nan, nanf, nanl − return ’Not a Number’ #include<math.h> );double nan(const char *tagp );float nanf(const char *tagp );long double nanl(const char *tagp Link with −lm. Feature Test Macro Requirements for glibc (see _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L These functions return a representation (determined by of a quiet NaN. If the implementation does not support quiet NaNs, these functions return zero. The call nan("char‐sequence") is equivalent to: strtod("NAN(char‐sequence)", NULL); Similarly, calls to and are equivalent to analogous calls to and The argument tagp is used in an unspecified manner. On IEEE 754 systems, there are many representations of NaN, and tagp selects one. On other systems it may do nothing. These functions first appeared in glibc in version 2.1. For an explanation of the terms used in this section, see ┌──────────────────────┬───────────────┬────────────────┐ │Interface │ Attribute │ Value │ ├──────────────────────┼───────────────┼────────────────┤ │ │ Thread safety │ MT‐Safe locale │ └──────────────────────┴───────────────┴────────────────┘ C99, POSIX.1‐2001, POSIX.1‐2008. See also IEC 559 and the appendix with recommended functions in IEEE 754/IEEE 854. This page is part of release 5.07 of the Linux man‐pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man−pages/.