From 5e5ea11658b30feeafb2f5bb075b23e6f8663a4d Mon Sep 17 00:00:00 2001
From: "Jeff Hill johill@lanl.gov" <johill@lanl.gov>
Date: Thu, 16 May 2013 11:17:47 -0600
Subject: [PATCH] fix for https://bugs.launchpad.net/epics-base/+bug/1179642

---
 src/ca/client/CASG.cpp                 | 103 ++++----
 src/ca/client/access.cpp               | 148 ++++++-----
 src/ca/client/ca_client_context.cpp    | 208 ++++++++-------
 src/ca/client/cac.cpp                  | 287 ++++++++++-----------
 src/ca/client/cac.h                    | 196 +++++++--------
 src/ca/client/cacIO.h                  | 127 ++++++----
 src/ca/client/nciu.cpp                 | 173 ++++++-------
 src/ca/client/nciu.h                   | 137 +++++-----
 src/ca/client/oldAccess.h              | 334 ++++++++++++++-----------
 src/ca/client/oldChannelNotify.cpp     | 107 ++++----
 src/ca/client/sgAutoPtr.h              |  23 +-
 src/ca/client/syncGroup.h              | 179 ++++++-------
 src/ca/client/syncGroupReadNotify.cpp  |  57 +++--
 src/ca/client/syncGroupWriteNotify.cpp |  53 ++--
 src/ca/client/syncgrp.cpp              | 137 +++++++---
 src/ioc/db/dbCAC.h                     |  14 +-
 src/ioc/db/dbChannelIO.cpp             |  11 +-
 src/ioc/db/dbChannelIO.h               |   9 +-
 src/ioc/db/dbContext.cpp               |  32 +--
 src/ioc/db/dbPutNotifyBlocker.cpp      |   6 +-
 src/ioc/db/dbPutNotifyBlocker.h        |   4 +-
 src/ioc/db/dbSubscriptionIO.cpp        |   8 +-
 22 files changed, 1287 insertions(+), 1066 deletions(-)

diff --git a/src/ca/client/CASG.cpp b/src/ca/client/CASG.cpp
index 4938d2364..b2dbaac4d 100644
--- a/src/ca/client/CASG.cpp
+++ b/src/ca/client/CASG.cpp
@@ -4,7 +4,7 @@
 * Copyright (c) 2002 The Regents of the University of California, as
 *     Operator of Los Alamos National Laboratory.
 * EPICS BASE is distributed subject to a Software License Agreement found
-* in file LICENSE that is included with this distribution. 
+* in file LICENSE that is included with this distribution.
 \*************************************************************************/
 
 /*
@@ -25,8 +25,6 @@
 #include "cac.h"
 #include "sgAutoPtr.h"
 
-casgRecycle::~casgRecycle () {}
-
 CASG::CASG ( epicsGuard < epicsMutex > & guard, ca_client_context & cacIn ) :
     client ( cacIn ), magic ( CASG_MAGIC )
 {
@@ -37,13 +35,14 @@ CASG::~CASG ()
 {
 }
 
-void CASG::destructor ( 
+void CASG::destructor (
+    CallbackGuard & cbGuard,
     epicsGuard < epicsMutex > & guard )
 {
     guard.assertIdenticalMutex ( this->client.mutexRef() );
 
     if ( this->verify ( guard ) ) {
-        this->reset ( guard );
+        this->reset ( cbGuard, guard );
         this->client.uninstallCASG ( guard, *this );
         this->magic = 0;
     }
@@ -61,9 +60,9 @@ bool CASG::verify ( epicsGuard < epicsMutex > & ) const
 /*
  * CASG::block ()
  */
-int CASG::block ( 
-    epicsGuard < epicsMutex > * pcbGuard, 
-    epicsGuard < epicsMutex > & guard, 
+int CASG::block (
+    epicsGuard < epicsMutex > * pcbGuard,
+    epicsGuard < epicsMutex > & guard,
     double timeout )
 {
     epicsTime cur_time;
@@ -75,7 +74,7 @@ int CASG::block (
     guard.assertIdenticalMutex ( this->client.mutexRef() );
 
     // prevent recursion nightmares by disabling blocking
-    // for IO from within a CA callback. 
+    // for IO from within a CA callback.
     if ( epicsThreadPrivateGet ( caClientCallbackThreadId ) ) {
         return ECA_EVDISALLOW;
     }
@@ -120,44 +119,45 @@ int CASG::block (
         }
 
         /*
-         * force a time update 
+         * force a time update
          */
         cur_time = epicsTime::getCurrent ();
 
         delay = cur_time - beg_time;
     }
 
-    this->reset ( guard );
-
     return status;
 }
 
-void CASG::reset ( 
+void CASG::reset (
+    CallbackGuard & cbGuard,
     epicsGuard < epicsMutex > & guard )
 {
     guard.assertIdenticalMutex ( this->client.mutexRef() );
-    this->destroyCompletedIO ( guard );
-    this->destroyPendingIO ( guard );
+    this->destroyCompletedIO ( cbGuard, guard );
+    this->destroyPendingIO ( cbGuard, guard );
 }
 
 // lock must be applied
-void CASG::destroyCompletedIO ( 
+void CASG::destroyCompletedIO (
+    CallbackGuard & cbGuard,
     epicsGuard < epicsMutex > & guard )
 {
     guard.assertIdenticalMutex ( this->client.mutexRef() );
     syncGroupNotify * pNotify;
     while ( ( pNotify = this->ioCompletedList.get () ) ) {
-        pNotify->destroy ( guard, * this );
+        pNotify->destroy ( cbGuard, guard );
     }
 }
 
-void CASG::destroyPendingIO ( 
+void CASG::destroyPendingIO (
+    CallbackGuard & cbGuard,
     epicsGuard < epicsMutex > & guard )
 {
     guard.assertIdenticalMutex ( this->client.mutexRef() );
     while ( syncGroupNotify * pNotify = this->ioPendingList.first () ) {
-        pNotify->cancel ( guard );
-        // cancel must release the guard while 
+        pNotify->cancel ( cbGuard, guard );
+        // cancel must release the guard while
         // canceling put callbacks so we
         // must double check list membership
         if ( pNotify->ioPending ( guard ) ) {
@@ -166,7 +166,7 @@ void CASG::destroyPendingIO (
         else {
             this->ioCompletedList.remove ( *pNotify );
         }
-        pNotify->destroy ( guard, *this );
+        pNotify->destroy ( cbGuard, guard );
     }
 }
 
@@ -176,7 +176,7 @@ void CASG::show ( unsigned level ) const
     this->show ( guard, level );
 }
 
-void CASG::show ( 
+void CASG::show (
     epicsGuard < epicsMutex > & guard, unsigned level ) const
 {
     guard.assertIdenticalMutex ( this->client.mutexRef() );
@@ -184,14 +184,14 @@ void CASG::show (
         this->getId (), this->magic, this->ioPendingList.count () );
     if ( level ) {
         ::printf ( "\tPending" );
-        tsDLIterConst < syncGroupNotify > notifyPending = 
+        tsDLIterConst < syncGroupNotify > notifyPending =
             this->ioPendingList.firstIter ();
         while ( notifyPending.valid () ) {
             notifyPending->show ( guard, level - 1u );
             notifyPending++;
         }
         ::printf ( "\tCompleted" );
-        tsDLIterConst < syncGroupNotify > notifyCompleted = 
+        tsDLIterConst < syncGroupNotify > notifyCompleted =
             this->ioCompletedList.firstIter ();
         while ( notifyCompleted.valid () ) {
             notifyCompleted->show ( guard, level - 1u );
@@ -201,10 +201,11 @@ void CASG::show (
 }
 
 bool CASG::ioComplete (
+    CallbackGuard & cbGuard,
     epicsGuard < epicsMutex > & guard )
 {
     guard.assertIdenticalMutex ( this->client.mutexRef() );
-    this->destroyCompletedIO ( guard );
+    this->destroyCompletedIO ( cbGuard, guard );
     return this->ioPendingList.count () == 0u;
 }
 
@@ -212,9 +213,9 @@ void CASG::put ( epicsGuard < epicsMutex > & guard, chid pChan,
     unsigned type, arrayElementCount count, const void * pValue )
 {
     guard.assertIdenticalMutex ( this->client.mutexRef() );
-    sgAutoPtr < syncGroupWriteNotify > pNotify ( guard, *this, this->ioPendingList );
+    sgAutoPtr < syncGroupWriteNotify > pNotify ( guard, *this );
     pNotify = syncGroupWriteNotify::factory ( 
-        this->freeListWriteOP, *this, pChan );
+        this->freeListWriteOP, *this, & CASG :: recycleWriteNotifyIO, pChan );
     pNotify->begin ( guard, type, count, pValue );
     pNotify.release ();
 }
@@ -223,14 +224,14 @@ void CASG::get ( epicsGuard < epicsMutex > & guard, chid pChan,
                 unsigned type, arrayElementCount count, void *pValue )
 {
     guard.assertIdenticalMutex ( this->client.mutexRef() );
-    sgAutoPtr < syncGroupReadNotify > pNotify ( guard, *this, this->ioPendingList );
+    sgAutoPtr < syncGroupReadNotify > pNotify ( guard, *this );
     pNotify = syncGroupReadNotify::factory ( 
-        this->freeListReadOP, *this, pChan, pValue );
+        this->freeListReadOP, *this, & CASG :: recycleReadNotifyIO, pChan, pValue );
     pNotify->begin ( guard, type, count );
     pNotify.release ();
 }
 
-void CASG::completionNotify ( 
+void CASG::completionNotify (
     epicsGuard < epicsMutex > & guard, syncGroupNotify & notify )
 {
     guard.assertIdenticalMutex ( this->client.mutexRef() );
@@ -241,56 +242,56 @@ void CASG::completionNotify (
     }
 }
 
-void CASG::recycleSyncGroupWriteNotify ( 
-    epicsGuard < epicsMutex > & guard, syncGroupWriteNotify & io )
-{
-    guard.assertIdenticalMutex ( this->client.mutexRef() );
-    this->freeListWriteOP.release ( & io );
-}
-
-void CASG::recycleSyncGroupReadNotify ( 
-    epicsGuard < epicsMutex > & guard, syncGroupReadNotify & io )
+void CASG :: recycleReadNotifyIO ( epicsGuard < epicsMutex > & guard, 
+                            syncGroupReadNotify & io )
 {
     guard.assertIdenticalMutex ( this->client.mutexRef() );
     this->freeListReadOP.release ( & io );
 }
 
+void CASG :: recycleWriteNotifyIO ( epicsGuard < epicsMutex > & guard, 
+                            syncGroupWriteNotify & io )
+{
+    guard.assertIdenticalMutex ( this->client.mutexRef() );
+    this->freeListWriteOP.release ( & io );
+}
+
 int CASG :: printFormated ( const char *pformat, ... )
 {
     va_list theArgs;
     int status;
 
     va_start ( theArgs, pformat );
-    
+
     status = this->client.varArgsPrintFormated ( pformat, theArgs );
-    
+
     va_end ( theArgs );
-    
+
     return status;
 }
 
-void CASG::exception ( 
-    epicsGuard < epicsMutex > & guard, 
-    int status, const char * pContext, 
+void CASG::exception (
+    epicsGuard < epicsMutex > & guard,
+    int status, const char * pContext,
     const char * pFileName, unsigned lineNo )
 {
     guard.assertIdenticalMutex ( this->client.mutexRef() );
     if ( status != ECA_CHANDESTROY ) {
-        this->client.exception ( 
+        this->client.exception (
             guard, status, pContext, pFileName, lineNo );
     }
 }
 
-void CASG::exception ( 
-    epicsGuard < epicsMutex > & guard, 
+void CASG::exception (
+    epicsGuard < epicsMutex > & guard,
     int status, const char * pContext,
-    const char * pFileName, unsigned lineNo, oldChannelNotify & chan, 
+    const char * pFileName, unsigned lineNo, oldChannelNotify & chan,
     unsigned type, arrayElementCount count, unsigned op )
 {
     guard.assertIdenticalMutex ( this->client.mutexRef() );
     if ( status != ECA_CHANDESTROY ) {
-        this->client.exception ( 
-            guard, status, pContext, pFileName, 
+        this->client.exception (
+            guard, status, pContext, pFileName,
             lineNo, chan, type, count, op );
     }
 }
diff --git a/src/ca/client/access.cpp b/src/ca/client/access.cpp
index 74278acd2..8c1f9901e 100644
--- a/src/ca/client/access.cpp
+++ b/src/ca/client/access.cpp
@@ -5,7 +5,7 @@
 *     Operator of Los Alamos National Laboratory.
 * EPICS BASE Versions 3.13.7
 * and higher are distributed subject to a Software License Agreement found
-* in file LICENSE that is included with this distribution. 
+* in file LICENSE that is included with this distribution.
 \*************************************************************************/
 
 /*
@@ -30,7 +30,7 @@
 
 
 /*
- * allocate error message string array 
+ * allocate error message string array
  * here so I can use sizeof
  */
 #define CA_ERROR_GLBLSOURCE
@@ -182,7 +182,7 @@ int epicsShareAPI ca_task_initialize ( void )
 }
 
 // extern "C"
-int epicsShareAPI ca_context_create ( 
+int epicsShareAPI ca_context_create (
             ca_preemptive_callback_select premptiveCallbackSelect )
 {
     ca_client_context *pcac;
@@ -202,7 +202,7 @@ int epicsShareAPI ca_context_create (
 		    return ECA_NORMAL;
 	    }
 
