991228-1.c

this is a gcc file, you can download it and learn its usetility, for further detail please look at t

__extension__ union { double d; int i[2]; } u = { d: -0.25 };

/* This assumes the endianness of words in a long long is the same as
   that for doubles, which doesn't hold for a few platforms, but we
   can probably special case them here, as appropriate.  */
long long endianness_test = 1;
#define MSW (*(int*)&endianness_test)

int
signbit(double x)
{
  __extension__ union { double d; int i[2]; } u = { d: x };
  return u.i[MSW] < 0;
}
    
int main(void)
{
  if (2*sizeof(int) != sizeof(double) || u.i[MSW] >= 0)
    exit(0);

  if (!signbit(-0.25))
    abort();

  exit(0);
}