isinf(), isinff() - test number for infinite value
#include <math.h>
int isinf (double x)
int isinf (float x)
The isinf(3) and isinff(3) functions test the argument x to see if it is effectively infinite. Both positive and negative infinities are tested.
The isinf(3) and isinff(3) functions return 1 if the argument x is positive or negative infinity, and returns 0 otherwise.
isnan(3)
math(3)