-        pcac = new ca_client_context ( 
+        pcac = new ca_client_context (
             premptiveCallbackSelect == ca_enable_preemptive_callback );
 	    if ( ! pcac ) {
 		    return ECA_ALLOCMEM;
@@ -275,7 +275,7 @@ int epicsShareAPI ca_task_exit ()
  */
 // extern "C"
 int epicsShareAPI ca_build_and_connect ( const char *name_str, chtype get_type,
-            arrayElementCount get_count, chid * chan, void *pvalue, 
+            arrayElementCount get_count, chid * chan, void *pvalue,
             caCh *conn_func, void *puser )
 {
     if ( get_type != TYPENOTCONN && pvalue != 0 && get_count != 0 ) {
@@ -286,14 +286,14 @@ int epicsShareAPI ca_build_and_connect ( const char *name_str, chtype get_type,
 }
 
 /*
- *  ca_search_and_connect() 
+ *  ca_search_and_connect()
  */
 // extern "C"
 int epicsShareAPI ca_search_and_connect (
     const char * name_str, chid * chanptr,
     caCh * conn_func, void * puser )
 {
-    return ca_create_channel ( name_str, conn_func, 
+    return ca_create_channel ( name_str, conn_func,
         puser, CA_PRIORITY_DEFAULT, chanptr );
 }
 
@@ -312,7 +312,7 @@ int epicsShareAPI ca_create_channel (
         CAFDHANDLER * pFunc = 0;
         void * pArg = 0;
         {
-            epicsGuard < epicsMutex > 
+            epicsGuard < epicsMutex >
                 guard ( pcac->mutex );
             if ( pcac->fdRegFuncNeedsToBeCalled ) {
                 pFunc = pcac->fdRegFunc;
@@ -327,9 +327,9 @@ int epicsShareAPI ca_create_channel (
 
     try {
         epicsGuard < epicsMutex > guard ( pcac->mutex );
-        oldChannelNotify * pChanNotify = 
-            new ( pcac->oldChannelNotifyFreeList ) 
-                oldChannelNotify ( guard, *pcac, name_str, 
+        oldChannelNotify * pChanNotify =
+            new ( pcac->oldChannelNotifyFreeList )
+                oldChannelNotify ( guard, *pcac, name_str,
                     conn_func, puser, priority );
         // make sure that their chan pointer is set prior to
         // calling connection call backs
@@ -352,9 +352,9 @@ int epicsShareAPI ca_create_channel (
         return ECA_UNAVAILINSERV;
     }
     catch ( std :: exception & except ) {
-        pcac->printFormated ( 
+        pcac->printFormated (
             "ca_create_channel: "
-            "unexpected exception was \"%s\"", 
+            "unexpected exception was \"%s\"",
             except.what () );
         return ECA_INTERNAL;
     }
@@ -369,22 +369,42 @@ int epicsShareAPI ca_create_channel (
  *  ca_clear_channel ()
  *
  * a known defect here is that there will be a
- * crash if they destroy the channel after destroying 
+ * crash if they destroy the channel after destroying
  * its context
  */
 // extern "C"
 int epicsShareAPI ca_clear_channel ( chid pChan )
 {
     ca_client_context & cac = pChan->getClientCtx ();
-    epicsGuard < epicsMutex > guard ( cac.mutex );
-    try {
-        pChan->eliminateExcessiveSendBacklog ( guard );
+    {
+        epicsGuard < epicsMutex > guard ( cac.mutex );
+        try {
+            pChan->eliminateExcessiveSendBacklog ( guard );
+        }
+        catch ( cacChannel::notConnected & ) {
+            // intentionally ignored
+        }
     }
-    catch ( cacChannel::notConnected & ) {
-        // intentionally ignored
+    if ( cac.pCallbackGuard.get() &&
+            cac.createdByThread == epicsThreadGetIdSelf () ) {
+        epicsGuard < epicsMutex > guard ( cac.mutex );
+        pChan->destructor ( *cac.pCallbackGuard.get(), guard );
+        cac.oldChannelNotifyFreeList.release ( pChan );
+    }
+    else {
+        //
+        // we will definately stall out here if all of the
+        // following are true
+        //
+        // o user creates non-preemtive mode client library context
+        // o user doesnt periodically call a ca function
+        // o user calls this function from an auxiillary thread
+        //
+        CallbackGuard cbGuard ( cac.cbMutex );
+        epicsGuard < epicsMutex > guard ( cac.mutex );
+        pChan->destructor ( *cac.pCallbackGuard.get(), guard );
+        cac.oldChannelNotifyFreeList.release ( pChan );
     }
-    pChan->destructor ( guard );
-    cac.oldChannelNotifyFreeList.release ( pChan );
     return ECA_NORMAL;
 }
 
@@ -399,7 +419,7 @@ int epicsShareAPI ca_add_exception_event ( caExceptionHandler *pfunc, void *arg
     if ( caStatus != ECA_NORMAL ) {
         return caStatus;
     }
-    
+
     pcac->changeExceptionEvent ( pfunc, arg );
 
     return ECA_NORMAL;
@@ -408,10 +428,10 @@ int epicsShareAPI ca_add_exception_event ( caExceptionHandler *pfunc, void *arg
 /*
  *  ca_add_masked_array_event
  */
-int epicsShareAPI ca_add_masked_array_event ( 
-        chtype type, arrayElementCount count, chid pChan, 
-        caEventCallBackFunc *pCallBack, void *pCallBackArg, 
-        ca_real, ca_real, ca_real, 
+int epicsShareAPI ca_add_masked_array_event (
+        chtype type, arrayElementCount count, chid pChan,
+        caEventCallBackFunc *pCallBack, void *pCallBackArg,
+        ca_real, ca_real, ca_real,
         evid *monixptr, long mask )
 {
     return ca_create_subscription ( type, count, pChan, mask,
@@ -429,7 +449,7 @@ int epicsShareAPI ca_clear_event ( evid pMon )
 // extern "C"
 chid epicsShareAPI ca_evid_to_chid ( evid pMon )
 {
-    return & pMon->channel (); 
+    return & pMon->channel ();
 }
 
 // extern "C"
@@ -498,7 +518,7 @@ int epicsShareAPI ca_pend_io ( ca_real timeout )
 
 /*
  *  ca_flush_io ()
- */ 
+ */
 int epicsShareAPI ca_flush_io ()
 {
     ca_client_context * pcac;
@@ -545,7 +565,7 @@ void epicsShareAPI ca_signal ( long ca_status, const char *message )
  * ca_message (long ca_status)
  *
  * - if it is an unknown error code then it possible
- * that the error string generated below 
+ * that the error string generated below
  * will be overwritten before (or while) the caller
  * of this routine is calling this routine
  * (if they call this routine again).
@@ -567,7 +587,7 @@ const char * epicsShareAPI ca_message ( long ca_status )
  * ca_signal_with_file_and_lineno()
  */
 // extern "C"
-void epicsShareAPI ca_signal_with_file_and_lineno ( long ca_status, 
+void epicsShareAPI ca_signal_with_file_and_lineno ( long ca_status,
             const char *message, const char *pfilenm, int lineno )
 {
     ca_signal_formated ( ca_status, pfilenm, lineno, message );
@@ -577,7 +597,7 @@ void epicsShareAPI ca_signal_with_file_and_lineno ( long ca_status,
  * ca_signal_formated()
  */
 // extern "C"
-void epicsShareAPI ca_signal_formated ( long ca_status, const char *pfilenm, 
+void epicsShareAPI ca_signal_formated ( long ca_status, const char *pfilenm,
                                        int lineno, const char *pFormat, ... )
 {
     ca_client_context *pcac;
@@ -590,12 +610,12 @@ void epicsShareAPI ca_signal_formated ( long ca_status, const char *pfilenm,
     }
 
     va_list theArgs;
-    va_start ( theArgs, pFormat );  
+    va_start ( theArgs, pFormat );
     if ( pcac ) {
         pcac->vSignal ( ca_status, pfilenm, lineno, pFormat, theArgs );
     }
     else {
-        fprintf ( stderr, "CA exception in thread w/o CA ctx: status=%s file=%s line=%d: \n", 
+        fprintf ( stderr, "CA exception in thread w/o CA ctx: status=%s file=%s line=%d: \n",
             ca_message ( ca_status ), pfilenm, lineno );
         if ( pFormat ) {
             vfprintf ( stderr, pFormat, theArgs );
@@ -607,7 +627,7 @@ void epicsShareAPI ca_signal_formated ( long ca_status, const char *pfilenm,
 /*
  *  CA_ADD_FD_REGISTRATION
  *
- *  call their function with their argument whenever 
+ *  call their function with their argument whenever
  *  a new fd is added or removed
  *  (for a manager of the select system call under UNIX)
  *
@@ -631,7 +651,7 @@ int epicsShareAPI ca_add_fd_registration ( CAFDHANDLER * func, void * arg )
  * function that returns the CA version string
  */
 // extern "C"
-const char * epicsShareAPI ca_version () 
+const char * epicsShareAPI ca_version ()
 {
     return CA_VERSION_STRING ( CA_MINOR_PROTOCOL_REVISION );
 }
@@ -660,7 +680,7 @@ int epicsShareAPI ca_replace_printf_handler ( caPrintfFunc *ca_printf_func )
  * (for testing purposes only)
  */
 // extern "C"
-unsigned epicsShareAPI ca_get_ioc_connection_count () 
+unsigned epicsShareAPI ca_get_ioc_connection_count ()
 {
     ca_client_context * pcac;
     int caStatus = fetchClientContext ( & pcac );
@@ -720,7 +740,7 @@ struct ca_client_context * epicsShareAPI ca_current_context ()
 {
     struct ca_client_context *pCtx;
     if ( caClientContextId ) {
-        pCtx = ( struct ca_client_context * ) 
+        pCtx = ( struct ca_client_context * )
             epicsThreadPrivateGet ( caClientContextId );
     }
     else {
@@ -749,7 +769,7 @@ int epicsShareAPI ca_attach_context ( struct ca_client_context * pCtx )
     return ECA_NORMAL;
 }
 
-void epicsShareAPI ca_detach_context () 
+void epicsShareAPI ca_detach_context ()
 {
     if ( caClientContextId ) {
         epicsThreadPrivateSet ( caClientContextId, 0 );
@@ -794,43 +814,43 @@ epicsShareDef const int epicsTypeToDBR_XXXX [lastEpicsType+1] = {
 // extern "C"
 epicsShareDef const epicsType DBR_XXXXToEpicsType [LAST_BUFFER_TYPE+1] = {
 	epicsOldStringT,
-	epicsInt16T,	
-	epicsFloat32T,	
-	epicsEnum16T,	
-	epicsUInt8T,	
-	epicsInt32T,	
+	epicsInt16T,
+	epicsFloat32T,
+	epicsEnum16T,
+	epicsUInt8T,
+	epicsInt32T,
 	epicsFloat64T,
 
 	epicsOldStringT,
-	epicsInt16T,	
-	epicsFloat32T,	
-	epicsEnum16T,	
-	epicsUInt8T,	
-	epicsInt32T,	
+	epicsInt16T,
+	epicsFloat32T,
+	epicsEnum16T,
+	epicsUInt8T,
+	epicsInt32T,
 	epicsFloat64T,
 
 	epicsOldStringT,
-	epicsInt16T,	
-	epicsFloat32T,	
-	epicsEnum16T,	
-	epicsUInt8T,	
-	epicsInt32T,	
+	epicsInt16T,
+	epicsFloat32T,
+	epicsEnum16T,
+	epicsUInt8T,
+	epicsInt32T,
 	epicsFloat64T,
 
 	epicsOldStringT,
-	epicsInt16T,	
-	epicsFloat32T,	
-	epicsEnum16T,	
-	epicsUInt8T,	
-	epicsInt32T,	
+	epicsInt16T,
+	epicsFloat32T,
+	epicsEnum16T,
+	epicsUInt8T,
+	epicsInt32T,
 	epicsFloat64T,
 
 	epicsOldStringT,
-	epicsInt16T,	
-	epicsFloat32T,	
-	epicsEnum16T,	
-	epicsUInt8T,	
-	epicsInt32T,	
+	epicsInt16T,
+	epicsFloat32T,
+	epicsEnum16T,
+	epicsUInt8T,
+	epicsInt32T,
 	epicsFloat64T,
 
 	epicsUInt16T,
@@ -939,7 +959,7 @@ epicsShareDef const unsigned short dbr_value_size[LAST_BUFFER_TYPE+1] = {
 	sizeof(dbr_string_t),	/* string max size		*/
 };
 
-//extern "C" 
+//extern "C"
 epicsShareDef const enum dbr_value_class dbr_value_class[LAST_BUFFER_TYPE+1] = {
 	dbr_class_string,	/* string max size		*/
 	dbr_class_int,		/* short			*/
diff --git a/src/ca/client/ca_client_context.cpp b/src/ca/client/ca_client_context.cpp
index c96c215f9..75ff80bb0 100644
--- a/src/ca/client/ca_client_context.cpp
+++ b/src/ca/client/ca_client_context.cpp
@@ -4,19 +4,19 @@
 * Copyright (c) 2002 The Regents of the University of California, as
 *     Operator of Los Alamos National Laboratory.
 * EPICS BASE is distributed subject to a Software License Agreement found
-* in file LICENSE that is included with this distribution. 
+* in file LICENSE that is included with this distribution.
 \*************************************************************************/
 
