Predefined Macros#

Several object-like macros are predefined; you use them without supplying their definitions. They fall into three classes: standard, common, and system-specific.

In C++, there is a fourth category, the named operators. They act like predefined macros, but you cannot undefine them.

Standard Predefined Macros#

The standard predefined macros are specified by the relevant language standards, so they are available with all compilers that implement those standards. Older compilers may not provide all of them. Their names all start with double underscores.

__FILE__#: This macro expands to the name of the current input file, in the form of a C string constant. This is the path by which the preprocessor opened the file, not the short name specified in #include or as the input file name argument. For example, "/usr/local/include/myheader.h" is a possible expansion of this macro.

__LINE__#

This macro expands to the current input line number, in the form of a decimal integer constant. While we call it a predefined macro, it’s a pretty strange macro, since its ‘definition’ changes with each new line of source code.

__FILE__ and __LINE__ are useful in generating an error message to report an inconsistency detected by the program; the message can state the source line at which the inconsistency was detected. For example,

fprintf (stderr, "Internal error: "
                 "negative string length "
                 "%d at %s, line %d.",
         length, __FILE__, __LINE__);

An #include directive changes the expansions of __FILE__ and __LINE__ to correspond to the included file. At the end of that file, when processing resumes on the input file that contained the #include directive, the expansions of __FILE__ and __LINE__ revert to the values they had before the #include (but __LINE__ is then incremented by one as processing moves to the line after the #include).

A #line directive changes __LINE__, and may change __FILE__ as well. See Line Control.

C99 introduced __func__, and GCC has provided __FUNCTION__ for a long time. Both of these are strings containing the name of the current function (there are slight semantic differences; see the GCC manual). Neither of them is a macro; the preprocessor does not know the name of the current function. They tend to be useful in conjunction with __FILE__ and __LINE__, though.

__DATE__#

This macro expands to a string constant that describes the date on which the preprocessor is being run. The string constant contains eleven characters and looks like "Feb 12 1996". If the day of the month is less than 10, it is padded with a space on the left.

If GCC cannot determine the current date, it will emit a warning message (once per compilation) and __DATE__ will expand to "??? ?? ????".

__TIME__#

This macro expands to a string constant that describes the time at which the preprocessor is being run. The string constant contains eight characters and looks like "23:59:01".

If GCC cannot determine the current time, it will emit a warning message (once per compilation) and __TIME__ will expand to "??:??:??".

__STDC__#

In normal operation, this macro expands to the constant 1, to signify that this compiler conforms to ISO Standard C. If GNU CPP is used with a compiler other than GCC, this is not necessarily true; however, the preprocessor always conforms to the standard unless the -traditional-cpp option is used.

This macro is not defined if the -traditional-cpp option is used.

On some hosts, the system compiler uses a different convention, where __STDC__ is normally 0, but is 1 if the user specifies strict conformance to the C Standard. CPP follows the host convention when processing system header files, but when processing user files __STDC__ is always 1. This has been reported to cause problems; for instance, some versions of Solaris provide X Windows headers that expect __STDC__ to be either undefined or 1. See Invocation.

__STDC_VERSION__#

This macro expands to the C Standard’s version number, a long integer constant of the form yyyymmL where yyyy and mm are the year and month of the Standard version. This signifies which version of the C Standard the compiler conforms to. Like __STDC__, this is not necessarily accurate for the entire implementation, unless GNU CPP is being used with GCC.

The value 199409L signifies the 1989 C standard as amended in 1994, which is the current default; the value 199901L signifies the 1999 revision of the C standard; the value 201112L signifies the 2011 revision of the C standard; the value 201710L signifies the 2017 revision of the C standard (which is otherwise identical to the 2011 version apart from correction of defects). An unspecified value larger than 201710L is used for the experimental -std=c2x and -std=gnu2x modes.

This macro is not defined if the -traditional-cpp option is used, nor when compiling C++ or Objective-C.

__STDC_HOSTED__#: This macro is defined, with value 1, if the compiler’s target is a hosted environment. A hosted environment has the complete facilities of the standard C library available.

