# acos man page

acos, acosf, acosl — arc cosine function

## Synopsis

**#include <math.h>**
**double acos(double** *x***);**
**float acosf(float** *x***);**
**long double acosl(long double** *x***);**

`Link with `

*-lm*.

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):**acosf**(), **acosl**():

_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L

|| /* Since glibc 2.19: */ _DEFAULT_SOURCE

|| /* Glibc versions <= 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) 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).

Interface | Attribute | Value |
---|---|---|

acos(), acosf(), acosl() | Thread safety | MT-Safe |

## Conforming to

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

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

## See Also

asin(3), atan(3), atan2(3), cacos(3), cos(3), sin(3), tan(3)

## Colophon

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## Referenced By

asin(3), atan(3), atan2(3), cos(3), sin(3), tan(3).

acosf(3) and acosl(3) are aliases of acos(3).