nan(3) -- Linux man page
NAMEnan, nanf, nanl - return 'Not a Number'
double nan(const char *tagp);
float nanf(const char *tagp);
DESCRIPTIONThese 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) and similarly calls to nanf and nanl are equivalent to analogous calls to strtof and strtold.
CONFORMING TOC99. See also IEC 559 and the appendix with recommended functions in IEEE 754/IEEE 854.
SEE ALSOisnan(3), strtod(3)