diff --git a/mps/code/lii4gc.gmk b/mps/code/lii4gc.gmk
index e51cc22a30d..2eceecb9827 100644
--- a/mps/code/lii4gc.gmk
+++ b/mps/code/lii4gc.gmk
@@ -8,7 +8,9 @@ PFM = lii4gc
 THREADSRC = lockli.c thlii4.c pthrdext.c
 THREADLIB = -lpthread
 
-PFMDEFS = -D_REENTRANT
+# _XOPEN_SOURCE is to get the modern POSIX signal handling
+# _GNU_SOURCE is to get register numbers in prmci3li.c
+PFMDEFS = -D_REENTRANT -D_XOPEN_SOURCE=500 -D_GNU_SOURCE
 
 MPMPF = ${THREADSRC} vmix.c \
         protix.c protlii3.c proti3.c prmci3li.c ssixi3.c span.c
@@ -23,7 +25,7 @@ CC = cc
 # Suppress some warnings (SuSE).
 #   .void: -Wpointer-arith cannot be used because the string.h header does
 #   arithmetic on void*.
-CFLAGSCOMPILER := $(subst -Wpointer-arith,,$(CFLAGSCOMPILER))
+CFLAGSCOMPILER := -pthread $(subst -Wpointer-arith,,$(CFLAGSCOMPILER))
 
 include comm.gmk
 
diff --git a/mps/code/prmci3li.c b/mps/code/prmci3li.c
index eb22ff78aa6..27d3ea9c565 100644
--- a/mps/code/prmci3li.c
+++ b/mps/code/prmci3li.c
@@ -21,8 +21,7 @@
  * storing into an MRef pointer.
  */
 
-/* prmcli.h will include mpm.h after defining open sesame magic */
-#include "prmcli.h"
+#include "prmcix.h"
 #include "prmci3.h"
 
 SRCID(prmci3li, "$Id$");
@@ -30,26 +29,26 @@ SRCID(prmci3li, "$Id$");
 
 /* Prmci3AddressHoldingReg -- return an address of a register in a context */
 
-MRef Prmci3AddressHoldingReg(MutatorFaultContext context, unsigned int regnum)
+MRef Prmci3AddressHoldingReg(MutatorFaultContext mfc, unsigned int regnum)
 {
-  struct sigcontext *scp;
+  gregset_t *gregs;
 
   AVER(regnum <= 7);
   AVER(regnum >= 0);
 
-  scp = context->scp;
+  gregs = &mfc->ucontext->uc_mcontext.gregs;
 
   /* .source.i486 */
   /* .assume.regref */
   switch (regnum) {
-    case 0: return (MRef)&scp->eax;
-    case 1: return (MRef)&scp->ecx;
-    case 2: return (MRef)&scp->edx;
-    case 3: return (MRef)&scp->ebx;
-    case 4: return (MRef)&scp->esp;
-    case 5: return (MRef)&scp->ebp;
-    case 6: return (MRef)&scp->esi;
-    case 7: return (MRef)&scp->edi;
+    case 0: return (MRef)&gregs[REG_EAX];
+    case 1: return (MRef)&gregs[REG_ECX];
+    case 2: return (MRef)&gregs[REG_EDX];
+    case 3: return (MRef)&gregs[REG_EBX];
+    case 4: return (MRef)&gregs[REG_ESP];
+    case 5: return (MRef)&gregs[REG_EBP];
+    case 6: return (MRef)&gregs[REG_ESI];
+    case 7: return (MRef)&gregs[REG_EDI];
   }
   NOTREACHED;
   return (MRef)NULL;  /* Avoids compiler warning. */
@@ -60,29 +59,19 @@ MRef Prmci3AddressHoldingReg(MutatorFaultContext context, unsigned int regnum)
 
 void Prmci3DecodeFaultContext(MRef *faultmemReturn,
                               Byte **insvecReturn,
-                              MutatorFaultContext context)
+                              MutatorFaultContext mfc)
 {
-  struct sigcontext *scp;
-
-  scp = context->scp;
-
-  /* Assert that this is a page fault exception. The computation of */
-  /* faultmem depends on this.  See .source.i486 (9.9.14). */
-  AVER(scp->trapno == 14);
-
-  /* cr2 contains the address which caused the fault. */
-  /* See .source.i486 (9.9.14) and */
   /* .source.linux.kernel (linux/arch/i386/mm/fault.c). */
-  *faultmemReturn = (MRef)scp->cr2;
-  *insvecReturn = (Byte*)scp->eip;
+  *faultmemReturn = (MRef)mfc->info->si_addr;
+  *insvecReturn = (Byte*)mfc->ucontext->uc_mcontext.gregs[REG_EIP];
 }
 
