# LOG1P

NAME

SYNOPSIS

DESCRIPTION

RETURN VALUE

ERRORS

ATTRIBUTES

CONFORMING TO

BUGS

SEE ALSO

COLOPHON

## 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

*errno* is set to
**EDOM** (but see BUGS). An invalid floating-point
exception (**FE_INVALID**) is raised.

Pole error: *x* is -1

*errno* is set to
**ERANGE** (but see BUGS). A divide-by-zero
floating-point exception (**FE_DIVBYZERO**) is
raised.

## ATTRIBUTES

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

## CONFORMING TO

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

## BUGS

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

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

## SEE ALSO

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

## COLOPHON

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