isnan - Man Page

test for a NaN


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.


#include <math.h>

int isnan(real-floating x);


The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2017 defers to the ISO C standard.

The isnan() macro shall determine whether its argument value is a NaN. First, an argument represented in a format wider than its semantic type is converted to its semantic type. Then determination is based on the type of the argument.

Return Value

The isnan() macro shall return a non-zero value if and only if its argument has a NaN value.


No errors are defined.

The following sections are informative.



Application Usage




Future Directions


See Also

fpclassify(), isfinite(), isinf(), isnormal(), signbit()

The Base Definitions volume of POSIX.1-2017, <math.h>

Referenced By

acos(3p), asin(3p), atan(3p), atan2(3p), ceil(3p), cos(3p), cosh(3p), erf(3p), erfc(3p), exp(3p), exp2(3p), fabs(3p), floor(3p), fmod(3p), fpclassify(3p), frexp(3p), hypot(3p), isfinite(3p), isinf(3p), isnormal(3p), j0(3p), ldexp(3p), lgamma(3p), log(3p), log10(3p), math.h(0p), modf(3p), pow(3p), rint(3p), signbit(3p), sin(3p), sinh(3p), sqrt(3p), tan(3p), tanh(3p), y0(3p).

2017 IEEE/The Open Group POSIX Programmer's Manual