or unsigned char, even though its behavior
is always just like one of those two.
-fsigned-char
char be signed, like signed char.
Note that this is equivalent to `-fno-unsigned-char', which is the negative form of `-funsigned-char'. Likewise, the option `-fno-signed-char' is equivalent to `-funsigned-char'.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
signed or unsigned. By
default, such a bit-field is signed, because this is consistent: the
basic integer types such as int are signed types.
-fwritable-strings
Writing into string constants is a very bad idea; "constants" should be constant.
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This section describes the command-line options that are only meaningful
for C++ programs; but you can also use most of the GNU compiler options
regardless of what language your program is in. For example, you
might compile a file firstClass.C like this:
g++ -g -frepo -O -c firstClass.C |
In this example, only `-frepo' is an option meant only for C++ programs; you can use the other options with any language supported by GCC.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n
The default is version 1.
-fno-access-control
-fcheck-new
operator new is non-null
before attempting to modify the storage allocated. The current Working
Paper requires that operator new never return a null pointer, so
this check is normally unnecessary.
An alternative to using this option is to specify that your
operator new does not throw any exceptions; if you declare it
`throw()', G++ will check the return value. See also `new
(nothrow)'.
-fconserve-space
main() has
completed, you may have an object that is being destroyed twice because
two definitions were merged.
This option is no longer useful on most targets, now that support has been added for putting variables into BSS without making them common.
-fno-const-strings
char * instead of type const
char *. By default, G++ uses type const char * as required by
the standard. Even if you use `-fno-const-strings', you cannot
actually modify the value of a string constant, unless you also use
`-fwritable-strings'.
This option might be removed in a future release of G++. For maximum
portability, you should structure your code so that it works with
string constants that have type const char *.
-fdollars-in-identifiers
-fno-elide-constructors
-fno-enforce-eh-specs
-fexternal-templates
Cause `#pragma interface' and `implementation' to apply to template instantiation; template instances are emitted or not according to the location of the template definition. See section 6.6 Where's the Template?, for more information.
This option is deprecated.
-falt-external-templates
This option is deprecated.
-ffor-scope
-fno-for-scope
The default if neither flag is given to follow the standard, but to allow and give a warning for old-style code that would otherwise be invalid, or have different behavior.
-fno-gnu-keywords
typeof as a keyword, so that code can use this
word as an identifier. You can use the keyword __typeof__ instead.
`-ansi' implies `-fno-gnu-keywords'.
-fno-implicit-templates
-fno-implicit-inline-templates
-fno-implement-inlines
-fms-extensions
-fno-nonansi-builtins
ffs, alloca, _exit,
index, bzero, conjf, and other related functions.
-fno-operator-names
and, bitand,
bitor, compl, not, or and xor as
synonyms as keywords.
-fno-optional-diags
-fpermissive
-frepo
-fno-rtti
-fstats
-ftemplate-depth-n
-fuse-cxa-atexit
__cxa_atexit function rather than the atexit function.
This option is required for fully standards-compliant handling of static
destructors, but will only work if your C library supports
__cxa_atexit.
-fvtable-gc
This optimization requires GNU as and GNU ld. Not all systems support this option. `-Wl,--gc-sections' is ignored without `-static'.
-fno-weak
-nostdinc++
In addition, these optimization, warning, and code generation options have meanings only for C++ programs:
-fno-default-inline
-Wabi (C++ only)
You should rewrite your code to avoid these warnings if you are concerned about the fact that code generated by G++ may not be binary compatible with code generated by other compilers.
The known incompatibilities at this point include:
struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
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In this case, G++ will place B::f2 into the same byte
asA::f1; other compilers will not. You can avoid this problem
by explicitly padding A so that its size is a multiple of the
byte size on your platform; that will cause G++ and other compilers to
layout B identically.
struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
struct C : public A, public virtual B {};
|
In this case, G++ will not place B into the tail-padding for
A; other compilers will. You can avoid this problem by
explicitly padding A so that its size is a multiple of its
alignment (ignoring virtual base classes); that will cause G++ and other
compilers to layout C identically.
union U { int i : 4096; };
|
Assuming that an int does not have 4096 bits, G++ will make the
union too small by the number of bits in an int.
struct A {};
struct B {
A a;
virtual void f ();
};
struct C : public B, public A {};
|
G++ will place the A base class of C at a nonzero offset;
it should be placed at offset zero. G++ mistakenly believes that the
A data member of B is already at offset zero.
typename or
template template parameters can be mangled incorrectly.
template <typename Q>
void f(typename Q::X) {}
template <template <typename> class Q>
void f(typename Q<int>::X) {}
|
Instantiations of these templates may be mangled incorrectly.
-Wctor-dtor-privacy (C++ only)
-Wnon-virtual-dtor (C++ only)
-Wreorder (C++ only)
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
|
Here the compiler will warn that the member initializers for `i' and `j' will be rearranged to match the declaration order of the members. This warning is enabled by `-Wall'.
The following `-W...' options are not affected by `-Wall'.
-Weffc++ (C++ only)
operator= return a reference to *this.
and about violations of the following style guidelines from Scott Meyers' More Effective C++ book:
&&, ||, or ,.
If you use this option, you should be aware that the standard library headers do not obey all of these guidelines; you can use `grep -v' to filter out those warnings.
-Wno-deprecated (C++ only)
-Wno-non-template-friend (C++ only)
-Wold-style-cast (C++ only)
-Woverloaded-virtual (C++ only)
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};
|
the A class version of f is hidden in B, and code
like this:
B* b; b->f(); |
will fail to compile.
-Wno-pmf-conversions (C++ only)
-Wsign-promo (C++ only)
-Wsynth (C++ only)
struct A {
operator int ();
A& operator = (int);
};
main ()
{
A a,b;
a = b;
}
|
In this example, G++ will synthesize a default `A& operator = (const A&);', while cfront will use the user-defined `operator ='.
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This section describes the command-line options that are only meaningful
for Objective-C programs; but you can also use most of the GNU compiler
options regardless of what language your program is in. For example,
you might compile a file some_class.m like this:
gcc -g -fgnu-runtime -O -c some_class.m |
In this example, only `-fgnu-runtime' is an option meant only for Objective-C programs; you can use the other options with any language supported by GCC.
Here is a list of options that are only for compiling Objective-C programs:
-fconstant-string-class=class-name
@"...". The default
class name is NXConstantString.
-fgnu-runtime
-fnext-runtime
__NEXT_RUNTIME__ is predefined if (and only if) this option is
used.
-gen-decls
-Wno-protocol
-Wno-protocol option, then
methods inherited from the superclass are considered to be implemented,
and no warning is issued for them.
-Wselector
@selector(...)
expression, a corresponding method with that selector has been found
during compilation. Because these checks scan the method table only at
the end of compilation, these warnings are not produced if the final
stage of compilation is not reached, for example because an error is
found during compilation, or because the -fsyntax-only option is
being used.
-Wundeclared-selector
@selector(...) expression referring to an
undeclared selector is found. A selector is considered undeclared if no
method with that name has been declared (explicitly, in an
@interface or @protocol declaration, or implicitly, in
an @implementation section) before the
@selector(...) expression. This option always performs its
checks as soon as a @selector(...) expression is found
(while -Wselector only performs its checks in the final stage of
compilation), and so additionally enforces the coding style convention
that methods and selectors must be declared before being used.
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Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (e.g. its width, ...). The options described below can be used to control the diagnostic messages formatting algorithm, e.g. how many characters per line, how often source location information should be reported. Right now, only the C++ front end can honor these options. However it is expected, in the near future, that the remaining front ends would be able to digest them correctly.
-fmessage-length=n
g++ and 0 for the rest of
the front ends supported by GCC. If n is zero, then no
line-wrapping will be done; each error message will appear on a single
line.
-fdiagnostics-show-location=once
-fdiagnostics-show-location=every-line
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Warnings are diagnostic messages that report constructions which are not inherently erroneous but which are risky or suggest there may have been an error.
You can request many specific warnings with options beginning `-W', for example `-Wimplicit' to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning `-Wno-' to turn off warnings; for example, `-Wno-implicit'. This manual lists only one of the two forms, whichever is not the default.
The following options control the amount and kinds of warnings produced by GCC; for further, language-specific options also refer to 3.5 Options Controlling C++ Dialect and 3.6 Options Controlling Objective-C Dialect.
-fsyntax-only
-pedantic
Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few will require `-ansi' or a `-std' option specifying the required version of ISO C). However, without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option, they are rejected.
`-pedantic' does not cause warning messages for use of the
alternate keywords whose names begin and end with `__'. Pedantic
warnings are also disabled in the expression that follows
__extension__. However, only system header files should use
these escape routes; application programs should avoid them.
See section 5.39 Alternate Keywords.
Some users try to use `-pedantic' to check programs for strict ISO C conformance. They soon find that it does not do quite what they want: it finds some non-ISO practices, but not all--only those for which ISO C requires a diagnostic, and some others for which diagnostics have been added.
A feature to report any failure to conform to ISO C might be useful in some instances, but would require considerable additional work and would be quite different from `-pedantic'. We don't have plans to support such a feature in the near future.
Where the standard specified with `-std' represents a GNU extended dialect of C, such as `gnu89' or `gnu99', there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based. Warnings from `-pedantic' are given where they are required by the base standard. (It would not make sense for such warnings to be given only for features not in the specified GNU C dialect, since by definition the GNU dialects of C include all features the compiler supports with the given option, and there would be nothing to warn about.)
-pedantic-errors
-w
-Wno-import
-Wchar-subscripts
char. This is a common cause
of error, as programmers often forget that this type is signed on some
machines.
-Wcomment
-Wformat
printf and scanf, etc., to make sure that
the arguments supplied have types appropriate to the format string
specified, and that the conversions specified in the format string make
sense. This includes standard functions, and others specified by format
attributes (see section 5.25 Declaring Attributes of Functions), in the printf,
scanf, strftime and strfmon (an X/Open extension,
not in the C standard) families.
The formats are checked against the format features supported by GNU
libc version 2.2. These include all ISO C90 and C99 features, as well
as features from the Single Unix Specification and some BSD and GNU
extensions. Other library implementations may not support all these
features; GCC does not support warning about features that go beyond a
particular library's limitations. However, if `-pedantic' is used
with `-Wformat', warnings will be given about format features not
in the selected standard version (but not for strfmon formats,
since those are not in any version of the C standard). See section Options Controlling C Dialect.
Since `-Wformat' also checks for null format arguments for several functions, `-Wformat' also implies `-Wnonnull'.
`-Wformat' is included in `-Wall'. For more control over some aspects of format checking, the options `-Wno-format-y2k', `-Wno-format-extra-args', `-Wno-format-zero-length', `-Wformat-nonliteral', `-Wformat-security', and `-Wformat=2' are available, but are not included in `-Wall'.
-Wno-format-y2k
strftime
formats which may yield only a two-digit year.
-Wno-format-extra-args
printf or scanf format function. The C standard specifies
that such arguments are ignored.
Where the unused arguments lie between used arguments that are
specified with `$' operand number specifications, normally
warnings are still given, since the implementation could not know what
type to pass to va_arg to skip the unused arguments. However,
in the case of scanf formats, this option will suppress the
warning if the unused arguments are all pointers, since the Single
Unix Specification says that such unused arguments are allowed.
-Wno-format-zero-length
-Wformat-nonliteral
va_list.
-Wformat-security
printf and scanf functions where the
format string is not a string literal and there are no format arguments,
as in printf (foo);. This may be a security hole if the format
string came from untrusted input and contains `%n'. (This is
currently a subset of what `-Wformat-nonliteral' warns about, but
in future warnings may be added to `-Wformat-security' that are not
included in `-Wformat-nonliteral'.)
-Wformat=2
-Wnonnull
nonnull function attribute.
`-Wnonnull' is included in `-Wall' and `-Wformat'. It can be disabled with the `-Wno-nonnull' option.
-Wimplicit-int
-Wimplicit-function-declaration
-Werror-implicit-function-declaration
-Wimplicit
-Wmain
-Wmissing-braces
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
|
-Wparentheses
Also warn about constructions where there may be confusion to which
if statement an else branch belongs. Here is an example of
such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
|
In C, every else branch belongs to the innermost possible if
statement, which in this example is if (b). This is often not
what the programmer expected, as illustrated in the above example by
indentation the programmer chose. When there is the potential for this
confusion, GCC will issue a warning when this flag is specified.
To eliminate the warning, add explicit braces around the innermost
if statement so there is no way the else could belong to
the enclosing if. The resulting code would look like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
|
-Wsequence-point
The C standard defines the order in which expressions in a C program are
evaluated in terms of sequence points, which represent a partial
ordering between the execution of parts of the program: those executed
before the sequence point, and those executed after it. These occur
after the evaluation of a full expression (one which is not part of a
larger expression), after the evaluation of the first operand of a
&&, ||, ? : or , (comma) operator, before a
function is called (but after the evaluation of its arguments and the
expression denoting the called function), and in certain other places.
Other than as expressed by the sequence point rules, the order of
evaluation of subexpressions of an expression is not specified. All
these rules describe only a partial order rather than a total order,
since, for example, if two functions are called within one expression
with no sequence point between them, the order in which the functions
are called is not specified. However, the standards committee have
ruled that function calls do not overlap.
It is not specified when between sequence points modifications to the values of objects take effect. Programs whose behavior depends on this have undefined behavior; the C standard specifies that "Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored.". If a program breaks these rules, the results on any particular implementation are entirely unpredictable.
Examples of code with undefined behavior are a = a++;, a[n]
= b[n++] and a[i++] = i;. Some more complicated cases are not
diagnosed by this option, and it may give an occasional false positive
result, but in general it has been found fairly effective at detecting
this sort of problem in programs.
The present implementation of this option only works for C programs. A future implementation may also work for C++ programs.
The C standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on our readings page, at http://gcc.gnu.org/readings.html.
-Wreturn-type
int. Also warn about any return statement with no
return-value in a function whose return-type is not void.
For C++, a function without return type always produces a diagnostic message, even when `-Wno-return-type' is specified. The only exceptions are `main' and functions defined in system headers.
-Wswitch
switch statement has an index of enumeral type
and lacks a case for one or more of the named codes of that
enumeration. (The presence of a default label prevents this
warning.) case labels outside the enumeration range also
provoke warnings when this option is used.
-Wswitch-default
switch statement does not have a default
case.
-Wswitch-enum
switch statement has an index of enumeral type
and lacks a case for one or more of the named codes of that
enumeration. case labels outside the enumeration range also
provoke warnings when this option is used.
-Wtrigraphs
-Wunused-function
-Wunused-label
To suppress this warning use the `unused' attribute (see section 5.32 Specifying Attributes of Variables).
-Wunused-parameter
To suppress this warning use the `unused' attribute (see section 5.32 Specifying Attributes of Variables).
-Wunused-variable
To suppress this warning use the `unused' attribute (see section 5.32 Specifying Attributes of Variables).
-Wunused-value
To suppress this warning cast the expression to `void'.
-Wunused
In order to get a warning about an unused function parameter, you must either specify `-W -Wunused' or separately specify `-Wunused-parameter'.
-Wuninitialized
setjmp call.
These warnings are possible only in optimizing compilation, because they require data flow information that is computed only when optimizing. If you don't specify `-O', you simply won't get these warnings.
These warnings occur only for variables that are candidates for
register allocation. Therefore, they do not occur for a variable that
is declared volatile, or whose address is taken, or whose size
is other than 1, 2, 4 or 8 bytes. Also, they do not occur for
structures, unions or arrays, even when they are in registers.
Note that there may be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed.
These warnings are made optional because GCC is not smart enough to see all the reasons why the code might be correct despite appearing to have an error. Here is one example of how this can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
|
If the value of y is always 1, 2 or 3, then x is
always initialized, but GCC doesn't know this. Here is
another common case:
{
int save_y;
if (change_y) save_y = y, y = new_y;
...
if (change_y) y = save_y;
}
|
This has no bug because save_y is used only if it is set.
This option also warns when a non-volatile automatic variable might be
changed by a call to longjmp. These warnings as well are possible
only in optimizing compilation.
The compiler sees only the calls to setjmp. It cannot know
where longjmp will be called; in fact, a signal handler could
call it at any point in the code. As a result, you may get a warning
even when there is in fact no problem because longjmp cannot
in fact be called at the place which would cause a problem.
Some spurious warnings can be avoided if you declare all the functions
you use that never return as noreturn. See section 5.25 Declaring Attributes of Functions.
-Wunknown-pragmas
-Wstrict-aliasing
-Wall
The following `-W...' options are not implied by `-Wall'. Some of them warn about constructions that users generally do not consider questionable, but which occasionally you might wish to check for; others warn about constructions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to suppress the warning.
-W
foo (a)
{
if (a > 0)
return a;
}
|
static are not the first things in
a declaration. According to the C Standard, this usage is obsolescent.
const.
Such a type qualifier has no effect, since the value returned by a
function is not an lvalue. (But don't warn about the GNU extension of
volatile void return types. That extension will be warned about
if `-pedantic' is specified.)
x.h:
struct s { int f, g; };
struct t { struct s h; int i; };
struct t x = { 1, 2, 3 };
|
x.h would be implicitly initialized to zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
|
-Wno-div-by-zero
-Wsystem-headers
-Wfloat-equal
The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers. If you are doing this, then you need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when performing comparisons (and when producing output, but that's a different problem). In particular, instead of testing for equality, you would check to see whether the two values have ranges that overlap; and this is done with the relational operators, so equality comparisons are probably mistaken.
-Wtraditional (C only)
<limits.h>.
Use of these macros in user code might normally lead to spurious
warnings, however gcc's integrated preprocessor has enough context to
avoid warning in these cases.
switch statement has an operand of type long.
static function declaration follows a static one.
This construct is not accepted by some traditional C compilers.
__STDC__ to avoid missing
initializer warnings and relies on default initialization to zero in the
traditional C case.
PARAMS and
VPARAMS. This warning is also bypassed for nested functions
because that feature is already a gcc extension and thus not relevant to
traditional C compatibility.
-Wundef
-Wendif-labels
-Wshadow
-Wlarger-than-len
-Wpointer-arith
void. GNU C assigns these types a size of 1, for
convenience in calculations with void * pointers and pointers
to functions.
-Wbad-function-cast (C only)
int malloc() is cast to anything *.
-Wcast-qual
const char * is cast
to an ordinary char *.
-Wcast-align
char * is cast to
an int * on machines where integers can only be accessed at
two- or four-byte boundaries.
-Wwrite-strings
const
char[length] so that
copying the address of one into a non-const char *
pointer will get a warning; when compiling C++, warn about the
deprecated conversion from string constants to char *.
These warnings will help you find at
compile time code that can try to write into a string constant, but
only if you have been very careful about using const in
declarations and prototypes. Otherwise, it will just be a nuisance;
this is why we did not make `-Wall' request these warnings.
-Wconversion
Also, warn if a negative integer constant expression is implicitly
converted to an unsigned type. For example, warn about the assignment
x = -1 if x is unsigned. But do not warn about explicit
casts like (unsigned) -1.
-Wsign-compare
-Waggregate-return
-Wstrict-prototypes (C only)
-Wmissing-prototypes (C only)
-Wmissing-declarations
-Wmissing-noreturn
noreturn.
Note these are only possible candidates, not absolute ones. Care should
be taken to manually verify functions actually do not ever return before
adding the noreturn attribute, otherwise subtle code generation
bugs could be introduced. You will not get a warning for main in
hosted C environments.
-Wmissing-format-attribute
format attributes. Note these are only possible
candidates, not absolute ones. GCC will guess that format
attributes might be appropriate for any function that calls a function
like vprintf or vscanf, but this might not always be the
case, and some functions for which format attributes are
appropriate may not be detected. This option has no effect unless
`-Wformat' is enabled (possibly by `-Wall').
-Wno-multichar
-Wno-deprecated-declarations
deprecated attribute.
(see section 5.25 Declaring Attributes of Functions, see section 5.32 Specifying Attributes of Variables,
see section 5.33 Specifying Attributes of Types.)
-Wpacked
f.x in struct bar
will be misaligned even though struct bar does not itself
have the packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
|
-Wpadded
-Wredundant-decls
-Wnested-externs (C only)
extern declaration is encountered within a function.
-Wunreachable-code
This option is intended to warn when the compiler detects that at least a whole line of source code will never be executed, because some condition is never satisfied or because it is after a procedure that never returns.
It is possible for this option to produce a warning even though there are circumstances under which part of the affected line can be executed, so care should be taken when removing apparently-unreachable code.
For instance, when a function is inlined, a warning may mean that the line is unreachable in only one inlined copy of the function.
This option is not made part of `-Wall' because in a debugging version of a program there is often substantial code which checks correct functioning of the program and is, hopefully, unreachable because the program does work. Another common use of unreachable code is to provide behavior which is selectable at compile-time.
-Winline
-Wlong-long
-Wdisabled-optimization
-Werror
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GCC has various special options that are used for debugging either your program or GCC:
-g
On most systems that use stabs format, `-g' enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but will probably make other debuggers crash or refuse to read the program. If you want to control for certain whether to generate the extra information, use `-gstabs+', `-gstabs', `-gxcoff+', `-gxcoff', `-gdwarf-1+', `-gdwarf-1', or `-gvms' (see below).
Unlike most other C compilers, GCC allows you to use `-g' with `-O'. The shortcuts taken by optimized code may occasionally produce surprising results: some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values were already at hand; some statements may execute in different places because they were moved out of loops.
Nevertheless it proves possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs.
The following options are useful when GCC is generated with the capability for more than one debugging format.
-ggdb
-gstabs
-gstabs+
-gcoff
-gxcoff
-gxcoff+
-gdwarf
This option is deprecated.
-gdwarf+
This option is deprecated.
-gdwarf-2
-gvms
-glevel
-ggdblevel
-gstabslevel
-gcofflevel
-gxcofflevel
-gvmslevel
Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, but no information about local variables and no line numbers.
Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use `-g3'.
Note that in order to avoid confusion between DWARF1 debug level 2, and DWARF2, neither `-gdwarf' nor `-gdwarf-2' accept a concatenated debug level. Instead use an additional `-glevel' option to change the debug level for DWARF1 or DWARF2.
-feliminate-dwarf2-dups
-p
prof. You must use this option when compiling
the source files you want data about, and you must also use it when
linking.
-Q
-ftime-report
-fmem-report
-fprofile-arcs
For profile-directed block ordering, compile the program with `-fprofile-arcs' plus optimization and code generation options, generate the arc profile information by running the program on a selected workload, and then compile the program again with the same optimization and code generation options plus `-fbranch-probabilities' (see section Options that Control Optimization).
The other use of `-fprofile-arcs' is for use with gcov,
when it is used with the `-ftest-coverage' option.
With `-fprofile-arcs', for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the number of times that these arcs are executed. When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation code.
-dletters
ADDRESSOF codes, to `file.10.addressof'.
-fdump-unnumbered
-fdump-translation-unit (C and C++ only)
-fdump-translation-unit-options (C and C++ only)
-fdump-class-hierarchy (C++ only)
-fdump-class-hierarchy-options (C++ only)
-fdump-tree-switch (C++ only)
-fdump-tree-switch-options (C++ only)
The following tree dumps are possible:
-fsched-verbose=n
For n greater than zero, `-fsched-verbose' outputs the same information as `-dRS'. For n greater than one, it also output basic block probabilities, detailed ready list information and unit/insn info. For n greater than two, it includes RTL at abort point, control-flow and regions info. And for n over four, `-fsched-verbose' also includes dependence info.
-save-temps
-time
# cc1 0.12 0.01 # as 0.00 0.01 |
The first number on each line is the "user time," that is time spent executing the program itself. The second number is "system time," time spent executing operating system routines on behalf of the program. Both numbers are in seconds.
-print-file-name=library
-print-multi-directory
GCC_EXEC_PREFIX.
-print-multi-lib
-print-prog-name=program
-print-libgcc-file-name
This is useful when you use `-nostdlib' or `-nodefaultlibs' but you do want to link with `libgcc.a'. You can do
gcc -nostdlib files... `gcc -print-libgcc-file-name` |
-print-search-dirs
This is useful when gcc prints the error message
`installation problem, cannot exec cpp0: No such file or directory'.
To resolve this you either need to put `cpp0' and the other compiler
components where gcc expects to find them, or you can set the environment
variable GCC_EXEC_PREFIX to the directory where you installed them.
Don't forget the trailing '/'.
See section 3.19 Environment Variables Affecting GCC.
-dumpmachine
-dumpversion
-dumpspecs
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These options control various sorts of optimizations.
Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you would expect from the source code.
Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program.
Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed.
-O
-O1
With `-O', the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time.
`-O' turns on the following optimization flags:
{-fdefer-pop
|
`-O' also turns on `-fomit-frame-pointer' on machines where doing so does not interfere with debugging.
-O2
`-O2' turns on all optimization flags specified by `-O'. It also turns on the following optimization flags:
{-fforce-mem
|
Please note the warning under `-fgcse' about invoking `-O2' on programs that use computed gotos.
-O3
-O0
-Os
`-Os' disables the following optimization flags:
{-falign-functions -falign-jumps -falign-loops
|
If you use multiple `-O' options, with or without level numbers, the last such option is the one that is effective.
Options of the form `-fflag' specify machine-independent flags. Most flags have both positive and negative forms; the negative form of `-ffoo' would be `-fno-foo'. In the table below, only one of the forms is listed--the one you typically will use. You can figure out the other form by either removing `no-' or adding it.
The following options control specific optimizations. They are either activated by `-O' options or are related to ones that are. You can use the following flags in the rare cases when "fine-tuning" of optimizations to be performed is desired.
-fno-default-inline
-fno-defer-pop
Disabled at levels `-O', `-O2', `-O3', `-Os'.
-fforce-mem
Enabled at levels `-O2', `-O3', `-Os'.
-fforce-addr
-fomit-frame-pointer
On some machines, such as the VAX, this flag has no effect, because
the standard calling sequence automatically handles the frame pointer
and nothing is saved by pretending it doesn't exist. The
machine-description macro FRAME_POINTER_REQUIRED controls
whether a target machine supports this flag. See section `Register Usage' in GNU Compiler Collection (GCC) Internals.
Enabled at levels `-O', `-O2', `-O3', `-Os'.
-foptimize-sibling-calls
Enabled at levels `-O2', `-O3', `-Os'.
-fno-inline
inline keyword. Normally this option
is used to keep the compiler from expanding any functions inline.
Note that if you are not optimizing, no functions can be expanded inline.
-finline-functions
If all calls to a given function are integrated, and the function is
declared static, then the function is normally not output as
assembler code in its own right.
Enabled at level `-O3'.
-finline-limit=n
Inlining is actually controlled by a number of parameters, which may be specified individually by using `--param name=value'. The `-finline-limit=n' option sets some of these parameters as follows:
max-inline-insns
max-inline-insns-single
max-inline-insns-single-auto
min-inline-insns
max-inline-insns-rtl
Using `-finline-limit=600' thus results in the default settings for these parameters. See below for a documentation of the individual parameters controlling inlining.
Note: pseudo instruction represents, in this particular context, an abstract measurement of function's size. In no way, it represents a count of assembly instructions and as such its exact meaning might change from one release to an another.
-fkeep-inline-functions
static, nevertheless output a separate run-time
callable version of the function. This switch does not affect
extern inline functions.
-fkeep-static-consts
static const when optimization isn't turned
on, even if the variables aren't referenced.
GCC enables this option by default. If you want to force the compiler to check if the variable was referenced, regardless of whether or not optimization is turned on, use the `-fno-keep-static-consts' option.
-fmerge-constants
This option is the default for optimized compilation if the assembler and linker support it. Use `-fno-merge-constants' to inhibit this behavior.
Enabled at levels `-O', `-O2', `-O3', `-Os'.
-fmerge-all-constants
This option implies `-fmerge-constants'. In addition to `-fmerge-constants' this considers e.g. even constant initialized arrays or initialized constant variables with integral or floating point types. Languages like C or C++ require each non-automatic variable to have distinct location, so using this option will result in non-conforming behavior.
-fno-branch-count-reg
The default is `-fbranch-count-reg', enabled when `-fstrength-reduce' is enabled.
-fno-function-cse
This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the optimizations performed when this option is not used.
The default is `-ffunction-cse'
-fno-zero-initialized-in-bss
This option turns off this behavior because some programs explicitly rely on variables going to the data section. E.g., so that the resulting executable can find the beginning of that section and/or make assumptions based on that.
The default is `-fzero-initialized-in-bss'.
-fstrength-reduce
Enabled at levels `-O2', `-O3', `-Os'.
-fthread-jumps
Enabled at levels `-O', `-O2', `-O3', `-Os'.
-fcse-follow-jumps
if statement with an
else clause, CSE will follow the jump when the condition
tested is false.
Enabled at levels `-O2', `-O3', `-Os'.
-fcse-skip-blocks
if statement with no else clause,
`-fcse-skip-blocks' causes CSE to follow the jump around the
body of the if.
Enabled at levels `-O2', `-O3', `-Os'.
-frerun-cse-after-loop
Enabled at levels `-O2', `-O3', `-Os'.
-frerun-loop-opt
Enabled at levels `-O2', `-O3', `-Os'.
-fgcse
Note: When compiling a program using computed gotos, a GCC extension, you may get better runtime performance if you disable the global common subexpression elimination pass by adding `-fno-gcse' to the command line.
Enabled at levels `-O2', `-O3', `-Os'.
-fgcse-lm
Enabled by default when gcse is enabled.
-fgcse-sm
Enabled by default when gcse is enabled.
-floop-optimize
Enabled at levels `-O', `-O2', `-O3', `-Os'.
-fcrossjumping
Enabled at levels `-O', `-O2', `-O3', `-Os'.
-fif-conversion
if-conversion2.
Enabled at levels `-O', `-O2', `-O3', `-Os'.
-fif-conversion2
Enabled at levels `-O', `-O2', `-O3', `-Os'.
-fdelete-null-pointer-checks
In some environments, this assumption is not true, and programs can safely dereference null pointers. Use `-fno-delete-null-pointer-checks' to disable this optimization for programs which depend on that behavior.
Enabled at levels `-O2', `-O3', `-Os'.
-fexpensive-optimizations
Enabled at levels `-O2', `-O3', `-Os'.
-foptimize-register-move
-fregmove
Note `-fregmove' and `-foptimize-register-move' are the same optimization.
Enabled at levels `-O2', `-O3', `-Os'.
-fdelayed-branch
Enabled at levels `-O', `-O2', `-O3', `-Os'.
-fschedule-insns
Enabled at levels `-O2', `-O3', `-Os'.
-fschedule-insns2
Enabled at levels `-O2', `-O3', `-Os'.
-fno-sched-interblock
-fno-sched-spec
-fsched-spec-load
-fsched-spec-load-dangerous
-fcaller-saves
This option is always enabled by default on certain machines, usually those which have no call-preserved registers to use instead.
Enabled at levels `-O2', `-O3', `-Os'.
-fmove-all-movables
-freduce-all-givs
Note: When compiling programs written in Fortran, `-fmove-all-movables' and `-freduce-all-givs' are enabled by default when you use the optimizer.
These options may generate better or worse code; results are highly dependent on the structure of loops within the source code.
These two options are intended to be removed someday, once they have helped determine the efficacy of various approaches to improving loop optimizations.
Please let us (gcc@gcc.gnu.org and fortran@gnu.org) know how use of these options affects the performance of your production code. We're very interested in code that runs slower when these options are enabled.
-fno-peephole
-fno-peephole2
`-fpeephole' is enabled by default. `-fpeephole2' enabled at levels `-O2', `-O3', `-Os'.
-fbranch-probabilities
-fno-guess-branch-probability
Sometimes gcc will opt to use a randomized model to guess branch probabilities, when none are available from either profiling feedback (`-fprofile-arcs') or `__builtin_expect'. This means that different runs of the compiler on the same program may produce different object code.
In a hard real-time system, people don't want different runs of the compiler to produce code that has different behavior; minimizing non-determinism is of paramount import. This switch allows users to reduce non-determinism, possibly at the expense of inferior optimization.
The default is `-fguess-branch-probability' at levels `-O', `-O2', `-O3', `-Os'.
-freorder-blocks
Enabled at levels `-O2', `-O3', `-Os'.
