# log1p - Man Page

logarithm of 1 plus argument

## Library

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

## Synopsis

#include <math.h> double log1p(doublex); float log1pf(floatx); long double log1pl(long doublex);

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 <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

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

_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L || /* Since glibc 2.19: */ _DEFAULT_SOURCE || /* glibc <= 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:

## Attributes

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

Interface | Attribute | Value |
---|---|---|

log1p(), log1pf(), log1pl() | Thread safety | MT-Safe |

## Standards

C11, POSIX.1-2008.

## History

C99, POSIX.1-2001.

## Bugs

Before glibc 2.22, the glibc implementation did not set *errno* to **EDOM** when a domain error occurred.

Before glibc 2.22, the glibc implementation did not set *errno* to **ERANGE** when a range error occurred.

## See Also

## Referenced By

The man pages log1pf(3) and log1pl(3) are aliases of log1p(3).