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.

Return Value

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

Conforming to

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).

See Also



This page is part of release 5.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at

Referenced By

copysign(3), fpclassify(3).

2017-09-15 GNU Linux Programmer's Manual