__cplusplus#: This macro is defined when the C++ compiler is in use. You can use __cplusplus to test whether a header is compiled by a C compiler or a C++ compiler. This macro is similar to __STDC_VERSION__, in that it expands to a version number. Depending on the language standard selected, the value of the macro is 199711L for the 1998 C++ standard, 201103L for the 2011 C++ standard, 201402L for the 2014 C++ standard, 201703L for the 2017 C++ standard, 202002L for the 2020 C++ standard, or an unspecified value strictly larger than 202002L for the experimental languages enabled by -std=c++23 and -std=gnu++23.

__OBJC__#: This macro is defined, with value 1, when the Objective-C compiler is in use. You can use __OBJC__ to test whether a header is compiled by a C compiler or an Objective-C compiler.

__ASSEMBLER__#: This macro is defined with value 1 when preprocessing assembly language.

Common Predefined Macros#

The common predefined macros are GNU C extensions. They are available with the same meanings regardless of the machine or operating system on which you are using GNU C or GNU Fortran. Their names all start with double underscores.

__COUNTER__#: This macro expands to sequential integral values starting from 0. In conjunction with the ## operator, this provides a convenient means to generate unique identifiers. Care must be taken to ensure that __COUNTER__ is not expanded prior to inclusion of precompiled headers which use it. Otherwise, the precompiled headers will not be used.

__GFORTRAN__#: The GNU Fortran compiler defines this.

__GNUC__#

__GNUC_MINOR__#

__GNUC_PATCHLEVEL__#

These macros are defined by all GNU compilers that use the C preprocessor: C, C++, Objective-C and Fortran. Their values are the major version, minor version, and patch level of the compiler, as integer constants. For example, GCC version x. y. z defines __GNUC__ to x, __GNUC_MINOR__ to y, and __GNUC_PATCHLEVEL__ to z. These macros are also defined if you invoke the preprocessor directly.

If all you need to know is whether or not your program is being compiled by GCC, or a non-GCC compiler that claims to accept the GNU C dialects, you can simply test __GNUC__. If you need to write code which depends on a specific version, you must be more careful. Each time the minor version is increased, the patch level is reset to zero; each time the major version is increased, the minor version and patch level are reset. If you wish to use the predefined macros directly in the conditional, you will need to write it like this:

/* Test for GCC > 3.2.0 */
#if __GNUC__ > 3 || \
    (__GNUC__ == 3 && (__GNUC_MINOR__ > 2 || \
                       (__GNUC_MINOR__ == 2 && \
                        __GNUC_PATCHLEVEL__ > 0))

Another approach is to use the predefined macros to calculate a single number, then compare that against a threshold:

#define GCC_VERSION (__GNUC__ * 10000 \
                     + __GNUC_MINOR__ * 100 \
                     + __GNUC_PATCHLEVEL__)
...
/* Test for GCC > 3.2.0 */
#if GCC_VERSION > 30200

Many people find this form easier to understand.

__GNUG__#: The GNU C++ compiler defines this. Testing it is equivalent to testing (__GNUC__ && __cplusplus).

__STRICT_ANSI__#: GCC defines this macro if and only if the -ansi switch, or a -std switch specifying strict conformance to some version of ISO C or ISO C++, was specified when GCC was invoked. It is defined to 1. This macro exists primarily to direct GNU libc’s header files to use only definitions found in standard C.

__BASE_FILE__#: This macro expands to the name of the main input file, in the form of a C string constant. This is the source file that was specified on the command line of the preprocessor or C compiler.

__FILE_NAME__#: This macro expands to the basename of the current input file, in the form of a C string constant. This is the last path component by which the preprocessor opened the file. For example, processing "/usr/local/include/myheader.h" would set this macro to "myheader.h".

__INCLUDE_LEVEL__#: This macro expands to a decimal integer constant that represents the depth of nesting in include files. The value of this macro is incremented on every #include directive and decremented at the end of every included file. It starts out at 0, its value within the base file specified on the command line.

__ELF__#: This macro is defined if the target uses the ELF object format.

__VERSION__#: This macro expands to a string constant which describes the version of the compiler in use. You should not rely on its contents having any particular form, but it can be counted on to contain at least the release number.

