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

Categorizes floating point value arg into the following categories: zero, subnormal, normal, infinite, NAN, or implementation-defined category. The macro returns an integral value.

FLT_EVAL_METHOD is ignored: even if the argument is evaluated with more range and precision than its type, it is first converted to its semantic type, and the classification is based on that: a normal long double value might become subnormal when converted to double and zero when converted to float.


arg - floating point value

Return value

One of FP_INFINITE, FP_NAN, FP_NORMAL, FP_SUBNORMAL, FP_ZERO or implementation-defined type, specifying the category of arg.


#include <stdio.h>
#include <math.h>
#include <float.h>
const char *show_classification(double x) {
    switch(fpclassify(x)) {
        case FP_INFINITE:  return "Inf";
        case FP_NAN:       return "NaN";
        case FP_NORMAL:    return "normal";
        case FP_SUBNORMAL: return "subnormal";
        case FP_ZERO:      return "zero";
        default:           return "unknown";
int main(void)
    printf("1.0/0.0 is %s\n", show_classification(1/0.0));
    printf("0.0/0.0 is %s\n", show_classification(0.0/0.0));
    printf("DBL_MIN/2 is %s\n", show_classification(DBL_MIN/2));
    printf("-0.0 is %s\n", show_classification(-0.0));
    printf("1.0 is %s\n", show_classification(1.0));


1.0/0.0 is Inf
0.0/0.0 is NaN
DBL_MIN/2 is subnormal
-0.0 is zero
1.0 is normal


  • C11 standard (ISO/IEC 9899:2011):
    • The fpclassify macro (p: 235)
  • C99 standard (ISO/IEC 9899:1999):
    • The fpclassify macro (p: 216)

See also

checks if the given number has finite value
(function macro)
checks if the given number is infinite
(function macro)
checks if the given number is NaN
(function macro)
checks if the given number is normal
(function macro)
C++ documentation for fpclassify

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.