Defined in header <math.h>
#define NAN /*implementation defined*/
(since C99)

The macro NAN expands to constant expression of type float which evaluates to a quiet not-a-number (QNaN) value. If the implementation does not support QNaNs, this macro constant is not defined.

The style used to print a NaN is implementation defined.


There are many different NaN values, differentiated by their payloads and their sign bits. The contents of the payload and the sign bit of the NaN generated by the macro NAN are implementation-defined.


Show style used to print a NaN and IEEE format.

#include <stdio.h>
#include <math.h>
#include <stdint.h>
#include <inttypes.h>
#include <string.h>
int main(void)
    double f = NAN;
    uint64_t fn; memcpy(&fn, &f, sizeof f);
    printf("NAN:   %f %" PRIx64 "\n", f, fn);

Possible output:

NAN:   nan 7ff8000000000000


  • C11 standard (ISO/IEC 9899:2011):
    • 7.12/5 NAN (p: 232)
    • F.10/11/13 NAN (p: 518)
  • C99 standard (ISO/IEC 9899:1999):
    • 7.12/5 NAN (p: 213)
    • F.9/11/13 NAN (p: 455)

See also

returns a NaN (not-a-number)
checks if the given number is NaN

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