#include <math.h>

double acos(double x);

float acosf(float x);

long double acosl(long double x);


Partially implemented


IEEE Std 1003.1-2017


These functions shall compute the principal value of the arc cosine of their argument x. The value of x should be in the range [-1,1].

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

Return value#

Upon successful completion, these functions shall 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 shall occur, and either a NaN (if supported) or an implementation-defined value shall be returned.

  • If x is NaN, a NaN shall be returned.

  • If x is +1, +0 shall be returned.

  • If x is ±Inf, a domain error shall occur, and a NaN shall be returned.


These functions shall fail if:

Domain Error The x argument is finite and is not in the range [-1,1], or is ±Inf. If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then errno shall be set to EDOM. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the invalid floating-point exception shall be raised.



Known bugs#


See Also#

  1. Standard library functions

  2. Table of Contents