---
name: "expm1"
section: "3"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/expm1"
---

# expm1(3)

exponential minus 1

## NAME

expm1, expm1f, expm1l - exponential minus 1

## LIBRARY

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

## SYNOPSIS

```
#include <math.h>
```

**double expm1(double** *x***);** **float expm1f(float** *x***);** **long double expm1l(long double** *x***);**

Feature Test Macro Requirements for glibc (see [**feature\_test\_macros**(7)](/feature_test_macros/7.md)):

**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)](/math_error/7.md) 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)](/attributes/7.md).

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **expm1**(), **expm1f**(), **expm1l**() | Thread safety | MT-Safe |

## STANDARDS

C11, POSIX.1-2008.

## HISTORY

C99, POSIX.1-2001. BSD.

## 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

[**exp**(3)](/exp/3.md), [**log**(3)](/log/3.md), [**log1p**(3)](/log1p/3.md)