### 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 || _ISOC99_SOURCE; or
*cc -std=c99* **expm1f**(), **expm1l**(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE; 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 | |

An overflow floating-point exception
(FE_OVERFLOW) is raised.
| |

*errno*.

### 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.

### SEE ALSO

### COLOPHON

This page is part of release 3.23 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/.