-freorder-functions
text.hot for most frequently executed functions and
text.unlikely for unlikely executed functions. Reordering is done by
the linker so object file format must support named sections and linker must
place them in a reasonable way.
Also profile feedback must be available in to make this option effective. See `-fprofile-arcs' for details.
Enabled at levels `-O2', `-O3', `-Os'.
-fstrict-aliasing
unsigned int can alias an int, but not a
void* or a double. A character type may alias any other
type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
a_union t;
t.d = 3.0;
return t.i;
}
|
int f() {
a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
|
Every language that wishes to perform language-specific alias analysis
should define a function that computes, given an tree
node, an alias set for the node. Nodes in different alias sets are not
allowed to alias. For an example, see the C front-end function
c_get_alias_set.
Enabled at levels `-O2', `-O3', `-Os'.
-falign-functions
-falign-functions=n
`-fno-align-functions' and `-falign-functions=1' are equivalent and mean that functions will not be aligned.
Some assemblers only support this flag when n is a power of two; in that case, it is rounded up.
If n is not specified, use a machine-dependent default.
Enabled at levels `-O2', `-O3'.
-falign-labels
-falign-labels=n
If `-falign-loops' or `-falign-jumps' are applicable and are greater than this value, then their values are used instead.
If n is not specified, use a machine-dependent default which is very likely to be `1', meaning no alignment.
Enabled at levels `-O2', `-O3'.
-falign-loops
-falign-loops=n
If n is not specified, use a machine-dependent default.
Enabled at levels `-O2', `-O3'.
-falign-jumps
-falign-jumps=n
If n is not specified, use a machine-dependent default.
Enabled at levels `-O2', `-O3'.
-frename-registers
Enabled at levels `-O3'.
-fno-cprop-registers
Disabled at levels `-O', `-O2', `-O3', `-Os'.
The following options control compiler behavior regarding floating point arithmetic. These options trade off between speed and correctness. All must be specifically enabled.
-ffloat-store
This option prevents undesirable excess precision on machines such as
the 68000 where the floating registers (of the 68881) keep more
precision than a double is supposed to have. Similarly for the
x86 architecture. For most programs, the excess precision does only
good, but a few programs rely on the precise definition of IEEE floating
point. Use `-ffloat-store' for such programs, after modifying
them to store all pertinent intermediate computations into variables.
-ffast-math
This option causes the preprocessor macro __FAST_MATH__ to be defined.
This option should never be turned on by any `-O' option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
-fno-math-errno
This option should never be turned on by any `-O' option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
The default is `-fmath-errno'.
-funsafe-math-optimizations
This option should never be turned on by any `-O' option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
The default is `-fno-unsafe-math-optimizations'.
-ffinite-math-only
This option should never be turned on by any `-O' option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications.
The default is `-fno-finite-math-only'.
-fno-trapping-math
This option should never be turned on by any `-O' option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
The default is `-ftrapping-math'.
-fsignaling-nans
This option causes the preprocessor macro __SUPPORT_SNAN__ to
be defined.
The default is `-fno-signaling-nans'.
This option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior.
-fsingle-precision-constant
The following options control optimizations that may improve performance, but are not enabled by any `-O' options. This section includes experimental options that may produce broken code.
-fbranch-probabilities
gcc), you can compile it a second time using
`-fbranch-probabilities', to improve optimizations based on
the number of times each branch was taken. When the program
compiled with `-fprofile-arcs' exits it saves arc execution
counts to a file called `sourcename.da' for each source
file The information in this data file is very dependent on the
structure of the generated code, so you must use the same source code
and the same optimization options for both compilations.
With `-fbranch-probabilities', GCC puts a `REG_BR_PROB' note on each `JUMP_INSN' and `CALL_INSN'. These can be used to improve optimization. Currently, they are only used in one place: in `reorg.c', instead of guessing which path a branch is mostly to take, the `REG_BR_PROB' values are used to exactly determine which path is taken more often.
-fnew-ra
-ftracer
-funroll-loops
-funroll-all-loops
-fprefetch-loop-arrays
Disabled at level `-Os'.
-ffunction-sections
-fdata-sections
Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. HPPA processors running HP-UX and SPARC processors running Solaris 2 have linkers with such optimizations. Other systems using the ELF object format as well as AIX may have these optimizations in the future.
Only use these options when there are significant benefits from doing
so. When you specify these options, the assembler and linker will
create larger object and executable files and will also be slower.
You will not be able to use gprof on all systems if you
specify this option and you may have problems with debugging if
you specify both this option and `-g'.
-fssa
-fssa-ccp
-fssa-dce
--param name=value
In each case, the value is an integer. The allowable choices for name are given in the following table:
max-crossjump-edges
max-delay-slot-insn-search
max-delay-slot-live-search
max-gcse-memory
max-gcse-passes
max-pending-list-length
max-inline-insns-single
max-inline-insns-auto
max-inline-insns
max-inline-slope
min-inline-insns
max-inline-insns-rtl
max-unrolled-insns
hot-bb-count-fraction
hot-bb-frequency-fraction
tracer-dynamic-coverage
tracer-dynamic-coverage-feedback
This value is used to limit superblock formation once the given percentage of executed instructions is covered. This limits unnecessary code size expansion.
The `tracer-dynamic-coverage-feedback' is used only when profile feedback is available. The real profiles (as opposed to statically estimated ones) are much less balanced allowing the threshold to be larger value.
tracer-max-code-growth
tracer-min-branch-ratio
Stop reverse growth when the reverse probability of best edge is less than this threshold (in percent).
tracer-min-branch-ratio
tracer-min-branch-ratio-feedback
Stop forward growth if the best edge do have probability lower than this threshold.
Similarly to `tracer-dynamic-coverage' two values are present, one for compilation for profile feedback and one for compilation without. The value for compilation with profile feedback needs to be more conservative (higher) in order to make tracer effective.
ggc-min-expand
GCC uses a garbage collector to manage its own memory allocation. This parameter specifies the minimum percentage by which the garbage collector's heap should be allowed to expand between collections. Tuning this may improve compilation speed; it has no effect on code generation.
The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when
RAM >= 1GB. If getrlimit is available, the notion of "RAM" is
the smallest of actual RAM, RLIMIT_RSS, RLIMIT_DATA and RLIMIT_AS. If
GCC is not able to calculate RAM on a particular platform, the lower
bound of 30% is used. Setting this parameter and
`ggc-min-heapsize' to zero causes a full collection to occur at
every opportunity. This is extremely slow, but can be useful for
debugging.
ggc-min-heapsize
Minimum size of the garbage collector's heap before it begins bothering to collect garbage. The first collection occurs after the heap expands by `ggc-min-expand'% beyond `ggc-min-heapsize'. Again, tuning this may improve compilation speed, and has no effect on code generation.
The default is RAM/8, with a lower bound of 4096 (four megabytes) and an
upper bound of 131072 (128 megabytes). If getrlimit is
available, the notion of "RAM" is the smallest of actual RAM,
RLIMIT_RSS, RLIMIT_DATA and RLIMIT_AS. If GCC is not able to calculate
RAM on a particular platform, the lower bound is used. Setting this
parameter very large effectively disables garbage collection. Setting
this parameter and `ggc-min-expand' to zero causes a full
collection to occur at every opportunity.
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These options control the C preprocessor, which is run on each C source file before actual compilation.
If you use the `-E' option, nothing is done except preprocessing. Some of these options make sense only together with `-E' because they cause the preprocessor output to be unsuitable for actual compilation.
You can use `-Wp,option' to bypass the compiler driver and pass option directly through to the preprocessor. If option contains commas, it is split into multiple options at the commas. However, many options are modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and `-Wp' forcibly bypasses this phase. The preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using `-Wp' and let the driver handle the options instead.
-D name
1.
-D name=definition
If you wish to define a function-like macro on the command line, write
its argument list with surrounding parentheses before the equals sign
(if any). Parentheses are meaningful to most shells, so you will need
to quote the option. With sh and csh,
`-D'name(args...)=definition'' works.
`-D' and `-U' options are processed in the order they are given on the command line. All `-imacros file' and `-include file' options are processed after all `-D' and `-U' options.
-U name
-undef
-I dir
-o file
cpp. gcc has a
different interpretation of a second non-option argument, so you must
use `-o' to specify the output file.
-Wall
-Wcomment
-Wcomments
-Wtrigraphs
-Wtraditional
-Wimport
-Wundef
-Wunused-macros
Built-in macros, macros defined on the command line, and macros defined in include files are not warned about.
Note: If a macro is actually used, but only used in skipped conditional blocks, then CPP will report it as unused. To avoid the warning in such a case, you might improve the scope of the macro's definition by, for example, moving it into the first skipped block. Alternatively, you could provide a dummy use with something like:
#if defined the_macro_causing_the_warning #endif |
-Wendif-labels
#if FOO ... #else FOO ... #endif FOO |
The second and third FOO should be in comments, but often are not
in older programs. This warning is on by default.
-Werror
-Wsystem-headers
-w
-pedantic
-pedantic-errors
-M
make describing the dependencies of the main
source file. The preprocessor outputs one make rule containing
the object file name for that source file, a colon, and the names of all
the included files, including those coming from `-include' or
`-imacros' command line options.
Unless specified explicitly (with `-MT' or `-MQ'), the object file name consists of the basename of the source file with any suffix replaced with object file suffix. If there are many included files then the rule is split into several lines using `\'-newline. The rule has no commands.
This option does not suppress the preprocessor's debug output, such as
`-dM'. To avoid mixing such debug output with the dependency
rules you should explicitly specify the dependency output file with
`-MF', or use an environment variable like
DEPENDENCIES_OUTPUT (see section 3.19 Environment Variables Affecting GCC). Debug output
will still be sent to the regular output stream as normal.
Passing `-M' to the driver implies `-E', and suppresses warnings with an implicit `-w'.
-MM
This implies that the choice of angle brackets or double quotes in an `#include' directive does not in itself determine whether that header will appear in `-MM' dependency output. This is a slight change in semantics from GCC versions 3.0 and earlier.
-MF file
When used with the driver options `-MD' or `-MMD', `-MF' overrides the default dependency output file.
-MG
#include directive without prepending any path. `-MG'
also suppresses preprocessed output, as a missing header file renders
this useless.
This feature is used in automatic updating of makefiles.
-MP
make gives if you remove header
files without updating the `Makefile' to match.
This is typical output:
test.o: test.c test.h test.h: |
-MT target
Change the target of the rule emitted by dependency generation. By default CPP takes the name of the main input file, including any path, deletes any file suffix such as `.c', and appends the platform's usual object suffix. The result is the target.
An `-MT' option will set the target to be exactly the string you specify. If you want multiple targets, you can specify them as a single argument to `-MT', or use multiple `-MT' options.
For example, `-MT '$(objpfx)foo.o'' might give
$(objpfx)foo.o: foo.c |
-MQ target
Same as `-MT', but it quotes any characters which are special to Make. `-MQ '$(objpfx)foo.o'' gives
$$(objpfx)foo.o: foo.c |
The default target is automatically quoted, as if it were given with `-MQ'.
-MD
If `-MD' is used in conjunction with `-E', any `-o' switch is understood to specify the dependency output file (but see -MF), but if used without `-E', each `-o' is understood to specify a target object file.
Since `-E' is not implied, `-MD' can be used to generate a dependency output file as a side-effect of the compilation process.
-MMD
-x c
-x c++
-x objective-c
-x assembler-with-cpp
Note: Previous versions of cpp accepted a `-lang' option which selected both the language and the standards conformance level. This option has been removed, because it conflicts with the `-l' option.
-std=standard
-ansi
standard may be one of:
iso9899:1990
c89
The `-ansi' option is equivalent to `-std=c89'.
iso9899:199409
iso9899:1999
c99
iso9899:199x
c9x
gnu89
gnu99
gnu9x
c++98
gnu++98
-I-
#include "file"; they are not searched for
#include <file>. If additional directories are
specified with `-I' options after the `-I-', those
directories are searched for all `#include' directives.
In addition, `-I-' inhibits the use of the directory of the current
file directory as the first search directory for #include
"file".
-nostdinc
-nostdinc++
-include file
#include "file" appeared as the first
line of the primary source file. However, the first directory searched
for file is the preprocessor's working directory instead of
the directory containing the main source file. If not found there, it
is searched for in the remainder of the #include "..." search
chain as normal.
If multiple `-include' options are given, the files are included in the order they appear on the command line.
-imacros file
All files specified by `-imacros' are processed before all files specified by `-include'.
-idirafter dir
-iprefix prefix
-iwithprefix dir
-iwithprefixbefore dir
Use of these options is discouraged.
-isystem dir
-fpreprocessed
`-fpreprocessed' is implicit if the input file has one of the extensions `.i', `.ii' or `.mi'. These are the extensions that GCC uses for preprocessed files created by `-save-temps'.
-ftabstop=width
-fno-show-column
dejagnu.
-A predicate=answer
-A -predicate=answer
-dCHARS
touch foo.h; cpp -dM foo.h |
will show all the predefined macros.
-P
-C
You should be prepared for side effects when using `-C'; it causes the preprocessor to treat comments as tokens in their own right. For example, comments appearing at the start of what would be a directive line have the effect of turning that line into an ordinary source line, since the first token on the line is no longer a `#'.
-CC
In addition to the side-effects of the `-C' option, the `-CC' option causes all C++-style comments inside a macro to be converted to C-style comments. This is to prevent later use of that macro from inadvertently commenting out the remainder of the source line.
The `-CC' option is generally used to support lint comments.
-gcc
gcc -E; you can turn them off in that case with
`-no-gcc'.
-traditional-cpp
-trigraphs
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~
|
-remap
--help
--target-help
-v
-H
-version
--version
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You can pass options to the assembler.
-Wa,option
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These options come into play when the compiler links object files into an executable output file. They are meaningless if the compiler is not doing a link step.
object-file-name
-c
-S
-E
-llibrary
-l library
It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, `foo.o -lz bar.o' searches library `z' after file `foo.o' but before `bar.o'. If `bar.o' refers to functions in `z', those functions may not be loaded.
The linker searches a standard list of directories for the library, which is actually a file named `liblibrary.a'. The linker then uses this file as if it had been specified precisely by name.
The directories searched include several standard system directories plus any that you specify with `-L'.
Normally the files found this way are library files--archive files whose members are object files. The linker handles an archive file by scanning through it for members which define symbols that have so far been referenced but not defined. But if the file that is found is an ordinary object file, it is linked in the usual fashion. The only difference between using an `-l' option and specifying a file name is that `-l' surrounds library with `lib' and `.a' and searches several directories.
-lobjc
-nostartfiles
-nodefaultlibs
-nostdlib
One of the standard libraries bypassed by `-nostdlib' and
`-nodefaultlibs' is `libgcc.a', a library of internal subroutines
that GCC uses to overcome shortcomings of particular machines, or special
needs for some languages.
(See section `Interfacing to GCC Output' in GNU Compiler Collection (GCC) Internals,
for more discussion of `libgcc.a'.)
In most cases, you need `libgcc.a' even when you want to avoid
other standard libraries. In other words, when you specify `-nostdlib'
or `-nodefaultlibs' you should usually specify `-lgcc' as well.
This ensures that you have no unresolved references to internal GCC
library subroutines. (For example, `__main', used to ensure C++
constructors will be called; see section `collect2' in GNU Compiler Collection (GCC) Internals.)
-s
-static
-shared
-shared-libgcc
-static-libgcc
There are several situations in which an application should use the shared `libgcc' instead of the static version. The most common of these is when the application wishes to throw and catch exceptions across different shared libraries. In that case, each of the libraries as well as the application itself should use the shared `libgcc'.
Therefore, the G++ and GCJ drivers automatically add `-shared-libgcc' whenever you build a shared library or a main executable, because C++ and Java programs typically use exceptions, so this is the right thing to do.
If, instead, you use the GCC driver to create shared libraries, you may find that they will not always be linked with the shared `libgcc'. If GCC finds, at its configuration time, that you have a GNU linker that does not support option `--eh-frame-hdr', it will link the shared version of `libgcc' into shared libraries by default. Otherwise, it will take advantage of the linker and optimize away the linking with the shared version of `libgcc', linking with the static version of libgcc by default. This allows exceptions to propagate through such shared libraries, without incurring relocation costs at library load time.
However, if a library or main executable is supposed to throw or catch exceptions, you must link it using the G++ or GCJ driver, as appropriate for the languages used in the program, or using the option `-shared-libgcc', such that it is linked with the shared `libgcc'.
-symbolic
-Xlinker option
If you want to pass an option that takes an argument, you must use `-Xlinker' twice, once for the option and once for the argument. For example, to pass `-assert definitions', you must write `-Xlinker -assert -Xlinker definitions'. It does not work to write `-Xlinker "-assert definitions"', because this passes the entire string as a single argument, which is not what the linker expects.
-Wl,option
-u symbol
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These options specify directories to search for header files, for libraries and for parts of the compiler:
-Idir
If a standard system include directory, or a directory specified with `-isystem', is also specified with `-I', the `-I' option will be ignored. The directory will still be searched but as a system directory at its normal position in the system include chain. This is to ensure that GCC's procedure to fix buggy system headers and the ordering for the include_next directive are not inadvertently changed. If you really need to change the search order for system directories, use the `-nostdinc' and/or `-isystem' options.
-I-
If additional directories are specified with `-I' options after the `-I-', these directories are searched for all `#include' directives. (Ordinarily all `-I' directories are used this way.)
In addition, the `-I-' option inhibits the use of the current directory (where the current input file came from) as the first search directory for `#include "file"'. There is no way to override this effect of `-I-'. With `-I.' you can specify searching the directory which was current when the compiler was invoked. That is not exactly the same as what the preprocessor does by default, but it is often satisfactory.
`-I-' does not inhibit the use of the standard system directories for header files. Thus, `-I-' and `-nostdinc' are independent.
-Ldir
-Bprefix
The compiler driver program runs one or more of the subprograms `cpp', `cc1', `as' and `ld'. It tries prefix as a prefix for each program it tries to run, both with and without `machine/version/' (see section 3.16 Specifying Target Machine and Compiler Version).
For each subprogram to be run, the compiler driver first tries the
`-B' prefix, if any. If that name is not found, or if `-B'
was not specified, the driver tries two standard prefixes, which are
`/usr/lib/gcc/' and `/usr/local/lib/gcc-lib/'. If neither of
those results in a file name that is found, the unmodified program
name is searched for using the directories specified in your
PATH environment variable.
The compiler will check to see if the path provided by the `-B' refers to a directory, and if necessary it will add a directory separator character at the end of the path.
`-B' prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into `-L' options for the linker. They also apply to includes files in the preprocessor, because the compiler translates these options into `-isystem' options for the preprocessor. In this case, the compiler appends `include' to the prefix.
The run-time support file `libgcc.a' can also be searched for using the `-B' prefix, if needed. If it is not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those means.
Another way to specify a prefix much like the `-B' prefix is to use
the environment variable GCC_EXEC_PREFIX. See section 3.19 Environment Variables Affecting GCC.
As a special kludge, if the path provided by `-B' is `[dir/]stageN/', where N is a number in the range 0 to 9, then it will be replaced by `[dir/]include'. This is to help with boot-strapping the compiler.
-specs=file
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gcc is a driver program. It performs its job by invoking a
sequence of other programs to do the work of compiling, assembling and
linking. GCC interprets its command-line parameters and uses these to
deduce which programs it should invoke, and which command-line options
it ought to place on their command lines. This behavior is controlled
by spec strings. In most cases there is one spec string for each
program that GCC can invoke, but a few programs have multiple spec
strings to control their behavior. The spec strings built into GCC can
be overridden by using the `-specs=' command-line switch to specify
a spec file.
Spec files are plaintext files that are used to construct spec strings. They consist of a sequence of directives separated by blank lines. The type of directive is determined by the first non-whitespace character on the line and it can be one of the following:
%command
%include <file>
%include_noerr <file>
%rename old_name new_name
*[spec_name]:
[suffix]:
.ZZ: z-compile -input %i |
This says that any input file whose name ends in `.ZZ' should be passed to the program `z-compile', which should be invoked with the command-line switch `-input' and with the result of performing the `%i' substitution. (See below.)
As an alternative to providing a spec string, the text that follows a suffix directive can be one of the following:
@language
.ZZ: @c++ |
Says that .ZZ files are, in fact, C++ source files.
#name
name compiler not installed on this system. |
GCC already has an extensive list of suffixes built into it. This directive will add an entry to the end of the list of suffixes, but since the list is searched from the end backwards, it is effectively possible to override earlier entries using this technique.
GCC has the following spec strings built into it. Spec files can override these strings or create their own. Note that individual targets can also add their own spec strings to this list.
asm Options to pass to the assembler
asm_final Options to pass to the assembler post-processor
cpp Options to pass to the C preprocessor
cc1 Options to pass to the C compiler
cc1plus Options to pass to the C++ compiler
endfile Object files to include at the end of the link
link Options to pass to the linker
lib Libraries to include on the command line to the linker
libgcc Decides which GCC support library to pass to the linker
linker Sets the name of the linker
predefines Defines to be passed to the C preprocessor
signed_char Defines to pass to CPP to say whether |
Here is a small example of a spec file:
%rename lib old_lib *lib: --start-group -lgcc -lc -leval1 --end-group %(old_lib) |
This example renames the spec called `lib' to `old_lib' and then overrides the previous definition of `lib' with a new one. The new definition adds in some extra command-line options before including the text of the old definition.
Spec strings are a list of command-line options to be passed to their corresponding program. In addition, the spec strings can contain `%'-prefixed sequences to substitute variable text or to conditionally insert text into the command line. Using these constructs it is possible to generate quite complex command lines.
Here is a table of all defined `%'-sequences for spec strings. Note that spaces are not generated automatically around the results of expanding these sequences. Therefore you can concatenate them together or combine them with constant text in a single argument.
%%
%i
%b
%B
%d
%gsuffix
%usuffix
%Usuffix
%jSUFFIX
HOST_BIT_BUCKET, if any, and if it is
writable, and if save-temps is off; otherwise, substitute the name
of a temporary file, just like `%u'. This temporary file is not
meant for communication between processes, but rather as a junk
disposal mechanism.
%.SUFFIX
%w
%o
%O
%p
cpp.
%P
%I
GCC_EXEC_PREFIX.
%s
%estr
%|
%(name)
%[name]
%x{option}
%X
%Y
%Z
%v1
%v2
%v3
%a
asm spec. This is used to compute the
switches to be passed to the assembler.
%A
asm_final spec. This is a spec string for
passing switches to an assembler post-processor, if such a program is
needed.
%l
link spec. This is the spec for computing the
command line passed to the linker. Typically it will make use of the
`%L %G %S %D and %E' sequences.
%D
%M
%L
lib spec. This is a spec string for deciding which
libraries should be included on the command line to the linker.
%G
libgcc spec. This is a spec string for deciding
which GCC support library should be included on the command line to the linker.
%S
startfile spec. This is a spec for deciding which
object files should be the first ones passed to the linker. Typically
this might be a file named `crt0.o'.
%E
endfile spec. This is a spec string that specifies
the last object files that will be passed to the linker.
%C
cpp spec. This is used to construct the arguments
to be passed to the C preprocessor.
%c
signed_char spec. This is intended to be used
to tell cpp whether a char is signed. It typically has the definition:
%{funsigned-char:-D__CHAR_UNSIGNED__}
|
%1
cc1 spec. This is used to construct the options to be
passed to the actual C compiler (`cc1').
%2
cc1plus spec. This is used to construct the options to be
passed to the actual C++ compiler (`cc1plus').
%*
%:function(args)
The following built-in spec functions are provided:
if-existsif-exists spec function takes one argument, an absolute
pathname to a file. If the file exists, if-exists returns the
pathname. Here is a small example of its usage:
*startfile: crt0%O%s %:if-exists(crti%O%s) crtbegin%O%s |
if-exists-elseif-exists-else spec function is similar to the if-exists
spec function, except that it takes two arguments. The first argument is
an absolute pathname to a file. If the file exists, if-exists-else
returns the pathname. If it does not exist, it returns the second argument.
This way, if-exists-else can be used to select one file or another,
based on the existence of the first. Here is a small example of its usage:
*startfile: crt0%O%s %:if-exists(crti%O%s) %:if-exists-else(crtbeginT%O%s crtbegin%O%s) |
%{S}
-S switch, if that switch was given to GCC.
If that switch was not specified, this substitutes nothing. Note that
the leading dash is omitted when specifying this option, and it is
automatically inserted if the substitution is performed. Thus the spec
string `%{foo}' would match the command-line option `-foo'
and would output the command line option `-foo'.
%W{S}
S} but mark last argument supplied within as a file to be
deleted on failure.
%{S*}
-S, but which also take an argument. This is used for
switches like `-o', `-D', `-I', etc.
GCC considers `-o foo' as being
one switch whose names starts with `o'. %{o*} would substitute this
text, including the space. Thus two arguments would be generated.
%{^S*}
S*}, but don't put a blank between a switch and its
argument. Thus %{^o*} would only generate one argument, not two.
%{S*&T*}
S*}, but preserve order of S and T options
(the order of S and T in the spec is not significant).
There can be any number of ampersand-separated variables; for each the
wild card is optional. Useful for CPP as `%{D*&U*&A*}'.
%{<S}
-S from the command line. Note--this
command is position dependent. `%' commands in the spec string
before this option will see -S, `%' commands in the spec
string after this option will not.
%{S*:X}
X if one or more switches whose names start with
-S are specified to GCC. Note that the tail part of the
-S option (i.e. the part matched by the `*') will be substituted
for each occurrence of `%*' within X.
%{S:X}
X, but only if the `-S' switch was given to GCC.
%{!S:X}
X, but only if the `-S' switch was not given to GCC.
%{|S:X}
S:X}, but if no S switch, substitute `-'.
%{|!S:X}
S:X}, but if there is an S switch, substitute `-'.
%{.S:X}
X, but only if processing a file with suffix S.
%{!.S:X}
X, but only if not processing a file with suffix S.
%{S|P:X}
X if either -S or -P was given to GCC. This may be
combined with `!' and `.' sequences as well, although they
have a stronger binding than the `|'. For example a spec string
like this:
%{.c:-foo} %{!.c:-bar} %{.c|d:-baz} %{!.c|d:-boggle}
|
will output the following command-line options from the following input command-line options:
fred.c -foo -baz jim.d -bar -boggle -d fred.c -foo -baz -boggle -d jim.d -bar -baz -boggle |
The conditional text X in a %{S:X} or
%{!S:X} construct may contain other nested `%' constructs
or spaces, or even newlines. They are processed as usual, as described
above.
The `-O', `-f', `-m', and `-W'
switches are handled specifically in these
constructs. If another value of `-O' or the negated form of a `-f', `-m', or
`-W' switch is found later in the command line, the earlier switch
value is ignored, except with {S*} where S is just one
letter, which passes all matching options.
The character `|' at the beginning of the predicate text is used to indicate that a command should be piped to the following command, but only if `-pipe' is specified.
It is built into GCC which switches take arguments and which do not. (You might think it would be useful to generalize this to allow each compiler's spec to say which switches take arguments. But this cannot be done in a consistent fashion. GCC cannot even decide which input files have been specified without knowing which switches take arguments, and it must know which input files to compile in order to tell which compilers to run).
GCC also knows implicitly that arguments starting in `-l' are to be treated as compiler output files, and passed to the linker in their proper position among the other output files.
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The usual way to run GCC is to run the executable called `gcc', or `<machine>-gcc' when cross-compiling, or `<machine>-gcc-<version>' to run a version other than the one that was installed last. Sometimes this is inconvenient, so GCC provides options that will switch to another cross-compiler or version.
-b machine
The value to use for machine is the same as was specified as the machine type when configuring GCC as a cross-compiler. For example, if a cross-compiler was configured with `configure i386v', meaning to compile for an 80386 running System V, then you would specify `-b i386v' to run that cross compiler.
-V version
The `-V' and `-b' options work by running the `<machine>-gcc-<version>' executable, so there's no real reason to use them if you can just run that directly.
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Earlier we discussed the standard option `-b' which chooses among different installed compilers for completely different target machines, such as VAX vs. 68000 vs. 80386.
In addition, each of these target machine types can have its own special options, starting with `-m', to choose among various hardware models or configurations--for example, 68010 vs 68020, floating coprocessor or none. A single installed version of the compiler can compile for any model or configuration, according to the options specified.
Some configurations of the compiler also support additional special options, usually for compatibility with other compilers on the same platform.
These options are defined by the macro TARGET_SWITCHES in the
machine description. The default for the options is also defined by
that macro, which enables you to change the defaults.
3.17.1 H8/300 Options 3.17.2 SH Options
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These `-m' options are defined for the H8/300 implementations:
-mrelax
ld and the H8/300' in Using ld, for a fuller description.
-mh
-ms
-mn
-ms2600
-mint32
int data 32 bits by default.
-malign-300
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These `-m' options are defined for the SH implementations:
-m1
-m2
-m2e
-m3
-m3e
-m4-nofpu
-m4-single-only
-m4-single
-m4
-mb
-ml
-mdalign
-mrelax
-mbigtable
switch tables. The default is to use
16-bit offsets.
-mfmovd
fmovd.
-mhitachi
-mnomacsave
MAC register as call-clobbered, even if
`-mhitachi' is given.
-mieee
-misize
-mpadstruct
-mspace
-mprefergot
-musermode
sh-*-linux*.
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These machine-independent options control the interface conventions used in code generation.
Most of them have both positive and negative forms; the negative form of `-ffoo' would be `-fno-foo'. In the table below, only one of the forms is listed--the one which is not the default. You can figure out the other form by either removing `no-' or adding it.
-fbounds-check
-ftrapv
-fexceptions
-fnon-call-exceptions
SIGALRM.
-funwind-tables
-fasynchronous-unwind-tables
-fpcc-struct-return
struct and union values in memory like
longer ones, rather than in registers. This convention is less
efficient, but it has the advantage of allowing intercallability between
GCC-compiled files and files compiled with other compilers, particularly
the Portable C Compiler (pcc).
The precise convention for returning structures in memory depends on the target configuration macros.
Short structures and unions are those whose size and alignment match that of some integer type.
Warning: code compiled with the `-fpcc-struct-return' switch is not binary compatible with code compiled with the `-freg-struct-return' switch. Use it to conform to a non-default application binary interface.
-freg-struct-return
struct and union values in registers when possible.
This is more efficient for small structures than
`-fpcc-struct-return'.
If you specify neither `-fpcc-struct-return' nor `-freg-struct-return', GCC defaults to whichever convention is standard for the target. If there is no standard convention, GCC defaults to `-fpcc-struct-return', except on targets where GCC is the principal compiler. In those cases, we can choose the standard, and we chose the more efficient register return alternative.
Warning: code compiled with the `-freg-struct-return' switch is not binary compatible with code compiled with the `-fpcc-struct-return' switch. Use it to conform to a non-default application binary interface.
-fshort-enums
enum type only as many bytes as it needs for the
declared range of possible values. Specifically, the enum type
will be equivalent to the smallest integer type which has enough room.
Warning: the `-fshort-enums' switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.
-fshort-double
double as for float.
Warning: the `-fshort-double' switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.
-fshort-wchar
Warning: the `-fshort-wchar' switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.
-fshared-data
const variables of this
compilation be shared data rather than private data. The distinction
makes sense only on certain operating systems, where shared data is
shared between processes running the same program, while private data
exists in one copy per process.
-fno-common
extern) in
two different compilations, you will get an error when you link them.
The only reason this might be useful is if you wish to verify that the
program will work on other systems which always work this way.
-fno-ident
-fno-gnu-linker
collect2 program to make sure the system linker includes
constructors and destructors. (collect2 is included in the GCC
distribution.) For systems which must use collect2, the
compiler driver gcc is configured to do this automatically.
-finhibit-size-directive
.size assembler directive, or anything else that
would cause trouble if the function is split in the middle, and the
two halves are placed at locations far apart in memory. This option is
used when compiling `crtstuff.c'; you should not need to use it
for anything else.
-fverbose-asm
`-fno-verbose-asm', the default, causes the extra information to be omitted and is useful when comparing two assembler files.
-fvolatile
-fvolatile-global
-fvolatile-static
-fpic
Position-independent code requires special support, and therefore works only on certain machines. For the 386, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent.
