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'\" t
.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\" and Copyright 2008, Linux Foundation, written by Michael Kerrisk
.\" <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" 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 (date) "Linux man-pages (unreleased)"
.SH NAME
log, logf, logl \- natural logarithmic function
.SH LIBRARY
Math library
.RI ( libm ", " \-lm )
.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
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR logf (),
.BR logl ():
.nf
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
.fi
.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;
lbx lb lb
l l l.
Interface Attribute Value
T{
.na
.nh
.BR log (),
.BR logf (),
.BR logl ()
T} Thread safety MT-Safe
.TE
.sp 1
.SH STANDARDS
C11, POSIX.1-2008.
.SH HISTORY
C99, POSIX.1-2001.
.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)
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