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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. Their names all start with double underscores.

`__GNUC__`
`__GNUC_MINOR__`
`__GNUC_PATCHLEVEL__`
  These macros are defined by all GNU compilers that use the C preprocessor: C, C++, and Objective-C. 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, it's 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.

`__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.
`__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` was used when compiling the file, then this macro will not be 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_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.