__OPTIMIZE__#

__OPTIMIZE_SIZE__#

__NO_INLINE__#

These macros describe the compilation mode. __OPTIMIZE__ is defined in all optimizing compilations. __OPTIMIZE_SIZE__ is defined if the compiler is optimizing for size, not speed. __NO_INLINE__ is defined if no functions will be inlined into their callers (when not optimizing, or when inlining has been specifically disabled by -fno-inline).

These macros cause certain GNU header files to provide optimized definitions, using macros or inline functions, of system library functions. You should not use these macros in any way unless you make sure that programs will execute with the same effect whether or not they are defined. If they are defined, their value is 1.

__GNUC_GNU_INLINE__#: GCC defines this macro if functions declared inline will be handled in GCC’s traditional gnu90 mode. Object files will contain externally visible definitions of all functions declared inline without extern or static. They will not contain any definitions of any functions declared extern inline.

__GNUC_STDC_INLINE__#

GCC defines this macro if functions declared inline will be handled according to the ISO C99 or later standards. Object files will contain externally visible definitions of all functions declared extern inline. They will not contain definitions of any functions declared inline without extern.

If this macro is defined, GCC supports the gnu_inline function attribute as a way to always get the gnu90 behavior.

__CHAR_UNSIGNED__#: GCC defines this macro if and only if the data type char is unsigned on the target machine. It exists to cause the standard header file limits.h to work correctly. You should not use this macro yourself; instead, refer to the standard macros defined in limits.h.

__WCHAR_UNSIGNED__#: Like __CHAR_UNSIGNED__, this macro is defined if and only if the data type wchar_t is unsigned and the front-end is in C++ mode.

__REGISTER_PREFIX__#: This macro expands to a single token (not a string constant) which is the prefix applied to CPU register names in assembly language for this target. You can use it to write assembly that is usable in multiple environments. For example, in the m68k-aout environment it expands to nothing, but in the m68k-coff environment it expands to a single %.

__USER_LABEL_PREFIX__#

This macro expands to a single token which is the prefix applied to user labels (symbols visible to C code) in assembly. For example, in the m68k-aout environment it expands to an _, but in the m68k-coff environment it expands to nothing.

This macro will have the correct definition even if -f(no-)underscores is in use, but it will not be correct if target-specific options that adjust this prefix are used (e.g. the OSF/rose -mno-underscores option).

__SIZE_TYPE__#
__PTRDIFF_TYPE__#
__WCHAR_TYPE__#
__WINT_TYPE__#
__INTMAX_TYPE__#
__UINTMAX_TYPE__#
__SIG_ATOMIC_TYPE__#
__INT8_TYPE__#
__INT16_TYPE__#
__INT32_TYPE__#
__INT64_TYPE__#
__UINT8_TYPE__#
__UINT16_TYPE__#
__UINT32_TYPE__#
__UINT64_TYPE__#
__INT_LEAST8_TYPE__#
__INT_LEAST16_TYPE__#
__INT_LEAST32_TYPE__#
__INT_LEAST64_TYPE__#
__UINT_LEAST8_TYPE__#
__UINT_LEAST16_TYPE__#
__UINT_LEAST32_TYPE__#
__UINT_LEAST64_TYPE__#
__INT_FAST8_TYPE__#
__INT_FAST16_TYPE__#
__INT_FAST32_TYPE__#
__INT_FAST64_TYPE__#
__UINT_FAST8_TYPE__#
__UINT_FAST16_TYPE__#
__UINT_FAST32_TYPE__#
__UINT_FAST64_TYPE__#
__INTPTR_TYPE__#
__UINTPTR_TYPE__#: These macros are defined to the correct underlying types for the size_t, ptrdiff_t, wchar_t, wint_t, intmax_t, uintmax_t, sig_atomic_t, int8_t, int16_t, int32_t, int64_t, uint8_t, uint16_t, uint32_t, uint64_t, int_least8_t, int_least16_t, int_least32_t, int_least64_t, uint_least8_t, uint_least16_t, uint_least32_t, uint_least64_t, int_fast8_t, int_fast16_t, int_fast32_t, int_fast64_t, uint_fast8_t, uint_fast16_t, uint_fast32_t, uint_fast64_t, intptr_t, and uintptr_t typedefs, respectively. They exist to make the standard header files stddef.h, stdint.h, and wchar.h work correctly. You should not use these macros directly; instead, include the appropriate headers and use the typedefs. Some of these macros may not be defined on particular systems if GCC does not provide a stdint.h header on those systems.

