acos

       #include <math.h>

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

       Link with -lm.

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       acosf(), acosl():
           _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 ||
           _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L;
           or cc -std=c99

DESCRIPTION
       These functions calculate the arc cosine of x; that is the value  whose
       cosine is x.

RETURN VALUE
       On  success, these functions return the arc cosine of x in radians; the
       return value is in the range [0, pi].

       If x is a NaN, a NaN is returned.

       If x is +1, +0 is returned.

       If x is positive infinity or negative infinity, a domain error  occurs,
       and a NaN is returned.

       If  x is outside the range [-1, 1], a domain error occurs, and a NaN is
       returned.

ERRORS
       See math_error(7) for information on how to determine whether an  error
       has occurred when calling these functions.

       The following errors can occur:

       Domain error: x is outside the range [-1, 1]
              errno  is  set  to  EDOM.   An  invalid floating-point exception
              (FE_INVALID) is raised.

ATTRIBUTES
       For  an  explanation  of  the  terms  used   in   this   section,   see
       attributes(7).

       +-------------------------+---------------+---------+
       |Interface                | Attribute     | Value   |
       +-------------------------+---------------+---------+
       |acos(), acosf(), acosl() | Thread safety | MT-Safe |
       +-------------------------+---------------+---------+
CONFORMING TO
       C99, POSIX.1-2001, POSIX.1-2008.
                                  2015-04-19                           ACOS(3)
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