3.4 Options Controlling C Dialect
- In C mode, this is equivalent to -std=c89. In C++ mode, it is
equivalent to -std=c++98.
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
typeofkeywords, and predefined macros such as
vaxthat 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
The alternate keywords
__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
__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 Warning Options.
__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 that would normally be built in but do not have semantics defined by ISO C (such as
ffs) are not built-in functions when -ansi is used. See Other built-in functions provided by GCC, for details of the functions affected.
- Determine the language standard. See Language Standards Supported by GCC, for details of these standard versions. This option
is currently only supported when compiling C or C++.
The compiler can accept several base standards, such as c89 or c++98, and GNU dialects of those standards, such as gnu89 or gnu++98. By specifying a base standard, the compiler will accept all programs following that standard and those using GNU extensions that do not contradict it. For example, -std=c89 turns off certain features of GCC that are incompatible with ISO C90, such as the
typeofkeywords, but not other GNU extensions that do not have a meaning in ISO C90, such as omitting the middle term of a
?:expression. On the other hand, by specifying a GNU dialect of a standard, all features the compiler support are enabled, even when those features change the meaning of the base standard and some strict-conforming programs may be rejected. The particular standard is used by -pedantic to identify which features are GNU extensions given that version of the standard. For example -std=gnu89 -pedantic would warn about C++ style // comments, while -std=gnu99 -pedantic would not.
A value for this option must be provided; possible values are
- Support all ISO C90 programs (certain GNU extensions that conflict
with ISO C90 are disabled). Same as -ansi for C code.
- ISO C90 as modified in amendment 1.
- ISO C99. Note that this standard is not yet fully supported; see
http://gcc.gnu.org/gcc-4.4/c99status.html for more information. The
names c9x and iso9899:199x are deprecated.
- GNU dialect of ISO C90 (including some C99 features). This
is the default for C code.
- GNU dialect of ISO C99. When ISO C99 is fully implemented in GCC,
this will become the default. The name gnu9x is deprecated.
- The 1998 ISO C++ standard plus amendments. Same as -ansi for
- GNU dialect of -std=c++98. This is the default for
- The working draft of the upcoming ISO C++0x standard. This option
enables experimental features that are likely to be included in
C++0x. The working draft is constantly changing, and any feature that is
enabled by this flag may be removed from future versions of GCC if it is
not part of the C++0x standard.
- GNU dialect of -std=c++0x. This option enables experimental features that may be removed in future versions of GCC.
- The option -fgnu89-inline tells GCC to use the traditional
GNU semantics for
inlinefunctions when in C99 mode. See An Inline Function is As Fast As a Macro. This option is accepted and ignored by GCC versions 4.1.3 up to but not including 4.3. In GCC versions 4.3 and later it changes the behavior of GCC in C99 mode. Using this option is roughly equivalent to adding the
gnu_inlinefunction attribute to all inline functions (see Function Attributes).
The option -fno-gnu89-inline explicitly tells GCC to use the C99 semantics for
inlinewhen in C99 or gnu99 mode (i.e., it specifies the default behavior). This option was first supported in GCC 4.3. This option is not supported in C89 or gnu89 mode.
The preprocessor macros
__GNUC_STDC_INLINE__may be used to check which semantics are in effect for
inlinefunctions. See Common Predefined Macros (The C Preprocessor).
- Output to the given filename prototyped declarations for all functions
declared and/or defined in a translation unit, including those in header
files. This option is silently ignored in any language other than C.
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.
- Do not recognize
typeofas a keyword, so that code can use these words as identifiers. You can use the keywords
__typeof__instead. -ansi implies -fno-asm.
In C++, this switch only affects the
inlineare 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
inlineis a standard keyword in ISO C99.
- Don't recognize built-in functions that do not begin with
__builtin_ as prefix. See Other built-in functions provided by GCC, for details of the functions affected,
including those which are not built-in functions when -ansi or
-std options for strict ISO C conformance are used because they
do not have an ISO standard meaning.
GCC normally generates special code to handle certain built-in functions more efficiently; for instance, calls to
allocamay become single instructions that adjust the stack directly, and calls to
memcpymay 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. In addition, when a function is recognized as a built-in function, GCC may use information about that function to warn about problems with calls to that function, or to generate more efficient code, even if the resulting code still contains calls to that function. For example, warnings are given with -Wformat for bad calls to
printfis built in, and
strlenis known not to modify global memory.
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 that 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))
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
mainhas a return type of
int. Examples are nearly everything except a kernel. This is equivalent to -fno-freestanding.
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 Language Standards Supported by GCC, for details of freestanding and hosted environments.
- Enable handling of OpenMP directives
#pragma ompin C/C++ and
!$ompin Fortran. When -fopenmp is specified, the compiler generates parallel code according to the OpenMP Application Program Interface v2.5 http://www.openmp.org/. This option implies -pthread, and thus is only supported on targets that have support for -pthread.
- Accept some non-standard constructs used in Microsoft header files.
Some cases of unnamed fields in structures and unions are only accepted with this option. See Unnamed struct/union fields within structs/unions, for details.
- Support ISO C trigraphs. The -ansi option (and -std
options for strict ISO C conformance) implies -trigraphs.
- Performs a compilation in two passes: preprocessing and compiling. This
option allows a user supplied "cc1", "cc1plus", or "cc1obj" via the
-B option. The user supplied compilation step can then add in
an additional preprocessing step after normal preprocessing but before
compiling. The default is to use the integrated cpp (internal cpp)
The semantics of this option will change if "cc1", "cc1plus", and "cc1obj" are merged.
- Formerly, these options caused GCC to attempt to emulate a pre-standard
C compiler. They are now only supported with the -E switch.
The preprocessor continues to support a pre-standard mode. See the GNU
CPP manual for details.
- Allow conditional expressions with mismatched types in the second and
third arguments. The value of such an expression is void. This option
is not supported for C++.
- Allow implicit conversions between vectors with differing numbers of
elements and/or incompatible element types. This option should not be
used for new code.
- Let the type
charbe unsigned, like
Each kind of machine has a default for what
charshould be. It is either like
unsigned charby default or like
signed charby default.
Ideally, a portable program should always use
unsigned charwhen it depends on the signedness of an object. But many programs have been written to use plain
charand 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.
charis always a distinct type from each of
unsigned char, even though its behavior is always just like one of those two.
- Let the type
charbe signed, like
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.
- These options control whether a bit-field is signed or unsigned, when the
declaration does not use either
unsigned. By default, such a bit-field is signed, because this is consistent: the basic integer types such as
intare signed types.