int signbit(fpval): Returns nonzero if fpval is negative (that is, its sign bit is set). : signbit « math.h « C Tutorial

1. C Tutorial
2. math.h
3. signbit

#include <stdio.h>
#include <math.h>

int main(void)
{
  printf("signbit(-1.0) %d\n", signbit(-1.0));
  printf("signbit(1.0) %d\n", signbit(1.0));
  printf("signbit(0.0) %d\n", signbit(0.0));
  return 0;
}

signbit(-1.0) 512
signbit(1.0) 0
signbit(0.0) 0