man3/signbit.3

   1 .\" Copyright 2002 Walter Harms (walter.harms@informatik.uni-oldenburg.de)
   2 .\" and Copyright 2008, Linux Foundation, written by Michael Kerrisk
   3 .\"     <mtk.manpages@gmail.com>
   4 .\"
   5 .\" SPDX-License-Identifier: GPL-1.0-or-later
   6 .\"
   7 .\" Based on glibc infopages, copyright Free Software Foundation
   8 .\"
   9 .TH SIGNBIT 3 2021-03-22 "Linux man-pages (unreleased)"
  10 .SH NAME
  11 signbit \- test sign of a real floating-point number
  12 .SH LIBRARY
  13 Math library
  14 .RI ( libm ", " \-lm )
  15 .SH SYNOPSIS
  16 .nf
  17 .B "#include <math.h>"
  18 .PP
  19 .BI  "int signbit(" x ");"
  20 .fi
  21 .PP
  22 .RS -4
  23 Feature Test Macro Requirements for glibc (see
  24 .BR feature_test_macros (7)):
  25 .RE
  26 .PP
  27 .BR signbit ():
  28 .nf
  29     _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
  30 .fi
  31 .SH DESCRIPTION
  32 .BR signbit ()
  33 is a generic macro which can work on all real floating-point types.
  34 It returns a nonzero value if the value of
  35 .I x
  36 has its sign bit set.
  37 .PP
  38 This is not the same as
  39 .IR "x < 0.0" ,
  40 because IEEE 754 floating point allows zero to be signed.
  41 The comparison
  42 .I \-0.0\~<\~0.0
  43 is false, but
  44 .I signbit(\-0.0)
  45 will return a nonzero value.
  46 .PP
  47 NaNs and infinities have a sign bit.
  48 .SH RETURN VALUE
  49 The
  50 .BR signbit ()
  51 macro returns nonzero if the sign of
  52 .I x
  53 is negative; otherwise it returns zero.
  54 .SH ERRORS
  55 No errors occur.
  56 .SH ATTRIBUTES
  57 For an explanation of the terms used in this section, see
  58 .BR attributes (7).
  59 .ad l
  60 .nh
  61 .TS
  62 allbox;
  63 lbx lb lb
  64 l l l.
  65 Interface       Attribute       Value
  66 T{
  67 .BR signbit ()
  68 T}      Thread safety   MT-Safe
  69 .TE
  70 .hy
  71 .ad
  72 .sp 1
  73 .SH STANDARDS
  74 POSIX.1-2001, POSIX.1-2008, C99.
  75 This function is defined in IEC 559 (and the appendix with
  76 recommended functions in IEEE 754/IEEE 854).
  77 .SH SEE ALSO
  78 .BR copysign (3)