GTK implementation of MessageBox

Hmm, ok. I'd suggest code like this, then:

typedef struct {    int type;    int result;} DialogData;    static gbooleandisplay_dialog(gpointer user_data){    DialogData *dialog_data = user_data;    GtkWidget *dialog;        if (dialog_data->type & MB_YESNO)        dialog = gtk_message_dialog_new(...);    else        dialog = gtk_message_dialog_new(...);        // Set title, etc.        dialog_data->result = gtk_dialog_run(...);        gtk_main_quit();  // Quits the main loop run in MessageBox()        return FALSE;}    int MessageBox(...){    DialogData dialog_data;        dialog_data.type = type;        gtk_idle_add(display_dialog, &dialog_data);        gtk_main();        // Do stuff based on dialog_data.result}

The struct is required because you need to pass around a couple pieces of data. The gtk_idle_add() call adds a method to be run when the main loop is running and idle, and the FALSE return value from the display_dialog() call means that it's only run once. After we get the result from the dialog, we quit the main loop. That'll cause the gtk_main() in your main MessageBox() method to return, and you'll be able to access the result from there.

To manage a dialog box with GTK+, use a GtkDialog and gtk_dialog_run() instead of managing a window and a main loop by yourself.

EDIT / ADDENDUM :

What I mean is "just use" : I don't understand why you create a windows you never use and a main loop which seems useless (at least from the piece of code you posted). You can write something as short as :

int MessageBox(HWND hwnd, const char* text, const char* caption, UINT type){    GtkWidget *dialog ;    /* Instead of 0, use GTK_DIALOG_MODAL to get a modal dialog box */    if (type & MB_YESNO)        dialog = gtk_message_dialog_new(NULL, 0, GTK_MESSAGE_QUESTION, GTK_BUTTONS_YES_NO, text );    else        dialog = gtk_message_dialog_new(NULL, 0, GTK_MESSAGE_INFO, GTK_BUTTONS_OK, text );    gtk_window_set_title(GTK_WINDOW(dialog), caption);    gint result = gtk_dialog_run(GTK_DIALOG(dialog));    gtk_widget_destroy( GTK_WIDGET(dialog) );    if (type & MB_YESNO)    {        switch (result)        {        default:        case GTK_RESPONSE_DELETE_EVENT:        case GTK_RESPONSE_NO:            return IDNO;        case GTK_RESPONSE_YES:            return IDYES;        }        return IDOK;    } }

A few things:

You are creating (and not using) an unnecessary toplevel window, named window. You can just delete these lines:

window = gtk_window_new(GTK_WINDOW_TOPLEVEL);g_signal_connect(G_OBJECT(window), "delete_event", G_CALLBACK(delete_event), NULL);g_signal_connect(G_OBJECT(window), "destroy", G_CALLBACK(destroy), NULL);

Also, the flow doesn't seem quite right. gtk_main() starts the GTK main loop, which blocks until something exits it. gtk_dialog_run() also starts a main loop, but it exits as soon as one of the buttons is clicked.

I think it might be enough for you to remove the gtk_init_add() and gtk_main() calls, and simply deal with the return value. Also the gtk_widget_destroy() call is unnecessary, as the dialog window is automatically destroyed when gtk_dialog_run() returns.