Return 'not a number'
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
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:
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.
C99, POSIX.1-2001. See also IEC 559 and the appendix with recommended functions in IEEE 754/IEEE 854.
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