When using Gnocl widgets only a single callback script is permitted. This differs somewhat to the underlying Gtk bindings api which allows for a chain of callback functions to be defined. Rather than trying to produce a convoluted means of setting complex callbacks in C, simple changes can be made during application runtime because Tcl, as a dynamic language, allows everything to be reconfigurable. Using a simple procedure which takes three arguments: the name of the procedure to be modified, the script modifications and a flag specifying append or prepend, the following proof of concept script can created. # !/bin/sh # the next line restarts using tclsh \ exec tclsh "$0" "$@" proc p1 {str} { puts $str } proc procModify {proc script {mode -append} } { set args [info args $proc] set body [string trim [info body $proc]] set box if { $mode == "-prepend" } { set body $script\n$body } else { set body $body\n$scrip