# log1p man page

log1p, log1pf, log1pl — logarithm of 1 plus argument

## Synopsis

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

`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).

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

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

## Conforming to

C99, POSIX.1-2001, POSIX.1-2008.

## See Also

## Colophon

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## Referenced By

log1pf(3) and log1pl(3) are aliases of log1p(3).