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Thread: using getopt in C

  1. #1
    Join Date
    Oct 2006

    using getopt in C

    Greetings, I am using the getopt function and I had a question. Is there anyway to easily, not require an argument after the r - example

    ./webserver -p 5000 -r

    rather than

    ./webserver -p 5000 -r /asd

    because it does this to me
    [bwhit132@csunix webserver]$ ./webserver -p 5000 -r
    ./webserver: option requires an argument -- r

    I want to allow the option but not have it as a necessity.

    this is what im doing with it
    void processargs(int argc, char **argv) {
      int dirSize = 100;
      int next_option;
      const char* short_options = "p:r:";
      do {
        next_option = getopt(argc, argv, short_options);
        if (next_option != -1) {
          switch (next_option)
            case 'p': /* -p -- port option  */
              port = optarg;
    	case 'r':
    	  myPath = malloc(dirSize);		//allocate because we are copying into a null ptr
    	  while (getcwd(myPath,dirSize) == NULL){   //if the directory has a long name, reallocate size
    		  dirSize *= 2;
    		  myPath = malloc(dirSize);		  
    	 downPath = optarg;	
    	 if (downPath == NULL){
    		  printf("\nHAHAAAAA NULL BOY\n");
    	 } else { printf("\n%s\n",downPath); }
              /* Unknown option detected. Print it to standard
                    output, and exit with exit code zero (normal termination). */
              fprintf(stderr, USAGE_STRING);
      } while (next_option != -1);
      if (port == NULL) {
              fprintf(stderr, USAGE_STRING);

    Edit: In case you were wondering I was just testing with the whole hahaha null boy thing..
    Last edited by g3k0; March 5th, 2007 at 12:44 AM.

  2. #2
    Join Date
    Feb 2007

    Re: using getopt in C

    man 3 getopt

    optstring is a string containing the legitimate option characters. If
    such a character is followed by a colon, the option requires an argu-
    ment, so getopt places a pointer to the following text in the same
    argv-element, or the text of the following argv-element, in optarg.
    Two colons mean an option takes an optional arg; if there is text in
    the current argv-element, it is returned in optarg, otherwise optarg is
    set to zero. This is a GNU extension. If optstring contains W fol-
    lowed by a semicolon, then -W foo is treated as the long option --foo.
    (The -W option is reserved by POSIX.2 for implementation extensions.)
    This behaviour is a GNU extension, not available with libraries before
    GNU libc 2.

  3. #3
    Join Date
    Oct 2006

    Re: using getopt in C

    ohhhh thanks. I was looking at the wrong man page. Thanks for your help


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