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
  10 .\"
  11 .TH SIGNBIT 3 2015-08-08 "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 .sp
  17 .BI  "int signbit(" x ");"
  18 .sp
  19 Link with \fI\-lm\fP.
  20 .sp
  21 .in -4n
  22 Feature Test Macro Requirements for glibc (see
  23 .BR feature_test_macros (7)):
  24 .in
  25 .sp
  26 .ad l
  27 .BR signbit ():
  28 .RS 4
  29 _XOPEN_SOURCE\ >=\ 600 || _ISOC99_SOURCE ||
  30 _POSIX_C_SOURCE\ >=\ 200112L;
  31 .br
  32 or
  33 .I cc\ -std=c99
  34 .RE
  35 .ad
  36 .SH DESCRIPTION
  37 .BR signbit ()
  38 is a generic macro which can work on all real floating-point types.
  39 It returns a nonzero value if the value of
  40 .I x
  41 has its sign bit set.
  42 .PP
  43 This is not the same as
  44 .IR "x < 0.0" ,
  45 because IEEE 754 floating point allows zero to be signed.
  46 The comparison
  47 .IR "-0.0 < 0.0"
  48 is false, but
  49 .IR "signbit(\-0.0)"
  50 will return a nonzero value.
  51
  52 NaNs and infinities have a sign bit.
  53 .SH RETURN VALUE
  54 The
  55 .BR signbit ()
  56 macro returns nonzero if the sign of
  57 .I x
  58 is negative; otherwise it returns zero.
  59 .SH ERRORS
  60 No errors occur.
  61 .SH ATTRIBUTES
  62 For an explanation of the terms used in this section, see
  63 .BR attributes (7).
  64 .TS
  65 allbox;
  66 lb lb lb
  67 l l l.
  68 Interface       Attribute       Value
  69 T{
  70 .BR signbit ()
  71 T}      Thread safety   MT-Safe
  72 .TE
  73 .SH CONFORMING TO
  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)