# expm1 man page

expm1, expm1f, expm1l — exponential minus 1

## Synopsis

#include <math.h>double expm1(doublex);float expm1f(floatx);long double expm1l(long doublex);

`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).

Interface | Attribute | Value |
---|---|---|

expm1(), expm1f(), expm1l() | Thread safety | MT-Safe |

## 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

## Colophon

This page is part of release 4.10 of the Linux *man-pages* project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

## Referenced By

expm1f(3) and expm1l(3) are aliases of expm1(3).