# expm1 - Man Page

exponential minus 1

## Library

Math library (*libm*, *-lm*)

## Synopsis

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

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 <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

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

_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L || /* Since glibc 2.19: */ _DEFAULT_SOURCE || /* Glibc <= 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 |

## Standards

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

## Bugs

Before glibc 2.17, on certain architectures (e.g., x86, but not x86_64) **expm1**() raised a bogus underflow floating-point exception for some large negative *x* values (where the function result approaches -1).

Before approximately glibc 2.11, **expm1**() raised a bogus invalid floating-point exception in addition to the expected overflow exception, and returned a NaN instead of positive infinity, for some large positive *x* values.

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

## See Also

## Referenced By

The man pages expm1f(3) and expm1l(3) are aliases of expm1(3).