### 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 || _ISOC99_SOURCE; or
*cc -std=c99* **log1pf**(), **log1pl**(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE; 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.
| |

*errno*.

### CONFORMING TO

C99, POSIX.1-2001.

### SEE ALSO

### COLOPHON

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