#!/usr/bin/stap

global var, varerr

probe $1 {
  t=tid() # or t=0 for thread-agnostic checking
  if (@defined($2)) {
     try {
         newvar = $2;
         if (var[t] != newvar) {
            printf("%s %s changed in thread %d from %s to %s\n", pp(), @2, t, sprint(var[t]), sprint(newvar));
            var[t] = newvar;
         }
     } catch { varerr ++ }  # error during $2 resolution or perhaps var[] assignment
  }
}

probe kprocess.release { # if using per-thread checking
  delete var[$p->pid] # thread
}

probe never {
  var[0]=""  # assigns a type to var in case no probes match $1 above
}

probe error,end {
  if (varerr) printf("%s %s access errors: %d", @1, @2, varerr);
}