diff --git a/C/absmi.c b/C/absmi.c
index d1e17ce60..79e455227 100644
--- a/C/absmi.c
+++ b/C/absmi.c
@@ -6313,7 +6313,7 @@ Yap_absmi(int inp)
       saveregs();
       {
 	PredEntry *ap = PredFromDefCode(PREG);
-	WRITE_LOCK(ap->PRWLock);
+ 	WRITE_LOCK(ap->PRWLock);
 #if defined(YAPOR) || defined(THREADS)
       /*
 	we do not lock access to the predicate,
@@ -6369,6 +6369,10 @@ Yap_absmi(int inp)
 	  UNLOCK(pe->PELock);
 	  JMPNext();
 	}
+	if (PP == NULL) {
+	  READ_LOCK(pe->PRWLock);
+	  PP = pe;
+	}
 #endif
  	saveregs();
 	pt0 = Yap_ExpandIndex(pe);
diff --git a/C/alloc.c b/C/alloc.c
index 8ae097ab8..f7ff7ad8c 100644
--- a/C/alloc.c
+++ b/C/alloc.c
@@ -12,7 +12,7 @@
 * Last rev:								 *
 * mods:									 *
 * comments:	allocating space					 *
-* version:$Id: alloc.c,v 1.46 2004-02-19 19:24:44 vsc Exp $		 *
+* version:$Id: alloc.c,v 1.47 2004-02-20 18:56:06 vsc Exp $		 *
 *************************************************************************/
 #ifdef SCCS
 static char SccsId[] = "%W% %G%";
@@ -189,7 +189,7 @@ Yap_InitExStacks(int Trail, int Stack)
 void
 Yap_KillStacks(int wid)
 {
-  ADDR gb = Yap_thread_gl[worker_id].global_base;
+  ADDR gb = Yap_thread_gl[wid].global_base;
   if (gb) {
     free(gb);
     Yap_thread_gl[wid].global_base = NULL;
diff --git a/C/cdmgr.c b/C/cdmgr.c
index 41e6c309e..082b194b2 100644
--- a/C/cdmgr.c
+++ b/C/cdmgr.c
@@ -1130,6 +1130,9 @@ addclause(Term t, yamop *cp, int mode, int mod)
   /* we are redefining a prolog module predicate */
   if (p->ModuleOfPred == PROLOG_MODULE && 
       mod != TermProlog && mod) {
+#if defined(YAPOR) || defined(THREADS)
+    WPP = NULL;
+#endif
     WRITE_UNLOCK(p->PRWLock);
     addcl_permission_error(RepAtom(at), Arity, FALSE);
     return TermNil;
@@ -1236,6 +1239,9 @@ Yap_EraseStaticClause(StaticClause *cl, Term mod) {
     }
   }
   WRITE_LOCK(ap->PRWLock);
+#if defined(YAPOR) || defined(THREADS)
+  WPP = NULL;
+#endif
   if (ap->PredFlags & IndexedPredFlag)
     RemoveIndexation(ap);
   ap->cs.p_code.NOfClauses--;
@@ -1277,6 +1283,9 @@ Yap_EraseStaticClause(StaticClause *cl, Term mod) {
     ap->OpcodeOfPred =
       ap->cs.p_code.TrueCodeOfPred->opc;
   }
+#if defined(YAPOR) || defined(THREADS)
+  WPP = NULL;
+#endif
   WRITE_UNLOCK(ap->PRWLock);
   if (cl->ClFlags & HasBlobsMask || static_in_use(ap,TRUE)) {
     DeadClause *dcl = (DeadClause *)cl;
diff --git a/C/dbase.c b/C/dbase.c
index 9810fffbb..b2bc6b644 100644
--- a/C/dbase.c
+++ b/C/dbase.c
@@ -3850,8 +3850,13 @@ static void
 EraseLogUpdCl(LogUpdClause *clau)
 {
   PredEntry *ap;
-  LOCK(clau->ClLock);
   ap = clau->ClPred;
+#if defined(YAPOR) || defined(THREADS)
+  if (WPP != ap) {
+    WRITE_LOCK(ap->PRWLock);
+  }
+#endif
+  LOCK(clau->ClLock);
   /* no need to erase what has been erased */ 
   if (!(clau->ClFlags & ErasedMask)) {
     /* get ourselves out of the list */
@@ -3865,7 +3870,7 @@ EraseLogUpdCl(LogUpdClause *clau)
       clau->ClPrev->ClNext = clau->ClNext;
       UNLOCK(clau->ClPrev->ClLock);
     }
-    WRITE_LOCK(ap->PRWLock);
+    UNLOCK(clau->ClLock);
     if (clau->ClCode == ap->cs.p_code.FirstClause) {
       if (clau->ClNext == NULL) {
 	ap->cs.p_code.FirstClause = NULL;
@@ -3881,7 +3886,6 @@ EraseLogUpdCl(LogUpdClause *clau)
       }
     }
     ap->cs.p_code.NOfClauses--;
-    WRITE_UNLOCK(ap->PRWLock);
     clau->ClFlags |= ErasedMask;
 #ifdef DEBUG
     {
@@ -3906,6 +3910,11 @@ EraseLogUpdCl(LogUpdClause *clau)
   }
   UNLOCK(clau->ClLock);
   complete_lu_erase(clau);
+#if defined(YAPOR) || defined(THREADS)
+    if (WPP != ap) {
+      WRITE_UNLOCK(ap->PRWLock);
+    }
+#endif
 }
 