__CHAR_BIT__#: Defined to the number of bits used in the representation of the char data type. It exists to make the standard header given numerical limits work correctly. You should not use this macro directly; instead, include the appropriate headers.

__SCHAR_MAX__#
__WCHAR_MAX__#
__SHRT_MAX__#
__INT_MAX__#
__LONG_MAX__#
__LONG_LONG_MAX__#
__WINT_MAX__#
__SIZE_MAX__#
__PTRDIFF_MAX__#
__INTMAX_MAX__#
__UINTMAX_MAX__#
__SIG_ATOMIC_MAX__#
__INT8_MAX__#
__INT16_MAX__#
__INT32_MAX__#
__INT64_MAX__#
__UINT8_MAX__#
__UINT16_MAX__#
__UINT32_MAX__#
__UINT64_MAX__#
__INT_LEAST8_MAX__#
__INT_LEAST16_MAX__#
__INT_LEAST32_MAX__#
__INT_LEAST64_MAX__#
__UINT_LEAST8_MAX__#
__UINT_LEAST16_MAX__#
__UINT_LEAST32_MAX__#
__UINT_LEAST64_MAX__#
__INT_FAST8_MAX__#
__INT_FAST16_MAX__#
__INT_FAST32_MAX__#
__INT_FAST64_MAX__#
__UINT_FAST8_MAX__#
__UINT_FAST16_MAX__#
__UINT_FAST32_MAX__#
__UINT_FAST64_MAX__#
__INTPTR_MAX__#
__UINTPTR_MAX__#
__WCHAR_MIN__#
__WINT_MIN__#
__SIG_ATOMIC_MIN__#: Defined to the maximum value of the signed char, wchar_t, signed short, signed int, signed long, signed long long, wint_t, size_t, ptrdiff_t, intmax_t, uintmax_t, sig_atomic_t, int8_t, int16_t, int32_t, int64_t, uint8_t, uint16_t, uint32_t, uint64_t, int_least8_t, int_least16_t, int_least32_t, int_least64_t, uint_least8_t, uint_least16_t, uint_least32_t, uint_least64_t, int_fast8_t, int_fast16_t, int_fast32_t, int_fast64_t, uint_fast8_t, uint_fast16_t, uint_fast32_t, uint_fast64_t, intptr_t, and uintptr_t types and to the minimum value of the wchar_t, wint_t, and sig_atomic_t types respectively. They exist to make the standard header given numerical limits work correctly. You should not use these macros directly; instead, include the appropriate headers. Some of these macros may not be defined on particular systems if GCC does not provide a stdint.h header on those systems.

__INT8_C#
__INT16_C#
__INT32_C#
__INT64_C#
__UINT8_C#
__UINT16_C#
__UINT32_C#
__UINT64_C#
__INTMAX_C#
__UINTMAX_C#: Defined to implementations of the standard stdint.h macros with the same names without the leading __. They exist the make the implementation of that header work correctly. You should not use these macros directly; instead, include the appropriate headers. Some of these macros may not be defined on particular systems if GCC does not provide a stdint.h header on those systems.