-fPIC
Position-independent code requires special support, and therefore works only on certain machines.
-ffixed-reg
reg must be the name of a register. The register names accepted
are machine-specific and are defined in the REGISTER_NAMES
macro in the machine description macro file.
This flag does not have a negative form, because it specifies a three-way choice.
-fcall-used-reg
It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model will produce disastrous results.
This flag does not have a negative form, because it specifies a three-way choice.
-fcall-saved-reg
It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model will produce disastrous results.
A different sort of disaster will result from the use of this flag for a register in which function values may be returned.
This flag does not have a negative form, because it specifies a three-way choice.
-fpack-struct
Warning: the `-fpack-struct' switch causes GCC to generate code that is not binary compatible with code generated without that switch. Additionally, it makes the code suboptimal. Use it to conform to a non-default application binary interface.
-finstrument-functions
__builtin_return_address does not work beyond the current
function, so the call site information may not be available to the
profiling functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
|
The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table.
This instrumentation is also done for functions expanded inline in other functions. The profiling calls will indicate where, conceptually, the inline function is entered and exited. This means that addressable versions of such functions must be available. If all your uses of a function are expanded inline, this may mean an additional expansion of code size. If you use `extern inline' in your C code, an addressable version of such functions must be provided. (This is normally the case anyways, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies.)
A function may be given the attribute no_instrument_function, in
which case this instrumentation will not be done. This can be used, for
example, for the profiling functions listed above, high-priority
interrupt routines, and any functions from which the profiling functions
cannot safely be called (perhaps signal handlers, if the profiling
routines generate output or allocate memory).
-fstack-check
Note that this switch does not actually cause checking to be done; the operating system must do that. The switch causes generation of code to ensure that the operating system sees the stack being extended.
-fstack-limit-register=reg
-fstack-limit-symbol=sym
-fno-stack-limit
For instance, if the stack starts at absolute address `0x80000000' and grows downwards, you can use the flags `-fstack-limit-symbol=__stack_limit' and `-Wl,--defsym,__stack_limit=0x7ffe0000' to enforce a stack limit of 128KB. Note that this may only work with the GNU linker.
-fargument-alias
-fargument-noalias
-fargument-noalias-global
`-fargument-alias' specifies that arguments (parameters) may
alias each other and may alias global storage.
`-fargument-noalias' specifies that arguments do not alias
each other, but may alias global storage.
`-fargument-noalias-global' specifies that arguments do not
alias each other and do not alias global storage.
Each language will automatically use whatever option is required by the language standard. You should not need to use these options yourself.
-fleading-underscore
Warning: the `-fleading-underscore' switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. Not all targets provide complete support for this switch.
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This section describes several environment variables that affect how GCC operates. Some of them work by specifying directories or prefixes to use when searching for various kinds of files. Some are used to specify other aspects of the compilation environment.
Note that you can also specify places to search using options such as `-B', `-I' and `-L' (see section 3.14 Options for Directory Search). These take precedence over places specified using environment variables, which in turn take precedence over those specified by the configuration of GCC. See section `Controlling the Compilation Driver `gcc'' in GNU Compiler Collection (GCC) Internals.
LANG
LC_CTYPE
LC_MESSAGES
LC_ALL
LC_CTYPE and LC_MESSAGES if it has been configured to do
so. These locale categories can be set to any value supported by your
installation. A typical value is `en_UK' for English in the United
Kingdom.
The LC_CTYPE environment variable specifies character
classification. GCC uses it to determine the character boundaries in
a string; this is needed for some multibyte encodings that contain quote
and escape characters that would otherwise be interpreted as a string
end or escape.
The LC_MESSAGES environment variable specifies the language to
use in diagnostic messages.
If the LC_ALL environment variable is set, it overrides the value
of LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE
and LC_MESSAGES default to the value of the LANG
environment variable. If none of these variables are set, GCC
defaults to traditional C English behavior.
TMPDIR
TMPDIR is set, it specifies the directory to use for temporary
files. GCC uses temporary files to hold the output of one stage of
compilation which is to be used as input to the next stage: for example,
the output of the preprocessor, which is the input to the compiler
proper.
GCC_EXEC_PREFIX
GCC_EXEC_PREFIX is set, it specifies a prefix to use in the
names of the subprograms executed by the compiler. No slash is added
when this prefix is combined with the name of a subprogram, but you can
specify a prefix that ends with a slash if you wish.
If GCC_EXEC_PREFIX is not set, GCC will attempt to figure out
an appropriate prefix to use based on the pathname it was invoked with.
If GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram.
The default value of GCC_EXEC_PREFIX is
`prefix/lib/gcc-lib/' where prefix is the value
of prefix when you ran the `configure' script.
Other prefixes specified with `-B' take precedence over this prefix.
This prefix is also used for finding files such as `crt0.o' that are used for linking.
In addition, the prefix is used in an unusual way in finding the
directories to search for header files. For each of the standard
directories whose name normally begins with `/usr/local/lib/gcc-lib'
(more precisely, with the value of GCC_INCLUDE_DIR), GCC tries
replacing that beginning with the specified prefix to produce an
alternate directory name. Thus, with `-Bfoo/', GCC will search
`foo/bar' where it would normally search `/usr/local/lib/bar'.
These alternate directories are searched first; the standard directories
come next.
COMPILER_PATH
COMPILER_PATH is a colon-separated list of
directories, much like PATH. GCC tries the directories thus
specified when searching for subprograms, if it can't find the
subprograms using GCC_EXEC_PREFIX.
LIBRARY_PATH
LIBRARY_PATH is a colon-separated list of
directories, much like PATH. When configured as a native compiler,
GCC tries the directories thus specified when searching for special
linker files, if it can't find them using GCC_EXEC_PREFIX. Linking
using GCC also uses these directories when searching for ordinary
libraries for the `-l' option (but directories specified with
`-L' come first).
LANG
LANG are recognized:
If LANG is not defined, or if it has some other value, then the
compiler will use mblen and mbtowc as defined by the default locale to
recognize and translate multibyte characters.
Some additional environments variables affect the behavior of the preprocessor.
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
PATH, in which to look for header files.
The special character, PATH_SEPARATOR, is target-dependent and
determined at GCC build time. For Windows-based targets it is a
semicolon, and for almost all other targets it is a colon.
CPATH specifies a list of directories to be searched as if
specified with `-I', but after any paths given with `-I'
options on the command line. This environment variable is used
regardless of which language is being preprocessed.
The remaining environment variables apply only when preprocessing the particular language indicated. Each specifies a list of directories to be searched as if specified with `-isystem', but after any paths given with `-isystem' options on the command line.
In all these variables, an empty element instructs the compiler to
search its current working directory. Empty elements can appear at the
beginning or end of a path. For instance, if the value of
CPATH is :/special/include, that has the same
effect as `-I. -I/special/include'.
DEPENDENCIES_OUTPUT
The value of DEPENDENCIES_OUTPUT can be just a file name, in
which case the Make rules are written to that file, guessing the target
name from the source file name. Or the value can have the form
`file target', in which case the rules are written to
file file using target as the target name.
In other words, this environment variable is equivalent to combining the options `-MM' and `-MF' (see section 3.11 Options Controlling the Preprocessor), with an optional `-MT' switch too.
SUNPRO_DEPENDENCIES
DEPENDENCIES_OUTPUT (see above),
except that system header files are not ignored, so it implies
`-M' rather than `-MM'. However, the dependence on the
main input file is omitted.
See section 3.11 Options Controlling the Preprocessor.
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The program protoize is an optional part of GCC. You can use
it to add prototypes to a program, thus converting the program to ISO
C in one respect. The companion program unprotoize does the
reverse: it removes argument types from any prototypes that are found.
When you run these programs, you must specify a set of source files as command line arguments. The conversion programs start out by compiling these files to see what functions they define. The information gathered about a file foo is saved in a file named `foo.X'.
After scanning comes actual conversion. The specified files are all eligible to be converted; any files they include (whether sources or just headers) are eligible as well.
But not all the eligible files are converted. By default,
protoize and unprotoize convert only source and header
files in the current directory. You can specify additional directories
whose files should be converted with the `-d directory'
option. You can also specify particular files to exclude with the
`-x file' option. A file is converted if it is eligible, its
directory name matches one of the specified directory names, and its
name within the directory has not been excluded.
Basic conversion with protoize consists of rewriting most
function definitions and function declarations to specify the types of
the arguments. The only ones not rewritten are those for varargs
functions.
protoize optionally inserts prototype declarations at the
beginning of the source file, to make them available for any calls that
precede the function's definition. Or it can insert prototype
declarations with block scope in the blocks where undeclared functions
are called.
Basic conversion with unprotoize consists of rewriting most
function declarations to remove any argument types, and rewriting
function definitions to the old-style pre-ISO form.
Both conversion programs print a warning for any function declaration or definition that they can't convert. You can suppress these warnings with `-q'.
The output from protoize or unprotoize replaces the
original source file. The original file is renamed to a name ending
with `.save' (for DOS, the saved filename ends in `.sav'
without the original `.c' suffix). If the `.save' (`.sav'
for DOS) file already exists, then the source file is simply discarded.
protoize and unprotoize both depend on GCC itself to
scan the program and collect information about the functions it uses.
So neither of these programs will work until GCC is installed.
Here is a table of the options you can use with protoize and
unprotoize. Each option works with both programs unless
otherwise stated.
-B directory
protoize.
-c compilation-options
gcc to
produce the `.X' files. The special option `-aux-info' is
always passed in addition, to tell gcc to write a `.X' file.
Note that the compilation options must be given as a single argument to
protoize or unprotoize. If you want to specify several
gcc options, you must quote the entire set of compilation options
to make them a single word in the shell.
There are certain gcc arguments that you cannot use, because they
would produce the wrong kind of output. These include `-g',
`-O', `-c', `-S', and `-o' If you include these in
the compilation-options, they are ignored.
-C
protoize.
-g
protoize.
-i string
protoize.
unprotoize converts prototyped function definitions to old-style
function definitions, where the arguments are declared between the
argument list and the initial `{'. By default, unprotoize
uses five spaces as the indentation. If you want to indent with just
one space instead, use `-i " "'.
-k
-l
protoize with `-l' inserts
a prototype declaration for each function in each block which calls the
function without any declaration. This option applies only to
protoize.
-n
-N
-p program
-q
-v
gcc.
If you need special compiler options to compile one of your program's
source files, then you should generate that file's `.X' file
specially, by running gcc on that source file with the
appropriate options and the option `-aux-info'. Then run
protoize on the entire set of files. protoize will use
the existing `.X' file because it is newer than the source file.
For example:
gcc -Dfoo=bar file1.c -aux-info file1.X protoize *.c |
You need to include the special files along with the rest in the
protoize command, even though their `.X' files already
exist, because otherwise they won't get converted.
See section 9.9 Caveats of using protoize, for more information on how to use
protoize successfully.
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A conforming implementation of ISO C is required to document its choice of behavior in each of the areas that are designated "implementation defined." The following lists all such areas, along with the section number from the ISO/IEC 9899:1999 standard.
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Diagnostics consist of all the output sent to stderr by GCC.
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The behavior of these points are dependent on the implementation of the C library, and are not defined by GCC itself.
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For internal names, all characters are significant. For external names, the number of significant characters are defined by the linker; for almost all targets, all characters are significant.
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char object into which has been stored any
character other than a member of the basic execution character set (6.2.5).
signed char or unsigned char has the same range,
representation, and behavior as "plain" char (6.2.5, 6.3.1.1).
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GCC supports only two's complement integer types, and all bit patterns are ordinary values.
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<math.h> and <complex.h> that return floating-point
results (5.2.4.2.2).
FLT_ROUNDS
(5.2.4.2.2).
FLT_EVAL_METHOD (5.2.4.2.2).
FP_CONTRACT pragma (6.5).
FENV_ACCESS pragma (7.6.1).
FP_CONTRACT pragma (7.12.2).
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A cast from pointer to integer discards most-significant bits if the pointer representation is larger than the integer type, sign-extends(2) if the pointer representation is smaller than the integer type, otherwise the bits are unchanged.
A cast from integer to pointer discards most-significant bits if the pointer representation is smaller than the integer type, extends according to the signedness of the integer type if the pointer representation is larger than the integer type, otherwise the bits are unchanged.
When casting from pointer to integer and back again, the resulting pointer must reference the same object as the original pointer, otherwise the behavior is undefined. That is, one may not use integer arithmetic to avoid the undefined behavior of pointer arithmetic as proscribed in 6.5.6/8.
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register
storage-class specifier are effective (6.7.1).
The register specifier affects code generation only in these ways:
register
storage-class specifier; if register is specified, the variable
may have a shorter lifespan than the code would indicate and may never
be placed in memory.
setjmp doesn't save the registers in
all circumstances. In those cases, GCC doesn't allocate any variables
in registers unless they are marked register.
GCC will not inline any functions if the `-fno-inline' option is used or if `-O0' is used. Otherwise, GCC may still be unable to inline a function for many reasons; the `-Winline' option may be used to determine if a function has not been inlined and why not.
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signed int
bit-field or as an unsigned int bit-field (6.7.2, 6.7.2.1).
_Bool, signed int,
and unsigned int (6.7.2.1).
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#include directive are combined into a header
name (6.10.2).
#include processing (6.10.2).
GCC imposes a limit of 200 nested #includes.
STDC #pragma
directive (6.10.6).
__DATE__ and __TIME__ when
respectively, the date and time of translation are not available (6.10.8).
If the date and time are not available, __DATE__ expands to
"??? ?? ????" and __TIME__ expands to
"??:??:??".
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The behavior of these points are dependent on the implementation of the C library, and are not defined by GCC itself.
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<float.h>, <limits.h>, and <stdint.h>
(5.2.4.2, 7.18.2, 7.18.3).
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The behavior of these points are dependent on the implementation of the C library, and are not defined by GCC itself.
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GNU C provides several language features not found in ISO standard C.
(The `-pedantic' option directs GCC to print a warning message if
any of these features is used.) To test for the availability of these
features in conditional compilation, check for a predefined macro
__GNUC__, which is always defined under GCC.
These extensions are available in C and Objective-C. Most of them are also available in C++. See section Extensions to the C++ Language, for extensions that apply only to C++.
Some features that are in ISO C99 but not C89 or C++ are also, as extensions, accepted by GCC in C89 mode and in C++.
(With them you can define "built-in" functions.)
5.1 Statements and Declarations in Expressions Putting statements and declarations inside expressions. 5.2 Locally Declared Labels Labels local to a statement-expression. 5.3 Labels as Values Getting pointers to labels, and computed gotos. 5.4 Nested Functions As in Algol and Pascal, lexical scoping of functions. 5.5 Constructing Function Calls Dispatching a call to another function. 5.6 Referring to a Type with typeoftypeof: referring to the type of an expression.5.7 Generalized Lvalues Using `?:', `,' and casts in lvalues. 5.8 Conditionals with Omitted Operands Omitting the middle operand of a `?:' expression. 5.9 Double-Word Integers Double-word integers--- long long int.5.10 Complex Numbers Data types for complex numbers. 5.11 Hex Floats Hexadecimal floating-point constants. 5.12 Arrays of Length Zero Zero-length arrays. 5.13 Arrays of Variable Length Arrays whose length is computed at run time. 5.14 Macros with a Variable Number of Arguments. Macros with a variable number of arguments. 5.15 Slightly Looser Rules for Escaped Newlines Slightly looser rules for escaped newlines. 5.16 String Literals with Embedded Newlines String literals with embedded newlines. 5.17 Non-Lvalue Arrays May Have Subscripts Any array can be subscripted, even if not an lvalue. 5.18 Arithmetic on void- and Function-PointersArithmetic on void-pointers and function pointers.5.19 Non-Constant Initializers Non-constant initializers. 5.20 Compound Literals Compound literals give structures, unions or arrays as values. 5.21 Designated Initializers Labeling elements of initializers. 5.23 Cast to a Union Type Casting to union type from any member of the union. 5.22 Case Ranges `case 1 ... 9' and such. 5.24 Mixed Declarations and Code Mixing declarations and code. 5.25 Declaring Attributes of Functions Declaring that functions have no side effects, or that they can never return. 5.26 Attribute Syntax Formal syntax for attributes. 5.27 Prototypes and Old-Style Function Definitions Prototype declarations and old-style definitions. 5.28 C++ Style Comments C++ comments are recognized. 5.29 Dollar Signs in Identifier Names Dollar sign is allowed in identifiers. 5.30 The Character ESC in Constants `\e' stands for the character ESC. 5.32 Specifying Attributes of Variables Specifying attributes of variables. 5.33 Specifying Attributes of Types Specifying attributes of types. 5.31 Inquiring on Alignment of Types or Variables Inquiring about the alignment of a type or variable. 5.34 An Inline Function is As Fast As a Macro Defining inline functions (as fast as macros). 5.35 Assembler Instructions with C Expression Operands Assembler instructions with C expressions as operands.
5.36 Constraints for asmOperandsConstraints for asm operands 5.37 Controlling Names Used in Assembler Code Specifying the assembler name to use for a C symbol. 5.38 Variables in Specified Registers Defining variables residing in specified registers. 5.39 Alternate Keywords __const__,__asm__, etc., for header files.5.40 Incomplete enumTypesenum foo;, with details to follow.5.41 Function Names as Strings Printable strings which are the name of the current function. 5.42 Getting the Return or Frame Address of a Function Getting the return or frame address of a function. 5.43 Using vector instructions through built-in functions 5.44 Other built-in functions provided by GCC Other built-in functions. 5.45 Unnamed struct/union fields within structs/unions.
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A compound statement enclosed in parentheses may appear as an expression in GNU C. This allows you to use loops, switches, and local variables within an expression.
Recall that a compound statement is a sequence of statements surrounded by braces; in this construct, parentheses go around the braces. For example:
({ int y = foo (); int z;
if (y > 0) z = y;
else z = - y;
z; })
|
is a valid (though slightly more complex than necessary) expression
for the absolute value of foo ().
The last thing in the compound statement should be an expression
followed by a semicolon; the value of this subexpression serves as the
value of the entire construct. (If you use some other kind of statement
last within the braces, the construct has type void, and thus
effectively no value.)
This feature is especially useful in making macro definitions "safe" (so that they evaluate each operand exactly once). For example, the "maximum" function is commonly defined as a macro in standard C as follows:
#define max(a,b) ((a) > (b) ? (a) : (b)) |
But this definition computes either a or b twice, with bad
results if the operand has side effects. In GNU C, if you know the
type of the operands (here let's assume int), you can define
the macro safely as follows:
#define maxint(a,b) \
({int _a = (a), _b = (b); _a > _b ? _a : _b; })
|
Embedded statements are not allowed in constant expressions, such as the value of an enumeration constant, the width of a bit-field, or the initial value of a static variable.
If you don't know the type of the operand, you can still do this, but you
must use typeof (see section 5.6 Referring to a Type with typeof).
Statement expressions are not supported fully in G++, and their fate there is unclear. (It is possible that they will become fully supported at some point, or that they will be deprecated, or that the bugs that are present will continue to exist indefinitely.) Presently, statement expressions do not work well as default arguments.
In addition, there are semantic issues with statement-expressions in C++. If you try to use statement-expressions instead of inline functions in C++, you may be surprised at the way object destruction is handled. For example:
#define foo(a) ({int b = (a); b + 3; })
|
does not work the same way as:
inline int foo(int a) { int b = a; return b + 3; }
|
In particular, if the expression passed into foo involves the
creation of temporaries, the destructors for those temporaries will be
run earlier in the case of the macro than in the case of the function.
These considerations mean that it is probably a bad idea to use statement-expressions of this form in header files that are designed to work with C++. (Note that some versions of the GNU C Library contained header files using statement-expression that lead to precisely this bug.)
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Each statement expression is a scope in which local labels can be
declared. A local label is simply an identifier; you can jump to it
with an ordinary goto statement, but only from within the
statement expression it belongs to.
A local label declaration looks like this:
__label__ label; |
or
__label__ label1, label2, /* ... */; |
Local label declarations must come at the beginning of the statement expression, right after the `({', before any ordinary declarations.
The label declaration defines the label name, but does not define
the label itself. You must do this in the usual way, with
label:, within the statements of the statement expression.
The local label feature is useful because statement expressions are
often used in macros. If the macro contains nested loops, a goto
can be useful for breaking out of them. However, an ordinary label
whose scope is the whole function cannot be used: if the macro can be
expanded several times in one function, the label will be multiply
defined in that function. A local label avoids this problem. For
example:
#define SEARCH(array, target) \
({ \
__label__ found; \
typeof (target) _SEARCH_target = (target); \
typeof (*(array)) *_SEARCH_array = (array); \
int i, j; \
int value; \
for (i = 0; i < max; i++) \
for (j = 0; j < max; j++) \
if (_SEARCH_array[i][j] == _SEARCH_target) \
{ value = i; goto found; } \
value = -1; \
found: \
value; \
})
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You can get the address of a label defined in the current function
(or a containing function) with the unary operator `&&'. The
value has type void *. This value is a constant and can be used
wherever a constant of that type is valid. For example:
void *ptr; /* ... */ ptr = &&foo; |
To use these values, you need to be able to jump to one. This is done
with the computed goto statement(3), goto *exp;. For example,
goto *ptr; |
Any expression of type void * is allowed.
One way of using these constants is in initializing a static array that will serve as a jump table:
static void *array[] = { &&foo, &&bar, &&hack };
|
Then you can select a label with indexing, like this:
goto *array[i]; |
Note that this does not check whether the subscript is in bounds--array indexing in C never does that.
Such an array of label values serves a purpose much like that of the
switch statement. The switch statement is cleaner, so
use that rather than an array unless the problem does not fit a
switch statement very well.
Another use of label values is in an interpreter for threaded code. The labels within the interpreter function can be stored in the threaded code for super-fast dispatching.
You may not use this mechanism to jump to code in a different function. If you do that, totally unpredictable things will happen. The best way to avoid this is to store the label address only in automatic variables and never pass it as an argument.
An alternate way to write the above example is
static const int array[] = { &&foo - &&foo, &&bar - &&foo,
&&hack - &&foo };
goto *(&&foo + array[i]);
|
This is more friendly to code living in shared libraries, as it reduces the number of dynamic relocations that are needed, and by consequence, allows the data to be read-only.
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A nested function is a function defined inside another function.
(Nested functions are not supported for GNU C++.) The nested function's
name is local to the block where it is defined. For example, here we
define a nested function named square, and call it twice:
foo (double a, double b)
{
double square (double z) { return z * z; }
return square (a) + square (b);
}
|
The nested function can access all the variables of the containing
function that are visible at the point of its definition. This is
called lexical scoping. For example, here we show a nested
function which uses an inherited variable named offset:
bar (int *array, int offset, int size)
{
int access (int *array, int index)
{ return array[index + offset]; }
int i;
/* ... */
for (i = 0; i < size; i++)
/* ... */ access (array, i) /* ... */
}
|
Nested function definitions are permitted within functions in the places where variable definitions are allowed; that is, in any block, before the first statement in the block.
It is possible to call the nested function from outside the scope of its name by storing its address or passing the address to another function:
hack (int *array, int size)
{
void store (int index, int value)
{ array[index] = value; }
intermediate (store, size);
}
|
Here, the function intermediate receives the address of
store as an argument. If intermediate calls store,
the arguments given to store are used to store into array.
But this technique works only so long as the containing function
(hack, in this example) does not exit.
If you try to call the nested function through its address after the containing function has exited, all hell will break loose. If you try to call it after a containing scope level has exited, and if it refers to some of the variables that are no longer in scope, you may be lucky, but it's not wise to take the risk. If, however, the nested function does not refer to anything that has gone out of scope, you should be safe.
GCC implements taking the address of a nested function using a technique called trampolines. A paper describing them is available as
http://people.debian.org/~aaronl/Usenix88-lexic.pdf.
A nested function can jump to a label inherited from a containing
function, provided the label was explicitly declared in the containing
function (see section 5.2 Locally Declared Labels). Such a jump returns instantly to the
containing function, exiting the nested function which did the
goto and any intermediate functions as well. Here is an example:
bar (int *array, int offset, int size)
{
__label__ failure;
int access (int *array, int index)
{
if (index > size)
goto failure;
return array[index + offset];
}
int i;
/* ... */
for (i = 0; i < size; i++)
/* ... */ access (array, i) /* ... */
/* ... */
return 0;
/* Control comes here from |
A nested function always has internal linkage. Declaring one with
extern is erroneous. If you need to declare the nested function
before its definition, use auto (which is otherwise meaningless
for function declarations).
bar (int *array, int offset, int size)
{
__label__ failure;
auto int access (int *, int);
/* ... */
int access (int *array, int index)
{
if (index > size)
goto failure;
return array[index + offset];
}
/* ... */
}
|
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Using the built-in functions described below, you can record the arguments a function received, and call another function with the same arguments, without knowing the number or types of the arguments.
You can also record the return value of that function call, and later return that value, without knowing what data type the function tried to return (as long as your caller expects that data type).
The function saves the arg pointer register, structure value address, and all registers that might be used to pass arguments to a function into a block of memory allocated on the stack. Then it returns the address of that block.
The value of arguments should be the value returned by
__builtin_apply_args. The argument size specifies the size
of the stack argument data, in bytes.
This function returns a pointer to data describing how to return whatever value was returned by function. The data is saved in a block of memory allocated on the stack.
It is not always simple to compute the proper value for size. The
value is used by __builtin_apply to compute the amount of data
that should be pushed on the stack and copied from the incoming argument
area.
__builtin_apply.
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typeof
Another way to refer to the type of an expression is with typeof.
The syntax of using of this keyword looks like sizeof, but the
construct acts semantically like a type name defined with typedef.
There are two ways of writing the argument to typeof: with an
expression or with a type. Here is an example with an expression:
typeof (x[0](1)) |
This assumes that x is an array of pointers to functions;
the type described is that of the values of the functions.
Here is an example with a typename as the argument:
typeof (int *) |
Here the type described is that of pointers to int.
If you are writing a header file that must work when included in ISO C
programs, write __typeof__ instead of typeof.
See section 5.39 Alternate Keywords.
A typeof-construct can be used anywhere a typedef name could be
used. For example, you can use it in a declaration, in a cast, or inside
of sizeof or typeof.
typeof is often useful in conjunction with the
statements-within-expressions feature. Here is how the two together can
be used to define a safe "maximum" macro that operates on any
arithmetic type and evaluates each of its arguments exactly once:
#define max(a,b) \
({ typeof (a) _a = (a); \
typeof (b) _b = (b); \
_a > _b ? _a : _b; })
|
The reason for using names that start with underscores for the local
variables is to avoid conflicts with variable names that occur within the
expressions that are substituted for a and b. Eventually we
hope to design a new form of declaration syntax that allows you to declare
variables whose scopes start only after their initializers; this will be a
more reliable way to prevent such conflicts.
Some more examples of the use of typeof:
y with the type of what x points to.
typeof (*x) y; |
y as an array of such values.
typeof (*x) y[4]; |
y as an array of pointers to characters:
typeof (typeof (char *)[4]) y; |
It is equivalent to the following traditional C declaration:
char *y[4]; |
To see the meaning of the declaration using typeof, and why it
might be a useful way to write, let's rewrite it with these macros:
#define pointer(T) typeof(T *) #define array(T, N) typeof(T [N]) |
Now the declaration can be rewritten this way:
array (pointer (char), 4) y; |
Thus, array (pointer (char), 4) is the type of arrays of 4
pointers to char.
Compatibility Note: In addition to typeof, GCC 2 supported
a more limited extension which permitted one to write
typedef T = expr; |
with the effect of declaring T to have the type of the expression
expr. This extension does not work with GCC 3 (versions between
3.0 and 3.2 will crash; 3.2.1 and later give an error). Code which
relies on it should be rewritten to use typeof:
typedef typeof(expr) T; |
This will work with all versions of GCC.
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Standard C++ allows compound expressions and conditional expressions as lvalues, and permits casts to reference type, so use of this extension is deprecated for C++ code.
For example, a compound expression can be assigned, provided the last expression in the sequence is an lvalue. These two expressions are equivalent:
(a, b) += 5 a, (b += 5) |
Similarly, the address of the compound expression can be taken. These two expressions are equivalent:
&(a, b) a, &b |
A conditional expression is a valid lvalue if its type is not void and the true and false branches are both valid lvalues. For example, these two expressions are equivalent:
(a ? b : c) = 5 (a ? b = 5 : (c = 5)) |
A cast is a valid lvalue if its operand is an lvalue. A simple
assignment whose left-hand side is a cast works by converting the
right-hand side first to the specified type, then to the type of the
inner left-hand side expression. After this is stored, the value is
converted back to the specified type to become the value of the
assignment. Thus, if a has type char *, the following two
expressions are equivalent:
(int)a = 5 (int)(a = (char *)(int)5) |
An assignment-with-arithmetic operation such as `+=' applied to a cast performs the arithmetic using the type resulting from the cast, and then continues as in the previous case. Therefore, these two expressions are equivalent:
(int)a += 5 (int)(a = (char *)(int) ((int)a + 5)) |
You cannot take the address of an lvalue cast, because the use of its
address would not work out coherently. Suppose that &(int)f were
permitted, where f has type float. Then the following
statement would try to store an integer bit-pattern where a floating
point number belongs:
*&(int)f = 1; |
This is quite different from what (int)f = 1 would do--that
would convert 1 to floating point and store it. Rather than cause this
inconsistency, we think it is better to prohibit use of `&' on a cast.
If you really do want an int * pointer with the address of
f, you can simply write (int *)&f.
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The middle operand in a conditional expression may be omitted. Then if the first operand is nonzero, its value is the value of the conditional expression.
Therefore, the expression
x ? : y |
has the value of x if that is nonzero; otherwise, the value of
y.
This example is perfectly equivalent to
x ? x : y |
In this simple case, the ability to omit the middle operand is not especially useful. When it becomes useful is when the first operand does, or may (if it is a macro argument), contain a side effect. Then repeating the operand in the middle would perform the side effect twice. Omitting the middle operand uses the value already computed without the undesirable effects of recomputing it.
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ISO C99 supports data types for integers that are at least 64 bits wide,
and as an extension GCC supports them in C89 mode and in C++.
Simply write long long int for a signed integer, or
unsigned long long int for an unsigned integer. To make an
integer constant of type long long int, add the suffix `LL'
to the integer. To make an integer constant of type unsigned long
long int, add the suffix `ULL' to the integer.
You can use these types in arithmetic like any other integer types. Addition, subtraction, and bitwise boolean operations on these types are open-coded on all types of machines. Multiplication is open-coded if the machine supports fullword-to-doubleword a widening multiply instruction. Division and shifts are open-coded only on machines that provide special support. The operations that are not open-coded use special library routines that come with GCC.
There may be pitfalls when you use long long types for function
arguments, unless you declare function prototypes. If a function
expects type int for its argument, and you pass a value of type
long long int, confusion will result because the caller and the
subroutine will disagree about the number of bytes for the argument.
Likewise, if the function expects long long int and you pass
int. The best way to avoid such problems is to use prototypes.
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ISO C99 supports complex floating data types, and as an extension GCC
supports them in C89 mode and in C++, and supports complex integer data
types which are not part of ISO C99. You can declare complex types
using the keyword _Complex. As an extension, the older GNU
keyword __complex__ is also supported.
For example, `_Complex double x;' declares x as a
variable whose real part and imaginary part are both of type
double. `_Complex short int y;' declares y to
have real and imaginary parts of type short int; this is not
likely to be useful, but it shows that the set of complex types is
complete.
To write a constant with a complex data type, use the suffix `i' or
`j' (either one; they are equivalent). For example, 2.5fi
has type _Complex float and 3i has type
_Complex int. Such a constant always has a pure imaginary
value, but you can form any complex value you like by adding one to a
real constant. This is a GNU extension; if you have an ISO C99
conforming C library (such as GNU libc), and want to construct complex
constants of floating type, you should include <complex.h> and
use the macros I or _Complex_I instead.
To extract the real part of a complex-valued expression exp, write
__real__ exp. Likewise, use __imag__ to
extract the imaginary part. This is a GNU extension; for values of
floating type, you should use the ISO C99 functions crealf,
creal, creall, cimagf, cimag and
cimagl, declared in <complex.h> and also provided as
built-in functions by GCC.
