Full working example from "Message Passing in C."

main.c

#include <glib.h>
#include <gtk/gtk.h>
#include <libsoup/soup.h>

#define PORT 42025


/* --- App --- */

typedef struct {
    GtkWidget *window;
    SoupServer *server;
    GAsyncQueue *channel;
} App;

/* --- Message --- */

typedef enum {
    MESSAGE_CHANGE_TITLE
} message_type_t;

typedef struct {
    message_type_t type;
    gpointer data;
} Message;

void free_message(Message *msg)
{
    if (msg->data)
        g_free(msg->data);

    g_free(msg);
}

/* --- Routes --- */

static void route_change_title(SoupServer *server,
                            SoupMessage *soup_msg,
                            const char *path,
                            GHashTable *query,
                            SoupClientContext *client,
                            gpointer user_data)
{
    App *app = (App*)user_data;

    char *new_title = g_strdup(path + 15);

    Message *msg = g_new(Message, 1);
    msg->type = MESSAGE_CHANGE_TITLE;
    msg->data = new_title;
    g_async_queue_push(app->channel, msg);

    soup_message_set_status(soup_msg, SOUP_STATUS_OK);
}

/* --- Idle --- */

static gboolean on_idle(gpointer user_data)
{
    App *app = (App*)user_data;
    Message *msg;

    if ((msg = g_async_queue_try_pop(app->channel)) != NULL) {
        switch (msg->type) {
            case MESSAGE_CHANGE_TITLE:
                gtk_window_set_title(GTK_WINDOW(app->window), (char*)msg->data);
                break;
        }
        free_message(msg);
    }

    return TRUE;
}

/* --- Server --- */

static gpointer server_main(gpointer user_data)
{
    App *app = (App*)user_data;
    GMainContext *context = g_main_context_new();

    app->server = soup_server_new(
            SOUP_SERVER_PORT, PORT,
            SOUP_SERVER_ASYNC_CONTEXT, context,
            NULL
    );

    if (!app->server)
        return NULL;

    g_async_queue_ref(app->channel);

    soup_server_add_handler(app->server, "/gui/set-title/", route_change_title, app, NULL);

    g_print("listening on port %d\n", soup_server_get_port(app->server));
    soup_server_run(app->server);

    g_print("shutting down server\n");
    g_async_queue_unref(app->channel);

    return NULL;
}

/* --- GUI --- */

static void gui_main(int *argc, char **argv[], App *app)
{
    gtk_init(argc, argv);

    app->window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
    g_signal_connect(app->window, "destroy", gtk_main_quit, NULL);
    gtk_widget_show_all(app->window);

    gtk_main();
}

/* --- Main --- */

int main(int argc, char *argv[])
{
    App *app = g_new(App, 1);
    app->channel = g_async_queue_new();
    g_async_queue_ref(app->channel);

    /* Start the server in a new thread */
    GThread *server_thread = g_thread_new("server", server_main, app);

    /* Idly listen for server requests */
    g_idle_add(on_idle, app);

    /* Start the GUI (needs to be done in the main thread) */
    gui_main(&argc, &argv, app);

    soup_server_quit(app->server);
    g_thread_join(server_thread);

    g_async_queue_unref(app->channel);
    g_free(app);

    return 0;
}