Description

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

      wcrtomb (buf, L'\0', ps)

   where buf is an internal buffer.

   If s is not a NULL pointer, the wcrtomb function determines the
   number of bytes needed to represent the multibyte character that
   corresponds to the wide character specified by wc (including any
   shift sequences), and stores the resulting bytes in the array
   whose first element is pointed to by s. At most MB_CUR_MAX bytes
   are stored.

   If wc is a null wide character, a null byte is stored preceded by
   any shift sequence needed to restore the initial shift state. The
   resulting state described is the initial conversion state.