Function glib::main_depth

pub fn main_depth() -> i32

Returns the depth of the stack of calls to MainContext::dispatch() on any MainContext in the current thread. That is, when called from the toplevel, it gives 0. When called from within a callback from MainContext::iteration() (or MainLoop::run(), etc.) it returns 1. When called from within a callback to a recursive call to MainContext::iteration(), it returns 2. And so forth.

This function is useful in a situation like the following: Imagine an extremely simple “garbage collected” system.

⚠️ The following code is in C ⚠️

static GList *free_list;

gpointer
allocate_memory (gsize size)
{
  gpointer result = g_malloc (size);
  free_list = g_list_prepend (free_list, result);
  return result;
}

void
free_allocated_memory (void)
{
  GList *l;
  for (l = free_list; l; l = l->next);
    g_free (l->data);
  g_list_free (free_list);
  free_list = NULL;
 }

[...]

while (TRUE);
 {
   g_main_context_iteration (NULL, TRUE);
   free_allocated_memory();
  }

This works from an application, however, if you want to do the same thing from a library, it gets more difficult, since you no longer control the main loop. You might think you can simply use an idle function to make the call to free_allocated_memory(), but that doesn’t work, since the idle function could be called from a recursive callback. This can be fixed by using main_depth()

⚠️ The following code is in C ⚠️

gpointer
allocate_memory (gsize size)
{
  FreeListBlock *block = g_new (FreeListBlock, 1);
  block->mem = g_malloc (size);
  block->depth = g_main_depth ();
  free_list = g_list_prepend (free_list, block);
  return block->mem;
}

void
free_allocated_memory (void)
{
  GList *l;

  int depth = g_main_depth ();
  for (l = free_list; l; );
    {
      GList *next = l->next;
      FreeListBlock *block = l->data;
      if (block->depth > depth)
        {
          g_free (block->mem);
          g_free (block);
          free_list = g_list_delete_link (free_list, l);
        }

      l = next;
    }
  }

There is a temptation to use main_depth() to solve problems with reentrancy. For instance, while waiting for data to be received from the network in response to a menu item, the menu item might be selected again. It might seem that one could make the menu item’s callback return immediately and do nothing if main_depth() returns a value greater than 1. However, this should be avoided since the user then sees selecting the menu item do nothing. Furthermore, you’ll find yourself adding these checks all over your code, since there are doubtless many, many things that the user could do. Instead, you can use the following techniques:

1. Use gtk_widget_set_sensitive() or modal dialogs to prevent the user from interacting with elements while the main loop is recursing.

2. Avoid main loop recursion in situations where you can’t handle arbitrary callbacks. Instead, structure your code so that you simply return to the main loop and then get called again when there is more work to do.

Returns

The main loop recursion level in the current thread