How do I add to a GLib.List from different Thread in Vala

Question

I have a GLib.List to which I want to add elements.
I want to add those elements concurrently using multiple GLib.Threads

I'm trying to synchronize access to the list using a GLib.Mutex. Synchronization seems to work but no elements are added.

public static void main() {
    var list = new GLib.List<string>();
    var mutex = GLib.Mutex();

    var workerA = new Worker("A", list, mutex);
    var workerB = new Worker("B", list, mutex);
    var workerC = new Worker("C", list, mutex);

    GLib.Thread<void*> tA = new GLib.Thread<void*>("WorkerThread", workerA.run);
    GLib.Thread<void*> tB = new GLib.Thread<void*>("WorkerThread", workerB.run);
    GLib.Thread<void*> tC = new GLib.Thread<void*>("WorkerThread", workerC.run);

    tA.join();
    tB.join();
    tC.join();

    stdout.printf("List:\n");
    foreach (string str in list) {
        stdout.printf(" - %s\n", str);
    }
}

class Worker : GLib.Object {
    private string name;
    private weak GLib.List<string> list;
    private weak GLib.Mutex mutex;

    public Worker(string name, GLib.List<string> list, GLib.Mutex mutex) {
        this.name = name;
        this.list = list;
        this.mutex = mutex;
    }

    public void* run() {
        mutex.lock();
        list.append(name);
        mutex.unlock();

        return null;
    }
}

When I look at the synchronization part it seems to work right (even with many more Threads), but no elements get added to the list!

Output:

List:

Can somebody please tell me how to do this ?

Answer 1

Thanks to apmasell pointing out the thing not working is actually GLib.List I took a look at the C source code.

He's right: The append method modifies the pointer - but only (!) if the GLib.List is empty!
So Apart from making the list a global variable or using another list implementation I think the best work workaround is to simply add one element before passing the list to a thread.
After all threads are done you can simply remove the element again.

Answer 2

GLib.List is a little weird. The append method actually modifies the pointer list, not the thing it is pointing to. If you want this to work you need to either:

1. Put the list in a shared place (e.g., make it a field of a class that all the threads share or a global variable).
2. Use Gee.List from the libgee package instead. In general, the data structures in libgee are much easier to use in Vala than their counter parts in glib.