TCL (Tool Command Language)

Writing TCL Extensions In C

A good description of TCL dual-ported objects and reference counts is here.

Trace

See the trace man page.

The following is a rather basic little example of putting a "trace" on a variable that will be executed everytime a variable is accessed. It doesn't do much other than print to stdout but could be augmented to write to a file or do something more complicated if required. It also only supports reading and writing to a simple variable: arrays are not supported and unsetting the variable is not supported. 

% proc jeh_trace {n1 n2 op} {
   upvar $n1 n1ref
   if {"$op" == "read"} {
      set var_exists [uplevel 1 "info exists $n1"]
      if {$var_exists} {
         puts "Reading $n1 as $n1ref"
      } else {
         puts "Trying to read $n1 but it does not exist"
      }
   } else {
      puts "Setting $n1 to $n1ref"
   }
}

% trace add variable jehtech {read write} jeh_trace

% set jehtech
Trying to read jehtech but it does not exist
can't read "jehtech": no such variable

% set jehtech "jehtech"
Setting jehtech to jehtech
jehtech

% set jehtech
Reading jehtech as jehtech
jehtech