Executing Gtk.ListBox.insert and Gtk.ListBox.append in another thread causes Vala program to segfault

Question

I am writing a program in Vala that uses Gtk.ListBox. And I have a function in it which uses insert() in it. And at some point (with different number of children, but eventually it happens) the program terminates with segmentation fault. This happens exactly when insert() function is executed, I tried to add some print() functions to understand what causes the segmentation fault. Also I tried to replace insert() with append() (I know they are different functions, I just wanted to check if the program will segfault) and the result is the same.

Here is the function I'm using:

public new void prepend(MediaInfo post)
{
    Gtk.Separator separator = new Gtk.Separator (Gtk.Orientation.HORIZONTAL);
    base.insert (separator, (int) this.get_children().length () - 1);
    PostBox box = new PostBox(post);
    base.insert (box, (int) this.get_children().length () - 1);
    boxes.append(box);      
}

(PostBox is a class inherited from Gtk.EventBox, MediaInfo is the class I'm using inside my program, it contains some information that is used inside PostBox constructor and boxes is a List of PostBox objects).

I am using this function in a for loop, so it adds not one box, but many, and at some point the program segfaults.

It seems to be not related to the MediaInfo class or PostBox class that I'm using, because a few times it happened when a Gtk.Separator was added.

Can you suggest me what is wrong with my code or or something else? I'm completely out of ideas how to deal with it.

UPD: Apparently this has something to do with threads, because I am using threads in my program and after replacing all threads creating with just function calls the problem was gone.

Answer 1

All calls to GDK and GTK+ should be made from the main thread. This is quite easy to do with Vala by using GLib.Idle.add, especially if you keep in mind that Vala supports closures:

// Do stuff in your thread (CPU-intensive processing, I/O, etc.)
var value = blocking_operation ();

GLib.Idle.add (() => {
    // You can access local variables, like value, here.
    widget.set_something (value);

    // Returning false means the idle callback will be removed automatically
    // after it is called (instead of being called repeatedly).
    return false;
  });

Basically, what you want to do is run your tasks in the other thread(s), and when they are complete switch back to the main thread (using an idle callback) to modify the GTK+ stuff.

That said, I think you can safely modify widgets which aren't part of any hierarchy GTK+ is drawing yet. So you could instantiate a container, add a bunch of children, etc., in your thread then just add it to your window in the main thread. Just don't change anything which GTK+ may currently be trying to draw.