Threads and idle functions

[David Buchan <pdbuchan yahoo com>]

My understanding is that child threads must never alter the UI in any way.

If I have a program which spawns a child thread to download some data and I want to be able to have a dialog 
pop up should an error occur, is it correct to say that I need an idle function to be running concurrently to 
monitor some global variable which would contain the status (set by the download thread), and then the idle 
function would create the dialog pop-up?

Put another way, if only the GTK+ main iteration is allowed to alter the GUI, then how does someone get 
information out of a child thread and to the UI?

Dave