# EXPM1

## 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**():

_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L

|| _XOPEN_SOURCE >= 500

|| /* Since glibc 2.19: */ _DEFAULT_SOURCE

|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

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

_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

exp(x) − 1

The result 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, POSIX.1-2008.

## 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

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