A simple example where the commandline is completely handled in the "command-line" handler. The launching instance exits once the signal handler in the primary instance has returned, and the return value of the signal handler becomes the exit status of the launching instance.

#include <gio/gio.h>
#include <stdlib.h>
#include <string.h>

static int
command_line (GApplication            *application,
              GApplicationCommandLine *cmdline)
{
  gchar **argv;
  gint argc;
  gint i;

  argv = g_application_command_line_get_arguments (cmdline, &argc);

  g_application_command_line_print (cmdline,
                                    "This text is written back\n"
                                    "to stdout of the caller\n");

  for (i = 0; i < argc; i++)
    g_print ("argument %d: %s\n", i, argv[i]);

  g_strfreev (argv);

  return 0;
}

int
main (int argc, char **argv)
{
  GApplication *app;
  int status;

  app = g_application_new ("org.gtk.TestApplication",
                           G_APPLICATION_HANDLES_COMMAND_LINE);
  g_signal_connect (app, "command-line", G_CALLBACK (command_line), NULL);
  g_application_set_inactivity_timeout (app, 10000);

  status = g_application_run (app, argc, argv);

  g_object_unref (app);

  return status;
}

An example of split commandline handling. Options that start with --local- are handled locally, all other options are passed to the "command-line" handler which runs in the primary instance.

#include <gio/gio.h>
#include <stdlib.h>
#include <string.h>

static int
command_line (GApplication            *application,
              GApplicationCommandLine *cmdline)
{
  gchar **argv;
  gint argc;
  gint i;

  argv = g_application_command_line_get_arguments (cmdline, &argc);

  for (i = 0; i < argc; i++)
    g_print ("handling argument %s remotely\n", argv[i]);

  g_strfreev (argv);

  return 0;
}

static gboolean
test_local_cmdline (GApplication   *application,
                    gchar        ***arguments,
                    gint           *exit_status)
{
  gint i, j;
  gchar **argv;

  argv = *arguments;

  i = 1;
  while (argv[i])
    {
      if (g_str_has_prefix (argv[i], "--local-"))
        {
          g_print ("handling argument %s locally\n", argv[i]);
          g_free (argv[i]);
          for (j = i; argv[j]; j++)
            argv[j] = argv[j + 1];
        }
      else
        {
          g_print ("not handling argument %s locally\n", argv[i]);
          i++;
        }
    }

  *exit_status = 0;

  return FALSE;
}

typedef GApplication TestApplication;
typedef GApplicationClass TestApplicationClass;

G_DEFINE_TYPE (TestApplication, test_application, G_TYPE_APPLICATION)

static void
test_application_finalize (GObject *object)
{
  G_OBJECT_CLASS (test_application_parent_class)->finalize (object);
}

static void
test_application_init (TestApplication *app)
{
}

static void
test_application_class_init (TestApplicationClass *class)
{
  G_OBJECT_CLASS (class)->finalize = test_application_finalize;
  G_APPLICATION_CLASS (class)->local_command_line = test_local_cmdline;
}

GApplication *
test_application_new (const gchar       *application_id,
                      GApplicationFlags  flags)
{
  g_return_val_if_fail (g_application_id_is_valid (application_id), NULL);

  g_type_init ();

  return g_object_new (test_application_get_type (),
                       "application-id", application_id,
                       "flags", flags,
                       NULL);
}

int
main (int argc, char **argv)
{
  GApplication *app;
  int status;

  app = test_application_new ("org.gtk.TestApplication", 0);
  g_application_set_inactivity_timeout (app, 10000);
  g_signal_connect (app, "command-line", G_CALLBACK (command_line), NULL);

  status = g_application_run (app, argc, argv);

  g_object_unref (app);

  return status;
}

An example of deferred commandline handling. Here, the commandline is not completely handled before the "command-line" handler returns. Instead, we keep a reference to the GApplicationCommandline object and handle it later(in this example, in an idle). Note that it is necessary to hold the application until you are done with the commandline.

This example also shows how to use GOptionContext for parsing the commandline arguments. Note that it is necessary to disable the built-in help-handling of GOptionContext, since it calls exit() after printing help, which is not what you want to happen in the primary instance.

#include <gio/gio.h>
#include <stdlib.h>
#include <string.h>

static gboolean
my_cmdline_handler (gpointer data)
{
  GApplicationCommandLine *cmdline = data;
  gchar **args;
  gchar **argv;
  gint argc;
  gint arg1;
  gboolean arg2;
  gboolean help;
  GOptionContext *context;
  GOptionEntry entries[] = {
    { "arg1", 0, 0, G_OPTION_ARG_INT, &arg1, NULL, NULL },
    { "arg2", 0, 0, G_OPTION_ARG_NONE, &arg2, NULL, NULL },
    { "help", '?', 0, G_OPTION_ARG_NONE, &help, NULL, NULL },
    { NULL }
  };
  GError *error;
  gint i;

  args = g_application_command_line_get_arguments (cmdline, &argc);

  /* We have to make an extra copy of the array, since g_option_context_parse()
   * assumes that it can remove strings from the array without freeing them.
   */
  argv = g_new (gchar*, argc + 1);
  for (i = 0; i <= argc; i++)
    argv[i] = args[i];

  context = g_option_context_new (NULL);
  g_option_context_set_help_enabled (context, FALSE);
  g_option_context_add_main_entries (context, entries, NULL);

  arg1 = 0;
  arg2 = FALSE;
  help = FALSE;
  error = NULL;
  if (!g_option_context_parse (context, &argc, &argv, &error))
    {
      g_application_command_line_printerr (cmdline, "%s\n", error->message);
      g_error_free (error);
      g_application_command_line_set_exit_status (cmdline, 1);
    }
  else if (help)
    {
      gchar *text;
      text = g_option_context_get_help (context, FALSE, NULL);
      g_application_command_line_print (cmdline, "%s",  text);
      g_free (text);
    }
  else
    {
      g_application_command_line_print (cmdline, "arg1 is %d and arg2 is %s\n",
                                        arg1, arg2 ? "TRUE" : "FALSE");
      g_application_command_line_set_exit_status (cmdline, 0);
    }

  g_free (argv);
  g_strfreev (args);

  g_option_context_free (context);

  /* we are done handling this commandline */
  g_object_unref (cmdline);

  return FALSE;
}

static int
command_line (GApplication            *application,
              GApplicationCommandLine *cmdline)
{
  /* keep the application running until we are done with this commandline */
  g_application_hold (application);

  g_object_set_data_full (G_OBJECT (cmdline),
                          "application", application,
                          (GDestroyNotify)g_application_release);

  g_object_ref (cmdline);
  g_idle_add (my_cmdline_handler, cmdline);

  return 0;
}

int
main (int argc, char **argv)
{
  GApplication *app;
  int status;

  app = g_application_new ("org.gtk.TestApplication",
                           G_APPLICATION_HANDLES_COMMAND_LINE);
  g_signal_connect (app, "command-line", G_CALLBACK (command_line), NULL);
  g_application_set_inactivity_timeout (app, 10000);

  status = g_application_run (app, argc, argv);

  g_object_unref (app);

  return status;
}