Description

   If the character is n or fewer bytes, the mbtowc function
   converts the multibyte character pointed to by s to its wide-
   character equivalent. If the character is invalid or greater than
   n bytes, the function returns -1 to indicate an error.

   If pwc is a NULL pointer and s is not a null pointer, the
   function determines the number of bytes that constitute the
   multibyte character pointed to by s (regardless of the value
   of n).

   This function is affected by the LC_CTYPE category of the
   program's current locale.