signbit(3) Library Functions Manual signbit(3)

signbit - test sign of a real floating-point number

Math library (libm, -lm)

#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.

No errors occur.

For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
signbit () Thread safety MT-Safe

C11, POSIX.1-2008.

POSIX.1-2001, C99.

This function is defined in IEC 559 (and the appendix with recommended functions in IEEE 754/IEEE 854).


2024-05-02 Linux man-pages 6.9.1