man-pages/man3/signbit.3

.\" Copyright 2002 Walter Harms (walter.harms@informatik.uni-oldenburg.de)
.\" and Copyright 2008, Linux Foundation, written by Michael Kerrisk
.\"     <mtk.manpages@gmail.com>
.\" Distributed under GPL
.\" Based on glibc infopages
.TH SIGNBIT 3 2008-08-05 "GNU" "Linux Programmer's Manual"
.SH NAME
signbit \- test sign of a real floating-point number
.SH SYNOPSIS
.B "#include <math.h>"
.sp
.BI  "int signbit(" x ");"
.sp
Link with \fI\-lm\fP.
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.ad l
.BR signbit ():
_XOPEN_SOURCE\ >=\ 600 || _ISOC99_SOURCE; or
.I cc\ -std=c99
.ad b
.SH DESCRIPTION
.BR signbit ()
is a generic macro which can work on all real floating-point types.
It returns a non-zero value if the value of
.I x
has its sign bit set.
.PP
This is not the same as
.IR "x < 0.0" ,
because IEEE 754 floating point allows zero to be signed.
The comparison
.IR "-0.0 < 0.0"
is false, but
.IR "signbit(\-0.0)"
will return a non-zero value.

NaNs and infinities have a sign bit.
.SH RETURN VALUE
The
.BR signbit ()
macro returns non-zero if the sign of
.I x
is negative; otherwise it returns zero.
.SH ERRORS
No errors occur.
.SH "CONFORMING TO"
C99, POSIX.1-2001.
This function is defined in IEC 559 (and the appendix with
recommended functions in IEEE 754/IEEE 854).
.SH "SEE ALSO"
.BR copysign (3)