-/*  
+/*
+ *
  *
- *                              
  *                    L O S  A L A M O S
  *              Los Alamos National Laboratory
  *               Los Alamos, New Mexico 87545
- *                                  
+ *
  *  Copyright, 1986, The Regents of the University of California.
- *                                  
- *           
+ *
+ *
  *	Author Jeffrey O. Hill
  *	johill@lanl.gov
  *	505 665 1831
@@ -27,7 +27,7 @@
 #endif
 
 #include <stdexcept>
-#include <string> // vxWorks 6.0 requires this include 
+#include <string> // vxWorks 6.0 requires this include
 #include <stdio.h>
 
 #include "epicsExit.h"
@@ -67,11 +67,11 @@ cacService * ca_client_context::pDefaultService = 0;
 epicsMutex * ca_client_context::pDefaultServiceInstallMutex;
 
 ca_client_context::ca_client_context ( bool enablePreemptiveCallback ) :
-    createdByThread ( epicsThreadGetIdSelf () ), 
-    ca_exception_func ( 0 ), ca_exception_arg ( 0 ), 
+    createdByThread ( epicsThreadGetIdSelf () ),
+    ca_exception_func ( 0 ), ca_exception_arg ( 0 ),
     pVPrintfFunc ( errlogVprintf ), fdRegFunc ( 0 ), fdRegArg ( 0 ),
-    pndRecvCnt ( 0u ), ioSeqNo ( 0u ), callbackThreadsPending ( 0u ), 
-    localPort ( 0 ), fdRegFuncNeedsToBeCalled ( false ), 
+    pndRecvCnt ( 0u ), ioSeqNo ( 0u ), callbackThreadsPending ( 0u ),
+    localPort ( 0 ), fdRegFuncNeedsToBeCalled ( false ),
     noWakeupSincePend ( true )
 {
     static const unsigned short PORT_ANY = 0u;
@@ -85,7 +85,7 @@ ca_client_context::ca_client_context ( bool enablePreemptiveCallback ) :
         epicsGuard < epicsMutex > guard ( *ca_client_context::pDefaultServiceInstallMutex );
         if ( ca_client_context::pDefaultService ) {
             this->pServiceContext.reset (
-                & ca_client_context::pDefaultService->contextCreate ( 
+                & ca_client_context::pDefaultService->contextCreate (
                     this->mutex, this->cbMutex, *this ) );
         }
         else {
@@ -125,7 +125,7 @@ ca_client_context::ca_client_context ( bool enablePreemptiveCallback ) :
         osiSockAddr addr;
         memset ( (char *)&addr, 0 , sizeof ( addr ) );
         addr.ia.sin_family = AF_INET;
-        addr.ia.sin_addr.s_addr = htonl ( INADDR_ANY ); 
+        addr.ia.sin_addr.s_addr = htonl ( INADDR_ANY );
         addr.ia.sin_port = htons ( PORT_ANY );
         int status = bind (this->sock, &addr.sa, sizeof (addr) );
         if ( status < 0 ) {
@@ -139,7 +139,7 @@ ca_client_context::ca_client_context ( bool enablePreemptiveCallback ) :
             throwWithLocation ( noSocket () );
         }
     }
-    
+
     {
         osiSockAddr tmpAddr;
         osiSocklen_t saddr_length = sizeof ( tmpAddr );
@@ -159,9 +159,9 @@ ca_client_context::ca_client_context ( bool enablePreemptiveCallback ) :
         this->localPort = htons ( tmpAddr.ia.sin_port );
     }
 
-    epics_auto_ptr < epicsGuard < epicsMutex > > pCBGuard;
+    epics_auto_ptr < CallbackGuard > pCBGuard;
     if ( ! enablePreemptiveCallback ) {
-        pCBGuard.reset ( new epicsGuard < epicsMutex > ( this->cbMutex ) );
+        pCBGuard.reset ( new CallbackGuard ( this->cbMutex ) );
     }
 
     // multiple steps ensure exception safety
@@ -171,7 +171,7 @@ ca_client_context::ca_client_context ( bool enablePreemptiveCallback ) :
 ca_client_context::~ca_client_context ()
 {
     if ( this->fdRegFunc ) {
-        ( *this->fdRegFunc ) 
+        ( *this->fdRegFunc )
             ( this->fdRegArg, this->sock, false );
     }
     epicsSocketDestroy ( this->sock );
@@ -179,9 +179,9 @@ ca_client_context::~ca_client_context ()
     osiSockRelease ();
 
     // force a logical shutdown order
-    // so that the cac class does not hang its 
+    // so that the cac class does not hang its
     // receive threads during their shutdown sequence
-    // and so that classes using this classes mutex 
+    // and so that classes using this classes mutex
     // are destroyed before the mutex is destroyed
     if ( this->pCallbackGuard.get() ) {
         epicsGuardRelease < epicsMutex > unguard ( *this->pCallbackGuard );
@@ -192,7 +192,7 @@ ca_client_context::~ca_client_context ()
     }
 }
 
-void ca_client_context::destroyGetCopy ( 
+void ca_client_context::destroyGetCopy (
     epicsGuard < epicsMutex > & guard, getCopy & gc )
 {
     guard.assertIdenticalMutex ( this->mutex );
@@ -200,7 +200,7 @@ void ca_client_context::destroyGetCopy (
     this->getCopyFreeList.release ( & gc );
 }
 
-void ca_client_context::destroyGetCallback ( 
+void ca_client_context::destroyGetCallback (
     epicsGuard < epicsMutex > & guard, getCallback & gcb )
 {
     guard.assertIdenticalMutex ( this->mutex );
@@ -208,7 +208,7 @@ void ca_client_context::destroyGetCallback (
     this->getCallbackFreeList.release ( & gcb );
 }
 
-void ca_client_context::destroyPutCallback ( 
+void ca_client_context::destroyPutCallback (
     epicsGuard < epicsMutex > & guard, putCallback & pcb )
 {
     guard.assertIdenticalMutex ( this->mutex );
@@ -216,7 +216,7 @@ void ca_client_context::destroyPutCallback (
     this->putCallbackFreeList.release ( & pcb );
 }
 
-void ca_client_context::destroySubscription ( 
+void ca_client_context::destroySubscription (
     epicsGuard < epicsMutex > & guard, oldSubscription & os )
 {
     guard.assertIdenticalMutex ( this->mutex );
@@ -224,7 +224,7 @@ void ca_client_context::destroySubscription (
     this->subscriptionFreeList.release ( & os );
 }
 
-void ca_client_context::changeExceptionEvent ( 
+void ca_client_context::changeExceptionEvent (
     caExceptionHandler * pfunc, void * arg )
 {
     epicsGuard < epicsMutex > guard ( this->mutex );
@@ -233,7 +233,7 @@ void ca_client_context::changeExceptionEvent (
 // should block here until releated callback in progress completes
 }
 
-void ca_client_context::replaceErrLogHandler ( 
+void ca_client_context::replaceErrLogHandler (
     caPrintfFunc * ca_printf_func )
 {
     epicsGuard < epicsMutex > guard ( this->mutex );
@@ -246,7 +246,7 @@ void ca_client_context::replaceErrLogHandler (
 // should block here until releated callback in progress completes
 }
 
-void ca_client_context::registerForFileDescriptorCallBack ( 
+void ca_client_context::registerForFileDescriptorCallBack (
     CAFDHANDLER *pFunc, void *pArg )
 {
     epicsGuard < epicsMutex > guard ( this->mutex );
@@ -261,22 +261,22 @@ void ca_client_context::registerForFileDescriptorCallBack (
 // should block here until releated callback in progress completes
 }
 
-int ca_client_context :: printFormated ( 
+int ca_client_context :: printFormated (
     const char *pformat, ... ) const
 {
     va_list theArgs;
     int status;
 
     va_start ( theArgs, pformat );
-    
+
     status = this->ca_client_context :: varArgsPrintFormated ( pformat, theArgs );
-    
+
     va_end ( theArgs );
-    
+
     return status;
 }
 
-int ca_client_context :: varArgsPrintFormated ( 
+int ca_client_context :: varArgsPrintFormated (
     const char *pformat, va_list args ) const
 {
     caPrintfFunc * pFunc;
@@ -292,8 +292,8 @@ int ca_client_context :: varArgsPrintFormated (
     }
 }
 
-void ca_client_context::exception ( 
-    epicsGuard < epicsMutex > & guard, int stat, const char * pCtx, 
+void ca_client_context::exception (
+    epicsGuard < epicsMutex > & guard, int stat, const char * pCtx,
         const char * pFile, unsigned lineNo )
 {
     struct exception_handler_args args;
@@ -321,9 +321,9 @@ void ca_client_context::exception (
     }
 }
 
-void ca_client_context::exception ( 
+void ca_client_context::exception (
     epicsGuard < epicsMutex > & guard, int status, const char * pContext,
-    const char * pFileName, unsigned lineNo, oldChannelNotify & chan, 
+    const char * pFileName, unsigned lineNo, oldChannelNotify & chan,
     unsigned type, arrayElementCount count, unsigned op )
 {
     struct exception_handler_args args;
@@ -346,29 +346,29 @@ void ca_client_context::exception (
             ( *pFunc ) ( args );
         }
         else {
-            this->signal ( status, pFileName, lineNo, 
-                "op=%u, channel=%s, type=%s, count=%lu, ctx=\"%s\"", 
-                op, ca_name ( &chan ), 
-                dbr_type_to_text ( static_cast <int> ( type ) ), 
+            this->signal ( status, pFileName, lineNo,
+                "op=%u, channel=%s, type=%s, count=%lu, ctx=\"%s\"",
+                op, ca_name ( &chan ),
+                dbr_type_to_text ( static_cast <int> ( type ) ),
                 count, pContext );
         }
     }
 }
 
-void ca_client_context::signal ( int ca_status, const char * pfilenm, 
+void ca_client_context::signal ( int ca_status, const char * pfilenm,
                      int lineno, const char * pFormat, ... )
 {
     va_list theArgs;
-    va_start ( theArgs, pFormat );  
+    va_start ( theArgs, pFormat );
     this->vSignal ( ca_status, pfilenm, lineno, pFormat, theArgs);
     va_end ( theArgs );
 }
 
-void ca_client_context :: vSignal ( 
-    int ca_status, const char *pfilenm, 
+void ca_client_context :: vSignal (
+    int ca_status, const char *pfilenm,
         int lineno, const char *pFormat, va_list args )
 {
-    static const char *severity[] = 
+    static const char *severity[] =
     {
         "Warning",
         "Success",
@@ -379,11 +379,11 @@ void ca_client_context :: vSignal (
         "Fatal",
         "Fatal"
     };
-    
+
     this->printFormated ( "CA.Client.Exception...............................................\n" );
-    
-    this->printFormated ( "    %s: \"%s\"\n", 
-        severity[ CA_EXTRACT_SEVERITY ( ca_status ) ], 
+
+    this->printFormated ( "    %s: \"%s\"\n",
+        severity[ CA_EXTRACT_SEVERITY ( ca_status ) ],
         ca_message ( ca_status ) );
 
     if  ( pFormat ) {
@@ -391,26 +391,26 @@ void ca_client_context :: vSignal (
         this->varArgsPrintFormated ( pFormat, args );
         this->printFormated ( "\"\n" );
     }
-        
+
     if ( pfilenm ) {
         this->printFormated ( "    Source File: %s line %d\n",
-            pfilenm, lineno );    
-    } 
+            pfilenm, lineno );
+    }
 
     epicsTime current = epicsTime::getCurrent ();
     char date[64];
     current.strftime ( date, sizeof ( date ), "%a %b %d %Y %H:%M:%S.%f");
     this->printFormated ( "    Current Time: %s\n", date );
-    
+
     /*
      *  Terminate execution if unsuccessful
      */
-    if( ! ( ca_status & CA_M_SUCCESS ) && 
+    if( ! ( ca_status & CA_M_SUCCESS ) &&
         CA_EXTRACT_SEVERITY ( ca_status ) != CA_K_WARNING ){
         errlogFlush ();
         abort ();
     }
-    
+
     this->printFormated ( "..................................................................\n" );
 }
 
@@ -418,7 +418,7 @@ void ca_client_context::show ( unsigned level ) const
 {
     epicsGuard < epicsMutex > guard ( this->mutex );
 
-    ::printf ( "ca_client_context at %p pndRecvCnt=%u ioSeqNo=%u\n", 
+    ::printf ( "ca_client_context at %p pndRecvCnt=%u ioSeqNo=%u\n",
         static_cast <const void *> ( this ),
         this->pndRecvCnt, this->ioSeqNo );
 
@@ -443,7 +443,7 @@ void ca_client_context::attachToClientCtx ()
     epicsThreadPrivateSet ( caClientContextId, this );
 }
 
-void ca_client_context::incrementOutstandingIO ( 
+void ca_client_context::incrementOutstandingIO (
     epicsGuard < epicsMutex > & guard, unsigned ioSeqNoIn )
 {
     guard.assertIdenticalMutex ( this->mutex );
@@ -453,7 +453,7 @@ void ca_client_context::incrementOutstandingIO (
     }
 }
 
-void ca_client_context::decrementOutstandingIO ( 
+void ca_client_context::decrementOutstandingIO (
     epicsGuard < epicsMutex > & guard, unsigned ioSeqNoIn )
 {
     guard.assertIdenticalMutex ( this->mutex );
@@ -466,16 +466,16 @@ void ca_client_context::decrementOutstandingIO (
     }
 }
 
