# LOG1P

NAME

SYNOPSIS

DESCRIPTION

RETURN VALUE

ERRORS

ATTRIBUTES

CONFORMING TO

SEE ALSO

COLOPHON

## NAME

log1p, log1pf,
log1pl − logarithm of 1 plus argument

## SYNOPSIS

**#include
<math.h>**

**double
log1p(double** *x***); **

float log1pf(float *x***); **

long double log1pl(long double *x***);**

Link with
*−lm*.

Feature Test
Macro Requirements for glibc (see
**feature_test_macros**(7)):

**log1p**():

_BSD_SOURCE || _SVID_SOURCE ||
_XOPEN_SOURCE >= 500 ||
_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED ||
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L;

or *cc -std=c99*

**log1pf**(),
**log1pl**():

_BSD_SOURCE || _SVID_SOURCE ||
_XOPEN_SOURCE >= 600 || _ISOC99_SOURCE ||
_POSIX_C_SOURCE >= 200112L;

or *cc -std=c99*

## DESCRIPTION

*log1p(x)*
returns a value equivalent to

log (1 +
*x*)

It is computed
in a way that is accurate even if the value of *x* is
near zero.

## RETURN VALUE

On success,
these functions return the natural logarithm of
*(1 + x)*.

If *x* is
a NaN, a NaN is returned.

If *x* is
positive infinity, positive infinity is returned.

If *x* is
−1, a pole error occurs, and the functions return
-**HUGE_VAL**, -**HUGE_VALF**, or -**HUGE_VALL**,
respectively.

If *x* is
less than −1 (including negative infinity), 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 less than −1

An invalid floating-point
exception (**FE_INVALID**) is raised.

Pole error: *x* is
−1

A divide-by-zero floating-point
exception (**FE_DIVBYZERO**) is raised.

These functions
do not set *errno*.

## ATTRIBUTES

For an
explanation of the terms used in this section, see
**attributes**(7).

## CONFORMING TO

C99,
POSIX.1-2001.

## SEE ALSO

**exp**(3),
**expm1**(3), **log**(3)

## COLOPHON

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part of release 3.53 of the Linux *man-pages* project.
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reporting bugs, can be found at
http://www.kernel.org/doc/man−pages/.