Question

I have written a simple code using getopt for understanding perspective.

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

int main(int argc, char* argv[])
{

 /* Here since c is followed with colon, so 'c' takes an argument */
  const char *optstring = "abc:d";

  int ret;

  while((ret=getopt(argc, argv, optstring))!= -1)
  {
    switch(ret)
    {
      case 'a':
        printf("Option found: a, optind = %d\n",optind);
        break;

      case 'b':
         printf("Option found: b, optind = %d\n",optind);
         break;

      case 'c':
         printf("Option found: c, optind = %d\n",optind);
         printf("Option argument: %s\n",optarg);
         break;

      case 'd':
         printf("Option found: d, optind = %d\n",optind);
         break;

      case ':':
         printf("The option takes an argument which is missing");
         break;

     //case '?':
       //   printf("Didn't you enter an invalid option?");
           // break;
     }
   }
 }

The problem is:

(1) Case 1:
If the case '?' is commented then:

[root@dhcppc0 getopt]# ./a.out -a -b -c
    Option found: a, optind = 2
    Option found: b, optind = 3
    ./a.out: option requires an argument -- c

So, as you can see, the case ':' did not take into effect, as normally we expect a missing argument to return a ‘:’ (colon) by getopt.

(2) Case 2:
AND, if i un-comment it, and then run the program, it hits the case '? even for missing argument.

enter code here
[root@dhcppc0 getopt]# ./a.out -a -b -c
   Option found: a, optind = 2
   Option found: b, optind = 3
      ./a.out: option requires an argument -- c
      Didn't you enter an invalid option?

What is the point i am missing here?

ADDED LATER:

Also why is the ./a.out: option requires an argument -- c the default error coming? How to handle it, since i am already taking care for it in the case ':', and don’t want the default error message?

ADDED AGAIN:
As suggested in the answer, i used the colon in beginning of optstring – const char *optstring = ":abc:d", then why is this happening?

./a.out -a -b -c -d returns -d as the argument to c?? -d is a separate optional character and not any argument

Answer 1

The POSIX version of the getopt() function specifies:

If getopt() encounters an option character that is not contained in optstring, it shall return the <question-mark> ( ‘?’ ) character. If it detects a missing option-argument, it shall return the <colon> character ( ‘:’ ) if the first character of optstring was a <colon>, or a <question-mark> character ( ‘?’ ) otherwise.

Since your optstr does not start with a :, it should return a ? instead.