 static void
diff --git a/C/exec.c b/C/exec.c
index 12a37f579..1ac5481be 100644
--- a/C/exec.c
+++ b/C/exec.c
@@ -1162,6 +1162,9 @@ do_goal(yamop *CodeAdr, int arity, CELL *pt, int top)
 #endif /* DEPTH_LIMIT */
   YENV = ASP = (CELL *)B;
   HB = H;
+#if defined(YAPOR) || defined(THREADS)
+  WPP = NULL;
+#endif
   YENV[E_CB] = Unsigned (B);
   P = (yamop *) CodeAdr;
   CP = YESCODE;
diff --git a/C/sysbits.c b/C/sysbits.c
index 4a1bd2c02..1d9224f01 100644
--- a/C/sysbits.c
+++ b/C/sysbits.c
@@ -1326,7 +1326,7 @@ InitSignals (void)
 #ifndef MPW
   my_signal (SIGFPE, HandleMatherr);
 #endif
-#if HAVE_SIGSEGV
+#if HAVE_SIGSEGV && !defined(THREADS)
   my_signal_info (SIGSEGV, HandleSIGSEGV);
 #endif
 #ifdef ACOW
diff --git a/C/tracer.c b/C/tracer.c
index 9aaa282e1..90a5d78b4 100644
--- a/C/tracer.c
+++ b/C/tracer.c
@@ -116,9 +116,6 @@ low_level_trace(yap_low_level_port port, PredEntry *pred, CELL *args)
 
   LOCK(heap_regs->low_level_trace_lock);
   vsc_count++;
-  if (vsc_count == 52404) vsc_xstop = 1;
-  UNLOCK(heap_regs->low_level_trace_lock);
-  return;
 #ifdef COMMENTED
   //  if (vsc_count == 218280)
   //    vsc_xstop = 1;
diff --git a/m4/Yap.h.m4 b/m4/Yap.h.m4
index 2559770bc..155f0c9bf 100644
--- a/m4/Yap.h.m4
+++ b/m4/Yap.h.m4
@@ -10,7 +10,7 @@
 * File:		Yap.h.m4						 *
 * mods:									 *
 * comments:	main header file for YAP				 *
-* version:      $Id: Yap.h.m4,v 1.53 2004-02-13 18:39:29 vsc Exp $	 *
+* version:      $Id: Yap.h.m4,v 1.54 2004-02-20 18:56:07 vsc Exp $	 *
 *************************************************************************/
 
 #include "config.h"
@@ -333,6 +333,11 @@ typedef CELL Term;
 #if !defined(YAPOR) && !defined(THREADS)
 #include <nolocks.h>
 #elif USE_PTHREAD_LOCKING
+
+#ifndef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 600
+#endif
+
 typedef pthread_mutex_t lockvar;
 typedef pthread_rwlock_t rwlock_t;
 #include <pthread_locks.h>
@@ -349,6 +354,11 @@ typedef volatile int lockvar;
 typedef volatile int lockvar;
 #include <alpha_locks.h>
 #else
+
+#ifndef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 600
+#endif
+
 typedef pthread_mutex_t lockvar;
 typedef pthread_rwlock_t rwlock_t;
 #include <pthread_locks.h>