另外查到 C99 其实是有个宏用来做这种事的 isnan(x)

参考:
http://linux.die.net/man/3/isnan
http://www.gnu.org/s/hello/manual/autoconf/Function-Portability.html

奇怪的是上面网址二里面的代码分情况讨论了….难道怕 “!=” 没正确重载?

#ifndef isnan
# define isnan(x) \
(sizeof (x) == sizeof (long double) ? isnan_ld (x) \
: sizeof (x) == sizeof (double) ? isnan_d (x) \
: isnan_f (x))
static inline int isnan_f (float x) { return x != x; }
static inline int isnan_d (double x) { return x != x; }
static inline int isnan_ld (long double x) { return x != x; }
#endif