# man pages section 3: Extended Library Functions, Volume 2

Updated: July 2017

## acos(3M)

### Name

acos, acosf, acosl - arc cosine functions

### Synopsis

```c99 [ flag... ] file... –lm [ library... ]
#include <math.h>

double acos(double x);```
`float acosf(float x);`
`long double acosl(long double x);`

### Description

These functions compute 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, these functions return the arc cosine of x in the range [0, π] radians.

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

If x is NaN, NaN is returned.

If x is +1, +0 is returned.

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

For exceptional cases, matherr(3M) tabulates the values to be returned by acos() as specified by SVID3 and XPG3.

### Errors

These functions will fail if:

Domain Error

The x argument is finite and not in the range [-1,1], or is ±Inf.

If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, the invalid floating-point exception is raised.

The acos() function sets errno to EDOM if x is not ±Inf or NaN and is not in the range [−1,1].

### Usage

An application wanting to check for exceptions should call feclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, if fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an exception has been raised. An application should either examine the return value or check the floating point exception flags to detect exceptions.

An application can also set errno to 0 before calling acos(). On return, if errno is non-zero, an error has occurred. The acosf() and acosl() functions do not set errno.

### Attributes

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
Committed
MT-Level
MT-Safe
Standard