-// !!!! This routine is only visible in the old interface - or in a new ST interface. 
-// !!!! In the old interface we restrict thread attach so that calls from threads 
-// !!!! other than the initializing thread are not allowed if preemptive callback 
+// !!!! This routine is only visible in the old interface - or in a new ST interface.
+// !!!! In the old interface we restrict thread attach so that calls from threads
+// !!!! other than the initializing thread are not allowed if preemptive callback
 // !!!! is disabled. This prevents the preemptive callback lock from being released
 // !!!! by other threads than the one that locked it.
 //
 int ca_client_context::pendIO ( const double & timeout )
 {
-    // prevent recursion nightmares by disabling calls to 
-    // pendIO () from within a CA callback. 
+    // prevent recursion nightmares by disabling calls to
+    // pendIO () from within a CA callback.
     if ( epicsThreadPrivateGet ( caClientCallbackThreadId ) ) {
         return ECA_EVDISALLOW;
     }
@@ -487,7 +487,7 @@ int ca_client_context::pendIO ( const double & timeout )
     epicsGuard < epicsMutex > guard ( this->mutex );
 
     this->flush ( guard );
-   
+
     while ( this->pndRecvCnt > 0 ) {
         if ( remaining < CAC_SIGNIFICANT_DELAY ) {
             status = ECA_TIMEOUT;
@@ -514,16 +514,16 @@ int ca_client_context::pendIO ( const double & timeout )
     return status;
 }
 
-// !!!! This routine is only visible in the old interface - or in a new ST interface. 
-// !!!! In the old interface we restrict thread attach so that calls from threads 
-// !!!! other than the initializing thread are not allowed if preemptive callback 
+// !!!! This routine is only visible in the old interface - or in a new ST interface.
+// !!!! In the old interface we restrict thread attach so that calls from threads
+// !!!! other than the initializing thread are not allowed if preemptive callback
 // !!!! is disabled. This prevents the preemptive callback lock from being released
 // !!!! by other threads than the one that locked it.
 //
 int ca_client_context::pendEvent ( const double & timeout )
 {
-    // prevent recursion nightmares by disabling calls to 
-    // pendIO () from within a CA callback. 
+    // prevent recursion nightmares by disabling calls to
+    // pendIO () from within a CA callback.
     if ( epicsThreadPrivateGet ( caClientCallbackThreadId ) ) {
         return ECA_EVDISALLOW;
     }
@@ -541,17 +541,17 @@ int ca_client_context::pendEvent ( const double & timeout )
         epicsGuard < epicsMutex > guard ( this->mutex );
 
         //
-        // This is needed because in non-preemptive callback mode 
-        // legacy applications that use file descriptor managers 
+        // This is needed because in non-preemptive callback mode
+        // legacy applications that use file descriptor managers
         // will register for ca receive thread activity and keep
         // calling ca_pend_event until all of the socket data has
-        // been read. We must guarantee that other threads get a 
+        // been read. We must guarantee that other threads get a
         // chance to run if there is data in any of the sockets.
         //
         if ( this->fdRegFunc ) {
             epicsGuardRelease < epicsMutex > unguard ( guard );
 
-            // remove short udp message sent to wake 
+            // remove short udp message sent to wake
             // up a file descriptor manager
             osiSockAddr tmpAddr;
             osiSocklen_t addrSize = sizeof ( tmpAddr.sa );
@@ -592,7 +592,7 @@ int ca_client_context::pendEvent ( const double & timeout )
     return ECA_TIMEOUT;
 }
 
-void ca_client_context::blockForEventAndEnableCallbacks ( 
+void ca_client_context::blockForEventAndEnableCallbacks (
     epicsEvent & event, const double & timeout )
 {
     if ( this->pCallbackGuard.get() ) {
@@ -659,12 +659,12 @@ void ca_client_context::callbackProcessingCompleteNotify ()
     }
 }
 
-cacChannel & ca_client_context::createChannel ( 
-    epicsGuard < epicsMutex > & guard, const char * pChannelName, 
+cacChannel & ca_client_context::createChannel (
+    epicsGuard < epicsMutex > & guard, const char * pChannelName,
     cacChannelNotify & chan, cacChannel::priLev pri )
 {
     guard.assertIdenticalMutex ( this->mutex );
-    return this->pServiceContext->createChannel ( 
+    return this->pServiceContext->createChannel (
         guard, pChannelName, chan, pri );
 }
 
@@ -685,21 +685,21 @@ unsigned ca_client_context::beaconAnomaliesSinceProgramStart () const
     return this->pServiceContext->beaconAnomaliesSinceProgramStart ( guard );
 }
 
-void ca_client_context::installCASG ( 
+void ca_client_context::installCASG (
     epicsGuard < epicsMutex > & guard, CASG & sg )
 {
     guard.assertIdenticalMutex ( this->mutex );
     this->sgTable.idAssignAdd ( sg );
 }
 
-void ca_client_context::uninstallCASG ( 
+void ca_client_context::uninstallCASG (
     epicsGuard < epicsMutex > & guard, CASG & sg )
 {
     guard.assertIdenticalMutex ( this->mutex );
     this->sgTable.remove ( sg );
 }
 
-CASG * ca_client_context::lookupCASG ( 
+CASG * ca_client_context::lookupCASG (
     epicsGuard < epicsMutex > & guard, unsigned idIn )
 {
     guard.assertIdenticalMutex ( this->mutex );
@@ -724,7 +724,7 @@ epicsMutex & ca_client_context::mutexRef () const
     return this->mutex;
 }
 
-cacContext & ca_client_context::createNetworkContext ( 
+cacContext & ca_client_context::createNetworkContext (
     epicsMutex & mutexIn, epicsMutex & cbMutexIn )
 {
     return * new cac ( mutexIn, cbMutexIn, *this );
@@ -751,34 +751,54 @@ epicsShareFunc int epicsShareAPI ca_clear_subscription ( evid pMon )
 {
     oldChannelNotify & chan = pMon->channel ();
     ca_client_context & cac = chan.getClientCtx ();
-    epicsGuard < epicsMutex > guard ( cac.mutex );
-    try {
-        // if this stalls out on a live circuit then an exception 
-        // can be forthcoming which we must ignore as the clear
-        // request must always be successful
-        chan.eliminateExcessiveSendBacklog ( guard );
+    // !!!! the order in which we take the mutex here prevents deadlocks
+    {
+      epicsGuard < epicsMutex > guard ( cac.mutex );
+      try {
+          // if this stalls out on a live circuit then an exception
+          // can be forthcoming which we must ignore as the clear
+          // request must always be successful
+          chan.eliminateExcessiveSendBacklog ( guard );
+      }
+      catch ( cacChannel::notConnected & ) {
+          // intentionally ignored
+      }
     }
-    catch ( cacChannel::notConnected & ) {
-        // intentionally ignored
+    if ( cac.pCallbackGuard.get() &&
+        cac.createdByThread == epicsThreadGetIdSelf () ) {
+      epicsGuard < epicsMutex > guard ( cac.mutex );
+      pMon->cancel ( *cac.pCallbackGuard.get(), guard );
+    }
+    else {
+      //
+      // we will definately stall out here if all of the
+      // following are true
+      //
+      // o user creates non-preemtive mode client library context
+      // o user doesnt periodically call a ca function
+      // o user calls this function from an auxiillary thread
+      //
+      CallbackGuard cbGuard ( cac.cbMutex );
+      epicsGuard < epicsMutex > guard ( cac.mutex );
+      pMon->cancel ( cbGuard, guard );
     }
-    pMon->cancel ( guard );
     return ECA_NORMAL;
 }
 
-void ca_client_context :: eliminateExcessiveSendBacklog ( 
+void ca_client_context :: eliminateExcessiveSendBacklog (
     epicsGuard < epicsMutex > & guard, cacChannel & chan )
 {
-    if ( chan.requestMessageBytesPending ( guard ) > 
+    if ( chan.requestMessageBytesPending ( guard ) >
             ca_client_context :: flushBlockThreshold ) {
-        if ( this->pCallbackGuard.get() && 
+        if ( this->pCallbackGuard.get() &&
             this->createdByThread == epicsThreadGetIdSelf () ) {
-            // we need to be very careful about lock hierarchy 
+            // we need to be very careful about lock hierarchy
             // inversion in this situation
             epicsGuardRelease < epicsMutex > unguard ( guard );
             {
                 epicsGuardRelease < epicsMutex > cbunguard (
                     * this->pCallbackGuard.get() );
-                { 
+                {
                     epicsGuard < epicsMutex > nestedGuard ( this->mutex );
                     chan.flush ( nestedGuard );
                 }
diff --git a/src/ca/client/cac.cpp b/src/ca/client/cac.cpp
index 99825f417..9d23cce62 100644
--- a/src/ca/client/cac.cpp
+++ b/src/ca/client/cac.cpp
@@ -4,7 +4,7 @@
 * Copyright (c) 2002 The Regents of the University of California, as
 *     Operator of Los Alamos National Laboratory.
 * EPICS BASE is distributed subject to a Software License Agreement found
-* in file LICENSE that is included with this distribution. 
+* in file LICENSE that is included with this distribution.
 \*************************************************************************/
 
 /*
@@ -22,7 +22,7 @@
 
 #include <new>
 #include <stdexcept>
-#include <string> // vxWorks 6.0 requires this include 
+#include <string> // vxWorks 6.0 requires this include
 
 #include "dbDefs.h"
 #include "epicsGuard.h"
@@ -50,12 +50,12 @@
 #include "autoPtrFreeList.h"
 #include "noopiiu.h"
 
-static const char pVersionCAC[] = 
-    "@(#) " EPICS_VERSION_STRING 
+static const char pVersionCAC[] =
+    "@(#) " EPICS_VERSION_STRING
     ", CA Client Library " __DATE__;
 
 // TCP response dispatch table
-const cac::pProtoStubTCP cac::tcpJumpTableCAC [] = 
+const cac::pProtoStubTCP cac::tcpJumpTableCAC [] =
 {
     &cac::versionAction,
     &cac::eventRespAction,
@@ -68,7 +68,7 @@ const cac::pProtoStubTCP cac::tcpJumpTableCAC [] =
     &cac::badTCPRespAction,
     &cac::badTCPRespAction,
     // legacy CA_PROTO_READ_SYNC used as an echo with legacy server
-    &cac::echoRespAction, 
+    &cac::echoRespAction,
     &cac::exceptionRespAction,
     &cac::clearChannelRespAction,
     &cac::badTCPRespAction,
@@ -89,7 +89,7 @@ const cac::pProtoStubTCP cac::tcpJumpTableCAC [] =
 };
 
 // TCP exception dispatch table
-const cac::pExcepProtoStubTCP cac::tcpExcepJumpTableCAC [] = 
+const cac::pExcepProtoStubTCP cac::tcpExcepJumpTableCAC [] =
 {
     &cac::defaultExcep,     // CA_PROTO_VERSION
     &cac::eventAddExcep,    // CA_PROTO_EVENT_ADD
@@ -109,7 +109,7 @@ const cac::pExcepProtoStubTCP cac::tcpExcepJumpTableCAC [] =
     &cac::readNotifyExcep,  // CA_PROTO_READ_NOTIFY
     &cac::defaultExcep,     // CA_PROTO_READ_BUILD
     &cac::defaultExcep,     // REPEATER_CONFIRM
-    &cac::defaultExcep,     // CA_PROTO_CREATE_CHAN 
+    &cac::defaultExcep,     // CA_PROTO_CREATE_CHAN
     &cac::writeNotifyExcep, // CA_PROTO_WRITE_NOTIFY
     &cac::defaultExcep,     // CA_PROTO_CLIENT_NAME
     &cac::defaultExcep,     // CA_PROTO_HOST_NAME
@@ -124,9 +124,9 @@ const cac::pExcepProtoStubTCP cac::tcpExcepJumpTableCAC [] =
 //
 // cac::cac ()
 //
-cac::cac ( 
-    epicsMutex & mutualExclusionIn, 
-    epicsMutex & callbackControlIn, 
+cac::cac (
+    epicsMutex & mutualExclusionIn,
+    epicsMutex & callbackControlIn,
     cacContextNotify & notifyIn ) :
     _refLocalHostName ( localHostNameCache.getReference () ),
     programBeginTime ( epicsTime::getCurrent() ),
@@ -134,7 +134,7 @@ cac::cac (
     mutex ( mutualExclusionIn ),
     cbMutex ( callbackControlIn ),
     ipToAEngine ( ipAddrToAsciiEngine::allocate () ),
-    timerQueue ( epicsTimerQueueActive::allocate ( false, 
+    timerQueue ( epicsTimerQueueActive::allocate ( false,
         lowestPriorityLevelAbove(epicsThreadGetPrioritySelf()) ) ),
     pUserName ( 0 ),
     pudpiiu ( 0 ),
@@ -157,8 +157,8 @@ cac::cac (
 	    long status;
 
         /*
-         * Certain os, such as HPUX, do not unblock a socket system call 
-         * when another thread asynchronously calls both shutdown() and 
+         * Certain os, such as HPUX, do not unblock a socket system call
+         * when another thread asynchronously calls both shutdown() and
          * close(). To solve this problem we need to employ OS specific
          * mechanisms.
          */
@@ -179,7 +179,7 @@ cac::cac (
             strncpy ( this->pUserName, tmp, len );
         }
 
-        this->_serverPort = 
+        this->_serverPort =
             envGetInetPortConfigParam ( &EPICS_CA_SERVER_PORT,
                                         static_cast <unsigned short> (CA_SERVER_PORT) );
 
