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3.4 Options Controlling C Dialect
The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and Objective-C++) that the compiler accepts:
- @gcctabopt -ansi
In C mode, support all ISO C90 programs. In C++ mode, remove GNU extensions that conflict with ISO C++.
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 section Options to Request or Suppress Warnings.
__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
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.
- @gcctabopt -std=
Determine the language standard. This option is currently only supported when compiling C or C++. A value for this option must be provided; possible values are
ISO C90 (same as ‘-ansi’).
ISO C90 as modified in amendment 1.
ISO C99. Note that this standard is not yet fully supported; see http://gcc.gnu.org/gcc-4.0/c99status.html for more information. The names ‘c9x’ and ‘iso9899:199x’ are deprecated.
Default, ISO C90 plus GNU extensions (including some C99 features).
ISO C99 plus GNU extensions. 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.
The same as ‘-std=c++98’ plus GNU extensions. This is the default for C++ code.
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
inlinekeyword in ISO C99) are not disabled.
See section Language Standards Supported by GCC, for details of these standard versions.
- @gcctabopt -aux-info filename
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.
- @gcctabopt -fno-asm
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.
- @gcctabopt -fno-builtin
- @gcctabopt -fno-builtin-function
Don't recognize built-in functions that do not begin with ‘__builtin_’ as prefix. See section 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 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))
- @gcctabopt -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
mainhas a return type of
int. Examples are nearly everything except a kernel. This is equivalent to ‘-fno-freestanding’.
- @gcctabopt -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.
- @gcctabopt -fms-extensions
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 section Unnamed struct/union fields within structs/unions, for details.
- @gcctabopt -trigraphs
Support ISO C trigraphs. The ‘-ansi’ option (and ‘-std’ options for strict ISO C conformance) implies ‘-trigraphs’.
- @gcctabopt -no-integrated-cpp
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.
- @gcctabopt -traditional
- @gcctabopt -traditional-cpp
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.
- @gcctabopt -fcond-mismatch
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++.
- @gcctabopt -funsigned-char
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.
- @gcctabopt -fsigned-char
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’.
- @gcctabopt -fsigned-bitfields
- @gcctabopt -funsigned-bitfields
- @gcctabopt -fno-signed-bitfields
- @gcctabopt -fno-unsigned-bitfields
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.
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