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FreeBSD 11.0 - man page for nanf (freebsd section 3)

NAN(3)							   BSD Library Functions Manual 						    NAN(3)

nan, nanf, nanl -- quiet NaNs
Math Library (libm, -lm)
#include <math.h> double nan(const char *s); float nanf(const char *s); long double nanl(const char *s);
The NAN macro expands to a quiet NaN (Not A Number). Similarly, each of the nan(), nanf(), and nanl() functions generate a quiet NaN value without raising an invalid exception. The argument s should point to either an empty string or a hexadecimal representation of a non-nega- tive integer (e.g., "0x1234".) In the latter case, the integer is encoded in some free bits in the representation of the NaN, which some- times store machine-specific information about why a particular NaN was generated. There are 22 such bits available for float variables, 51 bits for double variables, and at least 51 bits for a long double. If s is improperly formatted or represents an integer that is too large, then the particular encoding of the quiet NaN that is returned is indeterminate.
Calling these functions with a non-empty string isn't portable. Another operating system may translate the string into a different NaN encoding, and furthermore, the meaning of a given NaN encoding varies across machine architectures. If you understood the innards of a par- ticular platform well enough to know what string to use, then you would have no need for these functions anyway, so don't use them. Use the NAN macro instead.
fenv(3), ieee(3), isnan(3), math(3), strtod(3)
The nan(), nanf(), and nanl() functions and the NAN macro conform to ISO/IEC 9899:1999 (``ISO C99'').
December 16, 2007 BSD