Defined in header <math.h>
#define FP_NORMAL    /*implementation defined*/
(since C99)
#define FP_SUBNORMAL /*implementation defined*/
(since C99)
#define FP_ZERO      /*implementation defined*/
(since C99)
#define FP_INFINITE  /*implementation defined*/
(since C99)
#define FP_NAN       /*implementation defined*/
(since C99)

The FP_NORMAL, FP_SUBNORMAL, FP_ZERO, FP_INFINITE, FP_NAN macros each represent a distinct category of floating-point numbers. They all expand to an integer constant expression.

Constant Explanation
FP_NORMAL indicates that the value is normal, i.e. not an infinity, subnormal, not-a-number or zero
FP_SUBNORMAL indicates that the value is subnormal
FP_ZERO indicates that the value is positive or negative zero
FP_INFINITE indicates that the value is not representable by the underlying type (positive or negative infinity)
FP_NAN indicates that the value is not-a-number (NaN)


#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


  • C17 standard (ISO/IEC 9899:2018):
    • 7.12/6 FP_NORMAL, ... (p: 169-170)
  • C11 standard (ISO/IEC 9899:2011):
    • 7.12/6 FP_NORMAL, ... (p: 232)
  • C99 standard (ISO/IEC 9899:1999):
    • 7.12/6 FP_NORMAL, ... (p: 213)

See also

classifies the given floating-point value
(function macro)
C++ documentation for FP_categories

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