(update from mghello --> mgsimple)
"Executive summary" from the file below:
"This example creates a new Tk_Main and Tcl_AppInit. This example strips lots of extra code out of Tk_Main, and allows a cleaner startup of your own C + Tk application. This example will compile for Tk4.1/Tcl7.5 and Tk8.0+/Tcl8.0+.
This example illustrates a method of creating a GUI with Tk that will work with your C application. It illustrates the sharing of variables between C and Tk, and a method of registering your own C commands to call from Tk."
Go here: http://ftp.se.kde.org/pub/lang/tcl/sorted/packages-7.6/apps/mgsimple/mgsimple.README
[that URL is no longer accurage...]