__SCHAR_WIDTH__#
__SHRT_WIDTH__#
__INT_WIDTH__#
__LONG_WIDTH__#
__LONG_LONG_WIDTH__#
__PTRDIFF_WIDTH__#
__SIG_ATOMIC_WIDTH__#
__SIZE_WIDTH__#
__WCHAR_WIDTH__#
__WINT_WIDTH__#
__INT_LEAST8_WIDTH__#
__INT_LEAST16_WIDTH__#
__INT_LEAST32_WIDTH__#
__INT_LEAST64_WIDTH__#
__INT_FAST8_WIDTH__#
__INT_FAST16_WIDTH__#
__INT_FAST32_WIDTH__#
__INT_FAST64_WIDTH__#
__INTPTR_WIDTH__#
__INTMAX_WIDTH__#: Defined to the bit widths of the corresponding types. They exist to make the implementations of limits.h and stdint.h behave correctly. You should not use these macros directly; instead, include the appropriate headers. Some of these macros may not be defined on particular systems if GCC does not provide a stdint.h header on those systems.

__SIZEOF_INT__#
__SIZEOF_LONG__#
__SIZEOF_LONG_LONG__#
__SIZEOF_SHORT__#
__SIZEOF_POINTER__#
__SIZEOF_FLOAT__#
__SIZEOF_DOUBLE__#
__SIZEOF_LONG_DOUBLE__#
__SIZEOF_SIZE_T__#
__SIZEOF_WCHAR_T__#
__SIZEOF_WINT_T__#
__SIZEOF_PTRDIFF_T__#: Defined to the number of bytes of the C standard data types: int, long, long long, short, void *, float, double, long double, size_t, wchar_t, wint_t and ptrdiff_t.

__BYTE_ORDER__#

__ORDER_LITTLE_ENDIAN__#

__ORDER_BIG_ENDIAN__#

__ORDER_PDP_ENDIAN__#

__BYTE_ORDER__ is defined to one of the values __ORDER_LITTLE_ENDIAN__, __ORDER_BIG_ENDIAN__, or __ORDER_PDP_ENDIAN__ to reflect the layout of multi-byte and multi-word quantities in memory. If __BYTE_ORDER__ is equal to __ORDER_LITTLE_ENDIAN__ or __ORDER_BIG_ENDIAN__, then multi-byte and multi-word quantities are laid out identically: the byte (word) at the lowest address is the least significant or most significant byte (word) of the quantity, respectively. If __BYTE_ORDER__ is equal to __ORDER_PDP_ENDIAN__, then bytes in 16-bit words are laid out in a little-endian fashion, whereas the 16-bit subwords of a 32-bit quantity are laid out in big-endian fashion.

You should use these macros for testing like this:

/* Test for a little-endian machine */
#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__

__FLOAT_WORD_ORDER__#: __FLOAT_WORD_ORDER__ is defined to one of the values __ORDER_LITTLE_ENDIAN__ or __ORDER_BIG_ENDIAN__ to reflect the layout of the words of multi-word floating-point quantities.

__DEPRECATED#: This macro is defined, with value 1, when compiling a C++ source file with warnings about deprecated constructs enabled. These warnings are enabled by default, but can be disabled with -Wno-deprecated.

__EXCEPTIONS#: This macro is defined, with value 1, when compiling a C++ source file with exceptions enabled. If -fno-exceptions is used when compiling the file, then this macro is not defined.

__GXX_RTTI#: This macro is defined, with value 1, when compiling a C++ source file with runtime type identification enabled. If -fno-rtti is used when compiling the file, then this macro is not defined.

__USING_SJLJ_EXCEPTIONS__#: This macro is defined, with value 1, if the compiler uses the old mechanism based on setjmp and longjmp for exception handling.

__GXX_EXPERIMENTAL_CXX0X__#: This macro is defined when compiling a C++ source file with C++11 features enabled, i.e., for all C++ language dialects except -std=c++98 and -std=gnu++98. This macro is obsolete, but can be used to detect experimental C++0x features in very old versions of GCC. Since GCC 4.7.0 the __cplusplus macro is defined correctly, so most code should test __cplusplus >= 201103L instead of using this macro.

__GXX_WEAK__#: This macro is defined when compiling a C++ source file. It has the value 1 if the compiler will use weak symbols, COMDAT sections, or other similar techniques to collapse symbols with ‘vague linkage’ that are defined in multiple translation units. If the compiler will not collapse such symbols, this macro is defined with value 0. In general, user code should not need to make use of this macro; the purpose of this macro is to ease implementation of the C++ runtime library provided with G++.

