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

Determines if the given floating-point number arg is positive or negative infinity. 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.


arg - floating-point value

Return value

Nonzero integral value if arg has an infinite value, ​0​ otherwise.


#include <stdio.h>
#include <math.h>
#include <float.h>
int main(void)
    printf("isinf(NAN)         = %d\n", isinf(NAN));
    printf("isinf(INFINITY)    = %d\n", isinf(INFINITY));
    printf("isinf(0.0)         = %d\n", isinf(0.0));
    printf("isinf(DBL_MIN/2.0) = %d\n", isinf(DBL_MIN/2.0));
    printf("isinf(1.0)         = %d\n", isinf(1.0));
    printf("isinf(exp(800))    = %d\n", isinf(exp(800)));

Possible output:

isinf(NAN)         = 0
isinf(INFINITY)    = 1
isinf(0.0)         = 0
isinf(DBL_MIN/2.0) = 0
isinf(1.0)         = 0
isinf(exp(800))    = 1


  • C17 standard (ISO/IEC 9899:2018):
    • The isinf macro (p: 172)
  • C11 standard (ISO/IEC 9899:2011):
    • The isinf macro (p: 236)
  • C99 standard (ISO/IEC 9899:1999):
    • The isinf macro (p: 217)

See also

classifies the given floating-point value
(function macro)
checks if the given number has finite value
(function macro)
checks if the given number is NaN
(function macro)
checks if the given number is normal
(function macro)
C++ documentation for isinf

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