# EXPM1

NAME

SYNOPSIS

DESCRIPTION

RETURN VALUE

ERRORS

ATTRIBUTES

CONFORMING TO

BUGS

SEE ALSO

COLOPHON

## NAME

expm1, expm1f,
expm1l − exponential minus 1

## SYNOPSIS

**#include
<math.h>**

**double
expm1(double** *x***); **

float expm1f(float *x***); **

long double expm1l(long double *x***);**

Link with
*−lm*.

Feature Test
Macro Requirements for glibc (see
**feature_test_macros**(7)):

**expm1**():

_BSD_SOURCE || _SVID_SOURCE ||
_XOPEN_SOURCE >= 500 ||
_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED ||
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L;

or *cc -std=c99*

**expm1f**(),
**expm1l**():

_BSD_SOURCE || _SVID_SOURCE ||
_XOPEN_SOURCE >= 600 || _ISOC99_SOURCE ||
_POSIX_C_SOURCE >= 200112L;

or *cc -std=c99*

## DESCRIPTION

*expm1(x)*
returns a value equivalent to

exp(x) −
1

It is computed
in a way that is accurate even if the value of *x* is
near zero—a case where *exp(x) − 1* would
be inaccurate due to subtraction of two numbers that are
nearly equal.

## RETURN VALUE

On success,
these functions return
*exp(x) − 1*.

If *x* is
a NaN, a NaN is returned.

If *x* is
+0 (−0), +0 (−0) is returned.

If *x* is
positive infinity, positive infinity is returned.

If *x* is
negative infinity, −1 is returned.

If the result
overflows, a range error occurs, and the functions return
-**HUGE_VAL**, -**HUGE_VALF**, or -**HUGE_VALL**,
respectively.

## 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:

Range error, overflow

*errno* is set to
**ERANGE** (but see BUGS). An overflow floating-point
exception (**FE_OVERFLOW**) is raised.

## ATTRIBUTES

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

## CONFORMING TO

C99,
POSIX.1-2001.

## BUGS

For some large
negative *x* values (where the function result
approaches −1), **expm1**() raises a bogus
underflow floating-point exception.

For some large
positive *x* values, **expm1**() raises a bogus
invalid floating-point exception in addition to the expected
overflow exception, and returns a NaN instead of positive
infinity.

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

## SEE ALSO

**exp**(3),
**log**(3), **log1p**(3)

## COLOPHON

This page is
part of release 3.53 of the Linux *man-pages* project.
A description of the project, and information about
reporting bugs, can be found at
http://www.kernel.org/doc/man−pages/.