.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\" and Copyright 2008, Linux Foundation, written by Michael Kerrisk
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.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified 1993-07-24 by Rik Faith (faith@cs.unc.edu)
.\" Modified 1995-08-14 by Arnt Gulbrandsen <agulbra@troll.no>
.\" Modified 2002-07-27 by Walter Harms
.\" 	(walter.harms@informatik.uni-oldenburg.de)
.\"
.TH LOG 3  2017-09-15  "" "Linux Programmer's Manual"
.SH NAME
log, logf, logl \- natural logarithmic function
.SH SYNOPSIS
.nf
.B #include <math.h>
.PP
.BI "double log(double " x );
.BI "float logf(float " x );
.BI "long double logl(long double " x );
.fi
.PP
Link with \fI\-lm\fP.
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.ad l
.BR logf (),
.BR logl ():
.RS 4
_ISOC99_SOURCE || _POSIX_C_SOURCE\ >=\ 200112L
    || /* Since glibc 2.19: */ _DEFAULT_SOURCE
    || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
.RE
.ad
.SH DESCRIPTION
These functions return the natural logarithm of
.IR x .
.SH RETURN VALUE
On success, these functions return the natural logarithm of
.IR x .
.PP
If
.I x
is a NaN,
a NaN is returned.
.PP
If
.I x
is 1, the result is +0.
.PP
If
.I x
is positive infinity,
positive infinity is returned.
.PP
If
.I x
is zero,
then a pole error occurs, and the functions return
.RB - HUGE_VAL ,
.RB - HUGE_VALF ,
or
.RB - HUGE_VALL ,
respectively.
.PP
If
.I x
is negative (including negative infinity), then
a domain error occurs, and a NaN (not a number) is returned.
.SH ERRORS
See
.BR math_error (7)
for information on how to determine whether an error has occurred
when calling these functions.
.PP
The following errors can occur:
.TP
Domain error: \fIx\fP is negative
.I errno
is set to
.BR EDOM .
An invalid floating-point exception
.RB ( FE_INVALID )
is raised.
.TP
Pole error: \fIx\fP is zero
.I errno
is set to
.BR ERANGE .
A divide-by-zero floating-point exception
.RB ( FE_DIVBYZERO )
is raised.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbw21 lb lb
l l l.
Interface	Attribute	Value
T{
.BR log (),
.BR logf (),
.BR logl ()
T}	Thread safety	MT-Safe
.TE
.SH CONFORMING TO
C99, POSIX.1-2001, POSIX.1-2008.
.PP
The variant returning
.I double
also conforms to
SVr4, 4.3BSD, C89.
.SH BUGS
In glibc 2.5 and earlier,
taking the
.BR log ()
of a NaN produces a bogus invalid floating-point
.RB ( FE_INVALID )
exception.
.SH SEE ALSO
.BR cbrt (3),
.BR clog (3),
.BR log10 (3),
.BR log1p (3),
.BR log2 (3),
.BR sqrt (3)
.SH COLOPHON
This page is part of release 5.10 of the Linux
.I man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
\%https://www.kernel.org/doc/man\-pages/.