Description

   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.