 
 /* Prmci3StepOverIns -- modify context to step over instruction */
 
-void Prmci3StepOverIns(MutatorFaultContext context, Size inslen)
+void Prmci3StepOverIns(MutatorFaultContext mfc, Size inslen)
 {
-  context->scp->eip += (unsigned long)inslen;
+  mfc->ucontext->uc_mcontext.gregs[REG_EIP] += (unsigned long)inslen;
 }
 
 
diff --git a/mps/code/prmcfr.h b/mps/code/prmcix.h
similarity index 95%
rename from mps/code/prmcfr.h
rename to mps/code/prmcix.h
index 5514b683aca..a5781118607 100644
--- a/mps/code/prmcfr.h
+++ b/mps/code/prmcix.h
@@ -1,4 +1,4 @@
-/* prmcfr.h:  PROTECTION MUTATOR CONTEXT (FREEBSD)
+/* prmcix.h:  PROTECTION MUTATOR CONTEXT (UNIX)
  *
  * $Id$
  * Copyright (c) 2001 Ravenbrook Limited.  See end of file for license.
@@ -6,19 +6,20 @@
  * .readership: MPS developers.
  */
 
-#ifndef prmcfr_h
-#define prmcfr_h
+#ifndef prmcix_h
+#define prmcix_h
 
 #include "mpm.h"
 
 #include <ucontext.h>
 
 typedef struct MutatorFaultContextStruct { /* Protection fault context data */
+  siginfo_t *info;
   ucontext_t *ucontext;
 } MutatorFaultContextStruct;
 
 
-#endif /* prmcfr_h */
+#endif /* prmcix_h */
 
 
 /* C. COPYRIGHT AND LICENSE
diff --git a/mps/code/prmcli.h b/mps/code/prmcli.h
deleted file mode 100644
index 0defd356b68..00000000000
--- a/mps/code/prmcli.h
+++ /dev/null
@@ -1,68 +0,0 @@
-/* prmcli.h:  PROTECTION MUTATOR CONTEXT (Linux)
- *
- * $Id$
- * Copyright (c) 2001 Ravenbrook Limited.  See end of file for license.
- *
- * .readership: MPS developers.
- */
-
-#ifndef prmcli_h
-#define prmcli_h
-
-
-/* open sesame magic */
-#define _BSD_SOURCE 1
-#define _XOPEN_SOURCE 500
-
-#include "mpm.h"
-
-#include <signal.h>
-
-typedef struct MutatorFaultContextStruct { /* Protection fault context data */
-  struct sigcontext *scp;                   /* Linux sigcontext */
-} MutatorFaultContextStruct;
-
-
-#endif /* prmcli_h */
-
-
-/* C. COPYRIGHT AND LICENSE
- *
- * Copyright (C) 2001-2002 Ravenbrook Limited <http://www.ravenbrook.com/>.
- * All rights reserved.  This is an open source license.  Contact
- * Ravenbrook for commercial licensing options.
- * 
- * Redistribution and use in source and binary forms, with or without
- * modification, are permitted provided that the following conditions are
- * met:
- * 
- * 1. Redistributions of source code must retain the above copyright
- * notice, this list of conditions and the following disclaimer.
- * 
- * 2. Redistributions in binary form must reproduce the above copyright
- * notice, this list of conditions and the following disclaimer in the
- * documentation and/or other materials provided with the distribution.
- * 
- * 3. Redistributions in any form must be accompanied by information on how
- * to obtain complete source code for this software and any accompanying
- * software that uses this software.  The source code must either be
- * included in the distribution or be available for no more than the cost
- * of distribution plus a nominal fee, and must be freely redistributable
- * under reasonable conditions.  For an executable file, complete source
- * code means the source code for all modules it contains. It does not
- * include source code for modules or files that typically accompany the
- * major components of the operating system on which the executable file
- * runs.
- * 
- * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
- * IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
- * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
- * PURPOSE, OR NON-INFRINGEMENT, ARE DISCLAIMED. IN NO EVENT SHALL THE
- * COPYRIGHT HOLDERS AND CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
- * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
- * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
- * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
- * ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
- * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
- * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
- */
diff --git a/mps/code/protlii3.c b/mps/code/protlii3.c
index 4984230acaa..b6067a19114 100644
--- a/mps/code/protlii3.c
+++ b/mps/code/protlii3.c
@@ -11,7 +11,7 @@
  * .source.linux.kernel: Linux kernel source files.
  */
 
