# log - Man Page

natural logarithmic function

## Library

Math library (*libm*, *-lm*)

## Synopsis

#include <math.h> double log(doublex); float logf(floatx); long double logl(long doublex);

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 <= 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 |

## Standards

C11, POSIX.1-2008.

## History

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

## Referenced By

expm1(3), ilogb(3), log10(3), log1p(3), log2(3), logb(3), matherr(3), math_error(7), unu-1op(1).

The man pages logf(3) and logl(3) are aliases of log(3).