Re: Multithreaded application freezing

On Mon, Feb 2, 2015 at 5:07 PM, G Hasse <gorhas raditex nu> wrote:
Den 2015-02-02 22:57, Gulshan Singh skrev:

I strongly advice designing an application in this way.  Every window in an application should be a separate process. Between processes you should
find a good protocol. This protocol should be transported over some message buss. (example:

This will break under Wayland, which has a strong tie for one socket connection = one client. Threads can be used successfully, but they require care and expertise. The recommended approach is to use worker threads which share as little as possible with the main thread.

Regardless, this really should be using more event-based programming than threads for this sort of communication.


I've made a simplified example of this and asked it on StackOverflow:

Any help is appreciated.
On Fri Jan 30 2015 at 1:12:55 PM Gulshan Singh <gsingh2011 gmail com> wrote:
I'm working on a display manager here: (make sure you're on the `tutorial` branch, not `master`).

When a user successfully logs in, I fork a new process that starts the window manager and wait for that process to terminate. Since this is a long running operation, I need to do this in a new thread so I don't block the GTK thread.

However, I need to do various GTK operations during this long running thread, such as updating label text or hiding the window. I've read online that I should be using `gdk_threads_add_idle` to do this.

Here are the relevant two functions:
static void* login_func(void *data) {
    GtkWidget *widget = GTK_WIDGET(data);
    const gchar *username = gtk_entry_get_text(user_text_field);
    const gchar *password = gtk_entry_get_text(pass_text_field);

    gdk_threads_add_idle(set_status_label_text, "Logging in...");
    pid_t child_pid;
    if (login(username, password, &child_pid)) {
        gdk_threads_add_idle(hide_widget, widget);

        // Wait for child process to finish (wait for logout)
        int status;
        waitpid(child_pid, &status, 0);
        gdk_threads_add_idle(show_widget, widget);

        gdk_threads_add_idle(set_status_label_text, "");

    } else {
        gdk_threads_add_idle(set_status_label_text, "Login error");
    gdk_threads_add_idle(set_password_entry_text, "");

    return NULL;

static gboolean key_event(GtkWidget *widget, GdkEventKey *event) {
    if (event->keyval == ENTER_KEY) {
        pthread_create(&login_thread, NULL, login_func, (void*) widget);
    } else if (event->keyval == ESC_KEY) {
    return FALSE;
The problem is when the process I forked ends (the fork happens in the `login` function, I see my display manager screen again but I can no longer type in any of the text boxes.

If there isn't anything obvious wrong in the above code, I'd appreciate it if someone could run the display manager and take a look at what's wrong. You can run it with Xephyr. I start Xephyr with the command `Xephyr -ac -br -noreset -screen 800x600 :2 &` and then I run `DISPLAY=:2 ./display-manager`. Note that when you log in, it executes the contents of ~/.xinitrc. In my case, I have that set to `exec awesome`, and everything works fine.

Any help would be appreciated.

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