add callback for separate g_main_loop

Question

I have some misunderstanding about how GMainLoop work. Main thing - API which add some callbacks into g_main_loop (like g_timeout_add_seconds()) don't take pointer to which loop you want to add that callback.

It looks like you add callback's for all g_main_loop instances. Even if you have not yet created. Simple example for this:

#include <glib.h>

gboolean callback(gpointer data)
{
  static guint16 i=0; 
  g_print("Iter=%"G_GUINT16_FORMAT"\n",i++);
  if(i%5==0){
    g_print("try to stop loop1\n");
    g_main_loop_quit((GMainLoop*)data);
  }
  return TRUE;
}

int main()
{
  GMainLoop* loop1 = NULL;
  GMainLoop* loop2 = NULL;
  loop1 = g_main_loop_new (NULL, FALSE);

  g_timeout_add_seconds(1, callback,loop1);

  loop2 = g_main_loop_new (NULL, FALSE);
  g_print("run loop1\n");
  g_main_loop_run(loop1);
  g_free(loop1);
  g_print("run loop2\n");
  g_main_loop_run(loop2);
  g_free(loop2);
  return 0;
}

Result is:

run loop1
Iter=0
Iter=1
Iter=2
Iter=3
Iter=4
try to stop loop1
run loop2
Iter=5
Iter=6
Iter=7
Iter=8
Iter=9
try to stop loop1
Segmentation fault (core dumped)

Is it possible add callback() to loop1, and don't add it to loop2?

Answer 1

A quick look at the documentation for g_idle_add(), g_idle_add_full(), g_timeout_add(), or g_timeout_add_full() will tell you:

This internally creates a main loop source using g_timeout_source_new() and attaches it to the main loop context using g_source_attach(). You can do these steps manually if you need greater control.

Note that it says it attaches the source to the main loop context—i.e., a GMainContext, not a GMainLoop. When you create your GMainLoop instances you're passing NULL for the first argument. According to the the g_main_loop_new() documentation, that argument is

a GMainContext (if NULL, the default context will be used).

So, you are creating two main loops, both using the same context (which is the default context).

To get the result I think you're expecting, you should do something like:

#include <glib.h>

gboolean callback(gpointer data)
{
  static guint16 i=0; 
  g_print("Iter=%"G_GUINT16_FORMAT"\n",i++);
  if(i%5==0){
    g_print("try to stop loop1\n", data);
    g_main_loop_quit((GMainLoop*)data);
  }
  return TRUE;
}

int main()
{
  GMainContext* con1 = NULL;
  GMainContext* con2 = NULL;
  GMainLoop* loop1 = NULL;
  GMainLoop* loop2 = NULL;
  GSource* source1 = NULL;

  con1 = g_main_context_new ();
  con2 = g_main_context_new ();

  loop1 = g_main_loop_new (con1, FALSE);
  loop2 = g_main_loop_new (con2, FALSE);

  source1 = g_timeout_source_new_seconds (1);
  g_source_set_callback (source1, callback, loop1, NULL);
  g_source_attach (source1, con1);

  // We don't need the GMainContext anymore—the loop has an internal
  // reference so we'll drop ours.
  g_main_context_unref (con1);
  con1 = NULL;

  // Ditto for the GSource
  g_source_unref (source1);
  source1 = NULL;

  g_main_context_unref (con2);
  con2 = NULL;

  g_print("run loop1\n");
  g_main_loop_run(loop1);
  // Use g_main_loop_unref, not g_free
  g_main_loop_unref(loop1);
  loop1 = NULL;

  g_print("run loop2\n");
  // Note that there is no longer a callback attached here, so it will
  // run forever.
  g_main_loop_run(loop2);
  g_main_loop_unref(loop2);
  loop2 = NULL;

  return 0;
}