-#include "prmcli.h"
+#include "prmcix.h"
 
 #ifndef MPS_OS_LI
 #error "protlii3.c is Linux-specific, but MPS_OS_LI is not set"
@@ -27,20 +27,11 @@
 #include <stddef.h>
 #include <stdlib.h>
 #include <signal.h>
+#include <unistd.h>
 
 SRCID(protlii3, "$Id$");
 
 
-/* Useful stuff that doesn't appear to be in any header files. */
-
-/* Interrupt number 14 is Page Fault. */
-#define TRAPNO_PAGE_FAULT 14
-
-/* Bits in err field of sigcontext for interrupt 14 (page fault) */
-#define PAGE_FAULT_ERR_PAGEPROT   0x1
-#define PAGE_FAULT_ERR_WRITE      0x2
-#define PAGE_FAULT_ERR_USERMODE   0x4
-
 
 /* The previously-installed signal action, as returned by */
 /* sigaction(3).  See ProtSetup. */
@@ -62,39 +53,44 @@ typedef void (*__real_lii3_sighandler_t)(int, struct sigcontext);
  *  then sigHandle does its best to pass the signal on to the
  *  previously installed signal handler (sigNext).
  *
- *  .sigh.args: There is no officially documented way of getting the
- *  sigcontext, but on x86 Linux at least it is passed BY VALUE as a
- *  second argument to the signal handler.  The prototype doesn't
- *  include this arg.
- *  See .source.linux.kernel (linux/arch/i386/kernel/signal.c).
- *
- *  .sigh.context: We only know how to handle interrupt 14, where
- *  context.err gives the page fault error code and context.cr2 gives
- *  the fault address.  See .source.i486 (9.9.14) and
+ *  .sigh.context: We check si_code for being a memory access
+ *  si_addr gives the fault address.  See 
  *  .source.linux.kernel (linux/arch/i386/mm/fault.c).
  *
  *  .sigh.addr: We assume that the OS decodes the address to something
  *  sensible
  */
+/* This is defined here to keep the sources closer to those in protsgix.c
+ * They can't be merged yet because protsgix doesn't pass the context to
+ * ArenaAccess */
 
-static void sigHandle(int sig, struct sigcontext context)  /* .sigh.args */
+#define PROT_SIGNAL SIGSEGV
+
+static void sigHandle(int sig, siginfo_t *info, void *context)  /* .sigh.args */
 {
-  AVER(sig == SIGSEGV);
+  int e;
+  /* sigset renamed to asigset due to clash with global on Darwin. */
+  sigset_t asigset, oldset;
+  struct sigaction sa;
 
-  if(context.trapno == TRAPNO_PAGE_FAULT) {  /* .sigh.context */
+  AVER(sig == PROT_SIGNAL);
+
+  if(info->si_code == SEGV_ACCERR) {  /* .sigh.context */
     AccessSet mode;
-    Addr base, limit;
+    Addr base;
+    ucontext_t *ucontext;
     MutatorFaultContextStruct mfContext;
 
-    mfContext.scp = &context;
+    ucontext = (ucontext_t *)context;
+    mfContext.ucontext = ucontext;
+    mfContext.info = info;
 
-    mode = ((context.err & PAGE_FAULT_ERR_WRITE) != 0)  /* .sigh.context */
-             ? (AccessREAD | AccessWRITE)
-             : AccessREAD;
+    /* on linux we used to be able to tell whether this was a read or a write */
+    mode = AccessREAD | AccessWRITE;
 
