SIGNBIT(3) Linux Programmer's Manual SIGNBIT(3)

signbit - test sign of a real floating-point number

#include <math.h>
int signbit(x);
Link with -lm.

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

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.

No errors occur.

For an explanation of the terms used in this section, see attributes(7).
Interface Attribute Value
signbit () Thread safety MT-Safe

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).

2017-09-15 GNU