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__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);

}