The operator `~' performs complex conjugation when used on a value
with a complex type. This is a GNU extension; for values of
floating type, you should use the ISO C99 functions conjf,
conj and conjl, declared in <complex.h> and also
provided as built-in functions by GCC.
GCC can allocate complex automatic variables in a noncontiguous
fashion; it's even possible for the real part to be in a register while
the imaginary part is on the stack (or vice-versa). Only the DWARF2
debug info format can represent this, so use of DWARF2 is recommended.
If you are using the stabs debug info format, GCC describes a noncontiguous
complex variable as if it were two separate variables of noncomplex type.
If the variable's actual name is foo, the two fictitious
variables are named foo$real and foo$imag. You can
examine and set these two fictitious variables with your debugger.
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ISO C99 supports floating-point numbers written not only in the usual
decimal notation, such as 1.55e1, but also numbers such as
0x1.fp3 written in hexadecimal format. As a GNU extension, GCC
supports this in C89 mode (except in some cases when strictly
conforming) and in C++. In that format the
`0x' hex introducer and the `p' or `P' exponent field are
mandatory. The exponent is a decimal number that indicates the power of
2 by which the significant part will be multiplied. Thus `0x1.f' is
1 15/16,
`p3' multiplies it by 8, and the value of 0x1.fp3
is the same as 1.55e1.
Unlike for floating-point numbers in the decimal notation the exponent
is always required in the hexadecimal notation. Otherwise the compiler
would not be able to resolve the ambiguity of, e.g., 0x1.f. This
could mean 1.0f or 1.9375 since `f' is also the
extension for floating-point constants of type float.
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Zero-length arrays are allowed in GNU C. They are very useful as the last element of a structure which is really a header for a variable-length object:
struct line {
int length;
char contents[0];
};
struct line *thisline = (struct line *)
malloc (sizeof (struct line) + this_length);
thisline->length = this_length;
|
In ISO C90, you would have to give contents a length of 1, which
means either you waste space or complicate the argument to malloc.
In ISO C99, you would use a flexible array member, which is slightly different in syntax and semantics:
contents[] without
the 0.
sizeof
operator may not be applied. As a quirk of the original implementation
of zero-length arrays, sizeof evaluates to zero.
struct that is otherwise non-empty.
GCC versions before 3.0 allowed zero-length arrays to be statically initialized, as if they were flexible arrays. In addition to those cases that were useful, it also allowed initializations in situations that would corrupt later data. Non-empty initialization of zero-length arrays is now treated like any case where there are more initializer elements than the array holds, in that a suitable warning about "excess elements in array" is given, and the excess elements (all of them, in this case) are ignored.
Instead GCC allows static initialization of flexible array members.
This is equivalent to defining a new structure containing the original
structure followed by an array of sufficient size to contain the data.
I.e. in the following, f1 is constructed as if it were declared
like f2.
struct f1 {
int x; int y[];
} f1 = { 1, { 2, 3, 4 } };
struct f2 {
struct f1 f1; int data[3];
} f2 = { { 1 }, { 2, 3, 4 } };
|
The convenience of this extension is that f1 has the desired
type, eliminating the need to consistently refer to f2.f1.
This has symmetry with normal static arrays, in that an array of
unknown size is also written with [].
Of course, this extension only makes sense if the extra data comes at the end of a top-level object, as otherwise we would be overwriting data at subsequent offsets. To avoid undue complication and confusion with initialization of deeply nested arrays, we simply disallow any non-empty initialization except when the structure is the top-level object. For example:
struct foo { int x; int y[]; };
struct bar { struct foo z; };
struct foo a = { 1, { 2, 3, 4 } }; // Valid.
struct bar b = { { 1, { 2, 3, 4 } } }; // Invalid.
struct bar c = { { 1, { } } }; // Valid.
struct foo d[1] = { { 1 { 2, 3, 4 } } }; // Invalid.
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Variable-length automatic arrays are allowed in ISO C99, and as an extension GCC accepts them in C89 mode and in C++. (However, GCC's implementation of variable-length arrays does not yet conform in detail to the ISO C99 standard.) These arrays are declared like any other automatic arrays, but with a length that is not a constant expression. The storage is allocated at the point of declaration and deallocated when the brace-level is exited. For example:
FILE *
concat_fopen (char *s1, char *s2, char *mode)
{
char str[strlen (s1) + strlen (s2) + 1];
strcpy (str, s1);
strcat (str, s2);
return fopen (str, mode);
}
|
Jumping or breaking out of the scope of the array name deallocates the storage. Jumping into the scope is not allowed; you get an error message for it.
You can use the function alloca to get an effect much like
variable-length arrays. The function alloca is available in
many other C implementations (but not in all). On the other hand,
variable-length arrays are more elegant.
There are other differences between these two methods. Space allocated
with alloca exists until the containing function returns.
The space for a variable-length array is deallocated as soon as the array
name's scope ends. (If you use both variable-length arrays and
alloca in the same function, deallocation of a variable-length array
will also deallocate anything more recently allocated with alloca.)
You can also use variable-length arrays as arguments to functions:
struct entry
tester (int len, char data[len][len])
{
/* ... */
}
|
The length of an array is computed once when the storage is allocated
and is remembered for the scope of the array in case you access it with
sizeof.
If you want to pass the array first and the length afterward, you can use a forward declaration in the parameter list--another GNU extension.
struct entry
tester (int len; char data[len][len], int len)
{
/* ... */
}
|
The `int len' before the semicolon is a parameter forward
declaration, and it serves the purpose of making the name len
known when the declaration of data is parsed.
You can write any number of such parameter forward declarations in the parameter list. They can be separated by commas or semicolons, but the last one must end with a semicolon, which is followed by the "real" parameter declarations. Each forward declaration must match a "real" declaration in parameter name and data type. ISO C99 does not support parameter forward declarations.
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In the ISO C standard of 1999, a macro can be declared to accept a variable number of arguments much as a function can. The syntax for defining the macro is similar to that of a function. Here is an example:
#define debug(format, ...) fprintf (stderr, format, __VA_ARGS__) |
Here `...' is a variable argument. In the invocation of
such a macro, it represents the zero or more tokens until the closing
parenthesis that ends the invocation, including any commas. This set of
tokens replaces the identifier __VA_ARGS__ in the macro body
wherever it appears. See the CPP manual for more information.
GCC has long supported variadic macros, and used a different syntax that allowed you to give a name to the variable arguments just like any other argument. Here is an example:
#define debug(format, args...) fprintf (stderr, format, args) |
This is in all ways equivalent to the ISO C example above, but arguably more readable and descriptive.
GNU CPP has two further variadic macro extensions, and permits them to be used with either of the above forms of macro definition.
In standard C, you are not allowed to leave the variable argument out entirely; but you are allowed to pass an empty argument. For example, this invocation is invalid in ISO C, because there is no comma after the string:
debug ("A message")
|
GNU CPP permits you to completely omit the variable arguments in this way. In the above examples, the compiler would complain, though since the expansion of the macro still has the extra comma after the format string.
To help solve this problem, CPP behaves specially for variable arguments used with the token paste operator, `##'. If instead you write
#define debug(format, ...) fprintf (stderr, format, ## __VA_ARGS__) |
and if the variable arguments are omitted or empty, the `##' operator causes the preprocessor to remove the comma before it. If you do provide some variable arguments in your macro invocation, GNU CPP does not complain about the paste operation and instead places the variable arguments after the comma. Just like any other pasted macro argument, these arguments are not macro expanded.
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Recently, the preprocessor has relaxed its treatment of escaped newlines. Previously, the newline had to immediately follow a backslash. The current implementation allows whitespace in the form of spaces, horizontal and vertical tabs, and form feeds between the backslash and the subsequent newline. The preprocessor issues a warning, but treats it as a valid escaped newline and combines the two lines to form a single logical line. This works within comments and tokens, including multi-line strings, as well as between tokens. Comments are not treated as whitespace for the purposes of this relaxation, since they have not yet been replaced with spaces.
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As an extension, GNU CPP permits string literals to cross multiple lines without escaping the embedded newlines. Each embedded newline is replaced with a single `\n' character in the resulting string literal, regardless of what form the newline took originally.
CPP currently allows such strings in directives as well (other than the `#include' family). This is deprecated and will eventually be removed.
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In ISO C99, arrays that are not lvalues still decay to pointers, and may be subscripted, although they may not be modified or used after the next sequence point and the unary `&' operator may not be applied to them. As an extension, GCC allows such arrays to be subscripted in C89 mode, though otherwise they do not decay to pointers outside C99 mode. For example, this is valid in GNU C though not valid in C89:
struct foo {int a[4];};
struct foo f();
bar (int index)
{
return f().a[index];
}
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void- and Function-Pointers
In GNU C, addition and subtraction operations are supported on pointers to
void and on pointers to functions. This is done by treating the
size of a void or of a function as 1.
A consequence of this is that sizeof is also allowed on void
and on function types, and returns 1.
The option `-Wpointer-arith' requests a warning if these extensions are used.
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As in standard C++ and ISO C99, the elements of an aggregate initializer for an automatic variable are not required to be constant expressions in GNU C. Here is an example of an initializer with run-time varying elements:
foo (float f, float g)
{
float beat_freqs[2] = { f-g, f+g };
/* ... */
}
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ISO C99 supports compound literals. A compound literal looks like a cast containing an initializer. Its value is an object of the type specified in the cast, containing the elements specified in the initializer; it is an lvalue. As an extension, GCC supports compound literals in C89 mode and in C++.
Usually, the specified type is a structure. Assume that
struct foo and structure are declared as shown:
struct foo {int a; char b[2];} structure;
|
Here is an example of constructing a struct foo with a compound literal:
structure = ((struct foo) {x + y, 'a', 0});
|
This is equivalent to writing the following:
{
struct foo temp = {x + y, 'a', 0};
structure = temp;
}
|
You can also construct an array. If all the elements of the compound literal are (made up of) simple constant expressions, suitable for use in initializers of objects of static storage duration, then the compound literal can be coerced to a pointer to its first element and used in such an initializer, as shown here:
char **foo = (char *[]) { "x", "y", "z" };
|
Compound literals for scalar types and union types are is also allowed, but then the compound literal is equivalent to a cast.
As a GNU extension, GCC allows initialization of objects with static storage duration by compound literals (which is not possible in ISO C99, because the initializer is not a constant). It is handled as if the object was initialized only with the bracket enclosed list if compound literal's and object types match. The initializer list of the compound literal must be constant. If the object being initialized has array type of unknown size, the size is determined by compound literal size.
static struct foo x = (struct foo) {1, 'a', 'b'};
static int y[] = (int []) {1, 2, 3};
static int z[] = (int [3]) {1};
|
The above lines are equivalent to the following:
static struct foo x = {1, 'a', 'b'};
static int y[] = {1, 2, 3};
static int z[] = {1, 0, 0};
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Standard C89 requires the elements of an initializer to appear in a fixed order, the same as the order of the elements in the array or structure being initialized.
In ISO C99 you can give the elements in any order, specifying the array indices or structure field names they apply to, and GNU C allows this as an extension in C89 mode as well. This extension is not implemented in GNU C++.
To specify an array index, write `[index] =' before the element value. For example,
int a[6] = { [4] = 29, [2] = 15 };
|
is equivalent to
int a[6] = { 0, 0, 15, 0, 29, 0 };
|
The index values must be constant expressions, even if the array being initialized is automatic.
An alternative syntax for this which has been obsolete since GCC 2.5 but GCC still accepts is to write `[index]' before the element value, with no `='.
To initialize a range of elements to the same value, write `[first ... last] = value'. This is a GNU extension. For example,
int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 };
|
If the value in it has side-effects, the side-effects will happen only once, not for each initialized field by the range initializer.
Note that the length of the array is the highest value specified plus one.
In a structure initializer, specify the name of a field to initialize with `.fieldname =' before the element value. For example, given the following structure,
struct point { int x, y; };
|
the following initialization
struct point p = { .y = yvalue, .x = xvalue };
|
is equivalent to
struct point p = { xvalue, yvalue };
|
Another syntax which has the same meaning, obsolete since GCC 2.5, is `fieldname:', as shown here:
struct point p = { y: yvalue, x: xvalue };
|
The `[index]' or `.fieldname' is known as a designator. You can also use a designator (or the obsolete colon syntax) when initializing a union, to specify which element of the union should be used. For example,
union foo { int i; double d; };
union foo f = { .d = 4 };
|
will convert 4 to a double to store it in the union using
the second element. By contrast, casting 4 to type union foo
would store it into the union as the integer i, since it is
an integer. (See section 5.23 Cast to a Union Type.)
You can combine this technique of naming elements with ordinary C initialization of successive elements. Each initializer element that does not have a designator applies to the next consecutive element of the array or structure. For example,
int a[6] = { [1] = v1, v2, [4] = v4 };
|
is equivalent to
int a[6] = { 0, v1, v2, 0, v4, 0 };
|
Labeling the elements of an array initializer is especially useful
when the indices are characters or belong to an enum type.
For example:
int whitespace[256]
= { [' '] = 1, ['\t'] = 1, ['\h'] = 1,
['\f'] = 1, ['\n'] = 1, ['\r'] = 1 };
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You can also write a series of `.fieldname' and `[index]' designators before an `=' to specify a nested subobject to initialize; the list is taken relative to the subobject corresponding to the closest surrounding brace pair. For example, with the `struct point' declaration above:
struct point ptarray[10] = { [2].y = yv2, [2].x = xv2, [0].x = xv0 };
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If the same field is initialized multiple times, it will have value from the last initialization. If any such overridden initialization has side-effect, it is unspecified whether the side-effect happens or not. Currently, gcc will discard them and issue a warning.
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You can specify a range of consecutive values in a single case label,
like this:
case low ... high: |
This has the same effect as the proper number of individual case
labels, one for each integer value from low to high, inclusive.
This feature is especially useful for ranges of ASCII character codes:
case 'A' ... 'Z': |
Be careful: Write spaces around the ..., for otherwise
it may be parsed wrong when you use it with integer values. For example,
write this:
case 1 ... 5: |
rather than this:
case 1...5: |
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A cast to union type is similar to other casts, except that the type
specified is a union type. You can specify the type either with
union tag or with a typedef name. A cast to union is actually
a constructor though, not a cast, and hence does not yield an lvalue like
normal casts. (See section 5.20 Compound Literals.)
The types that may be cast to the union type are those of the members of the union. Thus, given the following union and variables:
union foo { int i; double d; };
int x;
double y;
|
both x and y can be cast to type union foo.
Using the cast as the right-hand side of an assignment to a variable of union type is equivalent to storing in a member of the union:
union foo u; /* ... */ u = (union foo) x == u.i = x u = (union foo) y == u.d = y |
You can also use the union cast as a function argument:
void hack (union foo); /* ... */ hack ((union foo) x); |
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ISO C99 and ISO C++ allow declarations and code to be freely mixed within compound statements. As an extension, GCC also allows this in C89 mode. For example, you could do:
int i; /* ... */ i++; int j = i + 2; |
Each identifier is visible from where it is declared until the end of the enclosing block.
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In GNU C, you declare certain things about functions called in your program which help the compiler optimize function calls and check your code more carefully.
The keyword __attribute__ allows you to specify special
attributes when making a declaration. This keyword is followed by an
attribute specification inside double parentheses. The following
attributes are currently defined for functions on all targets:
noreturn, noinline, always_inline,
pure, const, nothrow,
format, format_arg, no_instrument_function,
section, constructor, destructor, used,
unused, deprecated, weak, malloc,
alias, and nonnull. Several other attributes are defined
for functions on particular target systems. Other attributes, including
section are supported for variables declarations
(see section 5.32 Specifying Attributes of Variables) and for types (see section 5.33 Specifying Attributes of Types).
You may also specify attributes with `__' preceding and following
each keyword. This allows you to use them in header files without
being concerned about a possible macro of the same name. For example,
you may use __noreturn__ instead of noreturn.
See section 5.26 Attribute Syntax, for details of the exact syntax for using attributes.
noreturn
abort and exit,
cannot return. GCC knows this automatically. Some programs define
their own functions that never return. You can declare them
noreturn to tell the compiler this fact. For example,
void fatal () __attribute__ ((noreturn));
void
fatal (/* ... */)
{
/* ... */ /* Print error message. */ /* ... */
exit (1);
}
|
The noreturn keyword tells the compiler to assume that
fatal cannot return. It can then optimize without regard to what
would happen if fatal ever did return. This makes slightly
better code. More importantly, it helps avoid spurious warnings of
uninitialized variables.
Do not assume that registers saved by the calling function are
restored before calling the noreturn function.
It does not make sense for a noreturn function to have a return
type other than void.
The attribute noreturn is not implemented in GCC versions
earlier than 2.5. An alternative way to declare that a function does
not return, which works in the current version and in some older
versions, is as follows:
typedef void voidfn (); volatile voidfn fatal; |
noinline
always_inline
pure
pure. For example,
int square (int) __attribute__ ((pure)); |
says that the hypothetical function square is safe to call
fewer times than the program says.
Some of common examples of pure functions are strlen or memcmp.
Interesting non-pure functions are functions with infinite loops or those
depending on volatile memory or other system resource, that may change between
two consecutive calls (such as feof in a multithreading environment).
The attribute pure is not implemented in GCC versions earlier
than 2.96.
const
pure attribute above, since function is not
allowed to read global memory.
Note that a function that has pointer arguments and examines the data
pointed to must not be declared const. Likewise, a
function that calls a non-const function usually must not be
const. It does not make sense for a const function to
return void.
The attribute const is not implemented in GCC versions earlier
than 2.5. An alternative way to declare that a function has no side
effects, which works in the current version and in some older versions,
is as follows:
typedef int intfn (); extern const intfn square; |
This approach does not work in GNU C++ from 2.6.0 on, since the language specifies that the `const' must be attached to the return value.
nothrow
nothrow attribute is used to inform the compiler that a
function cannot throw an exception. For example, most functions in
the standard C library can be guaranteed not to throw an exception
with the notable exceptions of qsort and bsearch that
take function pointer arguments. The nothrow attribute is not
implemented in GCC versions earlier than 3.2.
format (archetype, string-index, first-to-check)
format attribute specifies that a function takes printf,
scanf, strftime or strfmon style arguments which
should be type-checked against a format string. For example, the
declaration:
extern int
my_printf (void *my_object, const char *my_format, ...)
__attribute__ ((format (printf, 2, 3)));
|
causes the compiler to check the arguments in calls to my_printf
for consistency with the printf style format string argument
my_format.
The parameter archetype determines how the format string is
interpreted, and should be printf, scanf, strftime
or strfmon. (You can also use __printf__,
__scanf__, __strftime__ or __strfmon__.) The
parameter string-index specifies which argument is the format
string argument (starting from 1), while first-to-check is the
number of the first argument to check against the format string. For
functions where the arguments are not available to be checked (such as
vprintf), specify the third parameter as zero. In this case the
compiler only checks the format string for consistency. For
strftime formats, the third parameter is required to be zero.
In the example above, the format string (my_format) is the second
argument of the function my_print, and the arguments to check
start with the third argument, so the correct parameters for the format
attribute are 2 and 3.
The format attribute allows you to identify your own functions
which take format strings as arguments, so that GCC can check the
calls to these functions for errors. The compiler always (unless
`-ffreestanding' is used) checks formats
for the standard library functions printf, fprintf,
sprintf, scanf, fscanf, sscanf, strftime,
vprintf, vfprintf and vsprintf whenever such
warnings are requested (using `-Wformat'), so there is no need to
modify the header file `stdio.h'. In C99 mode, the functions
snprintf, vsnprintf, vscanf, vfscanf and
vsscanf are also checked. Except in strictly conforming C
standard modes, the X/Open function strfmon is also checked as
are printf_unlocked and fprintf_unlocked.
See section Options Controlling C Dialect.
format_arg (string-index)
format_arg attribute specifies that a function takes a format
string for a printf, scanf, strftime or
strfmon style function and modifies it (for example, to translate
it into another language), so the result can be passed to a
printf, scanf, strftime or strfmon style
function (with the remaining arguments to the format function the same
as they would have been for the unmodified string). For example, the
declaration:
extern char *
my_dgettext (char *my_domain, const char *my_format)
__attribute__ ((format_arg (2)));
|
causes the compiler to check the arguments in calls to a printf,
scanf, strftime or strfmon type function, whose
format string argument is a call to the my_dgettext function, for
consistency with the format string argument my_format. If the
format_arg attribute had not been specified, all the compiler
could tell in such calls to format functions would be that the format
string argument is not constant; this would generate a warning when
`-Wformat-nonliteral' is used, but the calls could not be checked
without the attribute.
The parameter string-index specifies which argument is the format string argument (starting from 1).
The format-arg attribute allows you to identify your own
functions which modify format strings, so that GCC can check the
calls to printf, scanf, strftime or strfmon
type function whose operands are a call to one of your own function.
The compiler always treats gettext, dgettext, and
dcgettext in this manner except when strict ISO C support is
requested by `-ansi' or an appropriate `-std' option, or
`-ffreestanding' is used. See section Options Controlling C Dialect.
nonnull (arg-index, ...)
nonnull attribute specifies that some function parameters should
be non-null pointers. For instance, the declaration:
extern void * my_memcpy (void *dest, const void *src, size_t len) __attribute__((nonnull (1, 2))); |
causes the compiler to check that, in calls to my_memcpy,
arguments dest and src are non-null. If the compiler
determines that a null pointer is passed in an argument slot marked
as non-null, and the `-Wnonnull' option is enabled, a warning
is issued. The compiler may also choose to make optimizations based
on the knowledge that certain function arguments will not be null.
If no argument index list is given to the nonnull attribute,
all pointer arguments are marked as non-null. To illustrate, the
following declaration is equivalent to the previous example:
extern void * my_memcpy (void *dest, const void *src, size_t len) __attribute__((nonnull)); |
no_instrument_function
section ("section-name")
text section.
Sometimes, however, you need additional sections, or you need certain
particular functions to appear in special sections. The section
attribute specifies that a function lives in a particular section.
For example, the declaration:
extern void foobar (void) __attribute__ ((section ("bar")));
|
puts the function foobar in the bar section.
Some file formats do not support arbitrary sections so the section
attribute is not available on all platforms.
If you need to map the entire contents of a module to a particular
section, consider using the facilities of the linker instead.
constructor
destructor
constructor attribute causes the function to be called
automatically before execution enters main (). Similarly, the
destructor attribute causes the function to be called
automatically after main () has completed or exit () has
been called. Functions with these attributes are useful for
initializing data that will be used implicitly during the execution of
the program.
These attributes are not currently implemented for Objective-C.
unused
used
deprecated
deprecated attribute results in a warning if the function
is used anywhere in the source file. This is useful when identifying
functions that are expected to be removed in a future version of a
program. The warning also includes the location of the declaration
of the deprecated function, to enable users to easily find further
information about why the function is deprecated, or what they should
do instead. Note that the warnings only occurs for uses:
int old_fn () __attribute__ ((deprecated)); int old_fn (); int (*fn_ptr)() = old_fn; |
results in a warning on line 3 but not line 2.
The deprecated attribute can also be used for variables and
types (see section 5.32 Specifying Attributes of Variables, see section 5.33 Specifying Attributes of Types.)
weak
weak attribute causes the declaration to be emitted as a weak
symbol rather than a global. This is primarily useful in defining
library functions which can be overridden in user code, though it can
also be used with non-function declarations. Weak symbols are supported
for ELF targets, and also for a.out targets when using the GNU assembler
and linker.
malloc
malloc attribute is used to tell the compiler that a function
may be treated as if it were the malloc function. The compiler assumes
that calls to malloc result in a pointers that cannot alias anything.
This will often improve optimization.
alias ("target")
alias attribute causes the declaration to be emitted as an
alias for another symbol, which must be specified. For instance,
void __f () { /* Do something. */; }
void f () __attribute__ ((weak, alias ("__f")));
|
declares `f' to be a weak alias for `__f'. In C++, the mangled name for the target must be used.
Not all target machines support this attribute.
visibility ("visibility_type")
visibility attribute on ELF targets causes the declaration
to be emitted with default, hidden, protected or internal visibility.
void __attribute__ ((visibility ("protected")))
f () { /* Do something. */; }
int i __attribute__ ((visibility ("hidden")));
|
See the ELF gABI for complete details, but the short story is
Not all ELF targets support this attribute.
function_vector
You must use GAS and GLD from GNU binutils version 2.7 or later for this attribute to work correctly.
interrupt_handler
sp_switch
interrupt_handler
function should switch to an alternate stack. It expects a string
argument that names a global variable holding the address of the
alternate stack.
void *alt_stack;
void f () __attribute__ ((interrupt_handler,
sp_switch ("alt_stack")));
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trap_exit
interrupt_handle to return using
trapa instead of rte. This attribute expects an integer
argument specifying the trap number to be used.
eightbit_data
You must use GAS and GLD from GNU binutils version 2.7 or later for this attribute to work correctly.
tiny_data
You can specify multiple attributes in a declaration by separating them by commas within the double parentheses or by immediately following an attribute declaration with another attribute declaration.
Some people object to the __attribute__ feature, suggesting that
ISO C's #pragma should be used instead. At the time
__attribute__ was designed, there were two reasons for not doing
this.
#pragma commands from a macro.
#pragma might mean in another
compiler.
These two reasons applied to almost any application that might have been
proposed for #pragma. It was basically a mistake to use
#pragma for anything.
The ISO C99 standard includes _Pragma, which now allows pragmas
to be generated from macros. In addition, a #pragma GCC
namespace is now in use for GCC-specific pragmas. However, it has been
found convenient to use __attribute__ to achieve a natural
attachment of attributes to their corresponding declarations, whereas
#pragma GCC is of use for constructs that do not naturally form
part of the grammar. See section `Miscellaneous Preprocessing Directives' in The C Preprocessor.
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This section describes the syntax with which __attribute__ may be
used, and the constructs to which attribute specifiers bind, for the C
language. Some details may vary for C++ and Objective-C. Because of
infelicities in the grammar for attributes, some forms described here
may not be successfully parsed in all cases.
There are some problems with the semantics of attributes in C++. For
example, there are no manglings for attributes, although they may affect
code generation, so problems may arise when attributed types are used in
conjunction with templates or overloading. Similarly, typeid
does not distinguish between types with different attributes. Support
for attributes in C++ may be restricted in future to attributes on
declarations only, but not on nested declarators.
See section 5.25 Declaring Attributes of Functions, for details of the semantics of attributes applying to functions. See section 5.32 Specifying Attributes of Variables, for details of the semantics of attributes applying to variables. See section 5.33 Specifying Attributes of Types, for details of the semantics of attributes applying to structure, union and enumerated types.
An attribute specifier is of the form
__attribute__ ((attribute-list)). An attribute list
is a possibly empty comma-separated sequence of attributes, where
each attribute is one of the following:
unused, or a reserved
word such as const).
mode attributes use this form.
format attributes use this form.
format_arg attributes use this form with the list being a single
integer constant expression, and alias attributes use this form
with the list being a single string constant.
An attribute specifier list is a sequence of one or more attribute specifiers, not separated by any other tokens.
An attribute specifier list may appear after the colon following a
label, other than a case or default label. The only
attribute it makes sense to use after a label is unused. This
feature is intended for code generated by programs which contains labels
that may be unused but which is compiled with `-Wall'. It would
not normally be appropriate to use in it human-written code, though it
could be useful in cases where the code that jumps to the label is
contained within an #ifdef conditional.
An attribute specifier list may appear as part of a struct,
union or enum specifier. It may go either immediately
after the struct, union or enum keyword, or after
the closing brace. It is ignored if the content of the structure, union
or enumerated type is not defined in the specifier in which the
attribute specifier list is used--that is, in usages such as
struct __attribute__((foo)) bar with no following opening brace.
Where attribute specifiers follow the closing brace, they are considered
to relate to the structure, union or enumerated type defined, not to any
enclosing declaration the type specifier appears in, and the type
defined is not complete until after the attribute specifiers.
Otherwise, an attribute specifier appears as part of a declaration, counting declarations of unnamed parameters and type names, and relates to that declaration (which may be nested in another declaration, for example in the case of a parameter declaration), or to a particular declarator within a declaration. Where an attribute specifier is applied to a parameter declared as a function or an array, it should apply to the function or array rather than the pointer to which the parameter is implicitly converted, but this is not yet correctly implemented.
Any list of specifiers and qualifiers at the start of a declaration may
contain attribute specifiers, whether or not such a list may in that
context contain storage class specifiers. (Some attributes, however,
are essentially in the nature of storage class specifiers, and only make
sense where storage class specifiers may be used; for example,
section.) There is one necessary limitation to this syntax: the
first old-style parameter declaration in a function definition cannot
begin with an attribute specifier, because such an attribute applies to
the function instead by syntax described below (which, however, is not
yet implemented in this case). In some other cases, attribute
specifiers are permitted by this grammar but not yet supported by the
compiler. All attribute specifiers in this place relate to the
declaration as a whole. In the obsolescent usage where a type of
int is implied by the absence of type specifiers, such a list of
specifiers and qualifiers may be an attribute specifier list with no
other specifiers or qualifiers.
An attribute specifier list may appear immediately before a declarator (other than the first) in a comma-separated list of declarators in a declaration of more than one identifier using a single list of specifiers and qualifiers. Such attribute specifiers apply only to the identifier before whose declarator they appear. For example, in
__attribute__((noreturn)) void d0 (void),
__attribute__((format(printf, 1, 2))) d1 (const char *, ...),
d2 (void)
|
the noreturn attribute applies to all the functions
declared; the format attribute only applies to d1.
An attribute specifier list may appear immediately before the comma,
= or semicolon terminating the declaration of an identifier other
than a function definition. At present, such attribute specifiers apply
to the declared object or function, but in future they may attach to the
outermost adjacent declarator. In simple cases there is no difference,
but, for example, in
void (****f)(void) __attribute__((noreturn)); |
at present the noreturn attribute applies to f, which
causes a warning since f is not a function, but in future it may
apply to the function ****f. The precise semantics of what
attributes in such cases will apply to are not yet specified. Where an
assembler name for an object or function is specified (see section 5.37 Controlling Names Used in Assembler Code), at present the attribute must follow the asm
specification; in future, attributes before the asm specification
may apply to the adjacent declarator, and those after it to the declared
object or function.
An attribute specifier list may, in future, be permitted to appear after the declarator in a function definition (before any old-style parameter declarations or the function body).
Attribute specifiers may be mixed with type qualifiers appearing inside
the [] of a parameter array declarator, in the C99 construct by
which such qualifiers are applied to the pointer to which the array is
implicitly converted. Such attribute specifiers apply to the pointer,
not to the array, but at present this is not implemented and they are
ignored.
An attribute specifier list may appear at the start of a nested
declarator. At present, there are some limitations in this usage: the
attributes correctly apply to the declarator, but for most individual
attributes the semantics this implies are not implemented.
When attribute specifiers follow the * of a pointer
declarator, they may be mixed with any type qualifiers present.
The following describes the formal semantics of this syntax. It will make the
most sense if you are familiar with the formal specification of
declarators in the ISO C standard.
Consider (as in C99 subclause 6.7.5 paragraph 4) a declaration T
D1, where T contains declaration specifiers that specify a type
Type (such as int) and D1 is a declarator that
contains an identifier ident. The type specified for ident
for derived declarators whose type does not include an attribute
specifier is as in the ISO C standard.
If D1 has the form ( attribute-specifier-list D ),
and the declaration T D specifies the type
"derived-declarator-type-list Type" for ident, then
T D1 specifies the type "derived-declarator-type-list
attribute-specifier-list Type" for ident.
If D1 has the form *
type-qualifier-and-attribute-specifier-list D, and the
declaration T D specifies the type
"derived-declarator-type-list Type" for ident, then
T D1 specifies the type "derived-declarator-type-list
type-qualifier-and-attribute-specifier-list Type" for
ident.
For example,
void (__attribute__((noreturn)) ****f) (void); |
specifies the type "pointer to pointer to pointer to pointer to
non-returning function returning void". As another example,
char *__attribute__((aligned(8))) *f; |
specifies the type "pointer to 8-byte-aligned pointer to char".
Note again that this does not work with most attributes; for example,
the usage of `aligned' and `noreturn' attributes given above
is not yet supported.
