isnan()

Test for not-a-number (NaN)

Synopsis:

#include <math.h>

#define isnan ( x ) ...

Arguments:

x
The number you want to test.

Library:

libm
The general-purpose math library.
libm-sve
(QNX Neutrino 7.1 or later) A library that optimizes the code for ARMv8.2 chips that have Scalable Vector Extension hardware.

Your system requirements will determine how you should work with these libraries:

Note: Compile your program with the -fno-builtin option to prevent the compiler from using a built-in version of the function.

Description:

The isnan() macro determines if x is Not-A-Number (NaN).

Returns:

Nonzero
The value of x is NaN.
0
The value of x is a number.

Examples:

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

int main(int argc, char** argv) 
{
  double a, b, c, d;

  a = 2;
  b = -0.5;
  c = NAN;
  d = 1.0/0.0;
  printf("%f is %s \n", a, 
         (isnan(a)) ? "not a number" : "a number");
  printf("%f is %s \n", b,
         (isnan(b)) ? "not a number" : "a number");
  printf("%f is %s \n", c,
         (isnan(c)) ? "not a number" : "a number");
  printf("%f is %s \n", d,
         (isnan(d)) ? "not a number" : "a number");
  return EXIT_SUCCESS;
}

produces the output:

2.000000 is a number
-0.500000 is a number
nan is not a number
inf is a number

Classification:

POSIX 1003.1

Safety:  
Cancellation point No
Interrupt handler Yes
Signal handler Yes
Thread Yes