NAME

SYNOPSIS

DESCRIPTION

RETURN VALUE

ERRORS

ATTRIBUTES

CONFORMING TO

SEE ALSO

COLOPHON

## NAME

## SYNOPSIS

## DESCRIPTION

## RETURN VALUE

## ERRORS

## ATTRIBUTES

## CONFORMING TO

## SEE ALSO

## COLOPHON

SYNOPSIS

DESCRIPTION

RETURN VALUE

ERRORS

ATTRIBUTES

CONFORMING TO

SEE ALSO

COLOPHON

log1p, log1pf, log1pl − logarithm of 1 plus argument

**#include
<math.h>**

**double
log1p(double** *x***);
float log1pf(float**

long double log1pl(long double

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*

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.

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.

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*.

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

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

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

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