For compatibility with existing code written for compiler versions that did not implement attributes on nested declarators, some laxity is allowed in the placing of attributes. If an attribute that only applies to types is applied to a declaration, it will be treated as applying to the type of that declaration. If an attribute that only applies to declarations is applied to the type of a declaration, it will be treated as applying to that declaration; and, for compatibility with code placing the attributes immediately before the identifier declared, such an attribute applied to a function return type will be treated as applying to the function type, and such an attribute applied to an array element type will be treated as applying to the array type. If an attribute that only applies to function types is applied to a pointer-to-function type, it will be treated as applying to the pointer target type; if such an attribute is applied to a function return type that is not a pointer-to-function type, it will be treated as applying to the function type.
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GNU C extends ISO C to allow a function prototype to override a later old-style non-prototype definition. Consider the following example:
/* Use prototypes unless the compiler is old-fashioned. */
#ifdef __STDC__
#define P(x) x
#else
#define P(x) ()
#endif
/* Prototype function declaration. */
int isroot P((uid_t));
/* Old-style function definition. */
int
isroot (x) /* ??? lossage here ??? */
uid_t x;
{
return x == 0;
}
|
Suppose the type uid_t happens to be short. ISO C does
not allow this example, because subword arguments in old-style
non-prototype definitions are promoted. Therefore in this example the
function definition's argument is really an int, which does not
match the prototype argument type of short.
This restriction of ISO C makes it hard to write code that is portable
to traditional C compilers, because the programmer does not know
whether the uid_t type is short, int, or
long. Therefore, in cases like these GNU C allows a prototype
to override a later old-style definition. More precisely, in GNU C, a
function prototype argument type overrides the argument type specified
by a later old-style definition if the former type is the same as the
latter type before promotion. Thus in GNU C the above example is
equivalent to the following:
int isroot (uid_t);
int
isroot (uid_t x)
{
return x == 0;
}
|
GNU C++ does not support old-style function definitions, so this extension is irrelevant.
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In GNU C, you may use C++ style comments, which start with `//' and continue until the end of the line. Many other C implementations allow such comments, and they are included in the 1999 C standard. However, C++ style comments are not recognized if you specify an `-std' option specifying a version of ISO C before C99, or `-ansi' (equivalent to `-std=c89').
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In GNU C, you may normally use dollar signs in identifier names. This is because many traditional C implementations allow such identifiers. However, dollar signs in identifiers are not supported on a few target machines, typically because the target assembler does not allow them.
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You can use the sequence `\e' in a string or character constant to stand for the ASCII character ESC.
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The keyword __alignof__ allows you to inquire about how an object
is aligned, or the minimum alignment usually required by a type. Its
syntax is just like sizeof.
For example, if the target machine requires a double value to be
aligned on an 8-byte boundary, then __alignof__ (double) is 8.
This is true on many RISC machines. On more traditional machine
designs, __alignof__ (double) is 4 or even 2.
Some machines never actually require alignment; they allow reference to any
data type even at an odd addresses. For these machines, __alignof__
reports the recommended alignment of a type.
If the operand of __alignof__ is an lvalue rather than a type,
its value is the required alignment for its type, taking into account
any minimum alignment specified with GCC's __attribute__
extension (see section 5.32 Specifying Attributes of Variables). For example, after this
declaration:
struct foo { int x; char y; } foo1;
|
the value of __alignof__ (foo1.y) is 1, even though its actual
alignment is probably 2 or 4, the same as __alignof__ (int).
It is an error to ask for the alignment of an incomplete type.
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The keyword __attribute__ allows you to specify special
attributes of variables or structure fields. This keyword is followed
by an attribute specification inside double parentheses. Ten
attributes are currently defined for variables: aligned,
mode, nocommon, packed, section,
transparent_union, unused, deprecated,
vector_size, and weak. Some other attributes are defined
for variables on particular target systems. Other attributes are
available for functions (see section 5.25 Declaring Attributes of Functions) and for types
(see section 5.33 Specifying Attributes of Types). Other front ends might define more
attributes (see section Extensions to the C++ Language).
You may also specify attributes with `__' preceding and following
each keyword. This allows you to use them in header files without
being concerned about a possible macro of the same name. For example,
you may use __aligned__ instead of aligned.
See section 5.26 Attribute Syntax, for details of the exact syntax for using attributes.
aligned (alignment)
int x __attribute__ ((aligned (16))) = 0; |
causes the compiler to allocate the global variable x on a
16-byte boundary. On a 68040, this could be used in conjunction with
an asm expression to access the move16 instruction which
requires 16-byte aligned operands.
You can also specify the alignment of structure fields. For example, to
create a double-word aligned int pair, you could write:
struct foo { int x[2] __attribute__ ((aligned (8))); };
|
This is an alternative to creating a union with a double member
that forces the union to be double-word aligned.
As in the preceding examples, you can explicitly specify the alignment (in bytes) that you wish the compiler to use for a given variable or structure field. Alternatively, you can leave out the alignment factor and just ask the compiler to align a variable or field to the maximum useful alignment for the target machine you are compiling for. For example, you could write:
short array[3] __attribute__ ((aligned)); |
Whenever you leave out the alignment factor in an aligned attribute
specification, the compiler automatically sets the alignment for the declared
variable or field to the largest alignment which is ever used for any data
type on the target machine you are compiling for. Doing this can often make
copy operations more efficient, because the compiler can use whatever
instructions copy the biggest chunks of memory when performing copies to
or from the variables or fields that you have aligned this way.
The aligned attribute can only increase the alignment; but you
can decrease it by specifying packed as well. See below.
Note that the effectiveness of aligned attributes may be limited
by inherent limitations in your linker. On many systems, the linker is
only able to arrange for variables to be aligned up to a certain maximum
alignment. (For some linkers, the maximum supported alignment may
be very very small.) If your linker is only able to align variables
up to a maximum of 8 byte alignment, then specifying aligned(16)
in an __attribute__ will still only provide you with 8 byte
alignment. See your linker documentation for further information.
mode (mode)
You may also specify a mode of `byte' or `__byte__' to indicate the mode corresponding to a one-byte integer, `word' or `__word__' for the mode of a one-word integer, and `pointer' or `__pointer__' for the mode used to represent pointers.
nocommon
Specifying the nocommon attribute for a variable provides an
initialization of zeros. A variable may only be initialized in one
source file.
packed
packed attribute specifies that a variable or structure field
should have the smallest possible alignment--one byte for a variable,
and one bit for a field, unless you specify a larger value with the
aligned attribute.
Here is a structure in which the field x is packed, so that it
immediately follows a:
struct foo
{
char a;
int x[2] __attribute__ ((packed));
};
|
section ("section-name")
data and bss. Sometimes, however, you need additional sections,
or you need certain particular variables to appear in special sections,
for example to map to special hardware. The section
attribute specifies that a variable (or function) lives in a particular
section. For example, this small program uses several specific section names:
struct duart a __attribute__ ((section ("DUART_A"))) = { 0 };
struct duart b __attribute__ ((section ("DUART_B"))) = { 0 };
char stack[10000] __attribute__ ((section ("STACK"))) = { 0 };
int init_data __attribute__ ((section ("INITDATA"))) = 0;
main()
{
/* Initialize stack pointer */
init_sp (stack + sizeof (stack));
/* Initialize initialized data */
memcpy (&init_data, &data, &edata - &data);
/* Turn on the serial ports */
init_duart (&a);
init_duart (&b);
}
|
Use the section attribute with an initialized definition
of a global variable, as shown in the example. GCC issues
a warning and otherwise ignores the section attribute in
uninitialized variable declarations.
You may only use the section attribute with a fully initialized
global definition because of the way linkers work. The linker requires
each object be defined once, with the exception that uninitialized
variables tentatively go in the common (or bss) section
and can be multiply "defined". You can force a variable to be
initialized with the `-fno-common' flag or the nocommon
attribute.
Some file formats do not support arbitrary sections so the section
attribute is not available on all platforms.
If you need to map the entire contents of a module to a particular
section, consider using the facilities of the linker instead.
shared
shared and marking the section
shareable:
int foo __attribute__((section ("shared"), shared)) = 0;
int
main()
{
/* Read and write foo. All running
copies see the same value. */
return 0;
}
|
You may only use the shared attribute along with section
attribute with a fully initialized global definition because of the way
linkers work. See section attribute for more information.
The shared attribute is only available on Windows NT.
transparent_union
typedef for a union data type; then it
applies to all function parameters with that type.
unused
deprecated
deprecated attribute results in a warning if the variable
is used anywhere in the source file. This is useful when identifying
variables that are expected to be removed in a future version of a
program. The warning also includes the location of the declaration
of the deprecated variable, to enable users to easily find further
information about why the variable is deprecated, or what they should
do instead. Note that the warnings only occurs for uses:
extern int old_var __attribute__ ((deprecated));
extern int old_var;
int new_fn () { return old_var; }
|
results in a warning on line 3 but not line 2.
The deprecated attribute can also be used for functions and
types (see section 5.25 Declaring Attributes of Functions, see section 5.33 Specifying Attributes of Types.)
vector_size (bytes)
int foo __attribute__ ((vector_size (16))); |
causes the compiler to set the mode for foo, to be 16 bytes,
divided into int sized units. Assuming a 32-bit int (a vector of
4 units of 4 bytes), the corresponding mode of foo will be V4SI.
This attribute is only applicable to integral and float scalars, although arrays, pointers, and function return values are allowed in conjunction with this construct.
Aggregates with this attribute are invalid, even if they are of the same size as a corresponding scalar. For example, the declaration:
struct S { int a; };
struct S __attribute__ ((vector_size (16))) foo;
|
is invalid even if the size of the structure is the same as the size of
the int.
weak
weak attribute is described in See section 5.25 Declaring Attributes of Functions.
To specify multiple attributes, separate them by commas within the double parentheses: for example, `__attribute__ ((aligned (16), packed))'.
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The keyword __attribute__ allows you to specify special
attributes of struct and union types when you define such
types. This keyword is followed by an attribute specification inside
double parentheses. Six attributes are currently defined for types:
aligned, packed, transparent_union, unused,
deprecated and may_alias. Other attributes are defined for
functions (see section 5.25 Declaring Attributes of Functions) and for variables
(see section 5.32 Specifying Attributes of Variables).
You may also specify any one of these attributes with `__'
preceding and following its keyword. This allows you to use these
attributes in header files without being concerned about a possible
macro of the same name. For example, you may use __aligned__
instead of aligned.
You may specify the aligned and transparent_union
attributes either in a typedef declaration or just past the
closing curly brace of a complete enum, struct or union type
definition and the packed attribute only past the closing
brace of a definition.
You may also specify attributes between the enum, struct or union tag and the name of the type rather than after the closing brace.
See section 5.26 Attribute Syntax, for details of the exact syntax for using attributes.
aligned (alignment)
struct S { short f[3]; } __attribute__ ((aligned (8)));
typedef int more_aligned_int __attribute__ ((aligned (8)));
|
force the compiler to insure (as far as it can) that each variable whose
type is struct S or more_aligned_int will be allocated and
aligned at least on a 8-byte boundary.
Note that the alignment of any given struct or union type
is required by the ISO C standard to be at least a perfect multiple of
the lowest common multiple of the alignments of all of the members of
the struct or union in question. This means that you can
effectively adjust the alignment of a struct or union
type by attaching an aligned attribute to any one of the members
of such a type, but the notation illustrated in the example above is a
more obvious, intuitive, and readable way to request the compiler to
adjust the alignment of an entire struct or union type.
As in the preceding example, you can explicitly specify the alignment
(in bytes) that you wish the compiler to use for a given struct
or union type. Alternatively, you can leave out the alignment factor
and just ask the compiler to align a type to the maximum
useful alignment for the target machine you are compiling for. For
example, you could write:
struct S { short f[3]; } __attribute__ ((aligned));
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Whenever you leave out the alignment factor in an aligned
attribute specification, the compiler automatically sets the alignment
for the type to the largest alignment which is ever used for any data
type on the target machine you are compiling for. Doing this can often
make copy operations more efficient, because the compiler can use
whatever instructions copy the biggest chunks of memory when performing
copies to or from the variables which have types that you have aligned
this way.
In the example above, if the size of each short is 2 bytes, then
the size of the entire struct S type is 6 bytes. The smallest
power of two which is greater than or equal to that is 8, so the
compiler sets the alignment for the entire struct S type to 8
bytes.
Note that although you can ask the compiler to select a time-efficient alignment for a given type and then declare only individual stand-alone objects of that type, the compiler's ability to select a time-efficient alignment is primarily useful only when you plan to create arrays of variables having the relevant (efficiently aligned) type. If you declare or use arrays of variables of an efficiently-aligned type, then it is likely that your program will also be doing pointer arithmetic (or subscripting, which amounts to the same thing) on pointers to the relevant type, and the code that the compiler generates for these pointer arithmetic operations will often be more efficient for efficiently-aligned types than for other types.
The aligned attribute can only increase the alignment; but you
can decrease it by specifying packed as well. See below.
Note that the effectiveness of aligned attributes may be limited
by inherent limitations in your linker. On many systems, the linker is
only able to arrange for variables to be aligned up to a certain maximum
alignment. (For some linkers, the maximum supported alignment may
be very very small.) If your linker is only able to align variables
up to a maximum of 8 byte alignment, then specifying aligned(16)
in an __attribute__ will still only provide you with 8 byte
alignment. See your linker documentation for further information.
packed
enum, struct, or
union type definition, specified that the minimum required memory
be used to represent the type.
Specifying this attribute for struct and union types is
equivalent to specifying the packed attribute on each of the
structure or union members. Specifying the `-fshort-enums'
flag on the line is equivalent to specifying the packed
attribute on all enum definitions.
You may only specify this attribute after a closing curly brace on an
enum definition, not in a typedef declaration, unless that
declaration also contains the definition of the enum.
transparent_union
union type definition, indicates
that any function parameter having that union type causes calls to that
function to be treated in a special way.
First, the argument corresponding to a transparent union type can be of
any type in the union; no cast is required. Also, if the union contains
a pointer type, the corresponding argument can be a null pointer
constant or a void pointer expression; and if the union contains a void
pointer type, the corresponding argument can be any pointer expression.
If the union member type is a pointer, qualifiers like const on
the referenced type must be respected, just as with normal pointer
conversions.
Second, the argument is passed to the function using the calling conventions of first member of the transparent union, not the calling conventions of the union itself. All members of the union must have the same machine representation; this is necessary for this argument passing to work properly.
Transparent unions are designed for library functions that have multiple
interfaces for compatibility reasons. For example, suppose the
wait function must accept either a value of type int * to
comply with Posix, or a value of type union wait * to comply with
the 4.1BSD interface. If wait's parameter were void *,
wait would accept both kinds of arguments, but it would also
accept any other pointer type and this would make argument type checking
less useful. Instead, <sys/wait.h> might define the interface
as follows:
typedef union
{
int *__ip;
union wait *__up;
} wait_status_ptr_t __attribute__ ((__transparent_union__));
pid_t wait (wait_status_ptr_t);
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This interface allows either int * or union wait *
arguments to be passed, using the int * calling convention.
The program can call wait with arguments of either type:
int w1 () { int w; return wait (&w); }
int w2 () { union wait w; return wait (&w); }
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With this interface, wait's implementation might look like this:
pid_t wait (wait_status_ptr_t p)
{
return waitpid (-1, p.__ip, 0);
}
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unused
union or a struct),
this attribute means that variables of that type are meant to appear
possibly unused. GCC will not produce a warning for any variables of
that type, even if the variable appears to do nothing. This is often
the case with lock or thread classes, which are usually defined and then
not referenced, but contain constructors and destructors that have
nontrivial bookkeeping functions.
deprecated
deprecated attribute results in a warning if the type
is used anywhere in the source file. This is useful when identifying
types that are expected to be removed in a future version of a program.
If possible, the warning also includes the location of the declaration
of the deprecated type, to enable users to easily find further
information about why the type is deprecated, or what they should do
instead. Note that the warnings only occur for uses and then only
if the type is being applied to an identifier that itself is not being
declared as deprecated.
typedef int T1 __attribute__ ((deprecated)); T1 x; typedef T1 T2; T2 y; typedef T1 T3 __attribute__ ((deprecated)); T3 z __attribute__ ((deprecated)); |
results in a warning on line 2 and 3 but not lines 4, 5, or 6. No warning is issued for line 4 because T2 is not explicitly deprecated. Line 5 has no warning because T3 is explicitly deprecated. Similarly for line 6.
The deprecated attribute can also be used for functions and
variables (see section 5.25 Declaring Attributes of Functions, see section 5.32 Specifying Attributes of Variables.)
may_alias
char type. See
`-fstrict-aliasing' for more information on aliasing issues.
Example of use:
typedef short __attribute__((__may_alias__)) short_a;
int
main (void)
{
int a = 0x12345678;
short_a *b = (short_a *) &a;
b[1] = 0;
if (a == 0x12345678)
abort();
exit(0);
}
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If you replaced short_a with short in the variable
declaration, the above program would abort when compiled with
`-fstrict-aliasing', which is on by default at `-O2' or
above in recent GCC versions.
To specify multiple attributes, separate them by commas within the double parentheses: for example, `__attribute__ ((aligned (16), packed))'.
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By declaring a function inline, you can direct GCC to
integrate that function's code into the code for its callers. This
makes execution faster by eliminating the function-call overhead; in
addition, if any of the actual argument values are constant, their known
values may permit simplifications at compile time so that not all of the
inline function's code needs to be included. The effect on code size is
less predictable; object code may be larger or smaller with function
inlining, depending on the particular case. Inlining of functions is an
optimization and it really "works" only in optimizing compilation. If
you don't use `-O', no function is really inline.
Inline functions are included in the ISO C99 standard, but there are currently substantial differences between what GCC implements and what the ISO C99 standard requires.
To declare a function inline, use the inline keyword in its
declaration, like this:
inline int
inc (int *a)
{
(*a)++;
}
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(If you are writing a header file to be included in ISO C programs, write
__inline__ instead of inline. See section 5.39 Alternate Keywords.)
You can also make all "simple enough" functions inline with the option
`-finline-functions'.
Note that certain usages in a function definition can make it unsuitable
for inline substitution. Among these usages are: use of varargs, use of
alloca, use of variable sized data types (see section 5.13 Arrays of Variable Length),
use of computed goto (see section 5.3 Labels as Values), use of nonlocal goto,
and nested functions (see section 5.4 Nested Functions). Using `-Winline'
will warn when a function marked inline could not be substituted,
and will give the reason for the failure.
Note that in C and Objective-C, unlike C++, the inline keyword
does not affect the linkage of the function.
GCC automatically inlines member functions defined within the class
body of C++ programs even if they are not explicitly declared
inline. (You can override this with `-fno-default-inline';
see section Options Controlling C++ Dialect.)
When a function is both inline and static, if all calls to the
function are integrated into the caller, and the function's address is
never used, then the function's own assembler code is never referenced.
In this case, GCC does not actually output assembler code for the
function, unless you specify the option `-fkeep-inline-functions'.
Some calls cannot be integrated for various reasons (in particular,
calls that precede the function's definition cannot be integrated, and
neither can recursive calls within the definition). If there is a
nonintegrated call, then the function is compiled to assembler code as
usual. The function must also be compiled as usual if the program
refers to its address, because that can't be inlined.
When an inline function is not static, then the compiler must assume
that there may be calls from other source files; since a global symbol can
be defined only once in any program, the function must not be defined in
the other source files, so the calls therein cannot be integrated.
Therefore, a non-static inline function is always compiled on its
own in the usual fashion.
If you specify both inline and extern in the function
definition, then the definition is used only for inlining. In no case
is the function compiled on its own, not even if you refer to its
address explicitly. Such an address becomes an external reference, as
if you had only declared the function, and had not defined it.
This combination of inline and extern has almost the
effect of a macro. The way to use it is to put a function definition in
a header file with these keywords, and put another copy of the
definition (lacking inline and extern) in a library file.
The definition in the header file will cause most calls to the function
to be inlined. If any uses of the function remain, they will refer to
the single copy in the library.
For future compatibility with when GCC implements ISO C99 semantics for
inline functions, it is best to use static inline only. (The
existing semantics will remain available when `-std=gnu89' is
specified, but eventually the default will be `-std=gnu99' and
that will implement the C99 semantics, though it does not do so yet.)
GCC does not inline any functions when not optimizing unless you specify the `always_inline' attribute for the function, like this:
/* Prototype. */ inline void foo (const char) __attribute__((always_inline)); |
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In an assembler instruction using asm, you can specify the
operands of the instruction using C expressions. This means you need not
guess which registers or memory locations will contain the data you want
to use.
You must specify an assembler instruction template much like what appears in a machine description, plus an operand constraint string for each operand.
For example, here is how to use the 68881's fsinx instruction:
asm ("fsinx %1,%0" : "=f" (result) : "f" (angle));
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Here angle is the C expression for the input operand while
result is that of the output operand. Each has `"f"' as its
operand constraint, saying that a floating point register is required.
The `=' in `=f' indicates that the operand is an output; all
output operands' constraints must use `='. The constraints use the
same language used in the machine description (see section 5.36 Constraints for asm Operands).
Each operand is described by an operand-constraint string followed by the C expression in parentheses. A colon separates the assembler template from the first output operand and another separates the last output operand from the first input, if any. Commas separate the operands within each group. The total number of operands is currently limited to 30; this limitation may be lifted in some future version of GCC.
If there are no output operands but there are input operands, you must place two consecutive colons surrounding the place where the output operands would go.
As of GCC version 3.1, it is also possible to specify input and output
operands using symbolic names which can be referenced within the
assembler code. These names are specified inside square brackets
preceding the constraint string, and can be referenced inside the
assembler code using %[name] instead of a percentage sign
followed by the operand number. Using named operands the above example
could look like:
asm ("fsinx %[angle],%[output]"
: [output] "=f" (result)
: [angle] "f" (angle));
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Note that the symbolic operand names have no relation whatsoever to other C identifiers. You may use any name you like, even those of existing C symbols, but must ensure that no two operands within the same assembler construct use the same symbolic name.
Output operand expressions must be lvalues; the compiler can check this.
The input operands need not be lvalues. The compiler cannot check
whether the operands have data types that are reasonable for the
instruction being executed. It does not parse the assembler instruction
template and does not know what it means or even whether it is valid
assembler input. The extended asm feature is most often used for
machine instructions the compiler itself does not know exist. If
the output expression cannot be directly addressed (for example, it is a
bit-field), your constraint must allow a register. In that case, GCC
will use the register as the output of the asm, and then store
that register into the output.
The ordinary output operands must be write-only; GCC will assume that the values in these operands before the instruction are dead and need not be generated. Extended asm supports input-output or read-write operands. Use the constraint character `+' to indicate such an operand and list it with the output operands.
When the constraints for the read-write operand (or the operand in which
only some of the bits are to be changed) allows a register, you may, as
an alternative, logically split its function into two separate operands,
one input operand and one write-only output operand. The connection
between them is expressed by constraints which say they need to be in
the same location when the instruction executes. You can use the same C
expression for both operands, or different expressions. For example,
here we write the (fictitious) `combine' instruction with
bar as its read-only source operand and foo as its
read-write destination:
asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar));
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The constraint `"0"' for operand 1 says that it must occupy the same location as operand 0. A number in constraint is allowed only in an input operand and it must refer to an output operand.
Only a number in the constraint can guarantee that one operand will be in
the same place as another. The mere fact that foo is the value
of both operands is not enough to guarantee that they will be in the
same place in the generated assembler code. The following would not
work reliably:
asm ("combine %2,%0" : "=r" (foo) : "r" (foo), "g" (bar));
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Various optimizations or reloading could cause operands 0 and 1 to be in
different registers; GCC knows no reason not to do so. For example, the
compiler might find a copy of the value of foo in one register and
use it for operand 1, but generate the output operand 0 in a different
register (copying it afterward to foo's own address). Of course,
since the register for operand 1 is not even mentioned in the assembler
code, the result will not work, but GCC can't tell that.
As of GCC version 3.1, one may write [name] instead of
the operand number for a matching constraint. For example:
asm ("cmoveq %1,%2,%[result]"
: [result] "=r"(result)
: "r" (test), "r"(new), "[result]"(old));
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Some instructions clobber specific hard registers. To describe this, write a third colon after the input operands, followed by the names of the clobbered hard registers (given as strings). Here is a realistic example for the VAX:
asm volatile ("movc3 %0,%1,%2"
: /* no outputs */
: "g" (from), "g" (to), "g" (count)
: "r0", "r1", "r2", "r3", "r4", "r5");
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You may not write a clobber description in a way that overlaps with an
input or output operand. For example, you may not have an operand
describing a register class with one member if you mention that register
in the clobber list. Variables declared to live in specific registers
(see section 5.38 Variables in Specified Registers), and used as asm input or output operands must
have no part mentioned in the clobber description.
There is no way for you to specify that an input
operand is modified without also specifying it as an output
operand. Note that if all the output operands you specify are for this
purpose (and hence unused), you will then also need to specify
volatile for the asm construct, as described below, to
prevent GCC from deleting the asm statement as unused.
If you refer to a particular hardware register from the assembler code, you will probably have to list the register after the third colon to tell the compiler the register's value is modified. In some assemblers, the register names begin with `%'; to produce one `%' in the assembler code, you must write `%%' in the input.
If your assembler instruction can alter the condition code register, add `cc' to the list of clobbered registers. GCC on some machines represents the condition codes as a specific hardware register; `cc' serves to name this register. On other machines, the condition code is handled differently, and specifying `cc' has no effect. But it is valid no matter what the machine.
If your assembler instruction modifies memory in an unpredictable
fashion, add `memory' to the list of clobbered registers. This
will cause GCC to not keep memory values cached in registers across
the assembler instruction. You will also want to add the
volatile keyword if the memory affected is not listed in the
inputs or outputs of the asm, as the `memory' clobber does
not count as a side-effect of the asm.
You can put multiple assembler instructions together in a single
asm template, separated by the characters normally used in assembly
code for the system. A combination that works in most places is a newline
to break the line, plus a tab character to move to the instruction field
(written as `\n\t'). Sometimes semicolons can be used, if the
assembler allows semicolons as a line-breaking character. Note that some
assembler dialects use semicolons to start a comment.
The input operands are guaranteed not to use any of the clobbered
registers, and neither will the output operands' addresses, so you can
read and write the clobbered registers as many times as you like. Here
is an example of multiple instructions in a template; it assumes the
subroutine _foo accepts arguments in registers 9 and 10:
asm ("movl %0,r9\n\tmovl %1,r10\n\tcall _foo"
: /* no outputs */
: "g" (from), "g" (to)
: "r9", "r10");
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Unless an output operand has the `&' constraint modifier, GCC may allocate it in the same register as an unrelated input operand, on the assumption the inputs are consumed before the outputs are produced. This assumption may be false if the assembler code actually consists of more than one instruction. In such a case, use `&' for each output operand that may not overlap an input. See section 5.36.3 Constraint Modifier Characters.
If you want to test the condition code produced by an assembler
instruction, you must include a branch and a label in the asm
construct, as follows:
asm ("clr %0\n\tfrob %1\n\tbeq 0f\n\tmov #1,%0\n0:"
: "g" (result)
: "g" (input));
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This assumes your assembler supports local labels, as the GNU assembler and most Unix assemblers do.
Speaking of labels, jumps from one asm to another are not
supported. The compiler's optimizers do not know about these jumps, and
therefore they cannot take account of them when deciding how to
optimize.
Usually the most convenient way to use these asm instructions is to
encapsulate them in macros that look like functions. For example,
#define sin(x) \
({ double __value, __arg = (x); \
asm ("fsinx %1,%0": "=f" (__value): "f" (__arg)); \
__value; })
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Here the variable __arg is used to make sure that the instruction
operates on a proper double value, and to accept only those
arguments x which can convert automatically to a double.
Another way to make sure the instruction operates on the correct data
type is to use a cast in the asm. This is different from using a
variable __arg in that it converts more different types. For
example, if the desired type were int, casting the argument to
int would accept a pointer with no complaint, while assigning the
argument to an int variable named __arg would warn about
using a pointer unless the caller explicitly casts it.
If an asm has output operands, GCC assumes for optimization
purposes the instruction has no side effects except to change the output
operands. This does not mean instructions with a side effect cannot be
used, but you must be careful, because the compiler may eliminate them
if the output operands aren't used, or move them out of loops, or
replace two with one if they constitute a common subexpression. Also,
if your instruction does have a side effect on a variable that otherwise
appears not to change, the old value of the variable may be reused later
if it happens to be found in a register.
You can prevent an asm instruction from being deleted, moved
significantly, or combined, by writing the keyword volatile after
the asm. For example:
#define get_and_set_priority(new) \
({ int __old; \
asm volatile ("get_and_set_priority %0, %1" \
: "=g" (__old) : "g" (new)); \
__old; })
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If you write an asm instruction with no outputs, GCC will know
the instruction has side-effects and will not delete the instruction or
move it outside of loops.
The volatile keyword indicates that the instruction has
important side-effects. GCC will not delete a volatile asm if
it is reachable. (The instruction can still be deleted if GCC can
prove that control-flow will never reach the location of the
instruction.) In addition, GCC will not reschedule instructions
across a volatile asm instruction. For example:
*(volatile int *)addr = foo;
asm volatile ("eieio" : : );
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Assume addr contains the address of a memory mapped device
register. The PowerPC eieio instruction (Enforce In-order
Execution of I/O) tells the CPU to make sure that the store to that
device register happens before it issues any other I/O.
Note that even a volatile asm instruction can be moved in ways
that appear insignificant to the compiler, such as across jump
instructions. You can't expect a sequence of volatile asm
instructions to remain perfectly consecutive. If you want consecutive
output, use a single asm. Also, GCC will perform some
optimizations across a volatile asm instruction; GCC does not
"forget everything" when it encounters a volatile asm
instruction the way some other compilers do.
An asm instruction without any operands or clobbers (an "old
style" asm) will be treated identically to a volatile
asm instruction.
It is a natural idea to look for a way to give access to the condition code left by the assembler instruction. However, when we attempted to implement this, we found no way to make it work reliably. The problem is that output operands might need reloading, which would result in additional following "store" instructions. On most machines, these instructions would alter the condition code before there was time to test it. This problem doesn't arise for ordinary "test" and "compare" instructions because they don't have any output operands.
For reasons similar to those described above, it is not possible to give an assembler instruction access to the condition code left by previous instructions.
If you are writing a header file that should be includable in ISO C
programs, write __asm__ instead of asm. See section 5.39 Alternate Keywords.
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There are several rules on the usage of stack-like regs in asm_operands insns. These rules apply only to the operands that are stack-like regs:
An input reg that is implicitly popped by the asm must be explicitly clobbered, unless it is constrained to match an output operand.
All implicitly popped input regs must be closer to the top of the reg-stack than any input that is not implicitly popped.
It is possible that if an input dies in an insn, reload might use the input reg for an output reload. Consider this example:
asm ("foo" : "=t" (a) : "f" (b));
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This asm says that input B is not popped by the asm, and that the asm pushes a result onto the reg-stack, i.e., the stack is one deeper after the asm than it was before. But, it is possible that reload will think that it can use the same reg for both the input and the output, if input B dies in this insn.
If any input operand uses the f constraint, all output reg
constraints must use the & earlyclobber.
The asm above would be written as
asm ("foo" : "=&t" (a) : "f" (b));
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Output operands must specifically indicate which reg an output
appears in after an asm. =f is not allowed: the operand
constraints must select a class with a single reg.
Output operands must start at the top of the reg-stack: output operands may not "skip" a reg.
Here are a couple of reasonable asms to want to write. This asm takes one input, which is internally popped, and produces two outputs.
asm ("fsincos" : "=t" (cos), "=u" (sin) : "0" (inp));
|
This asm takes two inputs, which are popped by the fyl2xp1 opcode,
and replaces them with one output. The user must code the st(1)
clobber for reg-stack.c to know that fyl2xp1 pops both inputs.
asm ("fyl2xp1" : "=t" (result) : "0" (x), "u" (y) : "st(1)");
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asm Operands
Here are specific details on what constraint letters you can use with
asm operands.
