acos - arc cosine function

## SYNOPSIS

**#include** **<math.h>**
**double** **acos(double** *x***);**

## DESCRIPTION

The **acos()** function calculates the arc cosine of *x*; that
is the value whose cosine is *x*. If *x* falls outside the
range -1 to 1, **acos()** fails and *errno* is set.

## RETURN VALUE

The **acos()** function returns the arc cosine in radians and
the value is mathematically defined to be between 0 and PI
(inclusive).

## ERRORS

**EDOM** *x* is out of range.

## CONFORMING TO

SVID 3, POSIX, BSD 4.3, ISO 9899

## SEE ALSO

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