expm1, expm1f, expm1l - exponential minus 1

```
#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
```

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

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.

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.

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.

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

Interface | Attribute | Value |

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

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

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.

This page is part of release 4.15 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/.