Description

   If s is a NULL pointer, mbrtowc is equivalent to the call:

       mbrtowc(NULL, "", 1, ps)

   In this case, the values of pwc and n are ignored.

   If s is not a NULL pointer, mbrtowc inspects at most n bytes
   beginning with the byte pointed to by s to determine the
   number of bytes needed to complete the next multibyte character
   (including any shift sequences).

   If the function determines that the next multibyte character
   is completed, it determines the value of the corresponding wide
   character and then, if pwc is not a NULL pointer, stores that
   value in the object pointed to by pwc. If the corresponding
   wide character is the null wide character, the resulting state
   described is the initial conversion state.

   If mbrtowc is called as a counting function, which means that pwc
   is a NULL pointer and s is neither a NULL pointer nor a pointer
   to a null byte, the value of the internal mbstate_t object will
   remain unchanged.