The acos() function computes the principal value of the arc cosine of x. The value of x should be in the range [-1,1].

Upon successful completion, acos() returns the arc cosine of x, in the range [0,pi] radians. If the value of x is not in the range [-1,1], and is not +-Inf or NaN, either 0.0 or NaN is returned and errno is set to EDOM.

If x is NaN, NaN is returned. If x is +-Inf, either 0.0 is returned and errno is set to EDOM, or NaN is returned and errno may be set to EDOM.

For exceptional cases, matherr(3M) tabulates the values to be returned as dictated by Standards other than XPG4.

The acos() function will fail if:

EDOM

The value x is not +-Inf or NaN and is not in the range [-1,1].

The acos() function may fail if:

EDOM

The value x is +-Inf.

An application wishing to check for error situations should set errno to 0 before calling acos(). If errno is non-zero on return, or the value NaN is returned, an error has occurred.

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

cos(3M), isnan(3M), matherr(3M), attributes(5), standards(5)