int isnan(fpval): Returns nonzero if fpval is not a number. : isnan « math.h « C Tutorial

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

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

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

isnan(-1.0) 0
isnan(1.0) 0
isnan(0.0) 0