signbit - test sign of a real floating-point number
Link with -lm.
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
signbit() is a generic macro which can work on all real floating-point
types. It returns a nonzero value if the value of x has its sign bit
This is not the same as x < 0.0, because IEEE 754
floating point allows zero to be signed. The comparison -0.0 < 0.0
is false, but signbit(-0.0) will return a nonzero value.
NaNs and infinities have a sign bit.
The signbit() macro returns nonzero if the sign of x is negative;
otherwise it returns zero.
For an explanation of the terms used in this section, see attributes(7).
POSIX.1-2001, POSIX.1-2008, C99. This function is defined in IEC 559 (and the
appendix with recommended functions in IEEE 754/IEEE 854).
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