Constraints can say whether
an operand may be in a register, and which kinds of register; whether the
operand can be a memory reference, and which kinds of address; whether the
operand may be an immediate constant, and which possible values it may
have. Constraints can also require two operands to match.
5.36.1 Simple Constraints Basic use of constraints. 5.36.2 Multiple Alternative Constraints When an insn has two alternative constraint-patterns. 5.36.3 Constraint Modifier Characters More precise control over effects of constraints. 5.36.4 Constraints for Particular Machines Special constraints for some particular machines.
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The simplest kind of constraint is a string full of letters, each of which describes one kind of operand that is permitted. Here are the letters that are allowed:
For example, an address which is constant is offsettable; so is an address that is the sum of a register and a constant (as long as a slightly larger constant is also within the range of address-offsets supported by the machine); but an autoincrement or autodecrement address is not offsettable. More complicated indirect/indexed addresses may or may not be offsettable depending on the other addressing modes that the machine supports.
Note that in an output operand which can be matched by another operand, the constraint letter `o' is valid only when accompanied by both `<' (if the target machine has predecrement addressing) and `>' (if the target machine has preincrement addressing).
const_double) is
allowed, but only if the target floating point format is the same as
that of the host machine (on which the compiler is running).
const_double or
const_vector) is allowed.
This might appear strange; if an insn allows a constant operand with a value not known at compile time, it certainly must allow any known value. So why use `s' instead of `i'? Sometimes it allows better code to be generated.
For example, on the 68000 in a fullword instruction it is possible to use an immediate operand; but if the immediate value is between -128 and 127, better code results from loading the value into a register and using the register. This is because the load into the register can be done with a `moveq' instruction. We arrange for this to happen by defining the letter `K' to mean "any integer outside the range -128 to 127", and then specifying `Ks' in the operand constraints.
This number is allowed to be more than a single digit. If multiple digits are encountered consecutively, they are interpreted as a single decimal integer. There is scant chance for ambiguity, since to-date it has never been desirable that `10' be interpreted as matching either operand 1 or operand 0. Should this be desired, one can use multiple alternatives instead.
This is called a matching constraint and what it really means is
that the assembler has only a single operand that fills two roles
which asm distinguishes. For example, an add instruction uses
two input operands and an output operand, but on most CISC
machines an add instruction really has only two operands, one of them an
input-output operand:
addl #35,r12 |
Matching constraints are used in these circumstances. More precisely, the two operands that match must include one input-only operand and one output-only operand. Moreover, the digit must be a smaller number than the number of the operand that uses it in the constraint.
`p' in the constraint must be accompanied by address_operand
as the predicate in the match_operand. This predicate interprets
the mode specified in the match_operand as the mode of the memory
reference for which the address would be valid.
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Sometimes a single instruction has multiple alternative sets of possible operands. For example, on the 68000, a logical-or instruction can combine register or an immediate value into memory, or it can combine any kind of operand into a register; but it cannot combine one memory location into another.
These constraints are represented as multiple alternatives. An alternative can be described by a series of letters for each operand. The overall constraint for an operand is made from the letters for this operand from the first alternative, a comma, the letters for this operand from the second alternative, a comma, and so on until the last alternative.
If all the operands fit any one alternative, the instruction is valid. Otherwise, for each alternative, the compiler counts how many instructions must be added to copy the operands so that that alternative applies. The alternative requiring the least copying is chosen. If two alternatives need the same amount of copying, the one that comes first is chosen. These choices can be altered with the `?' and `!' characters:
?
!
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Here are constraint modifier characters.
When the compiler fixes up the operands to satisfy the constraints, it needs to know which operands are inputs to the instruction and which are outputs from it. `=' identifies an output; `+' identifies an operand that is both input and output; all other operands are assumed to be input only.
If you specify `=' or `+' in a constraint, you put it in the first character of the constraint string.
`&' applies only to the alternative in which it is written. In constraints with multiple alternatives, sometimes one alternative requires `&' while others do not. See, for example, the `movdf' insn of the 68000.
An input operand can be tied to an earlyclobber operand if its only use as an input occurs before the early result is written. Adding alternatives of this form often allows GCC to produce better code when only some of the inputs can be affected by the earlyclobber. See, for example, the `mulsi3' insn of the ARM.
`&' does not obviate the need to write `='.
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Whenever possible, you should use the general-purpose constraint letters
in asm arguments, since they will convey meaning more readily to
people reading your code. Failing that, use the constraint letters
that usually have very similar meanings across architectures. The most
commonly used constraints are `m' and `r' (for memory and
general-purpose registers respectively; see section 5.36.1 Simple Constraints), and
`I', usually the letter indicating the most common
immediate-constant format.
For each machine architecture, the
`config/machine/machine.h' file defines additional
constraints. These constraints are used by the compiler itself for
instruction generation, as well as for asm statements; therefore,
some of the constraints are not particularly interesting for asm.
The constraints are defined through these macros:
REG_CLASS_FROM_LETTER
CONST_OK_FOR_LETTER_P
CONST_DOUBLE_OK_FOR_LETTER_P
EXTRA_CONSTRAINT
Inspecting these macro definitions in the compiler source for your machine is the best way to be certain you have the right constraints. However, here is a summary of the machine-dependent constraints available on some particular machines.
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You can specify the name to be used in the assembler code for a C
function or variable by writing the asm (or __asm__)
keyword after the declarator as follows:
int foo asm ("myfoo") = 2;
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This specifies that the name to be used for the variable foo in
the assembler code should be `myfoo' rather than the usual
`_foo'.
On systems where an underscore is normally prepended to the name of a C function or variable, this feature allows you to define names for the linker that do not start with an underscore.
It does not make sense to use this feature with a non-static local variable since such variables do not have assembler names. If you are trying to put the variable in a particular register, see 5.38 Variables in Specified Registers. GCC presently accepts such code with a warning, but will probably be changed to issue an error, rather than a warning, in the future.
You cannot use asm in this way in a function definition; but
you can get the same effect by writing a declaration for the function
before its definition and putting asm there, like this:
extern func () asm ("FUNC");
func (x, y)
int x, y;
/* ... */
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It is up to you to make sure that the assembler names you choose do not conflict with any other assembler symbols. Also, you must not use a register name; that would produce completely invalid assembler code. GCC does not as yet have the ability to store static variables in registers. Perhaps that will be added.
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GNU C allows you to put a few global variables into specified hardware registers. You can also specify the register in which an ordinary register variable should be allocated.
These local variables are sometimes convenient for use with the extended
asm feature (see section 5.35 Assembler Instructions with C Expression Operands), if you want to write one
output of the assembler instruction directly into a particular register.
(This will work provided the register you specify fits the constraints
specified for that operand in the asm.)
5.38.1 Defining Global Register Variables 5.38.2 Specifying Registers for Local Variables
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You can define a global register variable in GNU C like this:
register int *foo asm ("a5");
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Here a5 is the name of the register which should be used. Choose a
register which is normally saved and restored by function calls on your
machine, so that library routines will not clobber it.
Naturally the register name is cpu-dependent, so you would need to
conditionalize your program according to cpu type. The register
a5 would be a good choice on a 68000 for a variable of pointer
type. On machines with register windows, be sure to choose a "global"
register that is not affected magically by the function call mechanism.
In addition, operating systems on one type of cpu may differ in how they
name the registers; then you would need additional conditionals. For
example, some 68000 operating systems call this register %a5.
Eventually there may be a way of asking the compiler to choose a register automatically, but first we need to figure out how it should choose and how to enable you to guide the choice. No solution is evident.
Defining a global register variable in a certain register reserves that register entirely for this use, at least within the current compilation. The register will not be allocated for any other purpose in the functions in the current compilation. The register will not be saved and restored by these functions. Stores into this register are never deleted even if they would appear to be dead, but references may be deleted or moved or simplified.
It is not safe to access the global register variables from signal handlers, or from more than one thread of control, because the system library routines may temporarily use the register for other things (unless you recompile them specially for the task at hand).
It is not safe for one function that uses a global register variable to
call another such function foo by way of a third function
lose that was compiled without knowledge of this variable (i.e. in a
different source file in which the variable wasn't declared). This is
because lose might save the register and put some other value there.
For example, you can't expect a global register variable to be available in
the comparison-function that you pass to qsort, since qsort
might have put something else in that register. (If you are prepared to
recompile qsort with the same global register variable, you can
solve this problem.)
If you want to recompile qsort or other source files which do not
actually use your global register variable, so that they will not use that
register for any other purpose, then it suffices to specify the compiler
option `-ffixed-reg'. You need not actually add a global
register declaration to their source code.
A function which can alter the value of a global register variable cannot safely be called from a function compiled without this variable, because it could clobber the value the caller expects to find there on return. Therefore, the function which is the entry point into the part of the program that uses the global register variable must explicitly save and restore the value which belongs to its caller.
On most machines, longjmp will restore to each global register
variable the value it had at the time of the setjmp. On some
machines, however, longjmp will not change the value of global
register variables. To be portable, the function that called setjmp
should make other arrangements to save the values of the global register
variables, and to restore them in a longjmp. This way, the same
thing will happen regardless of what longjmp does.
All global register variable declarations must precede all function definitions. If such a declaration could appear after function definitions, the declaration would be too late to prevent the register from being used for other purposes in the preceding functions.
Global register variables may not have initial values, because an executable file has no means to supply initial contents for a register.
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You can define a local register variable with a specified register like this:
register int *foo asm ("a5");
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Here a5 is the name of the register which should be used. Note
that this is the same syntax used for defining global register
variables, but for a local variable it would appear within a function.
Naturally the register name is cpu-dependent, but this is not a problem, since specific registers are most often useful with explicit assembler instructions (see section 5.35 Assembler Instructions with C Expression Operands). Both of these things generally require that you conditionalize your program according to cpu type.
In addition, operating systems on one type of cpu may differ in how they
name the registers; then you would need additional conditionals. For
example, some 68000 operating systems call this register %a5.
Defining such a register variable does not reserve the register; it remains available for other uses in places where flow control determines the variable's value is not live. However, these registers are made unavailable for use in the reload pass; excessive use of this feature leaves the compiler too few available registers to compile certain functions.
This option does not guarantee that GCC will generate code that has
this variable in the register you specify at all times. You may not
code an explicit reference to this register in an asm statement
and assume it will always refer to this variable.
Stores into local register variables may be deleted when they appear to be dead according to dataflow analysis. References to local register variables may be deleted or moved or simplified.
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`-ansi' and the various `-std' options disable certain
keywords. This causes trouble when you want to use GNU C extensions, or
a general-purpose header file that should be usable by all programs,
including ISO C programs. The keywords asm, typeof and
inline are not available in programs compiled with
`-ansi' or `-std' (although inline can be used in a
program compiled with `-std=c99'). The ISO C99 keyword
restrict is only available when `-std=gnu99' (which will
eventually be the default) or `-std=c99' (or the equivalent
`-std=iso9899:1999') is used.
The way to solve these problems is to put `__' at the beginning and
end of each problematical keyword. For example, use __asm__
instead of asm, and __inline__ instead of inline.
Other C compilers won't accept these alternative keywords; if you want to compile with another compiler, you can define the alternate keywords as macros to replace them with the customary keywords. It looks like this:
#ifndef __GNUC__ #define __asm__ asm #endif |
`-pedantic' and other options cause warnings for many GNU C extensions.
You can
prevent such warnings within one expression by writing
__extension__ before the expression. __extension__ has no
effect aside from this.
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enum Types
You can define an enum tag without specifying its possible values.
This results in an incomplete type, much like what you get if you write
struct foo without describing the elements. A later declaration
which does specify the possible values completes the type.
You can't allocate variables or storage using the type while it is incomplete. However, you can work with pointers to that type.
This extension may not be very useful, but it makes the handling of
enum more consistent with the way struct and union
are handled.
This extension is not supported by GNU C++.
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GCC predefines two magic identifiers to hold the name of the current
function. The identifier __FUNCTION__ holds the name of the function
as it appears in the source. The identifier __PRETTY_FUNCTION__
holds the name of the function pretty printed in a language specific
fashion.
These names are always the same in a C function, but in a C++ function they may be different. For example, this program:
extern "C" {
extern int printf (char *, ...);
}
class a {
public:
sub (int i)
{
printf ("__FUNCTION__ = %s\n", __FUNCTION__);
printf ("__PRETTY_FUNCTION__ = %s\n", __PRETTY_FUNCTION__);
}
};
int
main (void)
{
a ax;
ax.sub (0);
return 0;
}
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gives this output:
__FUNCTION__ = sub __PRETTY_FUNCTION__ = int a::sub (int) |
The compiler automagically replaces the identifiers with a string
literal containing the appropriate name. Thus, they are neither
preprocessor macros, like __FILE__ and __LINE__, nor
variables. This means that they catenate with other string literals, and
that they can be used to initialize char arrays. For example
char here[] = "Function " __FUNCTION__ " in " __FILE__; |
On the other hand, `#ifdef __FUNCTION__' does not have any special
meaning inside a function, since the preprocessor does not do anything
special with the identifier __FUNCTION__.
Note that these semantics are deprecated, and that GCC 3.2 will handle
__FUNCTION__ and __PRETTY_FUNCTION__ the same way as
__func__. __func__ is defined by the ISO standard C99:
The identifier __func__ is implicitly declared by the translator
as if, immediately following the opening brace of each function
definition, the declaration
static const char __func__[] = "function-name"; |
appeared, where function-name is the name of the lexically-enclosing function. This name is the unadorned name of the function.
By this definition, __func__ is a variable, not a string literal.
In particular, __func__ does not catenate with other string
literals.
In C++, __FUNCTION__ and __PRETTY_FUNCTION__ are
variables, declared in the same way as __func__.
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These functions may be used to get information about the callers of a function.
0 yields the return address
of the current function, a value of 1 yields the return address
of the caller of the current function, and so forth. When inlining
the expected behavior is that the function will return the address of
the function that will be returned to. To work around this behavior use
the noinline function attribute.
The level argument must be a constant integer.
On some machines it may be impossible to determine the return address of
any function other than the current one; in such cases, or when the top
of the stack has been reached, this function will return 0 or a
random value. In addition, __builtin_frame_address may be used
to determine if the top of the stack has been reached.
This function should only be used with a nonzero argument for debugging purposes.
__builtin_return_address, but it
returns the address of the function frame rather than the return address
of the function. Calling __builtin_frame_address with a value of
0 yields the frame address of the current function, a value of
1 yields the frame address of the caller of the current function,
and so forth.
The frame is the area on the stack which holds local variables and saved
registers. The frame address is normally the address of the first word
pushed on to the stack by the function. However, the exact definition
depends upon the processor and the calling convention. If the processor
has a dedicated frame pointer register, and the function has a frame,
then __builtin_frame_address will return the value of the frame
pointer register.
On some machines it may be impossible to determine the frame address of
any function other than the current one; in such cases, or when the top
of the stack has been reached, this function will return 0 if
the first frame pointer is properly initialized by the startup code.
This function should only be used with a nonzero argument for debugging purposes.
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On some targets, the instruction set contains SIMD vector instructions that operate on multiple values contained in one large register at the same time. For example, on the i386 the MMX, 3Dnow! and SSE extensions can be used this way.
The first step in using these extensions is to provide the necessary data
types. This should be done using an appropriate typedef:
typedef int v4si __attribute__ ((mode(V4SI))); |
The base type int is effectively ignored by the compiler, the
actual properties of the new type v4si are defined by the
__attribute__. It defines the machine mode to be used; for vector
types these have the form VnB; n should be the
number of elements in the vector, and B should be the base mode of the
individual elements. The following can be used as base modes:
QI
HI
SI
DI
SF
DF
Specifying a combination that is not valid for the current architecture
will cause gcc to synthesize the instructions using a narrower mode.
For example, if you specify a variable of type V4SI and your
architecture does not allow for this specific SIMD type, gcc will
produce code that uses 4 SIs.
The types defined in this manner can be used with a subset of normal C
operations. Currently, gcc will allow using the following operators on
these types: +, -, *, /, unary minus.
The operations behave like C++ valarrays. Addition is defined as
the addition of the corresponding elements of the operands. For
example, in the code below, each of the 4 elements in a will be
added to the corresponding 4 elements in b and the resulting
vector will be stored in c.
typedef int v4si __attribute__ ((mode(V4SI))); v4si a, b, c; c = a + b; |
Subtraction, multiplication, and division operate in a similar manner. Likewise, the result of using the unary minus operator on a vector type is a vector whose elements are the negative value of the corresponding elements in the operand.
You can declare variables and use them in function calls and returns, as well as in assignments and some casts. You can specify a vector type as a return type for a function. Vector types can also be used as function arguments. It is possible to cast from one vector type to another, provided they are of the same size (in fact, you can also cast vectors to and from other datatypes of the same size).
You cannot operate between vectors of different lengths or different signedness without a cast.
A port that supports hardware vector operations, usually provides a set of built-in functions that can be used to operate on vectors. For example, a function to add two vectors and multiply the result by a third could look like this:
v4si f (v4si a, v4si b, v4si c)
{
v4si tmp = __builtin_addv4si (a, b);
return __builtin_mulv4si (tmp, c);
}
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GCC provides a large number of built-in functions other than the ones mentioned above. Some of these are for internal use in the processing of exceptions or variable-length argument lists and will not be documented here because they may change from time to time; we do not recommend general use of these functions.
The remaining functions are provided for optimization purposes.
GCC includes built-in versions of many of the functions in the standard
C library. The versions prefixed with __builtin_ will always be
treated as having the same meaning as the C library function even if you
specify the `-fno-builtin' option. (see section 3.4 Options Controlling C Dialect)
Many of these functions are only optimized in certain cases; if they are
not optimized in a particular case, a call to the library function will
be emitted.
The functions abort, exit, _Exit and _exit
are recognized and presumed not to return, but otherwise are not built
in. _exit is not recognized in strict ISO C mode (`-ansi',
`-std=c89' or `-std=c99'). _Exit is not recognized in
strict C89 mode (`-ansi' or `-std=c89'). All these functions
have corresponding versions prefixed with __builtin_, which may be
used even in strict C89 mode.
Outside strict ISO C mode, the functions alloca, bcmp,
bzero, index, rindex, ffs, fputs_unlocked,
printf_unlocked and fprintf_unlocked may be handled as
built-in functions. All these functions have corresponding versions
prefixed with __builtin_, which may be used even in strict C89
mode.
The ISO C99 functions conj, conjf, conjl,
creal, crealf, creall, cimag, cimagf,
cimagl, imaxabs, llabs, snprintf,
vscanf, vsnprintf and vsscanf are handled as built-in
functions except in strict ISO C90 mode. There are also built-in
versions of the ISO C99 functions cosf, cosl,
expf, expl, fabsf, fabsl,
logf, logl, sinf, sinl, sqrtf, and
sqrtl, that are recognized in any mode since ISO C90 reserves
these names for the purpose to which ISO C99 puts them. All these
functions have corresponding versions prefixed with __builtin_.
The ISO C90 functions abs, cos, exp, fabs,
fprintf, fputs, labs, log,
memcmp, memcpy,
memset, printf, putchar, puts, scanf,
sin, snprintf, sprintf, sqrt, sscanf,
strcat,
strchr, strcmp, strcpy, strcspn,
strlen, strncat, strncmp, strncpy,
strpbrk, strrchr, strspn, strstr,
vprintf and vsprintf are all
recognized as built-in functions unless `-fno-builtin' is
specified (or `-fno-builtin-function' is specified for an
individual function). All of these functions have corresponding
versions prefixed with __builtin_.
GCC provides built-in versions of the ISO C99 floating point comparison
macros that avoid raising exceptions for unordered operands. They have
the same names as the standard macros ( isgreater,
isgreaterequal, isless, islessequal,
islessgreater, and isunordered) , with __builtin_
prefixed. We intend for a library implementor to be able to simply
#define each standard macro to its built-in equivalent.
You can use the built-in function __builtin_types_compatible_p to
determine whether two types are the same.
This built-in function returns 1 if the unqualified versions of the types type1 and type2 (which are types, not expressions) are compatible, 0 otherwise. The result of this built-in function can be used in integer constant expressions.
This built-in function ignores top level qualifiers (e.g., const,
volatile). For example, int is equivalent to const
int.
The type int[] and int[5] are compatible. On the other
hand, int and char * are not compatible, even if the size
of their types, on the particular architecture are the same. Also, the
amount of pointer indirection is taken into account when determining
similarity. Consequently, short * is not similar to
short **. Furthermore, two types that are typedefed are
considered compatible if their underlying types are compatible.
An enum type is considered to be compatible with another
enum type. For example, enum {foo, bar} is similar to
enum {hot, dog}.
You would typically use this function in code whose execution varies depending on the arguments' types. For example:
#define foo(x) \
({ \
typeof (x) tmp; \
if (__builtin_types_compatible_p (typeof (x), long double)) \
tmp = foo_long_double (tmp); \
else if (__builtin_types_compatible_p (typeof (x), double)) \
tmp = foo_double (tmp); \
else if (__builtin_types_compatible_p (typeof (x), float)) \
tmp = foo_float (tmp); \
else \
abort (); \
tmp; \
})
|
Note: This construct is only available for C.
You can use the built-in function __builtin_choose_expr to
evaluate code depending on the value of a constant expression. This
built-in function returns exp1 if const_exp, which is a
constant expression that must be able to be determined at compile time,
is nonzero. Otherwise it returns 0.
This built-in function is analogous to the `? :' operator in C, except that the expression returned has its type unaltered by promotion rules. Also, the built-in function does not evaluate the expression that was not chosen. For example, if const_exp evaluates to true, exp2 is not evaluated even if it has side-effects.
This built-in function can return an lvalue if the chosen argument is an lvalue.
If exp1 is returned, the return type is the same as exp1's type. Similarly, if exp2 is returned, its return type is the same as exp2.
Example:
#define foo(x) \
__builtin_choose_expr ( \
__builtin_types_compatible_p (typeof (x), double), \
foo_double (x), \
__builtin_choose_expr ( \
__builtin_types_compatible_p (typeof (x), float), \
foo_float (x), \
/* The void expression results in a compile-time error \
when assigning the result to something. */ \
(void)0))
|
Note: This construct is only available for C. Furthermore, the unused expression (exp1 or exp2 depending on the value of const_exp) may still generate syntax errors. This may change in future revisions.
__builtin_constant_p to
determine if a value is known to be constant at compile-time and hence
that GCC can perform constant-folding on expressions involving that
value. The argument of the function is the value to test. The function
returns the integer 1 if the argument is known to be a compile-time
constant and 0 if it is not known to be a compile-time constant. A
return of 0 does not indicate that the value is not a constant,
but merely that GCC cannot prove it is a constant with the specified
value of the `-O' option.
You would typically use this function in an embedded application where memory was a critical resource. If you have some complex calculation, you may want it to be folded if it involves constants, but need to call a function if it does not. For example:
#define Scale_Value(X) \ (__builtin_constant_p (X) \ ? ((X) * SCALE + OFFSET) : Scale (X)) |
You may use this built-in function in either a macro or an inline function. However, if you use it in an inlined function and pass an argument of the function as the argument to the built-in, GCC will never return 1 when you call the inline function with a string constant or compound literal (see section 5.20 Compound Literals) and will not return 1 when you pass a constant numeric value to the inline function unless you specify the `-O' option.
You may also use __builtin_constant_p in initializers for static
data. For instance, you can write
static const int table[] = {
__builtin_constant_p (EXPRESSION) ? (EXPRESSION) : -1,
/* ... */
};
|
This is an acceptable initializer even if EXPRESSION is not a constant expression. GCC must be more conservative about evaluating the built-in in this case, because it has no opportunity to perform optimization.
Previous versions of GCC did not accept this built-in in data initializers. The earliest version where it is completely safe is 3.0.1.
__builtin_expect to provide the compiler with
branch prediction information. In general, you should prefer to
use actual profile feedback for this (`-fprofile-arcs'), as
programmers are notoriously bad at predicting how their programs
actually perform. However, there are applications in which this
data is hard to collect.
The return value is the value of exp, which should be an integral expression. The value of c must be a compile-time constant. The semantics of the built-in are that it is expected that exp == c. For example:
if (__builtin_expect (x, 0)) foo (); |
would indicate that we do not expect to call foo, since
we expect x to be zero. Since you are limited to integral
expressions for exp, you should use constructions such as
if (__builtin_expect (ptr != NULL, 1)) error (); |
when testing pointer or floating-point values.
__builtin_prefetch into code for which
you know addresses of data in memory that is likely to be accessed soon.
If the target supports them, data prefetch instructions will be generated.
If the prefetch is done early enough before the access then the data will
be in the cache by the time it is accessed.
The value of addr is the address of the memory to prefetch. There are two optional arguments, rw and locality. The value of rw is a compile-time constant one or zero; one means that the prefetch is preparing for a write to the memory address and zero, the default, means that the prefetch is preparing for a read. The value locality must be a compile-time constant integer between zero and three. A value of zero means that the data has no temporal locality, so it need not be left in the cache after the access. A value of three means that the data has a high degree of temporal locality and should be left in all levels of cache possible. Values of one and two mean, respectively, a low or moderate degree of temporal locality. The default is three.
for (i = 0; i < n; i++)
{
a[i] = a[i] + b[i];
__builtin_prefetch (&a[i+j], 1, 1);
__builtin_prefetch (&b[i+j], 0, 1);
/* ... */
}
|
Data prefetch does not generate faults if addr is invalid, but
the address expression itself must be valid. For example, a prefetch
of p->next will not fault if p->next is not a valid
address, but evaluation will fault if p is not a valid address.
If the target does not support data prefetch, the address expression is evaluated if it includes side effects but no other code is generated and GCC does not issue a warning.
DBL_MAX. This function is suitable for implementing the
ISO C macro HUGE_VAL.
__builtin_huge_val, except the return type is float.
__builtin_huge_val, except the return
type is long double.
__builtin_huge_val, except a warning is generated
if the target floating-point format does not support infinities.
This function is suitable for implementing the ISO C99 macro INFINITY.
__builtin_inf, except the return type is float.
__builtin_inf, except the return
type is long double.
nan.
Since ISO C99 defines this function in terms of strtod, which we
do not implement, a description of the parsing is in order. The string
is parsed as by strtol; that is, the base is recognized by
leading `0' or `0x' prefixes. The number parsed is placed
in the significand such that the least significant bit of the number
is at the least significant bit of the significand. The number is
truncated to fit the significand field provided. The significand is
forced to be a quiet NaN.
This function, if given a string literal, is evaluated early enough that it is considered a compile-time constant.
__builtin_nan, except the return type is float.
__builtin_nan, except the return type is long double.
__builtin_nan, except the significand is forced
to be a signaling NaN. The nans function is proposed by
WG14 N965.
__builtin_nans, except the return type is float.
__builtin_nans, except the return type is long double.
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For compatibility with other compilers, GCC allows you to define a structure or union that contains, as fields, structures and unions without names. For example:
struct {
int a;
union {
int b;
float c;
};
int d;
} foo;
|
In this example, the user would be able to access members of the unnamed
union with code like `foo.b'. Note that only unnamed structs and
unions are allowed, you may not have, for example, an unnamed
int.
You must never create such structures that cause ambiguous field definitions. For example, this structure:
struct {
int a;
struct {
int a;
};
} foo;
|
It is ambiguous which a is being referred to with `foo.a'.
Such constructs are not supported and must be avoided. In the future,
such constructs may be detected and treated as compilation errors.
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The GNU compiler provides these extensions to the C++ language (and you
can also use most of the C language extensions in your C++ programs). If you
want to write code that checks whether these features are available, you can
test for the GNU compiler the same way as for C programs: check for a
predefined macro __GNUC__. You can also use __GNUG__ to
test specifically for GNU C++ (see section `Standard Predefined Macros' in The C Preprocessor).
6.1 Minimum and Maximum Operators in C++ C++ Minimum and maximum operators. 6.2 When is a Volatile Object Accessed? What constitutes an access to a volatile object. 6.3 Restricting Pointer Aliasing C99 restricted pointers and references. 6.4 Vague Linkage Where G++ puts inlines, vtables and such. 6.5 Declarations and Definitions in One Header You can use a single C++ header file for both declarations and definitions. 6.6 Where's the Template? Methods for ensuring that exactly one copy of each needed template instantiation is emitted. 6.7 Extracting the function pointer from a bound pointer to member function You can extract a function pointer to the method denoted by a `->*' or `.*' expression. 6.8 C++-Specific Variable, Function, and Type Attributes Variable, function, and type attributes for C++ only. 6.9 Java Exceptions Tweaking exception handling to work with Java. 6.10 Deprecated Features Things might disappear from g++. 6.11 Backwards Compatibility Compatibilities with earlier definitions of C++.
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It is very convenient to have operators which return the "minimum" or the "maximum" of two arguments. In GNU C++ (but not in GNU C),
a <? b
a >? b
These operations are not primitive in ordinary C++, since you can use a macro to return the minimum of two things in C++, as in the following example.
#define MIN(X,Y) ((X) < (Y) ? : (X) : (Y)) |
You might then use `int min = MIN (i, j);' to set min to the minimum value of variables i and j.
However, side effects in X or Y may cause unintended
behavior. For example, MIN (i++, j++) will fail, incrementing
the smaller counter twice. The GNU C typeof extension allows you
to write safe macros that avoid this kind of problem (see section 5.6 Referring to a Type with typeof).
However, writing MIN and MAX as macros also forces you to
use function-call notation for a fundamental arithmetic operation.
Using GNU C++ extensions, you can write `int min = i <? j;'
instead.
Since <? and >? are built into the compiler, they properly
handle expressions with side-effects; `int min = i++ <? j++;'
works correctly.
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Both the C and C++ standard have the concept of volatile objects. These are normally accessed by pointers and used for accessing hardware. The standards encourage compilers to refrain from optimizations concerning accesses to volatile objects that it might perform on non-volatile objects. The C standard leaves it implementation defined as to what constitutes a volatile access. The C++ standard omits to specify this, except to say that C++ should behave in a similar manner to C with respect to volatiles, where possible. The minimum either standard specifies is that at a sequence point all previous accesses to volatile objects have stabilized and no subsequent accesses have occurred. Thus an implementation is free to reorder and combine volatile accesses which occur between sequence points, but cannot do so for accesses across a sequence point. The use of volatiles does not allow you to violate the restriction on updating objects multiple times within a sequence point.
In most expressions, it is intuitively obvious what is a read and what is a write. For instance
volatile int *dst = somevalue; volatile int *src = someothervalue; *dst = *src; |
will cause a read of the volatile object pointed to by src and stores the
value into the volatile object pointed to by dst. There is no
guarantee that these reads and writes are atomic, especially for objects
larger than int.
Less obvious expressions are where something which looks like an access is used in a void context. An example would be,
volatile int *src = somevalue; *src; |
With C, such expressions are rvalues, and as rvalues cause a read of the object, GCC interprets this as a read of the volatile being pointed to. The C++ standard specifies that such expressions do not undergo lvalue to rvalue conversion, and that the type of the dereferenced object may be incomplete. The C++ standard does not specify explicitly that it is this lvalue to rvalue conversion which is responsible for causing an access. However, there is reason to believe that it is, because otherwise certain simple expressions become undefined. However, because it would surprise most programmers, G++ treats dereferencing a pointer to volatile object of complete type in a void context as a read of the object. When the object has incomplete type, G++ issues a warning.
struct S;
struct T {int m;};
volatile S *ptr1 = somevalue;
volatile T *ptr2 = somevalue;
*ptr1;
*ptr2;
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In this example, a warning is issued for *ptr1, and *ptr2
causes a read of the object pointed to. If you wish to force an error on
the first case, you must force a conversion to rvalue with, for instance
a static cast, static_cast<S>(*ptr1).
When using a reference to volatile, G++ does not treat equivalent expressions as accesses to volatiles, but instead issues a warning that no volatile is accessed. The rationale for this is that otherwise it becomes difficult to determine where volatile access occur, and not possible to ignore the return value from functions returning volatile references. Again, if you wish to force a read, cast the reference to an rvalue.
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As with gcc, g++ understands the C99 feature of restricted pointers,
specified with the __restrict__, or __restrict type
qualifier. Because you cannot compile C++ by specifying the `-std=c99'
language flag, restrict is not a keyword in C++.
