---
name: "acos"
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/acos"
---

# acos(3)

arc cosine function

## NAME

acos, acosf, acosl - arc cosine function

## LIBRARY

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

## SYNOPSIS

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

**double acos(double** *x***);** **float acosf(float** *x***);** **long double acosl(long double** *x***);**

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

**acosf**(), **acosl**():

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

## DESCRIPTION

These functions calculate the arc cosine of *x*; that is the value whose cosine is *x*.

## RETURN VALUE

On success, these functions return the arc cosine of *x* in radians; the return value is in the range \[0, pi\].

If *x* is a NaN, a NaN is returned.

If *x* is +1, +0 is returned.

If *x* is positive infinity or negative infinity, a domain error occurs, and a NaN is returned.

If *x* is outside the range \[-1, 1\], a domain error occurs, and a NaN is returned.

## 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:

- Domain error: *x* is outside the range \[-1, 1\]

  *errno* is set to **EDOM**. An invalid floating-point exception (**FE\_INVALID**) is raised.

## ATTRIBUTES

For an explanation of the terms used in this section, see [**attributes**(7)](/attributes/7.md).

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **acos**(), **acosf**(), **acosl**() | Thread safety | MT-Safe |

## STANDARDS

C11, POSIX.1-2008.

## HISTORY

C99, POSIX.1-2001.

The variant returning *double* also conforms to C89, SVr4, 4.3BSD.

## SEE ALSO

[**asin**(3)](/asin/3.md), [**atan**(3)](/atan/3.md), [**atan2**(3)](/atan2/3.md), [**cacos**(3)](/cacos/3.md), [**cos**(3)](/cos/3.md), [**sin**(3)](/sin/3.md), [**tan**(3)](/tan/3.md)