# LOG1P

## 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**():

_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L

|| _XOPEN_SOURCE >= 500

|| /* Since glibc 2.19: */ _DEFAULT_SOURCE

|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

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

_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L

|| /* Since glibc 2.19: */ _DEFAULT_SOURCE

|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

## DESCRIPTION

These functions return a value equivalent to

log (1 + *x*)

The result 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, POSIX.1-2008.

## SEE ALSO

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

## COLOPHON

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