isnan - test for NaN (not a number)

Syntax	#include <math.h> int isnan(double x);
Description	isnan() tests whether x is not NaN. Not NaN means that x is a valid bit pattern of a floating-point number.
Return val.	0	if x is not NaN.
Notes	C11 isnan() is implemented both as a function and as a macro. To generate a function call, the definition of the macro name must be first undefined (#undef isnan). (End) In this implementation, isnan() always returns the value 0, i.e. all bit patterns for floatingpoint numbers are valid.
See also	fpclassify, isfinite, isinf, isnormal, math.h.