How to dispaly an Widget during callback processing

["gtk rapp-informatik de" <gtk rapp-informatik de>]

In my callback have a few longer processes
Is ist possible in GTK 1.2 to show a  an information message in a widget 
like   "Please wait I am working"
I am searching way to force to call gtk_main to show this info widget.

I there any call to gtk_main to do this.
Thanks for help
günther
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