c - how to take integers as command line arguments?

Question

I've read a getopt() example but it doesn't show how to accept integers as argument options, like cvalue would be in the code from the example:

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

 int
 main (int argc, char **argv)
 {
   int aflag = 0;
   int bflag = 0;
   char *cvalue = NULL;
   int index;
   int c;

   opterr = 0;

   while ((c = getopt (argc, argv, "abc:")) != -1)
     switch (c)
       {
       case 'a':
         aflag = 1;
         break;
       case 'b':
         bflag = 1;
         break;
       case 'c':
         cvalue = optarg;
         break;
       case '?':
         if (optopt == 'c')
           fprintf (stderr, "Option -%c requires an argument.
", optopt);
         else if (isprint (optopt))
           fprintf (stderr, "Unknown option `-%c'.
", optopt);
         else
           fprintf (stderr,
                    "Unknown option character `\x%x'.
",
                    optopt);
         return 1;
       default:
         abort ();
       }

   printf ("aflag = %d, bflag = %d, cvalue = %s
",
           aflag, bflag, cvalue);

   for (index = optind; index < argc; index++)
     printf ("Non-option argument %s
", argv[index]);
   return 0;
 }

If I ran the above as testop -c foo, cvalue would be foo, but what if I wanted testop -c 42? Since cvalue is of type char *, could I just cast optarg to be (int)? I've tried doing this without using getopt() and accessing argv[whatever] directly, and casting it as an integer, but I always end up with a large negative number when printing with %d. I'm assuming I'm not dereferencing argv[] correctly or something, not sure...

Answer 1

You need to use atoi() to convert from string to integer.