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


  • 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
checks if the given number has finite value
checks if the given number is NaN
checks if the given number is normal

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