Getopt() newbie...

AMMullan · 02-10-2004, 05:24 PM

I have the following statement in my app to get getopt() arguments but I need to figure out a way to tell if an argument is passed or not.

I have static char stropts[] = "u:h"; defined but if there is no argument after the -u then I need the program to exit... This is the case statement:

Code:

while((optch = getopt(argc, argv, stropts)) != EOF)
    switch(optch) {
      case 'h':
        usage(argv[0]);
        return 0;
      case 'u':
          sprintf(name, "%s", optarg);
          break;
      default:
        printf("Unknown option -- %s.\n", optarg);
        printf("For help on usage please use: %s -h\n", argv[0]);
        return 1;

Thanks

Hko · 02-10-2004, 07:18 PM

When there is an argument missing for -u, getopt() itself also print a error message and exit the program. However, you can handle this case yourself if you have ':' as the very first character of your option string ("stropts" in your program). getopt() will then return ':' if an argument is missing for any option.

From "man 3 getopt":

Quote:

If getopt() finds an option character in argv that was not included in
optstring, or if it detects a missing option argument, it returns `?'
and sets the external variable optopt to the actual option character.
If the first character of optstring is a colon (`:'), then getopt()
returns `:' instead of `?' to indicate a missing option argument. If
an error was detected, and the first character of optstring is not a
colon, and the external variable opterr is nonzero (which is the
default), getopt() prints an error message.

You also have a few mistakes in your program. The biggest being that you try to use "optarg" to print the unknown options character. This should be "optopt". Note that your program prints "Unknown option -- (null).". If printf() wasn't so kind to check for the pointer to be NULL, it would segfault...
Another is that it's better to test for getopt() to return >= 0 in the while-statement instead of test for EOF.

Here's an improved version that also checks itself for a missing argument for -u.

Code:

#include <stdio.h>
#include <unistd.h>

#define MAXNAME 20

void usage(char *cmd)
{
     printf("Usage: %s [-h] [-u <name>]\n", cmd);
}


int main(int argc, char *argv[])
{
     int optch;
     char stropts[] = ":u:h";

     char name[MAXNAME + 1];

     while((optch = getopt(argc, argv, stropts)) >= 0) {
	  switch (optch) {
	  case 'h':
	       usage(argv[0]);
	       return 0;
	  case 'u':
	       snprintf(name, MAXNAME, "%s", optarg);
	       break;
	  case ':':
	       fprintf(stderr, "No argument specified for -%c option\n", optopt);
	       return 1;
	  case '?':
	       printf("Unknown option -%c.\n", optopt);
	       printf("For help on usage please use: %s -h\n", argv[0]);
	       return 1;
	  }
     }

     printf("User name = %s\n", name);

     /* Rest of program follows... */
}

AMMullan · 02-10-2004, 08:15 PM

Ummm that didn't work, it compiled fine but when I run it like ./pwdDiary -u it comes up with Unknown option -u (the ? argument in the switch)... any ideas?

Ummm another thing, now that I have errors setup I tried to get rid of the standard ones... In the man page it says to set opterr to 0 but when I do that I still get the errors.

AMMullan · 02-11-2004, 12:02 AM

K my bad - i left the other variable declarations at the top of my code - works perfectly now

Thanks for the help Hko

AMMullan · 02-11-2004, 12:05 AM

Ummm k i tried bob as a user name, that worked... but anything else (i.e. root, bo, foo etc) doesn't work

Hko · 02-11-2004, 11:06 AM

I copied / pasted the program from the forum from scratch, and it does work for me. For all names.

AMMullan · 02-11-2004, 01:02 PM

Yeah urs does but when i copied it into my program it doesn't seem to...

K just going through my code, my fault - I had a deugging test in their from when I was trying out getopt() before, it works now