Was this page helpful?
Your feedback about this content is important. Let us know what you think.
Additional feedback?
1500 characters remaining
Collapse the table of content
Expand the table of content


Checks a given double-precision floating-point value for not a number (NAN).

int _isnan( 
   double x  


Double-precision floating-point value.

_isnan returns a nonzero value if the argument x is a NAN; otherwise it returns 0.

The _isnan function tests a given double-precision floating-point value x, returning a nonzero value if x is a not a number (NAN). A NAN is generated when the result of a floating-point operation cannot be represented in Institute of Electrical and Electronics Engineers (IEEE) format. For information about how a NAN is represented for output, see printf.


Required header



For more compatibility information, see Compatibility in the Introduction.


All versions of the C run-time libraries.

Community Additions

© 2015 Microsoft