@@ -224,7 +224,7 @@ cac::cac (
         }
         unsigned bufsPerArray = this->maxRecvBytesTCP / comBuf::capacityBytes ();
         if ( bufsPerArray > 1u ) {
-            maxContigFrames = bufsPerArray * 
+            maxContigFrames = bufsPerArray *
                 contiguousMsgCountWhichTriggersFlowControl;
         }
     }
@@ -275,8 +275,8 @@ cac::~cac ()
     // this blocks until the UDP thread exits so that
     // it will not sneak in any new clients
     //
-    // lock intentionally not held here so that we dont deadlock 
-    // waiting for the UDP thread to exit while it is waiting to 
+    // lock intentionally not held here so that we dont deadlock
+    // waiting for the UDP thread to exit while it is waiting to
     // get the lock.
     {
         epicsGuard < epicsMutex > cbGuard ( this->cbMutex );
@@ -298,11 +298,11 @@ cac::~cac ()
             }
         }
     }
-    
+
     //
     // wait for all tcp threads to exit
     //
-    // this will block for oustanding sends to go out so dont 
+    // this will block for oustanding sends to go out so dont
     // hold a lock while waiting
     //
     {
@@ -323,7 +323,7 @@ cac::~cac ()
     delete [] this->pUserName;
 
     tsSLList < bhe > tmpBeaconList;
-    this->beaconTable.removeAll ( tmpBeaconList ); 
+    this->beaconTable.removeAll ( tmpBeaconList );
     while ( bhe * pBHE = tmpBeaconList.get() ) {
         pBHE->~bhe ();
         this->bheFreeList.release ( pBHE );
@@ -338,7 +338,7 @@ cac::~cac ()
     osiSockRelease ();
 
     // its ok for channels and subscriptions to still
-    // exist at this point. The user created them and 
+    // exist at this point. The user created them and
     // its his responsibility to clean them up.
 }
 
@@ -346,7 +346,7 @@ unsigned cac::lowestPriorityLevelAbove ( unsigned priority )
 {
     unsigned abovePriority;
     epicsThreadBooleanStatus tbs;
-    tbs = epicsThreadLowestPriorityLevelAbove ( 
+    tbs = epicsThreadLowestPriorityLevelAbove (
         priority, & abovePriority );
     if ( tbs != epicsThreadBooleanStatusSuccess ) {
         abovePriority = priority;
@@ -358,7 +358,7 @@ unsigned cac::highestPriorityLevelBelow ( unsigned priority )
 {
     unsigned belowPriority;
     epicsThreadBooleanStatus tbs;
-    tbs = epicsThreadHighestPriorityLevelBelow ( 
+    tbs = epicsThreadHighestPriorityLevelBelow (
         priority, & belowPriority );
     if ( tbs != epicsThreadBooleanStatusSuccess ) {
         belowPriority = priority;
@@ -379,21 +379,21 @@ void cac::flush ( epicsGuard < epicsMutex > & guard )
     }
 }
 
-unsigned cac::circuitCount ( 
+unsigned cac::circuitCount (
     epicsGuard < epicsMutex > & guard ) const
 {
     guard.assertIdenticalMutex ( this->mutex );
     return this->circuitList.count ();
 }
 
-void cac::show ( 
+void cac::show (
     epicsGuard < epicsMutex > & guard, unsigned level ) const
 {
     guard.assertIdenticalMutex ( this->mutex );
 
-    ::printf ( "Channel Access Client Context at %p for user %s\n", 
+    ::printf ( "Channel Access Client Context at %p for user %s\n",
         static_cast <const void *> ( this ), this->pUserName );
-    // this also supresses the "defined, but not used" 
+    // this also supresses the "defined, but not used"
     // warning message
     ::printf ( "\trevision \"%s\"\n", pVersionCAC );
 
@@ -451,7 +451,7 @@ void cac::beaconNotify ( const inetAddrID & addr, const epicsTime & currentTime,
         /*
          * return if the beacon period has not changed significantly
          */
-        if ( ! pBHE->updatePeriod ( guard, this->programBeginTime, 
+        if ( ! pBHE->updatePeriod ( guard, this->programBeginTime,
                 currentTime, beaconNumber, protocolRevision ) ) {
             return;
         }
@@ -488,8 +488,8 @@ void cac::beaconNotify ( const inetAddrID & addr, const epicsTime & currentTime,
 #   endif
 }
 
-cacChannel & cac::createChannel ( 
-    epicsGuard < epicsMutex > & guard, const char * pName, 
+cacChannel & cac::createChannel (
+    epicsGuard < epicsMutex > & guard, const char * pName,
     cacChannelNotify & chan, cacChannel::priLev pri )
 {
     guard.assertIdenticalMutex ( this->mutex );
@@ -503,19 +503,19 @@ cacChannel & cac::createChannel (
     }
 
     if ( ! this->pudpiiu ) {
-        this->pudpiiu = new udpiiu ( 
-            guard, this->timerQueue, this->cbMutex, 
+        this->pudpiiu = new udpiiu (
+            guard, this->timerQueue, this->cbMutex,
             this->mutex, this->notify, *this, this->_serverPort,
             this->searchDestList );
     }
 
-    nciu * pNetChan = new ( this->channelFreeList ) 
+    nciu * pNetChan = new ( this->channelFreeList )
             nciu ( *this, noopIIU, chan, pName, pri );
     this->chanTable.idAssignAdd ( *pNetChan );
     return *pNetChan;
 }
 
-bool cac::findOrCreateVirtCircuit ( 
+bool cac::findOrCreateVirtCircuit (
     epicsGuard < epicsMutex > & guard, const osiSockAddr & addr,
     unsigned priority, tcpiiu *& piiu, unsigned minorVersionNumber,
     SearchDestTCP * pSearchDest )
@@ -532,14 +532,14 @@ bool cac::findOrCreateVirtCircuit (
         try {
             autoPtrFreeList < tcpiiu, 32, epicsMutexNOOP > pnewiiu (
                     this->freeListVirtualCircuit,
-                    new ( this->freeListVirtualCircuit ) tcpiiu ( 
-                        *this, this->mutex, this->cbMutex, this->notify, this->connTMO, 
-                        this->timerQueue, addr, this->comBufMemMgr, minorVersionNumber, 
+                    new ( this->freeListVirtualCircuit ) tcpiiu (
+                        *this, this->mutex, this->cbMutex, this->notify, this->connTMO,
+                        this->timerQueue, addr, this->comBufMemMgr, minorVersionNumber,
                         this->ipToAEngine, priority, pSearchDest ) );
 
             bhe * pBHE = this->beaconTable.lookup ( addr.ia );
             if ( ! pBHE ) {
-                pBHE = new ( this->bheFreeList ) 
+                pBHE = new ( this->bheFreeList )
                                     bhe ( this->mutex, epicsTime (), 0u, addr.ia );
                 if ( this->beaconTable.add ( *pBHE ) < 0 ) {
                     return newIIU;
@@ -553,13 +553,13 @@ bool cac::findOrCreateVirtCircuit (
             newIIU = true;
         }
         catch ( std :: exception & except ) {
-            errlogPrintf ( 
+            errlogPrintf (
                 "CAC: exception during virtual circuit creation \"%s\"\n",
                 except.what () );
             return newIIU;
         }
         catch ( ... ) {
-            errlogPrintf ( 
+            errlogPrintf (
                 "CAC: Nonstandard exception during virtual circuit creation\n" );
             return newIIU;
         }
@@ -567,9 +567,9 @@ bool cac::findOrCreateVirtCircuit (
     return newIIU;
 }
 
-void cac::transferChanToVirtCircuit ( 
+void cac::transferChanToVirtCircuit (
     unsigned cid, unsigned sid,
-    ca_uint16_t typeCode, arrayElementCount count, 
+    ca_uint16_t typeCode, arrayElementCount count,
     unsigned minorVersionNumber, const osiSockAddr & addr,
     const epicsTime & currentTime )
 {
@@ -604,12 +604,12 @@ void cac::transferChanToVirtCircuit (
             char acc[64];
             pChan->getPIIU(guard)->getHostName ( guard, acc, sizeof ( acc ) );
             msgForMultiplyDefinedPV * pMsg = new ( this->mdpvFreeList )
-                msgForMultiplyDefinedPV ( this->ipToAEngine, 
+                msgForMultiplyDefinedPV ( this->ipToAEngine,
                     *this, pChan->pName ( guard ), acc );
             // It is possible for the ioInitiate call below to
             // call the callback directly if queue quota is exceeded.
             // This callback takes the callback lock and therefore we
-            // must release the primary mutex here to avoid a lock 
+            // must release the primary mutex here to avoid a lock
             // hierarchy inversion.
             epicsGuardRelease < epicsMutex > unguard ( guard );
             pMsg->ioInitiate ( addr );
@@ -625,9 +625,9 @@ void cac::transferChanToVirtCircuit (
         pChan->getPriority(guard), piiu, minorVersionNumber );
 
     // must occur before moving to new iiu
-    pChan->getPIIU(guard)->uninstallChanDueToSuccessfulSearchResponse ( 
+    pChan->getPIIU(guard)->uninstallChanDueToSuccessfulSearchResponse (
         guard, *pChan, currentTime );
-    piiu->installChannel ( 
+    piiu->installChannel (
         guard, *pChan, sid, typeCode, count );
 
     if ( newIIU ) {
@@ -635,14 +635,14 @@ void cac::transferChanToVirtCircuit (
     }
 }
 
-void cac::destroyChannel ( 
+void cac::destroyChannel (
     epicsGuard < epicsMutex > & guard,
-    nciu & chan ) 
+    nciu & chan )
 {
     guard.assertIdenticalMutex ( this->mutex );
 
     // uninstall channel so that recv threads
-    // will not start a new callback for this channel's IO. 
+    // will not start a new callback for this channel's IO.
     if ( this->chanTable.remove ( chan ) != & chan ) {
         throw std::logic_error ( "Invalid channel identifier" );
     }
@@ -650,9 +650,9 @@ void cac::destroyChannel (
     this->channelFreeList.release ( & chan );
 }
 
-void cac::disconnectAllIO ( 
+void cac::disconnectAllIO (
     epicsGuard < epicsMutex > & cbGuard,
-    epicsGuard < epicsMutex > & guard, 
+    epicsGuard < epicsMutex > & guard,
     nciu & chan, tsDLList < baseNMIU > & ioList )
 {
     cbGuard.assertIdenticalMutex ( this->cbMutex );
@@ -672,8 +672,8 @@ void cac::disconnectAllIO (
     }
 }
 
-int cac :: printFormated ( 
-    epicsGuard < epicsMutex > & callbackControl, 
+int cac :: printFormated (
+    epicsGuard < epicsMutex > & callbackControl,
         const char * pformat, ... ) const
 {
     va_list theArgs;
@@ -683,26 +683,26 @@ int cac :: printFormated (
     return status;
 }
 
-netWriteNotifyIO & cac::writeNotifyRequest ( 
+netWriteNotifyIO & cac::writeNotifyRequest (
     epicsGuard < epicsMutex > & guard, nciu & chan, privateInterfaceForIO & icni,
     unsigned type, arrayElementCount nElem, const void * pValue, cacWriteNotify & notifyIn )
 {
     guard.assertIdenticalMutex ( this->mutex );
-    autoPtrRecycle  < netWriteNotifyIO > pIO ( 
-        guard, this->ioTable, *this, 
+    autoPtrRecycle  < netWriteNotifyIO > pIO (
+        guard, this->ioTable, *this,
         netWriteNotifyIO::factory ( this->freeListWriteNotifyIO, icni, notifyIn ) );
     this->ioTable.idAssignAdd ( *pIO );
-    chan.getPIIU(guard)->writeNotifyRequest ( 
+    chan.getPIIU(guard)->writeNotifyRequest (
         guard, chan, *pIO, type, nElem, pValue );
     return *pIO.release();
 }
 
-netReadNotifyIO & cac::readNotifyRequest ( 
+netReadNotifyIO & cac::readNotifyRequest (
     epicsGuard < epicsMutex > & guard, nciu & chan, privateInterfaceForIO & icni,
     unsigned type, arrayElementCount nElem, cacReadNotify & notifyIn )
 {
     guard.assertIdenticalMutex ( this->mutex );
-    autoPtrRecycle  < netReadNotifyIO > pIO ( 
+    autoPtrRecycle  < netReadNotifyIO > pIO (
         guard, this->ioTable, *this,
         netReadNotifyIO::factory ( this->freeListReadNotifyIO, icni, notifyIn ) );
     this->ioTable.idAssignAdd ( *pIO );
@@ -711,7 +711,8 @@ netReadNotifyIO & cac::readNotifyRequest (
 }
 
 bool cac::destroyIO (
-    epicsGuard < epicsMutex > & guard, 
+    CallbackGuard & callbackGuard,
+    epicsGuard < epicsMutex > & guard,
     const cacChannel::ioid & idIn, nciu & chan )
 {
     guard.assertIdenticalMutex ( this->mutex );
@@ -720,18 +721,18 @@ bool cac::destroyIO (
     if ( pIO ) {
         class netSubscription * pSubscr = pIO->isSubscription ();
         if ( pSubscr ) {
-            pSubscr->unsubscribeIfRequired ( guard, chan );  
+            pSubscr->unsubscribeIfRequired ( guard, chan );
         }
 
         // this uninstalls from the list and destroys the IO
         pIO->exception ( guard, *this,
             ECA_CHANDESTROY, chan.pName ( guard ) );
         return true;
-    }       
+    }
     return false;
 }
 
-void cac::ioShow ( 
+void cac::ioShow (
     epicsGuard < epicsMutex > & guard,
     const cacChannel::ioid & idIn, unsigned level ) const
 {
@@ -741,8 +742,8 @@ void cac::ioShow (
     }
 }
 
