man3/log.3

   1 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
   2 .\" and Copyright 2008, Linux Foundation, written by Michael Kerrisk
   3 .\"     <mtk.manpages@gmail.com>
   4 .\"
   5 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   6 .\"
   7 .\" References consulted:
   8 .\"     Linux libc source code
   9 .\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
  10 .\"     386BSD man pages
  11 .\" Modified 1993-07-24 by Rik Faith (faith@cs.unc.edu)
  12 .\" Modified 1995-08-14 by Arnt Gulbrandsen <agulbra@troll.no>
  13 .\" Modified 2002-07-27 by Walter Harms
  14 .\"     (walter.harms@informatik.uni-oldenburg.de)
  15 .\"
  16 .TH LOG 3 2021-03-22 GNU "Linux Programmer's Manual"
  17 .SH NAME
  18 log, logf, logl \- natural logarithmic function
  19 .SH LIBRARY
  20 Math library
  21 .RI ( libm ", " \-lm )
  22 .SH SYNOPSIS
  23 .nf
  24 .B #include <math.h>
  25 .PP
  26 .BI "double log(double " x );
  27 .BI "float logf(float " x );
  28 .BI "long double logl(long double " x );
  29 .fi
  30 .PP
  31 .RS -4
  32 Feature Test Macro Requirements for glibc (see
  33 .BR feature_test_macros (7)):
  34 .RE
  35 .PP
  36 .BR logf (),
  37 .BR logl ():
  38 .nf
  39     _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
  40         || /* Since glibc 2.19: */ _DEFAULT_SOURCE
  41         || /* Glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
  42 .fi
  43 .SH DESCRIPTION
  44 These functions return the natural logarithm of
  45 .IR x .
  46 .SH RETURN VALUE
  47 On success, these functions return the natural logarithm of
  48 .IR x .
  49 .PP
  50 If
  51 .I x
  52 is a NaN,
  53 a NaN is returned.
  54 .PP
  55 If
  56 .I x
  57 is 1, the result is +0.
  58 .PP
  59 If
  60 .I x
  61 is positive infinity,
  62 positive infinity is returned.
  63 .PP
  64 If
  65 .I x
  66 is zero,
  67 then a pole error occurs, and the functions return
  68 .RB \- HUGE_VAL ,
  69 .RB \- HUGE_VALF ,
  70 or
  71 .RB \- HUGE_VALL ,
  72 respectively.
  73 .PP
  74 If
  75 .I x
  76 is negative (including negative infinity), then
  77 a domain error occurs, and a NaN (not a number) is returned.
  78 .SH ERRORS
  79 See
  80 .BR math_error (7)
  81 for information on how to determine whether an error has occurred
  82 when calling these functions.
  83 .PP
  84 The following errors can occur:
  85 .TP
  86 Domain error: \fIx\fP is negative
  87 .I errno
  88 is set to
  89 .BR EDOM .
  90 An invalid floating-point exception
  91 .RB ( FE_INVALID )
  92 is raised.
  93 .TP
  94 Pole error: \fIx\fP is zero
  95 .I errno
  96 is set to
  97 .BR ERANGE .
  98 A divide-by-zero floating-point exception
  99 .RB ( FE_DIVBYZERO )
 100 is raised.
 101 .SH ATTRIBUTES
 102 For an explanation of the terms used in this section, see
 103 .BR attributes (7).
 104 .ad l
 105 .nh
 106 .TS
 107 allbox;
 108 lbx lb lb
 109 l l l.
 110 Interface       Attribute       Value
 111 T{
 112 .BR log (),
 113 .BR logf (),
 114 .BR logl ()
 115 T}      Thread safety   MT-Safe
 116 .TE
 117 .hy
 118 .ad
 119 .sp 1
 120 .SH CONFORMING TO
 121 C99, POSIX.1-2001, POSIX.1-2008.
 122 .PP
 123 The variant returning
 124 .I double
 125 also conforms to
 126 SVr4, 4.3BSD, C89.
 127 .SH BUGS
 128 In glibc 2.5 and earlier,
 129 taking the
 130 .BR log ()
 131 of a NaN produces a bogus invalid floating-point
 132 .RB ( FE_INVALID )
 133 exception.
 134 .SH SEE ALSO
 135 .BR cbrt (3),
 136 .BR clog (3),
 137 .BR log10 (3),
 138 .BR log1p (3),
 139 .BR log2 (3),
 140 .BR sqrt (3)