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>
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   9 .\" Based on glibc infopages, copyright Free Software Foundation
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  11 .TH SIGNBIT 3 2017-09-15 "GNU" "Linux Programmer's Manual"
  12 .SH NAME
  13 signbit \- test sign of a real floating-point number
  14 .SH SYNOPSIS
  15 .B "#include <math.h>"
  16 .PP
  17 .BI  "int signbit(" x ");"
  18 .PP
  19 Link with \fI\-lm\fP.
  20 .PP
  21 .in -4n
  22 Feature Test Macro Requirements for glibc (see
  23 .BR feature_test_macros (7)):
  24 .in
  25 .PP
  26 .ad l
  27 .BR signbit ():
  28 .RS 4
  29 _ISOC99_SOURCE || _POSIX_C_SOURCE\ >=\ 200112L
  30 .RE
  31 .ad
  32 .SH DESCRIPTION
  33 .BR signbit ()
  34 is a generic macro which can work on all real floating-point types.
  35 It returns a nonzero value if the value of
  36 .I x
  37 has its sign bit set.
  38 .PP
  39 This is not the same as
  40 .IR "x < 0.0" ,
  41 because IEEE 754 floating point allows zero to be signed.
  42 The comparison
  43 .IR "-0.0 < 0.0"
  44 is false, but
  45 .IR "signbit(\-0.0)"
  46 will return a nonzero value.
  47 .PP
  48 NaNs and infinities have a sign bit.
  49 .SH RETURN VALUE
  50 The
  51 .BR signbit ()
  52 macro returns nonzero if the sign of
  53 .I x
  54 is negative; otherwise it returns zero.
  55 .SH ERRORS
  56 No errors occur.
  57 .SH ATTRIBUTES
  58 For an explanation of the terms used in this section, see
  59 .BR attributes (7).
  60 .TS
  61 allbox;
  62 lb lb lb
  63 l l l.
  64 Interface       Attribute       Value
  65 T{
  66 .BR signbit ()
  67 T}      Thread safety   MT-Safe
  68 .TE
  69 .SH CONFORMING TO
  70 POSIX.1-2001, POSIX.1-2008, C99.
  71 This function is defined in IEC 559 (and the appendix with
  72 recommended functions in IEEE 754/IEEE 854).
  73 .SH SEE ALSO
  74 .BR copysign (3)