2 files changed, 34 insertions, 0 deletions
diff --git a/testsuite/systemtap.examples/general/varwatch.meta b/testsuite/systemtap.examples/general/varwatch.meta
new file mode 100644
index 00000000..3d876bb1
--- /dev/null
+++ b/testsuite/systemtap.examples/general/varwatch.meta
@@ -0,0 +1,6 @@
+title: Watch a variable changing value in a thread.
+name: varwatch.stp
+keywords: monitoring
+description: This script places a set of probes (specified by $1), each of which monitors the state of some context $variable expression (specified by $2).  Whenever the value changes, with respect to the active thread, the event is traced.
+test_check: stap -wp4 varwatch.stp 'kernel.statement("do_sys_open@fs/open.c:*")' '$$vars'
+test_installcheck: stap -w varwatch.stp 'kernel.statement("do_sys_open@fs/open.c:*")' '$$vars' -c "sleep 0.2"
diff --git a/testsuite/systemtap.examples/general/varwatch.stp b/testsuite/systemtap.examples/general/varwatch.stp
new file mode 100644
index 00000000..845ca166
--- /dev/null
+++ b/testsuite/systemtap.examples/general/varwatch.stp
@@ -0,0 +1,28 @@
+#! /usr/bin/env 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);
+}