Using getopt_long

• The GNU C library provides a function, getopt_long, understands both short and long options. If you use this function, include the header file $<getopt.h>$.
• Suppose, for example, that you are writing a program that is to accept the three options shown in Table 1.
  

  Table 1: Example Program Options

  Short Form	Long Form	Purpose
  -h	-help	Display usage summary and exit
  -o filename	-output filename	Specify output filename
  -v	-verbose	Print verbose messages

  
  In addition, the program is to accept zero or more additional command-line arguments, which are the names of input files.
• To use getopt_long, you must provide two data structures; one for short options, each a single letter and one for long options, you construct an array of struct option elements. http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/getopt_long.cgetopt_long.c

  #include <getopt.h>
  #include <stdio.h>
  #include <stdlib.h>
  
  /* The name of this program.  */
  const char* program_name;
  
  /* Prints usage information for this program to STREAM (typically
     stdout or stderr), and exit the program with EXIT_CODE.  Does not
     return.  */
  
  void print_usage (FILE* stream, int exit_code)
  {
    fprintf (stream, "Usage:  %s options [ inputfile ... ]\n", program_name);
    fprintf (stream,
             "  -h  --help             Display this usage information.\n"
             "  -o  --output filename  Write output to file.\n"
             "  -v  --verbose          Print verbose messages.\n");
    exit (exit_code);
  }
  
  /* Main program entry point.  ARGC contains number of argument list
     elements; ARGV is an array of pointers to them.  */
  
  int main (int argc, char* argv[])
  {
    int next_option;
  
    /* A string listing valid short options letters.  */
    const char* const short_options = "ho:v";
    /* An array describing valid long options.  */
    const struct option long_options[] = {
      { "help",     0, NULL, 'h' },
      { "output",   1, NULL, 'o' },
      { "verbose",  0, NULL, 'v' },
      { NULL,       0, NULL, 0   }   /* Required at end of array.  */
    };
  
    /* The name of the file to receive program output, or NULL for
       standard output.  */
    const char* output_filename = NULL;
    /* Whether to display verbose messages.  */
    int verbose = 0;
  
    /* Remember the name of the program, to incorporate in messages.
       The name is stored in argv[0].  */
    program_name = argv[0];
  
    do {
      next_option = getopt_long (argc, argv, short_options,
                                 long_options, NULL);
      switch (next_option)
      {
      case 'h':   /* -h or --help */
        /* User has requested usage information.  Print it to standard
           output, and exit with exit code zero (normal termination).  */
        print_usage (stdout, 0);
  
      case 'o':   /* -o or --output */
        /* This option takes an argument, the name of the output file.  */
        output_filename = optarg;
        break;
  
      case 'v':   /* -v or --verbose */
        verbose = 1;
        break;
  
      case '?':   /* The user specified an invalid option.  */
        /* Print usage information to standard error, and exit with exit
           code one (indicating abnormal termination).  */
        print_usage (stderr, 1);
  
      case -1:    /* Done with options.  */
        break;
  
      default:    /* Something else: unexpected.  */
        abort ();
      }
    }
    while (next_option != -1);
  
    /* Done with options.  OPTIND points to first non-option argument.
       For demonstration purposes, print them if the verbose option was
       specified.  */
    if (verbose) {
      int i;
      for (i = optind; i < argc; ++i) 
        printf ("Argument: %s\n", argv[i]);
    }
  
    /* The main program goes here.  */
  
    return 0;
  }