Header File 

float.h 

Category 

Classification Routines, Math Routines 

Prototype 

int _isnan(double d); 

int _isnanl(long double ld); 

Description 

Tests whether a given double-precision floating-point value d is a NaN. 

_isnanl is the long double version; it takes a long double argument. 

Return Value 

Returns a nonzero value (TRUE) if the value passed in is a NaN; otherwise it returns 0 (FALSE). The non-zero return value corresponds to either _FPCLASS_SNAN, if the NaN is of the signaling type, or _FPCLASS_QNAN, if the NaN is of the quiet type. The values for _FPCLASS_SNAN and _FPCLASS_QNAN are in float.h. 

Portability