# acos(3M)

## NAME

acos - arc cosine function

## SYNOPSIS

cc [ *flag* ... ] *file* ... -lm [ *library* ... ]
#include <math.h>
double acos(double *x*);

## DESCRIPTION

The acos() function computes the principal value of the arc
cosine of *x*. The value of *x* should be in the range [-1,1].

## RETURN VALUES

Upon successful completion, acos() returns the arc cosine of
*x*, in the range [0,pi] radians. If the value of *x* is not in
the range [-1,1], and is not **_**Inf or NaN, either 0.0 or NaN
is returned and errno is set to EDOM.
If *x* is NaN, NaN is returned. If *x* is **_**Inf, either 0.0 is
returned and errno is set to EDOM, or NaN is returned and
errno may be set to EDOM.
For exceptional cases, **matherr(3M)** tabulates the values to
be returned as dictated by Standards other than XPG4.

## ERRORS

The acos() function will fail if:
EDOM The value *x* is not **_**Inf or NaN and is not in the range
[-1,1].
The acos() function may fail if:
EDOM The value *x* is **_**Inf.

## USAGE

An application wishing to check for error situations should
set errno to 0 before calling acos(). If errno is non-zero
on return, or the value NaN is returned, an error has
occurred.

## ATTRIBUTES

See **attributes(5)** for descriptions of the following attri-
butes:
____________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________*|*_____________________________*|*
| MT-Level | MT-Safe |
|_____________________________*|*_____________________________*|*

## SEE ALSO

**cos(3M)**, **isnan(3M)**, **matherr(3M)**, **attributes(5)**, **standards(5)**

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