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  2015-04-19  "" "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 .sp
  43 .BI "double log(double " x );
  44 .br
  45 .BI "float logf(float " x );
  46 .br
  47 .BI "long double logl(long double " x );
  48 .fi
  49 .sp
  50 Link with \fI\-lm\fP.
  51 .sp
  52 .in -4n
  53 Feature Test Macro Requirements for glibc (see
  54 .BR feature_test_macros (7)):
  55 .in
  56 .sp
  57 .ad l
  58 .BR logf (),
  59 .BR logl ():
  60 .RS 4
  61 _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE\ >=\ 600 || _ISOC99_SOURCE ||
  62 _POSIX_C_SOURCE\ >=\ 200112L;
  63 .br
  64 or
  65 .I cc\ -std=c99
  66 .RE
  67 .ad
  68 .SH DESCRIPTION
  69 These functions return the natural logarithm of
  70 .IR x .
  71 .SH RETURN VALUE
  72 On success, these functions return the natural logarithm of
  73 .IR x .
  74
  75 If
  76 .I x
  77 is a NaN,
  78 a NaN is returned.
  79
  80 If
  81 .I x
  82 is 1, the result is +0.
  83
  84 If
  85 .I x
  86 is positive infinity,
  87 positive infinity is returned.
  88
  89 If
  90 .I x
  91 is zero,
  92 then a pole error occurs, and the functions return
  93 .RB - HUGE_VAL ,
  94 .RB - HUGE_VALF ,
  95 or
  96 .RB - HUGE_VALL ,
  97 respectively.
  98
  99 If
 100 .I x
 101 is negative (including negative infinity), then
 102 a domain error occurs, and a NaN (not a number) is returned.
 103 .SH ERRORS
 104 See
 105 .BR math_error (7)
 106 for information on how to determine whether an error has occurred
 107 when calling these functions.
 108 .PP
 109 The following errors can occur:
 110 .TP
 111 Domain error: \fIx\fP is negative
 112 .I errno
 113 is set to
 114 .BR EDOM .
 115 An invalid floating-point exception
 116 .RB ( FE_INVALID )
 117 is raised.
 118 .TP
 119 Pole error: \fIx\fP is zero
 120 .I errno
 121 is set to
 122 .BR ERANGE .
 123 A divide-by-zero floating-point exception
 124 .RB ( FE_DIVBYZERO )
 125 is raised.
 126 .SH ATTRIBUTES
 127 For an explanation of the terms used in this section, see
 128 .BR attributes (7).
 129 .TS
 130 allbox;
 131 lbw21 lb lb
 132 l l l.
 133 Interface       Attribute       Value
 134 T{
 135 .BR log (),
 136 .BR logf (),
 137 .BR logl ()
 138 T}      Thread safety   MT-Safe
 139 .TE
 140 .SH CONFORMING TO
 141 C99, POSIX.1-2001, POSIX.1-2008.
 142
 143 The variant returning
 144 .I double
 145 also conforms to
 146 SVr4, 4.3BSD, C89.
 147 .SH BUGS
 148 In glibc 2.5 and earlier,
 149 taking the
 150 .BR log ()
 151 of a NaN produces a bogus invalid floating-point
 152 .RB ( FE_INVALID )
 153 exception.
 154 .SH SEE ALSO
 155 .BR cbrt (3),
 156 .BR clog (3),
 157 .BR log10 (3),
 158 .BR log1p (3),
 159 .BR log2 (3),
 160 .BR sqrt (3)