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#### 3.7.2 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 3.2.1 will define `__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__`
  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, 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__`
  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.
`__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 gnu89 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 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 gnu89 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 gnu89 mode, and the `gnu_inline` function attribute will not be recognized.

`__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__`
  These macros are defined to the correct underlying types for the `size_t`, `ptrdiff_t`, `wchar_t`, `wint_t`, `intmax_t`, and `uintmax_t` typedefs, respectively. They exist to make the standard header files stddef.h and wchar.h work correctly. You should not use these macros directly; instead, include the appropriate headers and use the typedefs.
`__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__`
`__INTMAX_MAX__`
  Defined to the maximum value of the `signed char`, `wchar_t`, `signed short`, `signed int`, `signed long`, `signed long long`, and `intmax_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.
`__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`.
`__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 the option -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__`
  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.
`__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.