isnan(), isnanf() - test argument for Not A Number
#include <math.h>
int isnan (double x)
int isnanf (float x)
The isnan(3) and isnanf(3) functions test the argument x to see if it is not a number.
The isnan(3) and isnanf(3) functions return 1 if the argument x is NaN, and return 0 otherwise.
isinf(3)
math(3)
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