### 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; 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), log1p(3), sqrt(3)

### COLOPHON

This page is part of release 3.23 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/.