     /* We assume that the access is for one word at the address. */
-    base = (Addr)context.cr2;   /* .sigh.addr */
-    limit = AddrAdd(base, (Size)sizeof(Addr));
+    base = (Addr)info->si_addr;   /* .sigh.addr */
+    /* limit = AddrAdd(base, (Size)sizeof(Addr)); */
 
     /* Offer each protection structure the opportunity to handle the */
     /* exception.  If it succeeds, then allow the mutator to continue. */
@@ -106,21 +102,17 @@ static void sigHandle(int sig, struct sigcontext context)  /* .sigh.args */
   /* The exception was not handled by any known protection structure, */
   /* so throw it to the previously installed handler. */
 
-  /* @@@@ This is really weak. */
-  /* Need to implement rest of the contract of sigaction */
-  /* We might also want to set SA_RESETHAND in the flags and explicitly */
-  /* reinstall the handler from withint itself so the SIG_DFL/SIG_IGN */
-  /* case can work properly by just returning. */
-  switch ((int)sigNext.sa_handler) {
-  case (int)SIG_DFL:
-  case (int)SIG_IGN:
-    abort();
-    NOTREACHED;
-    break;
-  default:
-    (*(__real_lii3_sighandler_t)sigNext.sa_handler)(sig, context);
-    break;
-  }
+  e = sigaction(PROT_SIGNAL, &sigNext, &sa);
+  AVER(e == 0);
+  sigemptyset(&asigset);
+  sigaddset(&asigset, PROT_SIGNAL);
+  e = sigprocmask(SIG_UNBLOCK, &asigset, &oldset);
+  AVER(e == 0);
+  kill(getpid(), PROT_SIGNAL);
+  e = sigprocmask(SIG_SETMASK, &oldset, NULL);
+  AVER(e == 0);
+  e = sigaction(PROT_SIGNAL, &sa, NULL);
+  AVER(e == 0);
 }
 
 
@@ -142,15 +134,14 @@ void ProtSetup(void)
   struct sigaction sa;
   int result;
 
-  sa.sa_handler = (__sighandler_t)sigHandle;  /* .sigh.args */
+  sa.sa_sigaction = sigHandle;
   sigemptyset(&sa.sa_mask);
-  sa.sa_flags = 0;
+  sa.sa_flags = SA_SIGINFO;
 
-  result = sigaction(SIGSEGV, &sa, &sigNext);
+  result = sigaction(PROT_SIGNAL, &sa, &sigNext);
   AVER(result == 0);
 }
 
-
 /* C. COPYRIGHT AND LICENSE
  *
  * Copyright (C) 2001-2002 Ravenbrook Limited <http://www.ravenbrook.com/>.
diff --git a/mps/code/pthrdext.c b/mps/code/pthrdext.c
index ff2986a1b6f..ab50a999ff5 100644
--- a/mps/code/pthrdext.c
+++ b/mps/code/pthrdext.c
@@ -15,12 +15,6 @@
 
 #include "mpm.h"
 
-#if defined(MPS_OS_LI)
-/* open sesame magic */
-#define _BSD_SOURCE 1
-#define _XOPEN_SOURCE 500
-#endif
-
 #include <pthread.h>
 #include <sched.h>
 #include <signal.h>
@@ -79,34 +73,7 @@ static RingStruct suspendedRing;            /* PThreadext suspend ring */
  * suspend protocol.
  */
 
-#if defined(MPS_OS_LI)
-
-#include "prmcli.h"
-
-static void suspendSignalHandler(int sig, struct sigcontext scp)
-{
-    sigset_t signal_set;
-    MutatorFaultContextStruct mfContext;
-
-    AVER(sig == PTHREADEXT_SIGSUSPEND);
-    UNUSED(sig);
-
-    AVER(suspendingVictim != NULL);
-    mfContext.scp = &scp;
-    suspendingVictim->suspendedMFC = &mfContext;
-    /* Block all signals except PTHREADEXT_SIGRESUME while suspended. */
-    sigfillset(&signal_set);
-    sigdelset(&signal_set, PTHREADEXT_SIGRESUME);
-    sem_post(&pthreadextSem);
-    sigsuspend(&signal_set);
-
-    /* Once here, the resume signal handler has run to completion. */
-    return;
-}
-
-#elif defined(MPS_OS_FR)
-
-#include "prmcfr.h"
+#include "prmcix.h"
 
 static void suspendSignalHandler(int sig,
                                  siginfo_t *info,
@@ -135,8 +102,6 @@ static void suspendSignalHandler(int sig,
     return;
 }
 