__NEXT_RUNTIME__#: This macro is defined, with value 1, if (and only if) the NeXT runtime (as in -fnext-runtime) is in use for Objective-C. If the GNU runtime is used, this macro is not defined, so that you can use this macro to determine which runtime (NeXT or GNU) is being used.

__LP64__#
_LP64#: These macros are defined, with value 1, if (and only if) the compilation is for a target where long int and pointer both use 64-bits and int uses 32-bit.

__SSP__#: This macro is defined, with value 1, when -fstack-protector is in use.

__SSP_ALL__#: This macro is defined, with value 2, when -fstack-protector-all is in use.

__SSP_STRONG__#: This macro is defined, with value 3, when -fstack-protector-strong is in use.

__SSP_EXPLICIT__#: This macro is defined, with value 4, when -fstack-protector-explicit is in use.

__SANITIZE_ADDRESS__: This macro is defined, with value 1, when -fsanitize=address or -fsanitize=kernel-address are in use.
__SANITIZE_THREAD__: This macro is defined, with value 1, when -fsanitize=thread is in use.

__TIMESTAMP__#

This macro expands to a string constant that describes the date and time of the last modification of the current source file. The string constant contains abbreviated day of the week, month, day of the month, time in hh:mm:ss form, year and looks like "Sun Sep 16 01:03:52 1973". If the day of the month is less than 10, it is padded with a space on the left.

If GCC cannot determine the current date, it will emit a warning message (once per compilation) and __TIMESTAMP__ will expand to "??? ??? ?? ??:??:?? ????".

__GCC_HAVE_SYNC_COMPARE_AND_SWAP_1#
__GCC_HAVE_SYNC_COMPARE_AND_SWAP_2#
__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4#
__GCC_HAVE_SYNC_COMPARE_AND_SWAP_8#
__GCC_HAVE_SYNC_COMPARE_AND_SWAP_16#: These macros are defined when the target processor supports atomic compare and swap operations on operands 1, 2, 4, 8 or 16 bytes in length, respectively.

__HAVE_SPECULATION_SAFE_VALUE#: This macro is defined with the value 1 to show that this version of GCC supports __builtin_speculation_safe_value.

__GCC_HAVE_DWARF2_CFI_ASM#: This macro is defined when the compiler is emitting DWARF CFI directives to the assembler. When this is defined, it is possible to emit those same directives in inline assembly.

__FP_FAST_FMA#
__FP_FAST_FMAF#
__FP_FAST_FMAL#: These macros are defined with value 1 if the backend supports the fma, fmaf, and fmal builtin functions, so that the include file math.h can define the macros FP_FAST_FMA, FP_FAST_FMAF, and FP_FAST_FMAL for compatibility with the 1999 C standard.

__FP_FAST_FMAF16#
__FP_FAST_FMAF32#
__FP_FAST_FMAF64#
__FP_FAST_FMAF128#
__FP_FAST_FMAF32X#
__FP_FAST_FMAF64X#
__FP_FAST_FMAF128X#: These macros are defined with the value 1 if the backend supports the fma functions using the additional _Floatn and _Floatnx types that are defined in ISO/IEC TS 18661-3:2015. The include file math.h can define the FP_FAST_FMAFn and FP_FAST_FMAFnx macros if the user defined __STDC_WANT_IEC_60559_TYPES_EXT__ before including math.h.

__GCC_IEC_559#

This macro is defined to indicate the intended level of support for IEEE 754 (IEC 60559) floating-point arithmetic. It expands to a nonnegative integer value. If 0, it indicates that the combination of the compiler configuration and the command-line options is not intended to support IEEE 754 arithmetic for float and double as defined in C99 and C11 Annex F (for example, that the standard rounding modes and exceptions are not supported, or that optimizations are enabled that conflict with IEEE 754 semantics). If 1, it indicates that IEEE 754 arithmetic is intended to be supported; this does not mean that all relevant language features are supported by GCC. If 2 or more, it additionally indicates support for IEEE 754-2008 (in particular, that the binary encodings for quiet and signaling NaNs are as specified in IEEE 754-2008).

