diff -ur common.h common.h
--- common.h	Fri Feb 22 12:55:46 2002
+++ common.h	Fri Jan 21 15:14:51 2005
@@ -190,6 +190,7 @@
   int on_ioq;                 /* Is it on ioq? */
 } _st_pollq_t;
 
+typedef void (*_st_switch_cb_t)(void);
 
 typedef struct _st_vp {
   _st_thread_t *idle_thread;  /* Idle thread for this vp */
@@ -204,6 +205,8 @@
 #endif
   st_utime_t sleep_max;
   int pagesize;
+  _st_switch_cb_t switch_out_cb; /* called when a thread is switched out */
+  _st_switch_cb_t switch_in_cb;  /* called when a thread is switched in */
 
 #ifndef USE_POLL
   int maxfd;
@@ -362,12 +365,22 @@
  * Switch away from the current thread context by saving its state and
  * calling the thread scheduler
  */
-#define _ST_SWITCH_CONTEXT(_thread)       \
-    ST_BEGIN_MACRO                        \
-    if (!MD_SETJMP((_thread)->context)) { \
-      _st_vp_schedule();                  \
-    }                                     \
-    ST_DEBUG_ITERATE_THREADS();           \
+#define _ST_SWITCH_CONTEXT(_thread)                     \
+    ST_BEGIN_MACRO                                      \
+    if (_st_this_vp.switch_out_cb != NULL &&            \
+	_thread != _st_this_vp.idle_thread &&		\
+	_thread->state != _ST_ST_ZOMBIE) {              \
+      _st_this_vp.switch_out_cb();                      \
+    }                                                   \
+    if (!MD_SETJMP((_thread)->context)) {               \
+      _st_vp_schedule();                                \
+    }                                                   \
+    ST_DEBUG_ITERATE_THREADS();                         \
+    if (_st_this_vp.switch_in_cb != NULL &&             \
+	_thread != _st_this_vp.idle_thread &&		\
+	_thread->state != _ST_ST_ZOMBIE) {              \
+      _st_this_vp.switch_in_cb();                       \
+    }                                                   \
     ST_END_MACRO
 
 /*