-#endif
-
 
 /* resumeSignalHandler -- signal handler called when resuming a thread
  *
@@ -184,15 +149,8 @@ static void PThreadextModuleInit(void)
     status = sigaddset(&pthreadext_sigsuspend.sa_mask, PTHREADEXT_SIGRESUME);
     AVER(status == 0);
 
-#if defined(MPS_OS_LI)
-    pthreadext_sigsuspend.sa_flags = 0;
-    pthreadext_sigsuspend.sa_handler = (__sighandler_t)suspendSignalHandler;
-
-#elif defined(MPS_OS_FR)
     pthreadext_sigsuspend.sa_flags = SA_SIGINFO;
     pthreadext_sigsuspend.sa_sigaction = suspendSignalHandler;
-#endif
-
     pthreadext_sigresume.sa_flags = 0;
     pthreadext_sigresume.sa_handler = resumeSignalHandler;
     status = sigemptyset(&pthreadext_sigresume.sa_mask);
diff --git a/mps/code/thfri4.c b/mps/code/thfri4.c
index 5c3d70e16c4..8e109d2b788 100644
--- a/mps/code/thfri4.c
+++ b/mps/code/thfri4.c
@@ -36,7 +36,7 @@
  * assumed to be recorded in the context at pointer-aligned boundaries.
  */
 
-#include "prmcfr.h"
+#include "prmcix.h"
 #include "mpm.h"
 
 #if !defined(MPS_OS_FR) || !defined(MPS_ARCH_I4)
diff --git a/mps/code/thlii4.c b/mps/code/thlii4.c
index fbd376c6352..4f96e9c5fb1 100644
--- a/mps/code/thlii4.c
+++ b/mps/code/thlii4.c
@@ -1,4 +1,4 @@
-/* thlii3.c: Threads Manager for Intel x86 systems with LinuxThreads
+/* thlii4.c: Threads Manager for Intel x86 systems with LinuxThreads
  *
  *  $Id$
  *  Copyright (c) 2001 Ravenbrook Limited.  See end of file for license.
@@ -36,7 +36,7 @@
  * assumed to be recorded in the context at pointer-aligned boundaries.
  */
 
-#include "prmcli.h"
+#include "prmcix.h"
 #include "mpm.h"
 
 #if !defined(MPS_OS_LI) || !defined(MPS_ARCH_I4)
@@ -243,18 +243,18 @@ Res ThreadScan(ScanState ss, Thread thread, void *stackBot)
     if(res != ResOK)
       return res;
   } else {
-    struct sigcontext *scp;
+    MutatorFaultContext mfc;
     Addr *stackBase, *stackLimit, stackPtr;
-
-    scp = thread->mfc->scp;
-    if(scp == NULL) {
+    mcontext_t *mc;
+    mfc = thread->mfc;
+    if(mfc == NULL) {
       /* .error.suspend */
       /* We assume that the thread must have been destroyed. */
       /* We ignore the situation by returning immediately. */
       return ResOK;
     }
 
-    stackPtr  = (Addr)scp->esp;   /* .i3.sp */
+    stackPtr  = (Addr)mfc->ucontext->uc_stack.ss_sp;   /* .i3.sp */
     /* .stack.align */
     stackBase  = (Addr *)AddrAlignUp(stackPtr, sizeof(Addr));
     stackLimit = (Addr *)stackBot;
@@ -273,8 +273,9 @@ Res ThreadScan(ScanState ss, Thread thread, void *stackBot)
      * unecessarily scans the rest of the context.  The optimisation
      * to scan only relevent parts would be machine dependent.
      */
-    res = TraceScanAreaTagged(ss, (Addr *)scp,
-           (Addr *)((char *)scp + sizeof(*scp)));
+    mc = &mfc->ucontext->uc_mcontext;
+    res = TraceScanAreaTagged(ss, (Addr *)mc,
+           (Addr *)((char *)mc + sizeof(*mc)));
     if(res != ResOK)
       return res;
   }