文章目录

• 定义
• 实例
• 定义

定义

GCC手册，第六章：
这个定义是gcc预定义的一个宏；
可以拿来判断当前编译器是否是GCC，是否支持GCC扩展的C功能。
GCC自己扩展了C语言的功能，而且这些功能并未在标准种提及。方便开发者使用的功能。

实例

#if (__GNUC__ >= 4)
#define memcpy(t, f, n) __builtin_memcpy(t, f, n)
#else
#define memcpy(t, f, n)				\
	(__builtin_constant_p((n))		\
	 ? __constant_memcpy((t), (f), (n))	\
	 : __memcpy((t), (f), (n)))
#endif

定义

gcc/cppbuiltin.c

/* Define __GNUC__, __GNUC_MINOR__, __GNUC_PATCHLEVEL__ and __VERSION__.  */
static void
define__GNUC__ (cpp_reader *pfile)
{
  int major, minor, patchlevel;

  parse_basever (&major, &minor, &patchlevel);
  cpp_define_formatted (pfile, "__GNUC__=%d", major);
  cpp_define_formatted (pfile, "__GNUC_MINOR__=%d", minor);
  cpp_define_formatted (pfile, "__GNUC_PATCHLEVEL__=%d", patchlevel);
  cpp_define_formatted (pfile, "__VERSION__=\"%s\"", version_string);
  cpp_define_formatted (pfile, "__ATOMIC_RELAXED=%d", MEMMODEL_RELAXED);
  cpp_define_formatted (pfile, "__ATOMIC_SEQ_CST=%d", MEMMODEL_SEQ_CST);
  cpp_define_formatted (pfile, "__ATOMIC_ACQUIRE=%d", MEMMODEL_ACQUIRE);
  cpp_define_formatted (pfile, "__ATOMIC_RELEASE=%d", MEMMODEL_RELEASE);
  cpp_define_formatted (pfile, "__ATOMIC_ACQ_REL=%d", MEMMODEL_ACQ_REL);
  cpp_define_formatted (pfile, "__ATOMIC_CONSUME=%d", MEMMODEL_CONSUME);
}