-void cac::ioExceptionNotify ( 
-    unsigned idIn, int status, const char * pContext, 
+void cac::ioExceptionNotify (
+    unsigned idIn, int status, const char * pContext,
     unsigned type, arrayElementCount count )
 {
     epicsGuard < epicsMutex > guard ( this->mutex );
@@ -752,8 +753,8 @@ void cac::ioExceptionNotify (
     }
 }
 
-void cac::ioExceptionNotifyAndUninstall ( 
-    unsigned idIn, int status, const char * pContext, 
+void cac::ioExceptionNotifyAndUninstall (
+    unsigned idIn, int status, const char * pContext,
     unsigned type, arrayElementCount count )
 {
     epicsGuard < epicsMutex > guard ( this->mutex );
@@ -763,38 +764,38 @@ void cac::ioExceptionNotifyAndUninstall (
     }
 }
 
-void cac::recycleReadNotifyIO ( 
+void cac::recycleReadNotifyIO (
     epicsGuard < epicsMutex > & guard, netReadNotifyIO & io )
 {
     guard.assertIdenticalMutex ( this->mutex );
     this->freeListReadNotifyIO.release ( & io );
 }
 
-void cac::recycleWriteNotifyIO ( 
+void cac::recycleWriteNotifyIO (
     epicsGuard < epicsMutex > & guard, netWriteNotifyIO & io )
 {
     guard.assertIdenticalMutex ( this->mutex );
     this->freeListWriteNotifyIO.release ( & io );
 }
 
-void cac::recycleSubscription ( 
+void cac::recycleSubscription (
     epicsGuard < epicsMutex > & guard, netSubscription & io )
 {
     guard.assertIdenticalMutex ( this->mutex );
     this->freeListSubscription.release ( & io );
 }
 
-netSubscription & cac::subscriptionRequest ( 
-    epicsGuard < epicsMutex > & guard, 
+netSubscription & cac::subscriptionRequest (
+    epicsGuard < epicsMutex > & guard,
     nciu & chan, privateInterfaceForIO & privChan,
     unsigned type,
-    arrayElementCount nElem, unsigned mask, 
+    arrayElementCount nElem, unsigned mask,
     cacStateNotify & notifyIn,
     bool chanIsInstalled )
 {
     guard.assertIdenticalMutex ( this->mutex );
-    autoPtrRecycle  < netSubscription > pIO ( 
-        guard, this->ioTable, *this, 
+    autoPtrRecycle  < netSubscription > pIO (
+        guard, this->ioTable, *this,
         netSubscription::factory ( this->freeListSubscription,
                                    privChan, type, nElem, mask, notifyIn ) );
     this->ioTable.idAssignAdd ( *pIO );
@@ -804,23 +805,23 @@ netSubscription & cac::subscriptionRequest (
     return *pIO.release ();
 }
 
-bool cac::versionAction ( callbackManager &, tcpiiu & iiu, 
+bool cac::versionAction ( callbackManager &, tcpiiu & iiu,
     const epicsTime &, const caHdrLargeArray & msg, void * )
 {
     iiu.versionRespNotify ( msg );
     return true;
 }
- 
-bool cac::echoRespAction ( 
-    callbackManager & mgr, tcpiiu & iiu, 
+
+bool cac::echoRespAction (
+    callbackManager & mgr, tcpiiu & iiu,
     const epicsTime & /* current */, const caHdrLargeArray &, void * )
 {
     iiu.probeResponseNotify ( mgr.cbGuard );
     return true;
 }
 
-bool cac::writeNotifyRespAction ( 
-    callbackManager &, tcpiiu &, 
+bool cac::writeNotifyRespAction (
+    callbackManager &, tcpiiu &,
     const epicsTime &, const caHdrLargeArray & hdr, void * )
 {
     epicsGuard < epicsMutex > guard ( this->mutex );
@@ -837,7 +838,7 @@ bool cac::writeNotifyRespAction (
     return true;
 }
 
-bool cac::readNotifyRespAction ( callbackManager &, tcpiiu & iiu, 
+bool cac::readNotifyRespAction ( callbackManager &, tcpiiu & iiu,
     const epicsTime &, const caHdrLargeArray & hdr, void * pMsgBdy )
 {
     epicsGuard < epicsMutex > guard ( this->mutex );
@@ -856,13 +857,13 @@ bool cac::readNotifyRespAction ( callbackManager &, tcpiiu & iiu,
 
     baseNMIU * pmiu = this->ioTable.remove ( hdr.m_available );
     //
-    // The IO destroy routines take the call back mutex 
-    // when uninstalling and deleting the baseNMIU so there is 
+    // The IO destroy routines take the call back mutex
+    // when uninstalling and deleting the baseNMIU so there is
     // no need to worry here about the baseNMIU being deleted while
     // it is in use here.
     //
     if ( pmiu ) {
-        // if its a circuit-becomes-responsive subscription update 
+        // if its a circuit-becomes-responsive subscription update
         // then we need to reinstall the IO into the table
         netSubscription * pSubscr = pmiu->isSubscription ();
         if ( pSubscr ) {
@@ -874,7 +875,7 @@ bool cac::readNotifyRespAction ( callbackManager &, tcpiiu & iiu,
              * convert the data buffer from net
              * format to host format
              */
-            caStatus = caNetConvert ( 
+            caStatus = caNetConvert (
                 hdr.m_dataType, pMsgBdy, pMsgBdy, false, hdr.m_count );
         }
         if ( caStatus == ECA_NORMAL ) {
@@ -882,7 +883,7 @@ bool cac::readNotifyRespAction ( callbackManager &, tcpiiu & iiu,
                 hdr.m_dataType, hdr.m_count, pMsgBdy );
         }
         else {
-            pmiu->exception ( guard, *this, 
+            pmiu->exception ( guard, *this,
                 caStatus, "read failed",
                 hdr.m_dataType, hdr.m_count );
         }
@@ -890,8 +891,8 @@ bool cac::readNotifyRespAction ( callbackManager &, tcpiiu & iiu,
     return true;
 }
 
-bool cac::searchRespAction ( callbackManager &, tcpiiu & iiu, 
-    const epicsTime & currentTime, const caHdrLargeArray & msg, 
+bool cac::searchRespAction ( callbackManager &, tcpiiu & iiu,
+    const epicsTime & currentTime, const caHdrLargeArray & msg,
         void * /* pMsgBdy */ )
 {
     assert ( this->pudpiiu );
@@ -899,13 +900,13 @@ bool cac::searchRespAction ( callbackManager &, tcpiiu & iiu,
     return true;
 }
 
-bool cac::eventRespAction ( callbackManager &, tcpiiu &iiu, 
+bool cac::eventRespAction ( callbackManager &, tcpiiu &iiu,
     const epicsTime &, const caHdrLargeArray & hdr, void * pMsgBdy )
-{   
+{
     int caStatus;
 
     /*
-     * m_postsize = 0 used to be a subscription cancel confirmation, 
+     * m_postsize = 0 used to be a subscription cancel confirmation,
      * but is now a noop because the IO block is immediately deleted
      */
     if ( ! hdr.m_postsize ) {
@@ -926,8 +927,8 @@ bool cac::eventRespAction ( callbackManager &, tcpiiu &iiu,
     }
 
     //
-    // The IO destroy routines take the call back mutex 
-    // when uninstalling and deleting the baseNMIU so there is 
+    // The IO destroy routines take the call back mutex
+    // when uninstalling and deleting the baseNMIU so there is
     // no need to worry here about the baseNMIU being deleted while
     // it is in use here.
     //
@@ -937,7 +938,7 @@ bool cac::eventRespAction ( callbackManager &, tcpiiu &iiu,
          * convert the data buffer from net format to host format
          */
         if ( caStatus == ECA_NORMAL ) {
-            caStatus = caNetConvert ( 
+            caStatus = caNetConvert (
                 hdr.m_dataType, pMsgBdy, pMsgBdy, false, hdr.m_count );
         }
         if ( caStatus == ECA_NORMAL ) {
@@ -945,7 +946,7 @@ bool cac::eventRespAction ( callbackManager &, tcpiiu &iiu,
                 hdr.m_dataType, hdr.m_count, pMsgBdy );
         }
         else {
-            pmiu->exception ( guard, *this, caStatus, 
+            pmiu->exception ( guard, *this, caStatus,
                 "subscription update read failed",
                 hdr.m_dataType, hdr.m_count );
         }
@@ -953,14 +954,14 @@ bool cac::eventRespAction ( callbackManager &, tcpiiu &iiu,
     return true;
 }
 
-bool cac::readRespAction ( callbackManager &, tcpiiu &, 
+bool cac::readRespAction ( callbackManager &, tcpiiu &,
     const epicsTime &, const caHdrLargeArray & hdr, void * pMsgBdy )
 {
     epicsGuard < epicsMutex > guard ( this->mutex );
     baseNMIU * pmiu = this->ioTable.remove ( hdr.m_available );
     //
-    // The IO destroy routines take the call back mutex 
-    // when uninstalling and deleting the baseNMIU so there is 
+    // The IO destroy routines take the call back mutex
+    // when uninstalling and deleting the baseNMIU so there is
     // no need to worry here about the baseNMIU being deleted while
     // it is in use here.
     //
@@ -971,14 +972,14 @@ bool cac::readRespAction ( callbackManager &, tcpiiu &,
     return true;
 }
 
-bool cac::clearChannelRespAction ( callbackManager &, tcpiiu &, 
+bool cac::clearChannelRespAction ( callbackManager &, tcpiiu &,
     const epicsTime &, const caHdrLargeArray &, void * /* pMsgBody */ )
 {
     return true; // currently a noop
 }
 
-bool cac::defaultExcep ( 
-    callbackManager &, tcpiiu & iiu, 
+bool cac::defaultExcep (
+    callbackManager &, tcpiiu & iiu,
     const caHdrLargeArray &, const char * pCtx, unsigned status )
 {
     epicsGuard < epicsMutex > guard ( this->mutex );
@@ -990,69 +991,69 @@ bool cac::defaultExcep (
     return true;
 }
 
-void cac::exception ( 
-    epicsGuard < epicsMutex > & cbGuard, 
-    epicsGuard < epicsMutex > & guard, int status, 
+void cac::exception (
+    epicsGuard < epicsMutex > & cbGuard,
+    epicsGuard < epicsMutex > & guard, int status,
     const char * pContext, const char * pFileName, unsigned lineNo )
 {
     cbGuard.assertIdenticalMutex ( this->cbMutex );
     guard.assertIdenticalMutex ( this->mutex );
-    this->notify.exception ( guard, status, pContext, 
+    this->notify.exception ( guard, status, pContext,
         pFileName, lineNo );
 }
 
-bool cac::eventAddExcep ( 
-    callbackManager &, tcpiiu &, 
-    const caHdrLargeArray &hdr, 
+bool cac::eventAddExcep (
+    callbackManager &, tcpiiu &,
+    const caHdrLargeArray &hdr,
     const char *pCtx, unsigned status )
 {
-    this->ioExceptionNotify ( hdr.m_available, status, pCtx, 
+    this->ioExceptionNotify ( hdr.m_available, status, pCtx,
         hdr.m_dataType, hdr.m_count );
     return true;
 }
 
-bool cac::readExcep ( callbackManager &, tcpiiu &, 
-                     const caHdrLargeArray & hdr, 
+bool cac::readExcep ( callbackManager &, tcpiiu &,
+                     const caHdrLargeArray & hdr,
                      const char * pCtx, unsigned status )
 {
-    this->ioExceptionNotifyAndUninstall ( hdr.m_available, 
+    this->ioExceptionNotifyAndUninstall ( hdr.m_available,
         status, pCtx, hdr.m_dataType, hdr.m_count );
     return true;
 }
 
-bool cac::writeExcep ( 
+bool cac::writeExcep (
     callbackManager & mgr,
-    tcpiiu &, const caHdrLargeArray & hdr, 
+    tcpiiu &, const caHdrLargeArray & hdr,
     const char * pCtx, unsigned status )
 {
     epicsGuard < epicsMutex > guard ( this->mutex );
     nciu * pChan = this->chanTable.lookup ( hdr.m_available );
     if ( pChan ) {
-        pChan->writeException ( mgr.cbGuard, guard, status, pCtx, 
+        pChan->writeException ( mgr.cbGuard, guard, status, pCtx,
             hdr.m_dataType, hdr.m_count );
     }
     return true;
 }
 
-bool cac::readNotifyExcep ( callbackManager &, tcpiiu &, 
-                           const caHdrLargeArray &hdr, 
+bool cac::readNotifyExcep ( callbackManager &, tcpiiu &,
+                           const caHdrLargeArray &hdr,
                            const char *pCtx, unsigned status )
 {
-    this->ioExceptionNotifyAndUninstall ( hdr.m_available, 
+    this->ioExceptionNotifyAndUninstall ( hdr.m_available,
         status, pCtx, hdr.m_dataType, hdr.m_count );
     return true;
 }
 
-bool cac::writeNotifyExcep ( callbackManager &, tcpiiu &, 
-                            const caHdrLargeArray &hdr, 
+bool cac::writeNotifyExcep ( callbackManager &, tcpiiu &,
+                            const caHdrLargeArray &hdr,
                             const char *pCtx, unsigned status )
 {
-    this->ioExceptionNotifyAndUninstall ( hdr.m_available, 
+    this->ioExceptionNotifyAndUninstall ( hdr.m_available,
         status, pCtx, hdr.m_dataType, hdr.m_count );
     return true;
 }
 
