'\" t
.\" Copyright 2002 Walter Harms (walter.harms@informatik.uni-oldenburg.de)
.\" and Copyright 2008, Linux Foundation, written by Michael Kerrisk
.\"     <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: GPL-1.0-or-later
.\"
.\" Based on glibc infopages, copyright Free Software Foundation
.\"
.TH signbit 3 2023-10-31 "Linux man-pages 6.7"
.SH NAME
signbit \- test sign of a real floating-point number
.SH LIBRARY
Math library
.RI ( libm ", " \-lm )
.SH SYNOPSIS
.nf
.B "#include <math.h>"
.P
.BI  "int signbit(" x ");"
.fi
.P
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.P
.BR signbit ():
.nf
    _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
.fi
.SH DESCRIPTION
.BR signbit ()
is a generic macro which can work on all real floating-point types.
It returns a nonzero value if the value of
.I x
has its sign bit set.
.P
This is not the same as
.IR "x < 0.0" ,
because IEEE 754 floating point allows zero to be signed.
The comparison
.I \-0.0\~<\~0.0
is false, but
.I signbit(\-0.0)
will return a nonzero value.
.P
NaNs and infinities have a sign bit.
.SH RETURN VALUE
The
.BR signbit ()
macro returns nonzero if the sign of
.I x
is negative; otherwise it returns zero.
.SH ERRORS
No errors occur.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR signbit ()
T}	Thread safety	MT-Safe
.TE
.SH STANDARDS
C11, POSIX.1-2008.
.SH HISTORY
POSIX.1-2001, C99.
.P
This function is defined in IEC 559 (and the appendix with
recommended functions in IEEE 754/IEEE 854).
.SH SEE ALSO
.BR copysign (3)