In C++, how to use only long options with a required argument?

Question

In a C++ program, I would like to have a "long-only" option with a required argument. Below is my minimal example using getopt_long(), but it's not working:

#include <getopt.h>
#include <cstdlib>
#include <iostream>
using namespace std;

void help (char ** argv)
{
  cout << "`" << argv[0] << "` experiments with long options." << endl;
}

void parse_args (int argc, char ** argv, int & verbose, int & param)
{
  int c = 0;
  while (1)
  {
    static struct option long_options[] =
      {
        {"help", no_argument, 0, 'h'},
        {"verbose", required_argument, 0, 'v'},
        {"param", required_argument, 0, 0}
      };
    int option_index = 0;
    c = getopt_long (argc, argv, "hv:",
                     long_options, &option_index);
    cout << "c=" << c << endl;
    if (c == -1)
      break;
    switch (c)
    {
    case 0:
      if (long_options[option_index].flag != 0)
        break;
      printf ("option %s", long_options[option_index].name);
      if (optarg)
        printf (" with arg %s", optarg);
      printf ("\n");
      break;
    case 'h':
      help (argv);
      exit (0);
    case 'v':
      verbose = atoi(optarg);
      break;
    case 'param':
      param = atoi(optarg);
      break;
    case '?':
      abort ();
    default:
      abort ();
    }
  }
}

int main (int argc, char ** argv)
{
  int verbose = 0;
  int param = 0;
  parse_args (argc, argv, verbose, param);
  cout << "verbose=" << verbose << " param=" << param << endl;
  return EXIT_SUCCESS;
}

I compile it with this command (gcc version 4.1.2 20080704 Red Hat 4.1.2-46):

g++ -Wall test.cpp

It tells me this:

test.cpp:44:10: warning: character constant too long for its type

And here is the result:

$ ./a.out -v 2 --param 3
c=118
c=0
option param with arg 3
c=-1
verbose=2 param=0

I tried to make it work on ideone but it doesn't even recognize the option -v.

As indicated by trojanfoe in his comments of another question, it should be possible to use "long-only" options because GNU tar does it. However, GNU tar uses argp and I have difficulty understanding its source code.

Could someone give me a minimal example that works, with GNU getopt_long() or argp()?

Answer 1

There are two problems:

1. According to the example code (your link), the final option defined in the struct must be {0,0,0,0}. I recommend changing the definition to

   static struct option long_options[] =
     {
       {"help", no_argument, 0, 'h'},
       {"verbose", required_argument, 0, 'v'},
       {"param", required_argument, 0, 0},
       {0,0,0,0}
     };
   

2. (And more importantly,) you must include code that actually processes the "param" option. You do that in the '0' case:

   case 0:
     if (long_options[option_index].flag != 0)
       break;
     if (strcmp(long_options[option_index].name,"param") == 0)
       param = atoi(optarg);
     break;
   

As you can see I use the strcmp function to compare the strings; for that you need to #include <cstring>. By the way, you also need #include <cstdio> for your use of printf.

With these changes in place, the program worked correctly for me (tested on GCC 4.5.1).

Answer 2

In your case statement, you are using:

case 'param':

which is giving you the warning, because compiler expects a single character in that place.

Answer 3

In fact, I realized that I should check the value of option_index when c has the value 0, like this. Such a case is not in the GNU libc examples, so here it is:

switch (c)
{
case 0:
  if (long_options[option_index].flag != 0)
    break;
  if (option_index == 2)
  {
    param = atoi(optarg);
    break;
  }
case 'h':
  help (argv);
  exit (0);
case 'v':
  verbose = atoi(optarg);
  break;
case '?':
  abort ();
default:
  abort ();
}

Answer 4

Another solution is to use 'flag' member value defined in struct option.

Setup your options like below:

int param_flag = 0;

{"param", required_argument, &param_flag, 1}

Then;

case 0:
    if (param_flag == 1) {
        do_something;
    }

See man getopt_long for details about struct option.