How to animate a progressbar?

[jca <jca mail phm vcu edu>]

I'm using a progressbar for the first time, and I've already got

 gtk_progress_set_percentage(progressbar,percentage);
 gtk_widget_show_now(progressbar);

installed in the middle of my time-consuming file read loop...

   But while the loop is running the progress bar never visibly updates
(and "percentage" does change).  How do we force a progressbar to
refresh itself in realtime & onscreen?

  jca