A command-line parser that can be used by applications that
    follow UNIX command-line conventions.

    Format

      #include  <unistd.h> (X/Open, POSIX-1)

      #include  <stdio.h> (X/Open, POSIX-2)

      int getopt  (int argc, char * const argv[], const char *optstring);

      extern  char *optarg;

      extern  int optind, opterr, optopt;