Manual Reference Pages - LOG (3)
NAME
log, logf, logl - natural logarithmic function
CONTENTS
Synopsis
Description
Return Value
Errors
Bugs
Colophon
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
The
log () function returns 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 floating-point exception
(FE_INVALID ) is raised.
Pole error: x is zero
errno is set to
ERANGE . A divide-by-zero floating-point exception
(FE_DIVBYZERO ) is raised.

CONFORMING TO
C99, POSIX.1-2001.
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 floating-point
(FE_INVALID ) exception.
SEE ALSO
cbrt (3),
clog (3),
log10 (3),
log1p (3),
log2 (3),
sqrt (3)
COLOPHON
This page is part of release 3.44 of the Linux
man-pages project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages /.

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