**logf**

**LOG(3)** Linux Programmer's Manual **LOG(3)**
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():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
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 oc-
curs, 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 excep-
tion (FE_DIVBYZERO) is raised.
ATTRIBUTES
For an explanation of the terms used in this section, see at-
**tributes(7)**.
+----------------------+---------------+---------+
|Interface | Attribute | Value |
+----------------------+---------------+---------+
|log(), logf(), logl() | Thread safety | MT-Safe |
+----------------------+---------------+---------+
CONFORMING TO
C99, POSIX.1-2001, POSIX.1-2008.
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 5.05 of the Linux 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/.
2017-09-15 **LOG(3)**

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