man3/logb.3

   1 .\" Copyright 2004 Andries Brouwer <aeb@cwi.nl>.
   2 .\" and Copyright 2008, Linux Foundation, written by Michael Kerrisk
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  27 .\" Inspired by a page by Walter Harms created 2002-08-10
  28 .\"
  29 .TH LOGB 3 2016-03-15 "" "Linux Programmer's Manual"
  30 .SH NAME
  31 logb, logbf, logbl \- get exponent of a floating-point value
  32 .SH SYNOPSIS
  33 .B #include <math.h>
  34 .sp
  35 .BI "double logb(double " x );
  36 .br
  37 .BI "float logbf(float " x );
  38 .br
  39 .BI "long double logbl(long double " x );
  40 .sp
  41 Link with \fI\-lm\fP.
  42 .sp
  43 .in -4n
  44 Feature Test Macro Requirements for glibc (see
  45 .BR feature_test_macros (7)):
  46 .in
  47 .sp
  48 .ad l
  49 .BR logb ():
  50 .RS 4
  51 _ISOC99_SOURCE || _POSIX_C_SOURCE\ >=\ 200112L
  52     || _XOPEN_SOURCE\ >=\ 500
  53 .\"    || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
  54     || /* Since glibc 2.19: */ _DEFAULT_SOURCE
  55     || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
  56 .RE
  57 .br
  58 .BR logbf (),
  59 .BR logbl ():
  60 .RS 4
  61 _ISOC99_SOURCE || _POSIX_C_SOURCE\ >=\ 200112L
  62     || /* Since glibc 2.19: */ _DEFAULT_SOURCE
  63     || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
  64 .RE
  65 .ad b
  66 .SH DESCRIPTION
  67 These functions extract the exponent from the
  68 internal floating-point representation of
  69 .I x
  70 and return it as a floating-point value.
  71 The integer constant
  72 .BR FLT_RADIX ,
  73 defined in
  74 .IR <float.h> ,
  75 indicates the radix used for the system's floating-point representation.
  76 If
  77 .B FLT_RADIX
  78 is 2,
  79 .BI logb( x )
  80 is equal to
  81 .BI floor(log2( x ))\fR,
  82 except that it is probably faster.
  83 .LP
  84 If
  85 .I x
  86 is subnormal,
  87 .BR logb ()
  88 returns the exponent
  89 .I x
  90 would have if it were normalized.
  91 .SH RETURN VALUE
  92 On success, these functions return the exponent of
  93 .IR x .
  94
  95 If
  96 .I x
  97 is a NaN,
  98 a NaN is returned.
  99
 100 If
 101 .I x
 102 is zero, then a pole error occurs, and the functions return
 103 .RB - HUGE_VAL ,
 104 .RB - HUGE_VALF ,
 105 or
 106 .RB - HUGE_VALL ,
 107 respectively.
 108
 109 If
 110 .I x
 111 is negative infinity or positive infinity, then
 112 positive infinity is returned.
 113 .SH ERRORS
 114 See
 115 .BR math_error (7)
 116 for information on how to determine whether an error has occurred
 117 when calling these functions.
 118 .PP
 119 The following errors can occur:
 120 .TP
 121 Pole error: \fIx\fP is 0
 122 .\" .I errno
 123 .\" is set to
 124 .\" .BR ERANGE .
 125 A divide-by-zero floating-point exception
 126 .RB ( FE_DIVBYZERO )
 127 is raised.
 128 .PP
 129 These functions do not set
 130 .IR errno .
 131 .\" FIXME . Is it intentional that these functions do not set errno?
 132 .\" log(), log2(), log10() do set errno
 133 .\" Bug raised: http://sources.redhat.com/bugzilla/show_bug.cgi?id=6793
 134 .\"
 135 .\" .SH HISTORY
 136 .\" The
 137 .\" .BR logb ()
 138 .\" function occurs in 4.3BSD.
 139 .\" see IEEE.3 in the 4.3BSD manual
 140 .SH ATTRIBUTES
 141 For an explanation of the terms used in this section, see
 142 .BR attributes (7).
 143 .TS
 144 allbox;
 145 lbw24 lb lb
 146 l l l.
 147 Interface       Attribute       Value
 148 T{
 149 .BR logb (),
 150 .BR logbf (),
 151 .BR logbl ()
 152 T}      Thread safety   MT-Safe
 153 .TE
 154 .SH CONFORMING TO
 155 C99, POSIX.1-2001, POSIX.1-2008.
 156 .SH SEE ALSO
 157 .BR ilogb (3),
 158 .BR log (3)