Gtk ProgressBar problem

[christophe meyer <christophe data-tools com>]

Hi all,
In my glade designed project i need to update a progressbar when a callback 
function is called like in this short example:

void on_button_clicked (GtkButton * button, gpointer user_data)
{
   GtkWidget *Progress_Bar = lookup_widget (GTK_WIDGET (button), 
"progressbar");

   action_function1();
   gtk_progress_bar_set_fraction (GTK_PROGRESS_BAR (Progress_Bar), 0.2);
   action_function2();
   gtk_progress_bar_set_fraction (GTK_PROGRESS_BAR (Progress_Bar), 0.4);
   action_function3();
   gtk_progress_bar_set_fraction (GTK_PROGRESS_BAR (Progress_Bar), 0.6);
   action_function4();
   gtk_progress_bar_set_fraction (GTK_PROGRESS_BAR (Progress_Bar), 0.8);
   action_function5();
   gtk_progress_bar_set_fraction (GTK_PROGRESS_BAR (Progress_Bar), 1.0);
}

Each action_function() takes 2 or 3 seconds to finish.
The matter is that when the callback function is called, nothing happens 
between the action_functions() with the bar, and at the end, once 
action_function5() is finished the progressbar updates directly to 100%.
Home can i gradualy update my progressbar while executing my callback 
function ?

Thanks for your help,
Chris