# LOG

NAME

SYNOPSIS

DESCRIPTION

RETURN VALUE

ERRORS

ATTRIBUTES

CONFORMING TO

BUGS

SEE ALSO

COLOPHON

## 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
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).

## 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

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