In addition to allowing restricted pointers, you can specify restricted references, which indicate that the reference is not aliased in the local context.
void fn (int *__restrict__ rptr, int &__restrict__ rref)
{
/* ... */
}
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In the body of fn, rptr points to an unaliased integer and
rref refers to a (different) unaliased integer.
You may also specify whether a member function's this pointer is
unaliased by using __restrict__ as a member function qualifier.
void T::fn () __restrict__
{
/* ... */
}
|
Within the body of T::fn, this will have the effective
definition T *__restrict__ const this. Notice that the
interpretation of a __restrict__ member function qualifier is
different to that of const or volatile qualifier, in that it
is applied to the pointer rather than the object. This is consistent with
other compilers which implement restricted pointers.
As with all outermost parameter qualifiers, __restrict__ is
ignored in function definition matching. This means you only need to
specify __restrict__ in a function definition, rather than
in a function prototype as well.
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There are several constructs in C++ which require space in the object file but are not clearly tied to a single translation unit. We say that these constructs have "vague linkage". Typically such constructs are emitted wherever they are needed, though sometimes we can be more clever.
Local static variables and string constants used in an inline function are also considered to have vague linkage, since they must be shared between all inlined and out-of-line instances of the function.
Note: If the chosen key method is later defined as inline, the vtable will still be emitted in every translation unit which defines it. Make sure that any inline virtuals are declared inline in the class body, even if they are not defined there.
When used with GNU ld version 2.8 or later on an ELF system such as Linux/GNU or Solaris 2, or on Microsoft Windows, duplicate copies of these constructs will be discarded at link time. This is known as COMDAT support.
On targets that don't support COMDAT, but do support weak symbols, GCC will use them. This way one copy will override all the others, but the unused copies will still take up space in the executable.
For targets which do not support either COMDAT or weak symbols, most entities with vague linkage will be emitted as local symbols to avoid duplicate definition errors from the linker. This will not happen for local statics in inlines, however, as having multiple copies will almost certainly break things.
See section Declarations and Definitions in One Header, for another way to control placement of these constructs.
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C++ object definitions can be quite complex. In principle, your source code will need two kinds of things for each object that you use across more than one source file. First, you need an interface specification, describing its structure with type declarations and function prototypes. Second, you need the implementation itself. It can be tedious to maintain a separate interface description in a header file, in parallel to the actual implementation. It is also dangerous, since separate interface and implementation definitions may not remain parallel.
With GNU C++, you can use a single header file for both purposes.
Warning: The mechanism to specify this is in transition. For the nonce, you must use one of two#pragmacommands; in a future release of GNU C++, an alternative mechanism will make these#pragmacommands unnecessary.
The header file contains the full definitions, but is marked with
`#pragma interface' in the source code. This allows the compiler
to use the header file only as an interface specification when ordinary
source files incorporate it with #include. In the single source
file where the full implementation belongs, you can use either a naming
convention or `#pragma implementation' to indicate this alternate
use of the header file.
#pragma interface
#pragma interface "subdir/objects.h"
The second form of this directive is useful for the case where you have multiple headers with the same name in different directories. If you use this form, you must specify the same string to `#pragma implementation'.
#pragma implementation
#pragma implementation "objects.h"
If you use `#pragma implementation' with no argument, it applies to an include file with the same basename(4) as your source file. For example, in `allclass.cc', giving just `#pragma implementation' by itself is equivalent to `#pragma implementation "allclass.h"'.
In versions of GNU C++ prior to 2.6.0 `allclass.h' was treated as an implementation file whenever you would include it from `allclass.cc' even if you never specified `#pragma implementation'. This was deemed to be more trouble than it was worth, however, and disabled.
If you use an explicit `#pragma implementation', it must appear in your source file before you include the affected header files.
Use the string argument if you want a single implementation file to include code from multiple header files. (You must also use `#include' to include the header file; `#pragma implementation' only specifies how to use the file--it doesn't actually include it.)
There is no way to split up the contents of a single header file into multiple implementation files.
`#pragma implementation' and `#pragma interface' also have an effect on function inlining.
If you define a class in a header file marked with `#pragma
interface', the effect on a function defined in that class is similar to
an explicit extern declaration--the compiler emits no code at
all to define an independent version of the function. Its definition
is used only for inlining with its callers.
Conversely, when you include the same header file in a main source file that declares it as `#pragma implementation', the compiler emits code for the function itself; this defines a version of the function that can be found via pointers (or by callers compiled without inlining). If all calls to the function can be inlined, you can avoid emitting the function by compiling with `-fno-implement-inlines'. If any calls were not inlined, you will get linker errors.
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C++ templates are the first language feature to require more intelligence from the environment than one usually finds on a UNIX system. Somehow the compiler and linker have to make sure that each template instance occurs exactly once in the executable if it is needed, and not at all otherwise. There are two basic approaches to this problem, which I will refer to as the Borland model and the Cfront model.
When used with GNU ld version 2.8 or later on an ELF system such as Linux/GNU or Solaris 2, or on Microsoft Windows, g++ supports the Borland model. On other systems, g++ implements neither automatic model.
A future version of g++ will support a hybrid model whereby the compiler will emit any instantiations for which the template definition is included in the compile, and store template definitions and instantiation context information into the object file for the rest. The link wrapper will extract that information as necessary and invoke the compiler to produce the remaining instantiations. The linker will then combine duplicate instantiations.
In the mean time, you have the following options for dealing with template instantiations:
This is your best option for application code written for the Borland
model, as it will just work. Code written for the Cfront model will
need to be modified so that the template definitions are available at
one or more points of instantiation; usually this is as simple as adding
#include <tmethods.cc> to the end of each template header.
For library code, if you want the library to provide all of the template instantiations it needs, just try to link all of its object files together; the link will fail, but cause the instantiations to be generated as a side effect. Be warned, however, that this may cause conflicts if multiple libraries try to provide the same instantiations. For greater control, use explicit instantiation as described in the next option.
#include "Foo.h"
#include "Foo.cc"
template class Foo<int>;
template ostream& operator <<
(ostream&, const Foo<int>&);
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for each of the instances you need, and create a template instantiation library from those.
If you are using Cfront-model code, you can probably get away with not using `-fno-implicit-templates' when compiling files that don't `#include' the member template definitions.
If you use one big file to do the instantiations, you may want to compile it without `-fno-implicit-templates' so you get all of the instances required by your explicit instantiations (but not by any other files) without having to specify them as well.
g++ has extended the template instantiation syntax outlined in the
Working Paper to allow forward declaration of explicit instantiations
(with extern), instantiation of the compiler support data for a
template class (i.e. the vtable) without instantiating any of its
members (with inline), and instantiation of only the static data
members of a template class, without the support data or member
functions (with (static):
extern template int max (int, int); inline template class Foo<int>; static template class Foo<int>; |
See section Declarations and Definitions in One Header, for more discussion of these pragmas.
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In C++, pointer to member functions (PMFs) are implemented using a wide pointer of sorts to handle all the possible call mechanisms; the PMF needs to store information about how to adjust the `this' pointer, and if the function pointed to is virtual, where to find the vtable, and where in the vtable to look for the member function. If you are using PMFs in an inner loop, you should really reconsider that decision. If that is not an option, you can extract the pointer to the function that would be called for a given object/PMF pair and call it directly inside the inner loop, to save a bit of time.
Note that you will still be paying the penalty for the call through a function pointer; on most modern architectures, such a call defeats the branch prediction features of the CPU. This is also true of normal virtual function calls.
The syntax for this extension is
extern A a; extern int (A::*fp)(); typedef int (*fptr)(A *); fptr p = (fptr)(a.*fp); |
For PMF constants (i.e. expressions of the form `&Klasse::Member'), no object is needed to obtain the address of the function. They can be converted to function pointers directly:
fptr p1 = (fptr)(&A::foo); |
You must specify `-Wno-pmf-conversions' to use this extension.
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Some attributes only make sense for C++ programs.
init_priority (priority)
In Standard C++, objects defined at namespace scope are guaranteed to be
initialized in an order in strict accordance with that of their definitions
in a given translation unit. No guarantee is made for initializations
across translation units. However, GNU C++ allows users to control the
order of initialization of objects defined at namespace scope with the
init_priority attribute by specifying a relative priority,
a constant integral expression currently bounded between 101 and 65535
inclusive. Lower numbers indicate a higher priority.
In the following example, A would normally be created before
B, but the init_priority attribute has reversed that order:
Some_Class A __attribute__ ((init_priority (2000))); Some_Class B __attribute__ ((init_priority (543))); |
Note that the particular values of priority do not matter; only their relative ordering.
java_interface
This type attribute informs C++ that the class is a Java interface. It may
only be applied to classes declared within an extern "Java" block.
Calls to methods declared in this interface will be dispatched using GCJ's
interface table mechanism, instead of regular virtual table dispatch.
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The Java language uses a slightly different exception handling model from C++. Normally, GNU C++ will automatically detect when you are writing C++ code that uses Java exceptions, and handle them appropriately. However, if C++ code only needs to execute destructors when Java exceptions are thrown through it, GCC will guess incorrectly. Sample problematic code is:
struct S { ~S(); };
extern void bar(); // is written in Java, and may throw exceptions
void foo()
{
S s;
bar();
}
|
The usual effect of an incorrect guess is a link failure, complaining of a missing routine called `__gxx_personality_v0'.
You can inform the compiler that Java exceptions are to be used in a translation unit, irrespective of what it might think, by writing `#pragma GCC java_exceptions' at the head of the file. This `#pragma' must appear before any functions that throw or catch exceptions, or run destructors when exceptions are thrown through them.
You cannot mix Java and C++ exceptions in the same translation unit. It is believed to be safe to throw a C++ exception from one file through another file compiled for the Java exception model, or vice versa, but there may be bugs in this area.
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In the past, the GNU C++ compiler was extended to experiment with new features, at a time when the C++ language was still evolving. Now that the C++ standard is complete, some of those features are superseded by superior alternatives. Using the old features might cause a warning in some cases that the feature will be dropped in the future. In other cases, the feature might be gone already.
While the list below is not exhaustive, it documents some of the options that are now deprecated:
-fexternal-templates
-falt-external-templates
-fstrict-prototype
-fno-strict-prototype
The named return value extension has been deprecated, and is now removed from g++.
The use of initializer lists with new expressions has been deprecated, and is now removed from g++.
Floating and complex non-type template parameters have been deprecated, and are now removed from g++.
The implicit typename extension has been deprecated and will be removed
from g++ at some point. In some cases g++ determines that a dependent
type such as TPL<T>::X is a type without needing a
typename keyword, contrary to the standard.
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Now that there is a definitive ISO standard C++, G++ has a specification to adhere to. The C++ language evolved over time, and features that used to be acceptable in previous drafts of the standard, such as the ARM [Annotated C++ Reference Manual], are no longer accepted. In order to allow compilation of C++ written to such drafts, G++ contains some backwards compatibilities. All such backwards compatibility features are liable to disappear in future versions of G++. They should be considered deprecated See section 6.10 Deprecated Features.
For scope
Implicit C language
extern "C" {...}
scope to set the language. On such systems, all header files are
implicitly scoped inside a C language scope. Also, an empty prototype
() will be treated as an unspecified number of arguments, rather
than no arguments, as C++ demands.
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This document is meant to describe some of the GNU Objective-C runtime features. It is not intended to teach you Objective-C, there are several resources on the Internet that present the language. Questions and comments about this document to Ovidiu Predescu ovidiu@cup.hp.com.
7.1 +load: Executing code before main7.2 Type encoding 7.3 Garbage Collection 7.4 Constant string objects 7.5 compatibility_alias
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+load: Executing code before main
The GNU Objective-C runtime provides a way that allows you to execute
code before the execution of the program enters the main
function. The code is executed on a per-class and a per-category basis,
through a special class method +load.
This facility is very useful if you want to initialize global variables
which can be accessed by the program directly, without sending a message
to the class first. The usual way to initialize global variables, in the
+initialize method, might not be useful because
+initialize is only called when the first message is sent to a
class object, which in some cases could be too late.
Suppose for example you have a FileStream class that declares
Stdin, Stdout and Stderr as global variables, like
below:
FileStream *Stdin = nil;
FileStream *Stdout = nil;
FileStream *Stderr = nil;
@implementation FileStream
+ (void)initialize
{
Stdin = [[FileStream new] initWithFd:0];
Stdout = [[FileStream new] initWithFd:1];
Stderr = [[FileStream new] initWithFd:2];
}
/* Other methods here */
@end
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In this example, the initialization of Stdin, Stdout and
Stderr in +initialize occurs too late. The programmer can
send a message to one of these objects before the variables are actually
initialized, thus sending messages to the nil object. The
+initialize method which actually initializes the global
variables is not invoked until the first message is sent to the class
object. The solution would require these variables to be initialized
just before entering main.
The correct solution of the above problem is to use the +load
method instead of +initialize:
@implementation FileStream
+ (void)load
{
Stdin = [[FileStream new] initWithFd:0];
Stdout = [[FileStream new] initWithFd:1];
Stderr = [[FileStream new] initWithFd:2];
}
/* Other methods here */
@end
|
The +load is a method that is not overridden by categories. If a
class and a category of it both implement +load, both methods are
invoked. This allows some additional initializations to be performed in
a category.
This mechanism is not intended to be a replacement for +initialize.
You should be aware of its limitations when you decide to use it
instead of +initialize.
7.1.1 What you can and what you cannot do in +load
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+load
The +load implementation in the GNU runtime guarantees you the following
things:
@"this is a
constant string");
+load implementation of all super classes of a class are executed before the +load of that class is executed;
+load implementation of a class is executed before the
+load implementation of any category.
In particular, the following things, even if they can work in a particular case, are not guaranteed:
You should make no assumptions about receiving +load in sibling
classes when you write +load of a class. The order in which
sibling classes receive +load is not guaranteed.
The order in which +load and +initialize are called could
be problematic if this matters. If you don't allocate objects inside
+load, it is guaranteed that +load is called before
+initialize. If you create an object inside +load the
+initialize method of object's class is invoked even if
+load was not invoked. Note if you explicitly call +load
on a class, +initialize will be called first. To avoid possible
problems try to implement only one of these methods.
The +load method is also invoked when a bundle is dynamically
loaded into your running program. This happens automatically without any
intervening operation from you. When you write bundles and you need to
write +load you can safely create and send messages to objects whose
classes already exist in the running program. The same restrictions as
above apply to classes defined in bundle.
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The Objective-C compiler generates type encodings for all the types. These type encodings are used at runtime to find out information about selectors and methods and about objects and classes.
The types are encoded in the following way:
char |
c
|
unsigned char |
C
|
short |
s
|
unsigned short |
S
|
int |
i
|
unsigned int |
I
|
long |
l
|
unsigned long |
L
|
long long |
q
|
unsigned long long |
Q
|
float |
f
|
double |
d
|
void |
v
|
id |
@
|
Class |
#
|
SEL |
:
|
char* |
*
|
| unknown type | ?
|
| bit-fields | b followed by the starting position of the bit-field, the type of the bit-field and the size of the bit-field (the bit-fields encoding was changed from the NeXT's compiler encoding, see below)
|
The encoding of bit-fields has changed to allow bit-fields to be properly handled by the runtime functions that compute sizes and alignments of types that contain bit-fields. The previous encoding contained only the size of the bit-field. Using only this information it is not possible to reliably compute the size occupied by the bit-field. This is very important in the presence of the Boehm's garbage collector because the objects are allocated using the typed memory facility available in this collector. The typed memory allocation requires information about where the pointers are located inside the object.
The position in the bit-field is the position, counting in bits, of the bit closest to the beginning of the structure.
The non-atomic types are encoded as follows:
| pointers | `^' followed by the pointed type. |
| arrays | `[' followed by the number of elements in the array followed by the type of the elements followed by `]' |
| structures | `{' followed by the name of the structure (or `?' if the structure is unnamed), the `=' sign, the type of the members and by `}' |
| unions | `(' followed by the name of the structure (or `?' if the union is unnamed), the `=' sign, the type of the members followed by `)' |
Here are some types and their encodings, as they are generated by the compiler on an i386 machine:
| Objective-C type | Compiler encoding | ||
int a[10]; |
[10i]
struct {
int i;
float f[3];
int a:3;
int b:2;
char c;
}
|
{?=i[3f]b128i3b131i2c}
In addition to the types the compiler also encodes the type specifiers. The table below describes the encoding of the current Objective-C type specifiers:
| Specifier | Encoding |
const |
r
|
in |
n
|
inout |
N
|
out |
o
|
bycopy |
O
|
oneway |
V
|
The type specifiers are encoded just before the type. Unlike types however, the type specifiers are only encoded when they appear in method argument types.
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Support for a new memory management policy has been added by using a powerful conservative garbage collector, known as the Boehm-Demers-Weiser conservative garbage collector. It is available from http://www.hpl.hp.com/personal/Hans_Boehm/gc/.
To enable the support for it you have to configure the compiler using an additional argument, `--enable-objc-gc'. You need to have garbage collector installed before building the compiler. This will build an additional runtime library which has several enhancements to support the garbage collector. The new library has a new name, `libobjc_gc.a' to not conflict with the non-garbage-collected library.
When the garbage collector is used, the objects are allocated using the so-called typed memory allocation mechanism available in the Boehm-Demers-Weiser collector. This mode requires precise information on where pointers are located inside objects. This information is computed once per class, immediately after the class has been initialized.
There is a new runtime function class_ivar_set_gcinvisible()
which can be used to declare a so-called weak pointer
reference. Such a pointer is basically hidden for the garbage collector;
this can be useful in certain situations, especially when you want to
keep track of the allocated objects, yet allow them to be
collected. This kind of pointers can only be members of objects, you
cannot declare a global pointer as a weak reference. Every type which is
a pointer type can be declared a weak pointer, including id,
Class and SEL.
Here is an example of how to use this feature. Suppose you want to implement a class whose instances hold a weak pointer reference; the following class does this:
@interface WeakPointer : Object
{
const void* weakPointer;
}
- initWithPointer:(const void*)p;
- (const void*)weakPointer;
@end
@implementation WeakPointer
+ (void)initialize
{
class_ivar_set_gcinvisible (self, "weakPointer", YES);
}
- initWithPointer:(const void*)p
{
weakPointer = p;
return self;
}
- (const void*)weakPointer
{
return weakPointer;
}
@end
|
Weak pointers are supported through a new type character specifier
represented by the `!' character. The
class_ivar_set_gcinvisible() function adds or removes this
specifier to the string type description of the instance variable named
as argument.
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GNU Objective-C provides constant string objects that are generated directly by the compiler. You declare a constant string object by prefixing a C constant string with the character `@':
id myString = @"this is a constant string object"; |
The constant string objects are by default instances of the
NXConstantString class which is provided by the GNU Objective-C
runtime. To get the definition of this class you must include the
`objc/NXConstStr.h' header file.
User defined libraries may want to implement their own constant string
class. To be able to support them, the GNU Objective-C compiler provides
a new command line options `-fconstant-string-class=class-name'.
The provided class should adhere to a strict structure, the same
as NXConstantString's structure:
@interface MyConstantStringClass
{
Class isa;
char *c_string;
unsigned int len;
}
@end
|
NXConstantString inherits from Object; user class
libraries may choose to inherit the customized constant string class
from a different class than Object. There is no requirement in
the methods the constant string class has to implement, but the final
ivar layout of the class must be the compatible with the given
structure.
When the compiler creates the statically allocated constant string
object, the c_string field will be filled by the compiler with
the string; the length field will be filled by the compiler with
the string length; the isa pointer will be filled with
NULL by the compiler, and it will later be fixed up automatically
at runtime by the GNU Objective-C runtime library to point to the class
which was set by the `-fconstant-string-class' option when the
object file is loaded (if you wonder how it works behind the scenes, the
name of the class to use, and the list of static objects to fixup, are
stored by the compiler in the object file in a place where the GNU
runtime library will find them at runtime).
As a result, when a file is compiled with the `-fconstant-string-class' option, all the constant string objects will be instances of the class specified as argument to this option. It is possible to have multiple compilation units referring to different constant string classes, neither the compiler nor the linker impose any restrictions in doing this.
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This is a feature of the Objective-C compiler rather than of the runtime, anyway since it is documented nowhere and its existence was forgotten, we are documenting it here.
The keyword @compatibility_alias allows you to define a class name
as equivalent to another class name. For example:
@compatibility_alias WOApplication GSWApplication; |
tells the compiler that each time it encounters WOApplication as
a class name, it should replace it with GSWApplication (that is,
WOApplication is just an alias for GSWApplication).
There are some constraints on how this can be used---
WOApplication (the alias) must not be an existing class;
GSWApplication (the real class) must be an existing class.
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Binary compatibility encompasses several related concepts:
The application binary interface implemented by a C or C++ compiler affects code generation and runtime support for:
In addition, the application binary interface implemented by a C++ compiler affects code generation and runtime support for:
Some GCC compilation options cause the compiler to generate code that does not conform to the platform's default ABI. Other options cause different program behavior for implementation-defined features that are not covered by an ABI. These options are provided for consistency with other compilers that do not follow the platform's default ABI or the usual behavior of implementation-defined features for the platform. Be very careful about using such options.
Most platforms have a well-defined ABI that covers C code, but ABIs that cover C++ functionality are not yet common.
Starting with GCC 3.2, GCC binary conventions for C++ are based on a
written, vendor-neutral C++ ABI that was designed to be specific to
64-bit Itanium but also includes generic specifications that apply to
any platform.
This C++ ABI is also implemented by other compiler vendors on some
platforms, notably GNU/Linux and BSD systems.
We have tried hard to provide a stable ABI that will be compatible with
future GCC releases, but it is possible that we will encounter problems
that make this difficult. Such problems could include different
interpretations of the C++ ABI by different vendors, bugs in the ABI, or
bugs in the implementation of the ABI in different compilers.
GCC's -Wabi switch warns when G++ generates code that is
probably not compatible with the C++ ABI.
The C++ library used with a C++ compiler includes the Standard C++ Library, with functionality defined in the C++ Standard, plus language runtime support. The runtime support is included in a C++ ABI, but there is no formal ABI for the Standard C++ Library. Two implementations of that library are interoperable if one follows the de-facto ABI of the other and if they are both built with the same compiler, or with compilers that conform to the same ABI for C++ compiler and runtime support.
When G++ and another C++ compiler conform to the same C++ ABI, but the implementations of the Standard C++ Library that they normally use do not follow the same ABI for the Standard C++ Library, object files built with those compilers can be used in the same program only if they use the same C++ library. This requires specifying the location of the C++ library header files when invoking the compiler whose usual library is not being used. The location of GCC's C++ header files depends on how the GCC build was configured, but can be seen by using the G++ `-v' option. With default configuration options for G++ 3.3 the compile line for a different C++ compiler needs to include
-Igcc_install_directory/include/c++/3.3 |
Similarly, compiling code with G++ that must use a C++ library other than the GNU C++ library requires specifying the location of the header files for that other library.
The most straightforward way to link a program to use a particular
C++ library is to use a C++ driver that specifies that C++ library by
default. The g++ driver, for example, tells the linker where
to find GCC's C++ library (`libstdc++') plus the other libraries
and startup files it needs, in the proper order.
If a program must use a different C++ library and it's not possible
to do the final link using a C++ driver that uses that library by default,
it is necessary to tell g++ the location and name of that
library. It might also be necessary to specify different startup files
and other runtime support libraries, and to suppress the use of GCC's
support libraries with one or more of the options `-nostdlib',
`-nostartfiles', and `-nodefaultlibs'.
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This section describes known problems that affect users of GCC. Most of these are not GCC bugs per se--if they were, we would fix them. But the result for a user may be like the result of a bug.
Some of these problems are due to bugs in other software, some are missing features that are too much work to add, and some are places where people's opinions differ as to what is best.
9.1 Actual Bugs We Haven't Fixed Yet Bugs we will fix later. 9.2 Cross-Compiler Problems Common problems of cross compiling with GCC. 9.3 Interoperation Problems using GCC with other compilers, and with certain linkers, assemblers and debuggers. 9.4 Incompatibilities of GCC GCC is incompatible with traditional C. 9.5 Fixed Header Files GCC uses corrected versions of system header files. This is necessary, but doesn't always work smoothly. 9.6 Standard Libraries GCC uses the system C library, which might not be compliant with the ISO C standard. 9.7 Disappointments and Misunderstandings Regrettable things we can't change, but not quite bugs. 9.8 Common Misunderstandings with GNU C++ Common misunderstandings with GNU C++. 9.9 Caveats of using protoizeThings to watch out for when using protoize.9.10 Certain Changes We Don't Want to Make Things we think are right, but some others disagree. 9.11 Warning Messages and Error Messages Which problems in your code get warnings, and which get errors.
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fixincludes script interacts badly with automounters; if the
directory of system header files is automounted, it tends to be
unmounted while fixincludes is running. This would seem to be a
bug in the automounter. We don't know any good way to work around it.
fixproto script will sometimes add prototypes for the
sigsetjmp and siglongjmp functions that reference the
jmp_buf type before that type is defined. To work around this,
edit the offending file and place the typedef in front of the
prototypes.
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You may run into problems with cross compilation on certain machines, for several reasons.
The compiler writes these integer constants by examining the floating point value as an integer and printing that integer, because this is simple to write and independent of the details of the floating point representation. But this does not work if the compiler is running on a different machine with an incompatible floating point format, or even a different byte-ordering.
In addition, correct constant folding of floating point values requires representing them in the target machine's format. (The C standard does not quite require this, but in practice it is the only way to win.)
It is now possible to overcome these problems by defining macros such
as REAL_VALUE_TYPE. But doing so is a substantial amount of
work for each target machine.
See section `Cross Compilation and Floating Point' in GNU Compiler Collection (GCC) Internals.
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This section lists various difficulties encountered in using GCC together with other compilers or with the assemblers, linkers, libraries and debuggers on certain systems.
An area where the difference is most apparent is name mangling. The use of different name mangling is intentional, to protect you from more subtle problems. Compilers differ as to many internal details of C++ implementation, including: how class instances are laid out, how multiple inheritance is implemented, and how virtual function calls are handled. If the name encoding were made the same, your programs would link against libraries provided from other compilers--but the programs would then crash when run. Incompatible libraries are then detected at link time, rather than at run time.
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There are several noteworthy incompatibilities between GNU C and K&R (non-ISO) versions of C.
One consequence is that you cannot call mktemp with a string
constant argument. The function mktemp always alters the
string its argument points to.
Another consequence is that sscanf does not work on some systems
when passed a string constant as its format control string or input.
This is because sscanf incorrectly tries to write into the string
constant. Likewise fscanf and scanf.
The best solution to these problems is to change the program to use
char-array variables with initialization strings for these
purposes instead of string constants. But if this is not possible,
you can use the `-fwritable-strings' flag, which directs GCC
to handle string constants the same way most C compilers do.
-2147483648 is positive.
This is because 2147483648 cannot fit in the type int, so
(following the ISO C rules) its data type is unsigned long int.
Negating this value yields 2147483648 again.
#define foo(a) "a" |
will produce output "a" regardless of what the argument a is.
setjmp and longjmp, the only automatic
variables guaranteed to remain valid are those declared
volatile. This is a consequence of automatic register
allocation. Consider this function:
jmp_buf j;
foo ()
{
int a, b;
a = fun1 ();
if (setjmp (j))
return a;
a = fun2 ();
/* |
Here a may or may not be restored to its first value when the
longjmp occurs. If a is allocated in a register, then
its first value is restored; otherwise, it keeps the last value stored
in it.
If you use the `-W' option with the `-O' option, you will get a warning when GCC thinks such a problem might be possible.
foobar (
#define luser
hack)
|
ISO C does not permit such a construct.
In some other C compilers, a extern declaration affects all the
rest of the file even if it happens within a block.
long, etc., with a typedef name,
as shown here:
typedef int foo; typedef long foo bar; |
In ISO C, this is not allowed: long and other type modifiers
require an explicit int.
typedef int foo; typedef foo foo; |
#if 0 You can't expect this to work. #endif |
The best solution to such a problem is to put the text into an actual C comment delimited by `/*...*/'.
time, so it did not matter what type your program declared it to
return. But in systems with ISO C headers, time is declared to
return time_t, and if that is not the same as long, then
`long time ();' is erroneous.
The solution is to change your program to use appropriate system headers
(<time.h> on systems with ISO C headers) and not to declare
time if the system header files declare it, or failing that to
use time_t as the return type of time.
float, PCC converts it to
a double. GCC actually returns a float. If you are concerned
with PCC compatibility, you should declare your functions to return
double; you might as well say what you mean.
The method used by GCC is as follows: a structure or union which is
1, 2, 4 or 8 bytes long is returned like a scalar. A structure or union
with any other size is stored into an address supplied by the caller
(usually in a special, fixed register, but on some machines it is passed
on the stack). The machine-description macros STRUCT_VALUE and
STRUCT_INCOMING_VALUE tell GCC where to pass this address.
By contrast, PCC on most target machines returns structures and unions of any size by copying the data into an area of static storage, and then returning the address of that storage as if it were a pointer value. The caller must copy the data from that memory area to the place where the value is wanted. GCC does not use this method because it is slower and nonreentrant.
On some newer machines, PCC uses a reentrant convention for all structure and union returning. GCC on most of these machines uses a compatible convention when returning structures and unions in memory, but still returns small structures and unions in registers.
You can tell GCC to use a compatible convention for all structure and union returning with the option `-fpcc-struct-return'.
A preprocessing token is a preprocessing number if it begins with a digit and is followed by letters, underscores, digits, periods and `e+', `e-', `E+', `E-', `p+', `p-', `P+', or `P-' character sequences. (In strict C89 mode, the sequences `p+', `p-', `P+' and `P-' cannot appear in preprocessing numbers.)
To make the above program fragment valid, place whitespace in front of the minus sign. This whitespace will end the preprocessing number.
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GCC needs to install corrected versions of some system header files. This is because most target systems have some header files that won't work with GCC unless they are changed. Some have bugs, some are incompatible with ISO C, and some depend on special features of other compilers.
Installing GCC automatically creates and installs the fixed header
files, by running a program called fixincludes (or for certain
targets an alternative such as fixinc.svr4). Normally, you
don't need to pay attention to this. But there are cases where it
doesn't do the right thing automatically.
The programs that fix the header files do not understand this special way of using symbolic links; therefore, the directory of fixed header files is good only for the machine model used to build it.
In SunOS 4, only programs that look inside the kernel will notice the difference between machine models. Therefore, for most purposes, you need not be concerned about this.
It is possible to make separate sets of fixed header files for the different machine models, and arrange a structure of symbolic links so as to use the proper set, but you'll have to do this by hand.
fixincludes script to fail.
This means you will encounter problems due to bugs in the system header files. It may be no comfort that they aren't GCC's fault, but it does mean that there's nothing for us to do about them.
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GCC by itself attempts to be a conforming freestanding implementation. See section Language Standards Supported by GCC, for details of what this means. Beyond the library facilities required of such an implementation, the rest of the C library is supplied by the vendor of the operating system. If that C library doesn't conform to the C standards, then your programs might get warnings (especially when using `-Wall') that you don't expect.
For example, the sprintf function on SunOS 4.1.3 returns
char * while the C standard says that sprintf returns an
int. The fixincludes program could make the prototype for
this function match the Standard, but that would be wrong, since the
function will still return char *.
If you need a Standard compliant library, then you need to find one, as
GCC does not provide one. The GNU C library (called glibc)
provides ISO C, POSIX, BSD, SystemV and X/Open compatibility for
GNU/Linux and HURD-based GNU systems; no recent version of it supports
other systems, though some very old versions did. Version 2.2 of the
GNU C library includes nearly complete C99 support. You could also ask
your operating system vendor if newer libraries are available.
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These problems are perhaps regrettable, but we don't know any practical way around them.
This occurs because sometimes GCC optimizes the variable out of existence. There is no way to tell the debugger how to compute the value such a variable "would have had", and it is not clear that would be desirable anyway. So GCC simply does not mention the eliminated variable when it writes debugging information.
You have to expect a certain amount of disagreement between the executable and your source code, when you use optimization.
int foo (struct mumble *);
struct mumble { ... };
int foo (struct mumble *x)
{ ... }
|
This code really is erroneous, because the scope of struct
mumble in the prototype is limited to the argument list containing it.
It does not refer to the struct mumble defined with file scope
immediately below--they are two unrelated types with similar names in
different scopes.
