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Section: Linux Programmer's Manual (3)
Updated: 20150419
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NAME
log, logf, logl  natural logarithmic function
SYNOPSIS
#include <math.h>
double log(double x);
float logf(float x);
long double logl(long double x);
Link with lm.
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
logf(),
logl():

_BSD_SOURCE  _SVID_SOURCE  _XOPEN_SOURCE >= 600  _ISOC99_SOURCE 
_POSIX_C_SOURCE >= 200112L;
or
cc std=c99
DESCRIPTION
These functions return the natural logarithm of
x.
RETURN VALUE
On success, these functions return the natural logarithm of
x.
If
x
is a NaN,
a NaN is returned.
If
x
is 1, the result is +0.
If
x
is positive infinity,
positive infinity is returned.
If
x
is zero,
then a pole error occurs, and the functions return
HUGE_VAL,
HUGE_VALF,
or
HUGE_VALL,
respectively.
If
x
is negative (including negative infinity), then
a domain error occurs, and a NaN (not a number) is returned.
ERRORS
See
math_error(7)
for information on how to determine whether an error has occurred
when calling these functions.
The following errors can occur:
 Domain error: x is negative

errno
is set to
EDOM.
An invalid floatingpoint exception
(FE_INVALID)
is raised.
 Pole error: x is zero

errno
is set to
ERANGE.
A dividebyzero floatingpoint exception
(FE_DIVBYZERO)
is raised.
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
Interface  Attribute  Value

log(),
logf(),
logl()
 Thread safety  MTSafe

CONFORMING TO
C99, POSIX.12001, POSIX.12008.
The variant returning
double
also conforms to
SVr4, 4.3BSD, C89.
BUGS
In glibc 2.5 and earlier,
taking the
log()
of a NaN produces a bogus invalid floatingpoint
(
FE_INVALID)
exception.
SEE ALSO
cbrt(3),
clog(3),
log10(3),
log1p(3),
log2(3),
sqrt(3)
COLOPHON
This page is part of release 4.04 of the Linux
manpages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
http://www.kernel.org/doc/manpages/.
Index
 NAME

 SYNOPSIS

 DESCRIPTION

 RETURN VALUE

 ERRORS

 ATTRIBUTES

 CONFORMING TO

 BUGS

 SEE ALSO

 COLOPHON

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