-bool cac::exceptionRespAction ( callbackManager & cbMutexIn, tcpiiu & iiu, 
+bool cac::exceptionRespAction ( callbackManager & cbMutexIn, tcpiiu & iiu,
     const epicsTime &, const caHdrLargeArray & hdr, void * pMsgBdy )
 {
     const caHdr * pReq = reinterpret_cast < const caHdr * > ( pMsgBdy );
@@ -1069,7 +1070,7 @@ bool cac::exceptionRespAction ( callbackManager & cbMutexIn, tcpiiu & iiu,
     req.m_available = AlignedWireRef < const epicsUInt32 > ( pReq->m_available );
     const ca_uint32_t * pLW = reinterpret_cast < const ca_uint32_t * > ( pReq + 1 );
     if ( req.m_postsize == 0xffff ) {
-        static const unsigned annexSize = 
+        static const unsigned annexSize =
             sizeof ( req.m_postsize ) + sizeof ( req.m_count );
         bytesSoFar += annexSize;
         if ( hdr.m_postsize < bytesSoFar ) {
@@ -1100,9 +1101,9 @@ bool cac::accessRightsRespAction (
     nciu * pChan = this->chanTable.lookup ( hdr.m_cid );
     if ( pChan ) {
         unsigned ar = hdr.m_available;
-        caAccessRights accessRights ( 
-            ( ar & CA_PROTO_ACCESS_RIGHT_READ ) ? true : false, 
-            ( ar & CA_PROTO_ACCESS_RIGHT_WRITE ) ? true : false); 
+        caAccessRights accessRights (
+            ( ar & CA_PROTO_ACCESS_RIGHT_READ ) ? true : false,
+            ( ar & CA_PROTO_ACCESS_RIGHT_WRITE ) ? true : false);
         pChan->accessRightsStateChange ( accessRights, mgr.cbGuard, guard );
     }
 
@@ -1125,11 +1126,11 @@ bool cac::createChannelRespAction (
         }
         bool wasExpected = iiu.connectNotify ( guard, *pChan );
         if ( wasExpected ) {
-            pChan->connect ( hdr.m_dataType, hdr.m_count, sidTmp, 
+            pChan->connect ( hdr.m_dataType, hdr.m_count, sidTmp,
                 mgr.cbGuard, guard );
         }
         else {
-            errlogPrintf ( 
+            errlogPrintf (
                 "CA Client Library: Ignored duplicate create channel "
                 "response from CA server?\n" );
         }
@@ -1140,10 +1141,10 @@ bool cac::createChannelRespAction (
         iiu.clearChannelRequest ( guard, hdr.m_available, hdr.m_cid );
     }
 
-    return true; 
+    return true;
 }
 
-bool cac::verifyAndDisconnectChan ( 
+bool cac::verifyAndDisconnectChan (
     callbackManager & mgr, tcpiiu &,
     const epicsTime &, const caHdrLargeArray & hdr, void * /* pMsgBody */ )
 {
@@ -1168,18 +1169,18 @@ void cac::disconnectChannel (
     chan.unresponsiveCircuitNotify ( cbGuard, guard );
 }
 
-bool cac::badTCPRespAction ( callbackManager &, tcpiiu & iiu, 
+bool cac::badTCPRespAction ( callbackManager &, tcpiiu & iiu,
     const epicsTime &, const caHdrLargeArray & hdr, void * /* pMsgBody */ )
 {
     epicsGuard < epicsMutex > guard ( this->mutex );
     char hostName[64];
     iiu.getHostName ( guard, hostName, sizeof ( hostName ) );
-    errlogPrintf ( "CAC: Undecipherable TCP message ( bad response type %u ) from %s\n", 
+    errlogPrintf ( "CAC: Undecipherable TCP message ( bad response type %u ) from %s\n",
         hdr.m_cmmd, hostName );
     return false;
 }
 
-bool cac::executeResponse ( callbackManager & mgr, tcpiiu & iiu, 
+bool cac::executeResponse ( callbackManager & mgr, tcpiiu & iiu,
     const epicsTime & currentTime, caHdrLargeArray & hdr, char * pMshBody )
 {
     // execute the response message
@@ -1193,7 +1194,7 @@ bool cac::executeResponse ( callbackManager & mgr, tcpiiu & iiu,
     return ( this->*pStub ) ( mgr, iiu, currentTime, hdr, pMshBody );
 }
 
-void cac::selfTest ( 
+void cac::selfTest (
     epicsGuard < epicsMutex > & guard ) const
 {
     guard.assertIdenticalMutex ( this->mutex );
@@ -1221,7 +1222,7 @@ void cac::destroyIIU ( tcpiiu & iiu )
                 pBHE->unregisterIIU ( guard, iiu );
             }
         }
-       
+
         assert ( this->pudpiiu );
         iiu.disconnectAllChannels ( mgr.cbGuard, guard, *this->pudpiiu );
 
@@ -1233,7 +1234,7 @@ void cac::destroyIIU ( tcpiiu & iiu )
     // so we must not hold the primary mutex here
     //
     // this waits for send/recv threads to exit
-    // this also uses the cac free lists so cac must wait 
+    // this also uses the cac free lists so cac must wait
     // for this to finish before it shuts down
 
     iiu.~tcpiiu ();
@@ -1248,8 +1249,8 @@ void cac::destroyIIU ( tcpiiu & iiu )
     // do not touch "this" after lock is released above
 }
 
-double cac::beaconPeriod ( 
-    epicsGuard < epicsMutex > & guard, 
+double cac::beaconPeriod (
+    epicsGuard < epicsMutex > & guard,
     const nciu & chan ) const
 {
     const netiiu * pIIU = chan.getConstPIIU ( guard );
@@ -1266,8 +1267,8 @@ double cac::beaconPeriod (
     return - DBL_MAX;
 }
 
-void cac::initiateConnect ( 
-    epicsGuard < epicsMutex > & guard, 
+void cac::initiateConnect (
+    epicsGuard < epicsMutex > & guard,
     nciu & chan, netiiu * & piiu )
 {
     guard.assertIdenticalMutex ( this->mutex );
@@ -1285,7 +1286,7 @@ void cacComBufMemoryManager::release ( void * pCadaver )
     this->freeList.release ( pCadaver );
 }
 
-void cac::pvMultiplyDefinedNotify ( msgForMultiplyDefinedPV & mfmdpv, 
+void cac::pvMultiplyDefinedNotify ( msgForMultiplyDefinedPV & mfmdpv,
     const char * pChannelName, const char * pAcc, const char * pRej )
 {
     char buf[256];
diff --git a/src/ca/client/cac.h b/src/ca/client/cac.h
index 2e95ed6bf..9f9d45a65 100644
--- a/src/ca/client/cac.h
+++ b/src/ca/client/cac.h
@@ -4,7 +4,7 @@
 * Copyright (c) 2002 The Regents of the University of California, as
 *     Operator of Los Alamos National Laboratory.
 * EPICS BASE is distributed subject to a Software License Agreement found
-* in file LICENSE that is included with this distribution. 
+* in file LICENSE that is included with this distribution.
 \*************************************************************************/
 
 /*
@@ -57,11 +57,11 @@ class netSubscription;
 // is applied
 class cacRecycle {
 public:
-    virtual void recycleReadNotifyIO ( 
+    virtual void recycleReadNotifyIO (
         epicsGuard < epicsMutex > &, netReadNotifyIO &io ) = 0;
-    virtual void recycleWriteNotifyIO ( 
+    virtual void recycleWriteNotifyIO (
         epicsGuard < epicsMutex > &, netWriteNotifyIO &io ) = 0;
-    virtual void recycleSubscription ( 
+    virtual void recycleSubscription (
         epicsGuard < epicsMutex > &, netSubscription &io ) = 0;
 protected:
     virtual ~cacRecycle() {}
@@ -96,85 +96,81 @@ private:
 
 class callbackManager : public notifyGuard {
 public:
-    callbackManager ( 
-        cacContextNotify &, 
+    callbackManager (
+        cacContextNotify &,
         epicsMutex & callbackControl );
     epicsGuard < epicsMutex > cbGuard;
 };
 
-class cac : 
+class cac :
     public cacContext,
-    private cacRecycle, 
+    private cacRecycle,
     private callbackForMultiplyDefinedPV
 {
 public:
-    cac ( 
-        epicsMutex & mutualExclusion, 
-        epicsMutex & callbackControl, 
+    cac (
+        epicsMutex & mutualExclusion,
+        epicsMutex & callbackControl,
         cacContextNotify & );
     virtual ~cac ();
 
     // beacon management
-    void beaconNotify ( const inetAddrID & addr, const epicsTime & currentTime, 
+    void beaconNotify ( const inetAddrID & addr, const epicsTime & currentTime,
         ca_uint32_t beaconNumber, unsigned protocolRevision );
     unsigned beaconAnomaliesSinceProgramStart (
         epicsGuard < epicsMutex > & ) const;
 
     // IO management
     void flush ( epicsGuard < epicsMutex > & guard );
-    bool executeResponse ( callbackManager &, tcpiiu &, 
+    bool executeResponse ( callbackManager &, tcpiiu &,
         const epicsTime & currentTime, caHdrLargeArray &, char *pMsgBody );
 
     // channel routines
-    void transferChanToVirtCircuit ( 
-        unsigned cid, unsigned sid, 
-        ca_uint16_t typeCode, arrayElementCount count, 
+    void transferChanToVirtCircuit (
+        unsigned cid, unsigned sid,
+        ca_uint16_t typeCode, arrayElementCount count,
         unsigned minorVersionNumber, const osiSockAddr &,
         const epicsTime & currentTime );
-    cacChannel & createChannel ( 
-        epicsGuard < epicsMutex > & guard, const char * pChannelName, 
+    cacChannel & createChannel (
+        epicsGuard < epicsMutex > & guard, const char * pChannelName,
         cacChannelNotify &, cacChannel::priLev );
-    void destroyChannel ( 
+    void destroyChannel (
         epicsGuard < epicsMutex > &, nciu & );
-    void initiateConnect ( 
+    void initiateConnect (
         epicsGuard < epicsMutex > &, nciu &, netiiu * & );
     nciu * lookupChannel (
         epicsGuard < epicsMutex > &, const cacChannel::ioid & );
 
     // IO requests
-    netWriteNotifyIO & writeNotifyRequest ( 
+    netWriteNotifyIO & writeNotifyRequest (
         epicsGuard < epicsMutex > &, nciu &, privateInterfaceForIO &,
-        unsigned type, arrayElementCount nElem, const void * pValue, 
+        unsigned type, arrayElementCount nElem, const void * pValue,
         cacWriteNotify & );
-    netReadNotifyIO & readNotifyRequest ( 
+    netReadNotifyIO & readNotifyRequest (
         epicsGuard < epicsMutex > &, nciu &, privateInterfaceForIO &,
-        unsigned type, arrayElementCount nElem, 
+        unsigned type, arrayElementCount nElem,
         cacReadNotify & );
-    netSubscription & subscriptionRequest ( 
+    netSubscription & subscriptionRequest (
         epicsGuard < epicsMutex > &, nciu &, privateInterfaceForIO &,
-        unsigned type, arrayElementCount nElem, unsigned mask, 
+        unsigned type, arrayElementCount nElem, unsigned mask,
         cacStateNotify &, bool channelIsInstalled );
     bool destroyIO (
-        epicsGuard < epicsMutex > & guard, 
-        const cacChannel::ioid & idIn, 
+        CallbackGuard & callbackGuard,
+        epicsGuard < epicsMutex > & mutualExclusionGuard,
+        const cacChannel::ioid & idIn,
         nciu & chan );
-    void disconnectAllIO ( 
+    void disconnectAllIO (
         epicsGuard < epicsMutex > & cbGuard,
-        epicsGuard < epicsMutex > & guard, 
+        epicsGuard < epicsMutex > & guard,
         nciu &, tsDLList < baseNMIU > & ioList );
 
-    void ioShow ( 
+    void ioShow (
         epicsGuard < epicsMutex > & guard,
         const cacChannel::ioid &id, unsigned level ) const;
 
-    // sync group routines
-    CASG * lookupCASG ( epicsGuard < epicsMutex > &, unsigned id );
-    void installCASG ( epicsGuard < epicsMutex > &, CASG & );
-    void uninstallCASG ( epicsGuard < epicsMutex > &, CASG & );
-
     // exception generation
-    void exception ( 
-        epicsGuard < epicsMutex > & cbGuard, 
+    void exception (
+        epicsGuard < epicsMutex > & cbGuard,
         epicsGuard < epicsMutex > & guard,
         int status, const char * pContext,
         const char * pFileName, unsigned lineNo );
@@ -189,11 +185,11 @@ public:
     // diagnostics
     unsigned circuitCount ( epicsGuard < epicsMutex > & ) const;
     void show ( epicsGuard < epicsMutex > &, unsigned level ) const;
-    int printFormated ( 
-        epicsGuard < epicsMutex > & callbackControl, 
+    int printFormated (
+        epicsGuard < epicsMutex > & callbackControl,
         const char *pformat, ... ) const;
-    int varArgsPrintFormated ( 
-        epicsGuard < epicsMutex > & callbackControl, 
+    int varArgsPrintFormated (
+        epicsGuard < epicsMutex > & callbackControl,
         const char *pformat, va_list args ) const;
     double connectionTimeout ( epicsGuard < epicsMutex > & );
 
