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# Technical Reference: Base Operating System and Extensions, Volume 1

## acos, acosf, or acosl Subroutine

### Purpose

Computes the inverse cosine of a given value.

### Syntax

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

### Description

The acosf, acosl, and acos subroutines compute the principal value of the arc cosine of the x parameter. The value of x should
be in the range [-1,1].

An application wishing to check for error situations should set the errno global variable to zero and call fetestexcept(FE_ALL_EXCEPT) before calling these functions. On return,
if errno is nonzero or fetestexcept(FE_INVALID
| FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is nonzero, an error has
occurred.

### Parameters

x |
Specifies the value to be computed. |

### Return Values

Upon successful completion, these subroutines return the arc cosine of x, in the range [0, pi] radians.

For finite values of x not in the range [-1,1],
a domain error occurs, and a NaN is returned.

If x is NaN, a NaN is returned.

If x is +1, 0 is returned.

If x is ±Inf, a domain error occurs, and
a NaN is returned.

### Related Information

The acosh, acoshf, or acoshl Subroutine.

math.h in AIX 5L Version 5.2 Files Reference.

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