## NAME

acos, acosf, acosl − arc cosine function

## SYNOPSIS

**#include
<math.h>**

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

*x*

**);**

long double acosl(long double

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).

## 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

This page is
part of release 4.16 of the Linux *man-pages* project.
A description of the project, information about reporting
bugs, and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.