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Linux 2.6 - man page for nan (linux section 3)

NAN(3)							     Linux Programmer's Manual							    NAN(3)

NAME
nan, nanf, nanl - return 'Not a Number'
SYNOPSIS
#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 feature_test_macros(7)): nan(), nanf(), nanl(): _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L; or cc -std=c99
DESCRIPTION
These functions return a representation (determined by tagp) 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 nanf() and nanl() are equivalent to analogous calls to strtof(3) and strtold(3). 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.
VERSIONS
These functions first appeared in glibc in version 2.1.
CONFORMING TO
C99, POSIX.1-2001. See also IEC 559 and the appendix with recommended functions in IEEE 754/IEEE 854.
SEE ALSO
isnan(3), strtod(3), math_error(7)
COLOPHON
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.
GNU
2010-09-20 NAN(3)