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__## 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____GNUC____GNUC_MINOR____GNUC_PATCHLEVEL____GNUC__ to 3, __GNUC_MINOR__ to 2, and __GNUC_PATCHLEVEL__ to 1. These macros are also defined if you invoke the preprocessor directly. __GNUC_PATCHLEVEL__ is new to GCC 3.0; it is also present in the widely-used development snapshots leading up to 3.0 (which identify themselves as GCC 2.96 or 2.97, depending on which snapshot you have).
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 (which happens rarely), 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__(__GNUC__ && __cplusplus). __STRICT_ANSI__-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 restrict their definitions to the minimal set found in the 1989 C standard. __BASE_FILE____INCLUDE_LEVEL__#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____VERSION____OPTIMIZE____OPTIMIZE_SIZE____NO_INLINE____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__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__inline will be handled according to the ISO C99 standard. 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. Support for this and __GNUC_GNU_INLINE__ was added in GCC 4.1.3. If neither macro is defined, an older version of GCC is being used: inline functions will be compiled in gnu90 mode, and the gnu_inline function attribute will not be recognized.
__CHAR_UNSIGNED__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____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__m68k-aout environment it expands to nothing, but in the m68k-coff environment it expands to a single ‘%’. __USER_LABEL_PREFIX__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__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__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__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_Cstdint.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. __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__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-Wno-deprecated. __EXCEPTIONS-fno-exceptions is used when compiling the file, then this macro is not defined. __GXX_RTTI-fno-rtti is used when compiling the file, then this macro is not defined. __USING_SJLJ_EXCEPTIONS__setjmp and longjmp for exception handling. __GXX_EXPERIMENTAL_CXX0X__-std=c++0x or -std=gnu++0x. It indicates that some features likely to be included in C++0x are available. Note that these features are experimental, and may change or be removed in future versions of GCC. __GXX_WEAK____NEXT_RUNTIME__-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___LP64long int and pointer both use 64-bits and int uses 32-bit. __SSP__-fstack-protector is in use. __SSP_ALL__-fstack-protector-all is in use. __SSP_STRONG__-fstack-protector-strong is in use. __SSP_EXPLICIT__-fstack-protector-explicit is in use. __SANITIZE_ADDRESS__-fsanitize=address or -fsanitize=kernel-address are in use. __TIMESTAMP__"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__GCC_HAVE_DWARF2_CFI_ASM__FP_FAST_FMA__FP_FAST_FMAF__FP_FAST_FMALfma, 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. __GCC_IEC_559float 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-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__-fno-math-errno is used, or enabled by another option such as -ffast-math or by default.
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https://gcc.gnu.org/onlinedocs/gcc-6.3.0/cpp/Common-Predefined-Macros.html