man3/log.3

   1 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
   2 .\" and Copyright 2008, Linux Foundation, written by Michael Kerrisk
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  27 .\" References consulted:
  28 .\"     Linux libc source code
  29 .\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
  30 .\"     386BSD man pages
  31 .\" Modified 1993-07-24 by Rik Faith (faith@cs.unc.edu)
  32 .\" Modified 1995-08-14 by Arnt Gulbrandsen <agulbra@troll.no>
  33 .\" Modified 2002-07-27 by Walter Harms
  34 .\"     (walter.harms@informatik.uni-oldenburg.de)
  35 .\"
  36 .TH LOG 3  2017-09-15  "" "Linux Programmer's Manual"
  37 .SH NAME
  38 log, logf, logl \- natural logarithmic function
  39 .SH SYNOPSIS
  40 .nf
  41 .B #include <math.h>
  42 .PP
  43 .BI "double log(double " x );
  44 .BI "float logf(float " x );
  45 .BI "long double logl(long double " x );
  46 .fi
  47 .PP
  48 Link with \fI\-lm\fP.
  49 .PP
  50 .in -4n
  51 Feature Test Macro Requirements for glibc (see
  52 .BR feature_test_macros (7)):
  53 .in
  54 .PP
  55 .ad l
  56 .BR logf (),
  57 .BR logl ():
  58 .RS 4
  59 _ISOC99_SOURCE || _POSIX_C_SOURCE\ >=\ 200112L
  60     || /* Since glibc 2.19: */ _DEFAULT_SOURCE
  61     || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
  62 .RE
  63 .ad
  64 .SH DESCRIPTION
  65 These functions return the natural logarithm of
  66 .IR x .
  67 .SH RETURN VALUE
  68 On success, these functions return the natural logarithm of
  69 .IR x .
  70 .PP
  71 If
  72 .I x
  73 is a NaN,
  74 a NaN is returned.
  75 .PP
  76 If
  77 .I x
  78 is 1, the result is +0.
  79 .PP
  80 If
  81 .I x
  82 is positive infinity,
  83 positive infinity is returned.
  84 .PP
  85 If
  86 .I x
  87 is zero,
  88 then a pole error occurs, and the functions return
  89 .RB - HUGE_VAL ,
  90 .RB - HUGE_VALF ,
  91 or
  92 .RB - HUGE_VALL ,
  93 respectively.
  94 .PP
  95 If
  96 .I x
  97 is negative (including negative infinity), then
  98 a domain error occurs, and a NaN (not a number) is returned.
  99 .SH ERRORS
 100 See
 101 .BR math_error (7)
 102 for information on how to determine whether an error has occurred
 103 when calling these functions.
 104 .PP
 105 The following errors can occur:
 106 .TP
 107 Domain error: \fIx\fP is negative
 108 .I errno
 109 is set to
 110 .BR EDOM .
 111 An invalid floating-point exception
 112 .RB ( FE_INVALID )
 113 is raised.
 114 .TP
 115 Pole error: \fIx\fP is zero
 116 .I errno
 117 is set to
 118 .BR ERANGE .
 119 A divide-by-zero floating-point exception
 120 .RB ( FE_DIVBYZERO )
 121 is raised.
 122 .SH ATTRIBUTES
 123 For an explanation of the terms used in this section, see
 124 .BR attributes (7).
 125 .TS
 126 allbox;
 127 lbw21 lb lb
 128 l l l.
 129 Interface       Attribute       Value
 130 T{
 131 .BR log (),
 132 .BR logf (),
 133 .BR logl ()
 134 T}      Thread safety   MT-Safe
 135 .TE
 136 .SH CONFORMING TO
 137 C99, POSIX.1-2001, POSIX.1-2008.
 138 .PP
 139 The variant returning
 140 .I double
 141 also conforms to
 142 SVr4, 4.3BSD, C89.
 143 .SH BUGS
 144 In glibc 2.5 and earlier,
 145 taking the
 146 .BR log ()
 147 of a NaN produces a bogus invalid floating-point
 148 .RB ( FE_INVALID )
 149 exception.
 150 .SH SEE ALSO
 151 .BR cbrt (3),
 152 .BR clog (3),
 153 .BR log10 (3),
 154 .BR log1p (3),
 155 .BR log2 (3),
 156 .BR sqrt (3)