signbit - test sign of a real floating-point number

#include <math.h>
int signbit(x);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

    _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 set.

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.

copysign(3)