The mbrlen function is equivalent to the call:

        mbrtowc(NULL, s, n, ps != NULL ? ps : &internal)

    Where internal is the mbstate_t object for the mbrlen function.

    If the multibyte character pointed to by s is of n bytes or less,
    the function returns the number of bytes comprising the character
    (including any shift sequences).

    If either an encoding error occurs or the next n bytes contribute
    to an incomplete but potentially valid multibyte character, the
    function returns -1 or -2, respectively.

    See also mbrtowc.