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**():

_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 |

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).

**copysign**(3)