# log - Man Page

natural logarithmic function

## Library

Math library (libm, -lm)

## Synopsis

```#include <math.h>

double log(double x);
float logf(float x);
long double logl(long double x);```

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).

InterfaceAttributeValue
log(), logf(), logl()Thread safetyMT-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.

## Referenced By

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

2023-10-31 Linux man-pages 6.7