This macro does not indicate the default state of command-line options that control optimizations that C99 and C11 permit to be controlled by standard pragmas, where those standards do not require a particular default state. It does not indicate whether optimizations respect signaling NaN semantics (the macro for that is __SUPPORT_SNAN__). It does not indicate support for decimal floating point or the IEEE 754 binary16 and binary128 types.

__GCC_IEC_559_COMPLEX#: This macro is defined to indicate the intended level of support for IEEE 754 (IEC 60559) floating-point arithmetic for complex numbers, as defined in C99 and C11 Annex G. It expands to a nonnegative integer value. If 0, it indicates that the combination of the compiler configuration and the command-line options is not intended to support Annex G requirements (for example, because -fcx-limited-range was used). If 1 or more, it indicates that it is intended to support those requirements; this does not mean that all relevant language features are supported by GCC.

__NO_MATH_ERRNO__#: This macro is defined if -fno-math-errno is used, or enabled by another option such as -ffast-math or by default.

__RECIPROCAL_MATH__#: This macro is defined if -freciprocal-math is used, or enabled by another option such as -ffast-math or by default.

__NO_SIGNED_ZEROS__#: This macro is defined if -fno-signed-zeros is used, or enabled by another option such as -ffast-math or by default.

__NO_TRAPPING_MATH__#: This macro is defined if -fno-trapping-math is used.

__ASSOCIATIVE_MATH__#: This macro is defined if -fassociative-math is used, or enabled by another option such as -ffast-math or by default.

__ROUNDING_MATH__#: This macro is defined if -frounding-math is used.

__GNUC_EXECUTION_CHARSET_NAME#
__GNUC_WIDE_EXECUTION_CHARSET_NAME#: These macros are defined to expand to a narrow string literal of the name of the narrow and wide compile-time execution character set used. It directly reflects the name passed to the options -fexec-charset and -fwide-exec-charset, or the defaults documented for those options (that is, it can expand to something like "UTF-8"). See Invocation.

System-specific Predefined Macros#

The C preprocessor normally predefines several macros that indicate what type of system and machine is in use. They are obviously different on each target supported by GCC. This manual, being for all systems and machines, cannot tell you what their names are, but you can use cpp -dM to see them all. See Invocation. All system-specific predefined macros expand to a constant value, so you can test them with either #ifdef or #if.

The C standard requires that all system-specific macros be part of the reserved namespace. All names which begin with two underscores, or an underscore and a capital letter, are reserved for the compiler and library to use as they wish. However, historically system-specific macros have had names with no special prefix; for instance, it is common to find unix defined on Unix systems. For all such macros, GCC provides a parallel macro with two underscores added at the beginning and the end. If unix is defined, __unix__ will be defined too. There will never be more than two underscores; the parallel of _mips is __mips__.

When the -ansi option, or any -std option that requests strict conformance, is given to the compiler, all the system-specific predefined macros outside the reserved namespace are suppressed. The parallel macros, inside the reserved namespace, remain defined.

We are slowly phasing out all predefined macros which are outside the reserved namespace. You should never use them in new programs, and we encourage you to correct older code to use the parallel macros whenever you find it. We don’t recommend you use the system-specific macros that are in the reserved namespace, either. It is better in the long run to check specifically for features you need, using a tool such as autoconf.

C++ Named Operators#

In C++, there are eleven keywords which are simply alternate spellings of operators normally written with punctuation. These keywords are treated as such even in the preprocessor. They function as operators in #if, and they cannot be defined as macros or poisoned. In C, you can request that those keywords take their C++ meaning by including iso646.h. That header defines each one as a normal object-like macro expanding to the appropriate punctuator.

These are the named operators and their corresponding punctuators:

Named Operator	Punctuator
`and`	`&&`
`and_eq`	`&=`
`bitand`	`&`
`bitor`	`\|`
`compl`	`~`
`not`	`!`
`not_eq`	`!=`
`or`	`\|\|`
`or_eq`	`\|=`
`xor`	`^`
`xor_eq`	`^=`