# log man page

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 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.

## Attributes

For an explanation of the terms used in this section, see attributes(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

## Colophon

This page is part of release 4.09 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/.

## Referenced By

beh.3alc(3), expm1(3), ilogb(3), log10(3), log1p(3), log2(3), logb(3), matherr(3), math_error(7).

logf(3) and logl(3) are aliases of log(3).