But in the definition of foo, the file-scope type is used
because that is available to be inherited. Thus, the definition and
the prototype do not match, and you get an error.
This behavior may seem silly, but it's what the ISO standard specifies.
It is easy enough for you to make your code work by moving the
definition of struct mumble above the prototype. It's not worth
being incompatible with ISO C just to avoid an error for the example
shown above.
If you care about controlling the amount of memory that is accessed, use volatile but do not use bit-fields.
If new system header files are installed, nothing automatically arranges
to update the corrected header files. You will have to reinstall GCC
to fix the new header files. More specifically, go to the build
directory and delete the files `stmp-fixinc' and
`stmp-headers', and the subdirectory include; then do
`make install' again.
double in memory.
Compiled code moves values between memory and floating point registers
at its convenience, and moving them into memory truncates them.
You can partially avoid this problem by using the `-ffloat-store' option (see section 3.10 Options That Control Optimization).
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C++ is a complex language and an evolving one, and its standard definition (the ISO C++ standard) was only recently completed. As a result, your C++ compiler may occasionally surprise you, even when its behavior is correct. This section discusses some areas that frequently give rise to questions of this sort.
9.8.1 Declare and Define Static Members Static member declarations are not definitions 9.8.2 Temporaries May Vanish Before You Expect Temporaries may vanish before you expect 9.8.3 Implicit Copy-Assignment for Virtual Bases Copy Assignment operators copy virtual bases twice
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When a class has static data members, it is not enough to declare the static member; you must also define it. For example:
class Foo
{
...
void method();
static int bar;
};
|
This declaration only establishes that the class Foo has an
int named Foo::bar, and a member function named
Foo::method. But you still need to define both
method and bar elsewhere. According to the ISO
standard, you must supply an initializer in one (and only one) source
file, such as:
int Foo::bar = 0; |
Other C++ compilers may not correctly implement the standard behavior.
As a result, when you switch to g++ from one of these compilers,
you may discover that a program that appeared to work correctly in fact
does not conform to the standard: g++ reports as undefined
symbols any static data members that lack definitions.
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It is dangerous to use pointers or references to portions of a
temporary object. The compiler may very well delete the object before
you expect it to, leaving a pointer to garbage. The most common place
where this problem crops up is in classes like string classes,
especially ones that define a conversion function to type char *
or const char *---which is one reason why the standard
string class requires you to call the c_str member
function. However, any class that returns a pointer to some internal
structure is potentially subject to this problem.
For example, a program may use a function strfunc that returns
string objects, and another function charfunc that
operates on pointers to char:
string strfunc ();
void charfunc (const char *);
void
f ()
{
const char *p = strfunc().c_str();
...
charfunc (p);
...
charfunc (p);
}
|
In this situation, it may seem reasonable to save a pointer to the C
string returned by the c_str member function and use that rather
than call c_str repeatedly. However, the temporary string
created by the call to strfunc is destroyed after p is
initialized, at which point p is left pointing to freed memory.
Code like this may run successfully under some other compilers, particularly obsolete cfront-based compilers that delete temporaries along with normal local variables. However, the GNU C++ behavior is standard-conforming, so if your program depends on late destruction of temporaries it is not portable.
The safe way to write such code is to give the temporary a name, which forces it to remain until the end of the scope of the name. For example:
string& tmp = strfunc (); charfunc (tmp.c_str ()); |
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When a base class is virtual, only one subobject of the base class belongs to each full object. Also, the constructors and destructors are invoked only once, and called from the most-derived class. However, such objects behave unspecified when being assigned. For example:
struct Base{
char *name;
Base(char *n) : name(strdup(n)){}
Base& operator= (const Base& other){
free (name);
name = strdup (other.name);
}
};
struct A:virtual Base{
int val;
A():Base("A"){}
};
struct B:virtual Base{
int bval;
B():Base("B"){}
};
struct Derived:public A, public B{
Derived():Base("Derived"){}
};
void func(Derived &d1, Derived &d2)
{
d1 = d2;
}
|
The C++ standard specifies that `Base::Base' is only called once when constructing or copy-constructing a Derived object. It is unspecified whether `Base::operator=' is called more than once when the implicit copy-assignment for Derived objects is invoked (as it is inside `func' in the example).
g++ implements the "intuitive" algorithm for copy-assignment: assign all
direct bases, then assign all members. In that algorithm, the virtual
base subobject can be encountered many times. In the example, copying
proceeds in the following order: `val', `name' (via
strdup), `bval', and `name' again.
If application code relies on copy-assignment, a user-defined copy-assignment operator removes any uncertainties. With such an operator, the application can define whether and how the virtual base subobject is assigned.
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protoize
The conversion programs protoize and unprotoize can
sometimes change a source file in a way that won't work unless you
rearrange it.
protoize can insert references to a type name or type tag before
the definition, or in a file where they are not defined.
If this happens, compiler error messages should show you where the new references are, so fixing the file by hand is straightforward.
protoize cannot figure out.
For example, it can't determine argument types for declaring a
pointer-to-function variable; this you must do by hand. protoize
inserts a comment containing `???' each time it finds such a
variable; so you can find all such variables by searching for this
string. ISO C does not require declaring the argument types of
pointer-to-function types.
unprotoize can easily introduce bugs. If the program
relied on prototypes to bring about conversion of arguments, these
conversions will not take place in the program without prototypes.
One case in which you can be sure unprotoize is safe is when
you are removing prototypes that were made with protoize; if
the program worked before without any prototypes, it will work again
without them.
You can find all the places where this problem might occur by compiling the program with the `-Wconversion' option. It prints a warning whenever an argument is converted.
protoize cannot get the argument types for a function whose
definition was not actually compiled due to preprocessing conditionals.
When this happens, protoize changes nothing in regard to such
a function. protoize tries to detect such instances and warn
about them.
You can generally work around this problem by using protoize step
by step, each time specifying a different set of `-D' options for
compilation, until all of the functions have been converted. There is
no automatic way to verify that you have got them all, however.
If you plan on converting source files which contain such code, it is recommended that you first make sure that each conditionally compiled region of source code which contains an alternative function header also contains at least one additional follower token (past the final right parenthesis of the function header). This should circumvent the problem.
unprotoize can become confused when trying to convert a function
definition or declaration which contains a declaration for a
pointer-to-function formal argument which has the same name as the
function being defined or declared. We recommend you avoid such choices
of formal parameter names.
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This section lists changes that people frequently request, but which we do not make because we think GCC is better without them.
Such a feature would work only occasionally--only for calls that appear in the same file as the called function, following the definition. The only way to check all calls reliably is to add a prototype for the function. But adding a prototype eliminates the motivation for this feature. So the feature is not worthwhile.
Shift count operands are probably signed more often than unsigned. Warning about this would cause far more annoyance than good.
Such assignments must be very common; warning about them would cause more annoyance than good.
Coming as I do from a Lisp background, I balk at the idea that there is
something dangerous about discarding a value. There are functions that
return values which some callers may find useful; it makes no sense to
clutter the program with a cast to void whenever the value isn't
useful.
This would cause storage layout to be incompatible with most other C compilers. And it doesn't seem very important, given that you can get the same result in other ways. The case where it matters most is when the enumeration-valued object is inside a structure, and in that case you can specify a field width explicitly.
The ISO C standard leaves it up to the implementation whether a bit-field
declared plain int is signed or not. This in effect creates two
alternative dialects of C.
The GNU C compiler supports both dialects; you can specify the signed dialect with `-fsigned-bitfields' and the unsigned dialect with `-funsigned-bitfields'. However, this leaves open the question of which dialect to use by default.
Currently, the preferred dialect makes plain bit-fields signed, because
this is simplest. Since int is the same as signed int in
every other context, it is cleanest for them to be the same in bit-fields
as well.
Some computer manufacturers have published Application Binary Interface standards which specify that plain bit-fields should be unsigned. It is a mistake, however, to say anything about this issue in an ABI. This is because the handling of plain bit-fields distinguishes two dialects of C. Both dialects are meaningful on every type of machine. Whether a particular object file was compiled using signed bit-fields or unsigned is of no concern to other object files, even if they access the same bit-fields in the same data structures.
A given program is written in one or the other of these two dialects. The program stands a chance to work on most any machine if it is compiled with the proper dialect. It is unlikely to work at all if compiled with the wrong dialect.
Many users appreciate the GNU C compiler because it provides an environment that is uniform across machines. These users would be inconvenienced if the compiler treated plain bit-fields differently on certain machines.
Occasionally users write programs intended only for a particular machine type. On these occasions, the users would benefit if the GNU C compiler were to support by default the same dialect as the other compilers on that machine. But such applications are rare. And users writing a program to run on more than one type of machine cannot possibly benefit from this kind of compatibility.
This is why GCC does and will treat plain bit-fields in the same fashion on all types of machines (by default).
There are some arguments for making bit-fields unsigned by default on all machines. If, for example, this becomes a universal de facto standard, it would make sense for GCC to go along with it. This is something to be considered in the future.
(Of course, users strongly concerned about portability should indicate explicitly in each bit-field whether it is signed or not. In this way, they write programs which have the same meaning in both C dialects.)
__STDC__ when `-ansi' is not used.
Currently, GCC defines __STDC__ unconditionally. This provides
good results in practice.
Programmers normally use conditionals on __STDC__ to ask whether
it is safe to use certain features of ISO C, such as function
prototypes or ISO token concatenation. Since plain gcc supports
all the features of ISO C, the correct answer to these questions is
"yes".
Some users try to use __STDC__ to check for the availability of
certain library facilities. This is actually incorrect usage in an ISO
C program, because the ISO C standard says that a conforming
freestanding implementation should define __STDC__ even though it
does not have the library facilities. `gcc -ansi -pedantic' is a
conforming freestanding implementation, and it is therefore required to
define __STDC__, even though it does not come with an ISO C
library.
Sometimes people say that defining __STDC__ in a compiler that
does not completely conform to the ISO C standard somehow violates the
standard. This is illogical. The standard is a standard for compilers
that claim to support ISO C, such as `gcc -ansi'---not for other
compilers such as plain gcc. Whatever the ISO C standard says
is relevant to the design of plain gcc without `-ansi' only
for pragmatic reasons, not as a requirement.
GCC normally defines __STDC__ to be 1, and in addition
defines __STRICT_ANSI__ if you specify the `-ansi' option,
or a `-std' option for strict conformance to some version of ISO C.
On some hosts, system include files use a different convention, where
__STDC__ is normally 0, but is 1 if the user specifies strict
conformance to the C Standard. GCC follows the host convention when
processing system include files, but when processing user files it follows
the usual GNU C convention.
__STDC__ in C++.
Programs written to compile with C++-to-C translators get the
value of __STDC__ that goes with the C compiler that is
subsequently used. These programs must test __STDC__
to determine what kind of C preprocessor that compiler uses:
whether they should concatenate tokens in the ISO C fashion
or in the traditional fashion.
These programs work properly with GNU C++ if __STDC__ is defined.
They would not work otherwise.
In addition, many header files are written to provide prototypes in ISO
C but not in traditional C. Many of these header files can work without
change in C++ provided __STDC__ is defined. If __STDC__
is not defined, they will all fail, and will all need to be changed to
test explicitly for C++ as well.
Historically, GCC has not deleted "empty" loops under the assumption that the most likely reason you would put one in a program is to have a delay, so deleting them will not make real programs run any faster.
However, the rationale here is that optimization of a nonempty loop cannot produce an empty one, which holds for C but is not always the case for C++.
Moreover, with `-funroll-loops' small "empty" loops are already removed, so the current behavior is both sub-optimal and inconsistent and will change in the future.
It is never safe to depend on the order of evaluation of side effects. For example, a function call like this may very well behave differently from one compiler to another:
void func (int, int); int i = 2; func (i++, i++); |
There is no guarantee (in either the C or the C++ standard language
definitions) that the increments will be evaluated in any particular
order. Either increment might happen first. func might get the
arguments `2, 3', or it might get `3, 2', or even `2, 2'.
Strictly speaking, there is no prohibition in the ISO C standard against allowing structures with volatile fields in registers, but it does not seem to make any sense and is probably not what you wanted to do. So the compiler will give an error message in this case.
Some ISO C testsuites report failure when the compiler does not produce an error message for a certain program.
ISO C requires a "diagnostic" message for certain kinds of invalid programs, but a warning is defined by GCC to count as a diagnostic. If GCC produces a warning but not an error, that is correct ISO C support. If test suites call this "failure", they should be run with the GCC option `-pedantic-errors', which will turn these warnings into errors.
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The GNU compiler can produce two kinds of diagnostics: errors and warnings. Each kind has a different purpose:
Warnings may indicate danger points where you should check to make sure that your program really does what you intend; or the use of obsolete features; or the use of nonstandard features of GNU C or C++. Many warnings are issued only if you ask for them, with one of the `-W' options (for instance, `-Wall' requests a variety of useful warnings).
GCC always tries to compile your program if possible; it never gratuitously rejects a program whose meaning is clear merely because (for instance) it fails to conform to a standard. In some cases, however, the C and C++ standards specify that certain extensions are forbidden, and a diagnostic must be issued by a conforming compiler. The `-pedantic' option tells GCC to issue warnings in such cases; `-pedantic-errors' says to make them errors instead. This does not mean that all non-ISO constructs get warnings or errors.
See section Options to Request or Suppress Warnings, for more detail on these and related command-line options.
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Your bug reports play an essential role in making GCC reliable.
When you encounter a problem, the first thing to do is to see if it is already known. See section 9. Known Causes of Trouble with GCC. If it isn't known, then you should report the problem.
Reporting a bug may help you by bringing a solution to your problem, or it may not. (If it does not, look in the service directory; see 11. How To Get Help with GCC.) In any case, the principal function of a bug report is to help the entire community by making the next version of GCC work better. Bug reports are your contribution to the maintenance of GCC.
Since the maintainers are very overloaded, we cannot respond to every bug report. However, if the bug has not been fixed, we are likely to send you a patch and ask you to tell us whether it works.
In order for a bug report to serve its purpose, you must include the information that makes for fixing the bug.
10.1 Have You Found a Bug? Have you really found a bug? 10.2 Where to Report Bugs Where to send your bug report. 10.3 How to Report Bugs How to report a bug effectively. 10.4 The gccbug script You can use a bug reporting tool. 9. Known Causes of Trouble with GCC Known problems. 11. How To Get Help with GCC Where to ask for help.
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If you are not sure whether you have found a bug, here are some guidelines:
asm statement), that is a compiler bug, unless the
compiler reports errors (not just warnings) which would ordinarily
prevent the assembler from being run.
However, you must double-check to make sure, because you may have a program whose behavior is undefined, which happened by chance to give the desired results with another C or C++ compiler.
For example, in many nonoptimizing compilers, you can write `x;'
at the end of a function instead of `return x;', with the same
results. But the value of the function is undefined if return
is omitted; it is not a bug when GCC produces different results.
Problems often result from expressions with two increment operators,
as in f (*p++, *p++). Your previous compiler might have
interpreted that expression the way you intended; GCC might
interpret it another way. Neither compiler is wrong. The bug is
in your code.
After you have localized the error to a single source line, it should be easy to check for these things. If your program is correct and well defined, you have found a compiler bug.
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Please read http://gcc.gnu.org/bugs.html for additional and/or more up-to-date bug reporting instructions before you post a bug report.
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The fundamental principle of reporting bugs usefully is this: report all the facts. If you are not sure whether to state a fact or leave it out, state it!
Often people omit facts because they think they know what causes the problem and they conclude that some details don't matter. Thus, you might assume that the name of the variable you use in an example does not matter. Well, probably it doesn't, but one cannot be sure. Perhaps the bug is a stray memory reference which happens to fetch from the location where that name is stored in memory; perhaps, if the name were different, the contents of that location would fool the compiler into doing the right thing despite the bug. Play it safe and give a specific, complete example. That is the easiest thing for you to do, and the most helpful.
Keep in mind that the purpose of a bug report is to enable someone to fix the bug if it is not known. It isn't very important what happens if the bug is already known. Therefore, always write your bug reports on the assumption that the bug is not known.
Sometimes people give a few sketchy facts and ask, "Does this ring a bell?" This cannot help us fix a bug, so it is basically useless. We respond by asking for enough details to enable us to investigate. You might as well expedite matters by sending them to begin with.
Try to make your bug report self-contained. If we have to ask you for more information, it is best if you include all the previous information in your response, as well as the information that was missing.
Please report each bug in a separate message. This makes it easier for us to track which bugs have been fixed and to forward your bugs reports to the appropriate maintainer.
To enable someone to investigate the bug, you should include all these things:
Without this, we won't know whether there is any point in looking for the bug in the current version of GCC.
A single statement is not enough of an example. In order to compile it, it must be embedded in a complete file of compiler input; and the bug might depend on the details of how this is done.
Without a real example one can compile, all anyone can do about your bug report is wish you luck. It would be futile to try to guess how to provoke the bug. For example, bugs in register allocation and reloading frequently depend on every little detail of the function they happen in.
Even if the input file that fails comes from a GNU program, you should still send the complete test case. Don't ask the GCC maintainers to do the extra work of obtaining the program in question--they are all overworked as it is. Also, the problem may depend on what is in the header files on your system; it is unreliable for the GCC maintainers to try the problem with the header files available to them. By sending CPP output, you can eliminate this source of uncertainty and save us a certain percentage of wild goose chases.
If we were to try to guess the arguments, we would probably guess wrong and then we would not encounter the bug.
configure command when you installed
the compiler.
Be precise about these changes. A description in English is not enough--send a context diff for them.
Adding files of your own (such as a machine description for a machine we don't support) is a modification of the compiler source.
Of course, if the bug is that the compiler gets a fatal signal, then one can't miss it. But if the bug is incorrect output, the maintainer might not notice unless it is glaringly wrong. None of us has time to study all the assembler code from a 50-line C program just on the chance that one instruction might be wrong. We need you to do this part!
Even if the problem you experience is a fatal signal, you should still say so explicitly. Suppose something strange is going on, such as, your copy of the compiler is out of synch, or you have encountered a bug in the C library on your system. (This has happened!) Your copy might crash and the copy here would not. If you said to expect a crash, then when the compiler here fails to crash, we would know that the bug was not happening. If you don't say to expect a crash, then we would not know whether the bug was happening. We would not be able to draw any conclusion from our observations.
If the problem is a diagnostic when compiling GCC with some other compiler, say whether it is a warning or an error.
Often the observed symptom is incorrect output when your program is run. Sad to say, this is not enough information unless the program is short and simple. None of us has time to study a large program to figure out how it would work if compiled correctly, much less which line of it was compiled wrong. So you will have to do that. Tell us which source line it is, and what incorrect result happens when that line is executed. A person who understands the program can find this as easily as finding a bug in the program itself.
The line numbers in the development sources don't match those in your sources. Your line numbers would convey no useful information to the maintainers.
For example, many people send just a backtrace, but that is never useful by itself. A simple backtrace with arguments conveys little about GCC because the compiler is largely data-driven; the same functions are called over and over for different RTL insns, doing different things depending on the details of the insn.
Most of the arguments listed in the backtrace are useless because they are pointers to RTL list structure. The numeric values of the pointers, which the debugger prints in the backtrace, have no significance whatever; all that matters is the contents of the objects they point to (and most of the contents are other such pointers).
In addition, most compiler passes consist of one or more loops that scan the RTL insn sequence. The most vital piece of information about such a loop--which insn it has reached--is usually in a local variable, not in an argument.
What you need to provide in addition to a backtrace are the values of
the local variables for several stack frames up. When a local
variable or an argument is an RTX, first print its value and then use
the GDB command pr to print the RTL expression that it points
to. (If GDB doesn't run on your machine, use your debugger to call
the function debug_rtx with the RTX as an argument.) In
general, whenever a variable is a pointer, its value is no use
without the data it points to.
Here are some things that are not necessary:
Often people who encounter a bug spend a lot of time investigating which changes to the input file will make the bug go away and which changes will not affect it.
This is often time consuming and not very useful, because the way we will find the bug is by running a single example under the debugger with breakpoints, not by pure deduction from a series of examples. You might as well save your time for something else.
Of course, if you can find a simpler example to report instead of the original one, that is a convenience. Errors in the output will be easier to spot, running under the debugger will take less time, etc. Most GCC bugs involve just one function, so the most straightforward way to simplify an example is to delete all the function definitions except the one where the bug occurs. Those earlier in the file may be replaced by external declarations if the crucial function depends on them. (Exception: inline functions may affect compilation of functions defined later in the file.)
However, simplification is not vital; if you don't want to do this, report the bug anyway and send the entire test case you used.
A patch for the bug is useful if it is a good one. But don't omit the necessary information, such as the test case, on the assumption that a patch is all we need. We might see problems with your patch and decide to fix the problem another way, or we might not understand it at all.
Sometimes with a program as complicated as GCC it is very hard to construct an example that will make the program follow a certain path through the code. If you don't send the example, we won't be able to construct one, so we won't be able to verify that the bug is fixed.
And if we can't understand what bug you are trying to fix, or why your patch should be an improvement, we won't install it. A test case will help us to understand.
See http://gcc.gnu.org/contribute.html for guidelines on how to make it easy for us to understand and install your patches.
Such guesses are usually wrong. Even I can't guess right about such things without first using the debugger to find the facts.
We have no way of examining a core dump for your type of machine unless we have an identical system--and if we do have one, we should be able to reproduce the crash ourselves.
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To simplify creation of bug reports, and to allow better tracking of
reports, we use the GNATS bug tracking system. Part of that system is
the gccbug script. This is a Unix shell script, so you need a
shell to run it. It is normally installed in the same directory where
gcc is installed.
The gccbug script is derived from send-pr, see section `Creating new Problem Reports' in Reporting Problems. When invoked, it starts a text editor so you can fill out the various fields of the report. When the you quit the editor, the report is automatically send to the bug reporting address.
A number of fields in this bug report form are specific to GCC, and are explained at http://gcc.gnu.org/gnats.html.
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If you need help installing, using or changing GCC, there are two ways to find it:
For further information, see http://gcc.gnu.org/faq.html#support.
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If you would like to help pretest GCC releases to assure they work well, our current development sources are available by CVS (see http://gcc.gnu.org/cvs.html). Source and binary snapshots are also available for FTP; see http://gcc.gnu.org/snapshots.html.
If you would like to work on improvements to GCC, please read the advice at these URLs:
http://gcc.gnu.org/contribute.html http://gcc.gnu.org/contributewhy.html |
for information on how to make useful contributions and avoid duplication of effort. Suggested projects are listed at http://gcc.gnu.org/projects/.
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If you want to have more free software a few years from now, it makes sense for you to help encourage people to contribute funds for its development. The most effective approach known is to encourage commercial redistributors to donate.
Users of free software systems can boost the pace of development by encouraging for-a-fee distributors to donate part of their selling price to free software developers--the Free Software Foundation, and others.
The way to convince distributors to do this is to demand it and expect it from them. So when you compare distributors, judge them partly by how much they give to free software development. Show distributors they must compete to be the one who gives the most.
To make this approach work, you must insist on numbers that you can compare, such as, "We will donate ten dollars to the Frobnitz project for each disk sold." Don't be satisfied with a vague promise, such as "A portion of the profits are donated," since it doesn't give a basis for comparison.
Even a precise fraction "of the profits from this disk" is not very meaningful, since creative accounting and unrelated business decisions can greatly alter what fraction of the sales price counts as profit. If the price you pay is $50, ten percent of the profit is probably less than a dollar; it might be a few cents, or nothing at all.
Some redistributors do development work themselves. This is useful too; but to keep everyone honest, you need to inquire how much they do, and what kind. Some kinds of development make much more long-term difference than others. For example, maintaining a separate version of a program contributes very little; maintaining the standard version of a program for the whole community contributes much. Easy new ports contribute little, since someone else would surely do them; difficult ports such as adding a new CPU to the GNU Compiler Collection contribute more; major new features or packages contribute the most.
By establishing the idea that supporting further development is "the proper thing to do" when distributing free software for a fee, we can assure a steady flow of resources into making more free software.
Copyright (C) 1994 Free Software Foundation, Inc. Verbatim copying and redistribution of this section is permitted without royalty; alteration is not permitted.
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The GNU Project was launched in 1984 to develop a complete Unix-like operating system which is free software: the GNU system. (GNU is a recursive acronym for "GNU's Not Unix"; it is pronounced "guh-NEW".) Variants of the GNU operating system, which use the kernel Linux, are now widely used; though these systems are often referred to as "Linux", they are more accurately called GNU/Linux systems.
For more information, see:
http://www.gnu.org/ http://www.gnu.org/gnu/linux-and-gnu.html |
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Copyright (C) 1989, 1991 Free Software Foundation, Inc. 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
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The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software--to make sure the software is free for all its users. This General Public License applies to most of the Free Software Foundation's software and to any other program whose authors commit to using it. (Some other Free Software Foundation software is covered by the GNU Library General Public License instead.) You can apply it to your programs, too.
When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things.
To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights. These restrictions translate to certain responsibilities for you if you distribute copies of the software, or if you modify it.
For example, if you distribute copies of such a program, whether gratis or for a fee, you must give the recipients all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights.
We protect your rights with two steps: (1) copyright the software, and (2) offer you this license which gives you legal permission to copy, distribute and/or modify the software.
Also, for each author's protection and ours, we want to make certain that everyone understands that there is no warranty for this free software. If the software is modified by someone else and passed on, we want its recipients to know that what they have is not the original, so that any problems introduced by others will not reflect on the original authors' reputations.
Finally, any free program is threatened constantly by software patents. We wish to avoid the danger that redistributors of a free program will individually obtain patent licenses, in effect making the program proprietary. To prevent this, we have made it clear that any patent must be licensed for everyone's free use or not licensed at all.
The precise terms and conditions for copying, distribution and modification follow.
Activities other than copying, distribution and modification are not covered by this License; they are outside its scope. The act of running the Program is not restricted, and the output from the Program is covered only if its contents constitute a work based on the Program (independent of having been made by running the Program). Whether that is true depends on what the Program does.
You may charge a fee for the physical act of transferring a copy, and you may at your option offer warranty protection in exchange for a fee.
These requirements apply to the modified work as a whole. If identifiable sections of that work are not derived from the Program, and can be reasonably considered independent and separate works in themselves, then this License, and its terms, do not apply to those sections when you distribute them as separate works. But when you distribute the same sections as part of a whole which is a work based on the Program, the distribution of the whole must be on the terms of this License, whose permissions for other licensees extend to the entire whole, and thus to each and every part regardless of who wrote it.
Thus, it is not the intent of this section to claim rights or contest your rights to work written entirely by you; rather, the intent is to exercise the right to control the distribution of derivative or collective works based on the Program.
In addition, mere aggregation of another work not based on the Program with the Program (or with a work based on the Program) on a volume of a storage or distribution medium does not bring the other work under the scope of this License.
The source code for a work means the preferred form of the work for making modifications to it. For an executable work, complete source code means all the source code for all modules it contains, plus any associated interface definition files, plus the scripts used to control compilation and installation of the executable. However, as a special exception, the source code distributed need not include anything that is normally distributed (in either source or binary form) with the major components (compiler, kernel, and so on) of the operating system on which the executable runs, unless that component itself accompanies the executable.
If distribution of executable or object code is made by offering access to copy from a designated place, then offering equivalent access to copy the source code from the same place counts as distribution of the source code, even though third parties are not compelled to copy the source along with the object code.
If any portion of this section is held invalid or unenforceable under any particular circumstance, the balance of the section is intended to apply and the section as a whole is intended to apply in other circumstances.
It is not the purpose of this section to induce you to infringe any patents or other property right claims or to contest validity of any such claims; this section has the sole purpose of protecting the integrity of the free software distribution system, which is implemented by public license practices. Many people have made generous contributions to the wide range of software distributed through that system in reliance on consistent application of that system; it is up to the author/donor to decide if he or she is willing to distribute software through any other system and a licensee cannot impose that choice.
This section is intended to make thoroughly clear what is believed to be a consequence of the rest of this License.
Each version is given a distinguishing version number. If the Program specifies a version number of this License which applies to it and "any later version", you have the option of following the terms and conditions either of that version or of any later version published by the Free Software Foundation. If the Program does not specify a version number of this License, you may choose any version ever published by the Free Software Foundation.
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If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.
To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the "copyright" line and a pointer to where the full notice is found.
one line to give the program's name and a brief idea of what it does. Copyright (C) year name of author This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. |
Also add information on how to contact you by electronic and paper mail.
If the program is interactive, make it output a short notice like this when it starts in an interactive mode:
Gnomovision version 69, Copyright (C) year name of author Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'. This is free software, and you are welcome to redistribute it under certain conditions; type `show c' for details. |
The hypothetical commands `show w' and `show c' should show the appropriate parts of the General Public License. Of course, the commands you use may be called something other than `show w' and `show c'; they could even be mouse-clicks or menu items--whatever suits your program.
You should also get your employer (if you work as a programmer) or your school, if any, to sign a "copyright disclaimer" for the program, if necessary. Here is a sample; alter the names:
Yoyodyne, Inc., hereby disclaims all copyright interest in the program `Gnomovision' (which makes passes at compilers) written by James Hacker. signature of Ty Coon, 1 April 1989 Ty Coon, President of Vice |
This General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Library General Public License instead of this License.
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Copyright (C) 2000,2001,2002 Free Software Foundation, Inc. 59 Temple Place, Suite 330, Boston, MA 02111-1307, USA
Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.
This License is a kind of "copyleft", which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.
We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.
This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The "Document", below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as "you". You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law.
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The "Invariant Sections" are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License. If a section does not fit the above definition of Secondary then it is not allowed to be designated as Invariant. The Document may contain zero Invariant Sections. If the Document does not identify any Invariant Sections then there are none.
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Copyright (C) year your name. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License''. |
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The GCC project would like to thank its many contributors. Without them the project would not have been nearly as successful as it has been. Any omissions in this list are accidental. Feel free to contact law@redhat.com or gerald@pfeifer.com if you have been left out or some of your contributions are not listed. Please keep this list in alphabetical order.
protoize and unprotoize
tools, the support for Dwarf symbolic debugging information, and much of
the support for System V Release 4. He has also worked heavily on the
Intel 386 and 860 support.
restrict support, and serving as release manager for GCC 3.x.
We'd also like to thank the folks who have contributed time and energy in testing GCC:
And finally we'd like to thank everyone who uses the compiler, submits bug reports and generally reminds us why we're doing this work in the first place.
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GCC's command line options are indexed here without any initial `-' or `--'. Where an option has both positive and negative forms (such as `-foption' and `-fno-option'), relevant entries in the manual are indexed under the most appropriate form; it may sometimes be useful to look up both forms.
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On some systems, `gcc -shared' needs to build supplementary stub code for constructors to work. On multi-libbed systems, `gcc -shared' must select the correct support libraries to link against. Failing to supply the correct flags may lead to subtle defects. Supplying them in cases where they are not necessary is innocuous.
Future versions of GCC may zero-extend, or use
a target-defined ptr_extend pattern. Do not rely on sign extension.
The analogous feature in Fortran is called an assigned goto, but that name seems inappropriate in C, where one can do more than simply store label addresses in label variables.
A file's basename was the name stripped of all leading path information and of trailing suffixes, such as `.h' or `.C' or `.cc'.
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typeof
void- and Function-Pointers
asm Operands
enum Types
+load: Executing code before main
protoize
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1. Compile C, C++, Objective-C, Ada, Fortran, Java, or treelang
2. Language Standards Supported by GCC
3. GCC Command Options
4. C Implementation-defined behavior
5. Extensions to the C Language Family
6. Extensions to the C++ Language
7. GNU Objective-C runtime features
8. Binary Compatibility
9. Known Causes of Trouble with GCC
10. Reporting Bugs
11. How To Get Help with GCC
12. Contributing to GCC Development
Funding Free Software
The GNU Project and GNU/Linux
GNU GENERAL PUBLIC LICENSE
GNU Free Documentation License
Contributors to GCC
Option Index
Keyword Index
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| Button | Name | Go to | From 1.2.3 go to |
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| [ << ] | FastBack | previous or up-and-previous section | 1.1 |
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| [Top] | Top | cover (top) of document | |
| [Contents] | Contents | table of contents | |
| [Index] | Index | concept index | |
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