# C++ Programming/Code/Standard C Library/Functions/acos

## acos

editSyntax |
```
#include <cmath>
double acos( double arg );
``` |

The acos() function returns the arc cosine of *arg*, which will be in the range [0, pi]. *arg* should be between -1 and 1. If *arg* is outside this range, acos() returns NAN and raises a floating-point exception.