Description

   The variable optind is the index of the next element of the
   argv vector to be processed. It is initialized to 1 by the
   system, and it is updated by getopt when it finishes with each
   element of argv. When an element of argv contains multiple option
   characters, it is unspecified how getopt determines which options
   have already been processed.

   The getopt function returns the next option character (if one is
   found) from argv that matches a character in optstring, if there
   is one that matches. If the option takes an argument, getopt sets
   the variable optarg to point to the option-argument as follows:

   o  If the option was the last character in the string pointed to
      by an element of argv, then optarg contains the next element
      of argv, and optind is incremented by 2. If the resulting
      value of optind is not less than argc, getopt returns an
      error, indicating a missing option-argument.

   o  Otherwise, optarg points to the string following the option
      character in that element of argv, and optind is incremented
      by 1.

   If one of the following is true, getopt returns -1 without
   changing optind:

      argv[optind] is a NULL pointer
      *argv[optind] is not the character -
      argv[optind] points to the string "-"

   If argv[optind] points to the string "- -" getopt returns -1
   after incrementing optind.

   If getopt encounters an option character not contained in
   optstring, the question-mark character (?)  is returned.

   If getopt detects a missing argument, the colon character (:)
   is returned if the first character of optstring is a colon;
   otherwise, a question-mark character is returned.

   In either of the previous two cases, getopt sets the variable
   optopt to the option character that caused the error. If the
   application has not set the variable opterr to 0 and the first
   character of optstring is not a colon, getopt also prints a
   diagnostic message to stderr.