### NAME

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**(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE; or
*cc -std=c99*

### DESCRIPTION

The
**acos**() function calculates 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.
| |

### CONFORMING TO

C99, POSIX.1-2001.
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 3.23 of the Linux
*man-pages* project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.