@@ -210,17 +206,17 @@ public:
     unsigned getInitializingThreadsPriority () const;
     epicsMutex & mutexRef ();
     void attachToClientCtx ();
-    void selfTest ( 
+    void selfTest (
         epicsGuard < epicsMutex > & ) const;
-    double beaconPeriod ( 
+    double beaconPeriod (
         epicsGuard < epicsMutex > &,
         const nciu & chan ) const;
     static unsigned lowestPriorityLevelAbove ( unsigned priority );
     static unsigned highestPriorityLevelBelow ( unsigned priority );
-    void destroyIIU ( tcpiiu & iiu ); 
+    void destroyIIU ( tcpiiu & iiu );
 
     const char * pLocalHostName ();
-    
+
 private:
     epicsSingleton < localHostName > :: reference _refLocalHostName;
     chronIntIdResTable < nciu > chanTable;
@@ -229,11 +225,11 @@ private:
     // !!!! to maintain one IO table for all types of
     // !!!! IO. It would probably be better to maintain
     // !!!! an independent table for each IO type. The
-    // !!!! new adaptive sized hash table will not 
-    // !!!! waste memory. Making this change will 
-    // !!!! avoid virtual function overhead when 
+    // !!!! new adaptive sized hash table will not
+    // !!!! waste memory. Making this change will
+    // !!!! avoid virtual function overhead when
     // !!!! accessing the different types of IO. This
-    // !!!! approach would also probably be safer in 
+    // !!!! approach would also probably be safer in
     // !!!! terms of detecting damaged protocol.
     //
     chronIntIdResTable < baseNMIU > ioTable;
@@ -241,23 +237,23 @@ private:
     resTable < tcpiiu, caServerID > serverTable;
     tsDLList < tcpiiu > circuitList;
     tsDLList < SearchDest > searchDestList;
-    tsFreeList 
-        < class tcpiiu, 32, epicsMutexNOOP > 
+    tsFreeList
+        < class tcpiiu, 32, epicsMutexNOOP >
             freeListVirtualCircuit;
-    tsFreeList 
-        < class netReadNotifyIO, 1024, epicsMutexNOOP > 
+    tsFreeList
+        < class netReadNotifyIO, 1024, epicsMutexNOOP >
             freeListReadNotifyIO;
-    tsFreeList 
-        < class netWriteNotifyIO, 1024, epicsMutexNOOP > 
+    tsFreeList
+        < class netWriteNotifyIO, 1024, epicsMutexNOOP >
             freeListWriteNotifyIO;
-    tsFreeList 
-        < class netSubscription, 1024, epicsMutexNOOP > 
+    tsFreeList
+        < class netSubscription, 1024, epicsMutexNOOP >
             freeListSubscription;
-    tsFreeList 
-        < class nciu, 1024, epicsMutexNOOP > 
+    tsFreeList
+        < class nciu, 1024, epicsMutexNOOP >
             channelFreeList;
-    tsFreeList 
-        < class msgForMultiplyDefinedPV, 16 > 
+    tsFreeList
+        < class msgForMultiplyDefinedPV, 16 >
             mdpvFreeList;
     cacComBufMemoryManager comBufMemMgr;
     bheFreeStore bheFreeList;
@@ -285,74 +281,74 @@ private:
     unsigned iiuExistenceCount;
     bool cacShutdownInProgress;
 
-    void recycleReadNotifyIO ( 
+    void recycleReadNotifyIO (
         epicsGuard < epicsMutex > &, netReadNotifyIO &io );
-    void recycleWriteNotifyIO ( 
+    void recycleWriteNotifyIO (
         epicsGuard < epicsMutex > &, netWriteNotifyIO &io );
-    void recycleSubscription ( 
+    void recycleSubscription (
         epicsGuard < epicsMutex > &, netSubscription &io );
 
-    void disconnectChannel ( 
-        epicsGuard < epicsMutex > & cbGuard, 
+    void disconnectChannel (
+        epicsGuard < epicsMutex > & cbGuard,
         epicsGuard < epicsMutex > & guard, nciu & chan );
 
-    void ioExceptionNotify ( unsigned id, int status, 
+    void ioExceptionNotify ( unsigned id, int status,
         const char * pContext, unsigned type, arrayElementCount count );
-    void ioExceptionNotifyAndUninstall ( unsigned id, int status, 
+    void ioExceptionNotifyAndUninstall ( unsigned id, int status,
         const char * pContext, unsigned type, arrayElementCount count );
 
-    void pvMultiplyDefinedNotify ( msgForMultiplyDefinedPV & mfmdpv, 
+    void pvMultiplyDefinedNotify ( msgForMultiplyDefinedPV & mfmdpv,
         const char * pChannelName, const char * pAcc, const char * pRej );
 
     // recv protocol stubs
-    bool versionAction ( callbackManager &, tcpiiu &, 
+    bool versionAction ( callbackManager &, tcpiiu &,
         const epicsTime & currentTime, const caHdrLargeArray &, void *pMsgBdy );
-    bool echoRespAction ( callbackManager &, tcpiiu &, 
+    bool echoRespAction ( callbackManager &, tcpiiu &,
         const epicsTime & currentTime, const caHdrLargeArray &, void *pMsgBdy );
-    bool writeNotifyRespAction ( callbackManager &, tcpiiu &, 
+    bool writeNotifyRespAction ( callbackManager &, tcpiiu &,
         const epicsTime & currentTime, const caHdrLargeArray &, void *pMsgBdy );
-    bool searchRespAction ( callbackManager &, tcpiiu &, 
+    bool searchRespAction ( callbackManager &, tcpiiu &,
         const epicsTime & currentTime, const caHdrLargeArray &, void *pMsgBdy );
-    bool readNotifyRespAction ( callbackManager &, tcpiiu &, 
+    bool readNotifyRespAction ( callbackManager &, tcpiiu &,
         const epicsTime & currentTime, const caHdrLargeArray &, void *pMsgBdy );
-    bool eventRespAction ( callbackManager &, tcpiiu &, 
+    bool eventRespAction ( callbackManager &, tcpiiu &,
         const epicsTime & currentTime, const caHdrLargeArray &, void *pMsgBdy );
-    bool readRespAction ( callbackManager &, tcpiiu &, 
+    bool readRespAction ( callbackManager &, tcpiiu &,
         const epicsTime & currentTime, const caHdrLargeArray &, void *pMsgBdy );
-    bool clearChannelRespAction ( callbackManager &, tcpiiu &, 
+    bool clearChannelRespAction ( callbackManager &, tcpiiu &,
         const epicsTime & currentTime, const caHdrLargeArray &, void *pMsgBdy );
-    bool exceptionRespAction ( callbackManager &, tcpiiu &, 
+    bool exceptionRespAction ( callbackManager &, tcpiiu &,
         const epicsTime & currentTime, const caHdrLargeArray &, void *pMsgBdy );
-    bool accessRightsRespAction ( callbackManager &, tcpiiu &, 
+    bool accessRightsRespAction ( callbackManager &, tcpiiu &,
         const epicsTime & currentTime, const caHdrLargeArray &, void *pMsgBdy );
-    bool createChannelRespAction ( callbackManager &, tcpiiu &, 
+    bool createChannelRespAction ( callbackManager &, tcpiiu &,
         const epicsTime & currentTime, const caHdrLargeArray &, void *pMsgBdy );
-    bool verifyAndDisconnectChan ( callbackManager &, tcpiiu &, 
+    bool verifyAndDisconnectChan ( callbackManager &, tcpiiu &,
         const epicsTime & currentTime, const caHdrLargeArray &, void *pMsgBdy );
-    bool badTCPRespAction ( callbackManager &, tcpiiu &, 
+    bool badTCPRespAction ( callbackManager &, tcpiiu &,
         const epicsTime & currentTime, const caHdrLargeArray &, void *pMsgBdy );
 
-    typedef bool ( cac::*pProtoStubTCP ) ( 
-        callbackManager &, tcpiiu &, 
+    typedef bool ( cac::*pProtoStubTCP ) (
+        callbackManager &, tcpiiu &,
         const epicsTime & currentTime, const caHdrLargeArray &, void *pMsgBdy );
     static const pProtoStubTCP tcpJumpTableCAC [];
 
-    bool defaultExcep ( callbackManager &, tcpiiu &iiu, const caHdrLargeArray &hdr, 
+    bool defaultExcep ( callbackManager &, tcpiiu &iiu, const caHdrLargeArray &hdr,
                     const char *pCtx, unsigned status );
-    bool eventAddExcep ( callbackManager &, tcpiiu &iiu, const caHdrLargeArray &hdr, 
+    bool eventAddExcep ( callbackManager &, tcpiiu &iiu, const caHdrLargeArray &hdr,
                     const char *pCtx, unsigned status );
-    bool readExcep ( callbackManager &, tcpiiu &iiu, const caHdrLargeArray &hdr, 
+    bool readExcep ( callbackManager &, tcpiiu &iiu, const caHdrLargeArray &hdr,
                     const char *pCtx, unsigned status );
-    bool writeExcep ( callbackManager &, tcpiiu &iiu, const caHdrLargeArray &hdr, 
+    bool writeExcep ( callbackManager &, tcpiiu &iiu, const caHdrLargeArray &hdr,
                     const char *pCtx, unsigned status );
-    bool clearChanExcep ( callbackManager &, tcpiiu &iiu, const caHdrLargeArray &hdr, 
+    bool clearChanExcep ( callbackManager &, tcpiiu &iiu, const caHdrLargeArray &hdr,
                     const char *pCtx, unsigned status );
-    bool readNotifyExcep ( callbackManager &, tcpiiu &iiu, const caHdrLargeArray &hdr, 
+    bool readNotifyExcep ( callbackManager &, tcpiiu &iiu, const caHdrLargeArray &hdr,
                     const char *pCtx, unsigned status );
-    bool writeNotifyExcep ( callbackManager &, tcpiiu &iiu, const caHdrLargeArray &hdr, 
+    bool writeNotifyExcep ( callbackManager &, tcpiiu &iiu, const caHdrLargeArray &hdr,
                     const char *pCtx, unsigned status );
-    typedef bool ( cac::*pExcepProtoStubTCP ) ( 
-                    callbackManager &, tcpiiu &iiu, const caHdrLargeArray &hdr, 
+    typedef bool ( cac::*pExcepProtoStubTCP ) (
+                    callbackManager &, tcpiiu &iiu, const caHdrLargeArray &hdr,
                     const char *pCtx, unsigned status );
     static const pExcepProtoStubTCP tcpExcepJumpTableCAC [];
 
@@ -375,8 +371,8 @@ inline epicsMutex & cac::mutexRef ()
     return this->mutex;
 }
 
-inline int cac :: varArgsPrintFormated ( 
-    epicsGuard < epicsMutex > & callbackControl, 
+inline int cac :: varArgsPrintFormated (
+    epicsGuard < epicsMutex > & callbackControl,
     const char *pformat, va_list args ) const
 {
     callbackControl.assertIdenticalMutex ( this->cbMutex );
@@ -435,14 +431,14 @@ inline notifyGuard::~notifyGuard ()
     this->notify.callbackProcessingCompleteNotify ();
 }
 
-inline callbackManager::callbackManager ( 
+inline callbackManager::callbackManager (
     cacContextNotify & notify, epicsMutex & callbackControl ) :
     notifyGuard ( notify ), cbGuard ( callbackControl )
 {
 }
 
 inline nciu * cac::lookupChannel (
-    epicsGuard < epicsMutex > & guard, 
+    epicsGuard < epicsMutex > & guard,
     const cacChannel::ioid & idIn )
 {
     guard.assertIdenticalMutex ( this->mutex );
@@ -458,11 +454,11 @@ inline unsigned cac ::
     maxContiguousFrames ( epicsGuard < epicsMutex > & ) const
 {
     return maxContigFrames;
-}    
+}
 
 inline double cac ::
     connectionTimeout ( epicsGuard < epicsMutex > & guard )
-{    
+{
     guard.assertIdenticalMutex ( this->mutex );
     return this->connTMO;
 }
diff --git a/src/ca/client/cacIO.h b/src/ca/client/cacIO.h
index f1cf81f47..4e8af4a05 100644
--- a/src/ca/client/cacIO.h
+++ b/src/ca/client/cacIO.h
@@ -5,18 +5,18 @@
 *     Operator of Los Alamos National Laboratory.
 * EPICS BASE Versions 3.13.7
 * and higher are distributed subject to a Software License Agreement found
-* in file LICENSE that is included with this distribution. 
+* in file LICENSE that is included with this distribution.
 \*************************************************************************/
-/*  
+/*
+ *
  *
- *                              
  *                    L O S  A L A M O S
  *              Los Alamos National Laboratory
  *               Los Alamos, New Mexico 87545
- *                                  
+ *
  *  Copyright, 1986, The Regents of the University of California.
- *                                  
- *           
+ *
+ *
  *	Author Jeffrey O. Hill
  *	johill@lanl.gov
  *	505 665 1831
@@ -74,9 +74,9 @@ public:
     virtual ~cacWriteNotify () = 0;
     virtual void completion ( epicsGuard < epicsMutex > & ) = 0;
 // we should probably have a different vf for each type of exception ????
-    virtual void exception ( 
+    virtual void exception (
         epicsGuard < epicsMutex > &,
-        int status, const char * pContext, 
+        int status, const char * pContext,
         unsigned type, arrayElementCount count ) = 0;
 };
 
@@ -85,13 +85,13 @@ public:
 class epicsShareClass cacReadNotify {
 public:
     virtual ~cacReadNotify () = 0;
-    virtual void completion ( 
-        epicsGuard < epicsMutex > &, unsigned type, 
+    virtual void completion (
+        epicsGuard < epicsMutex > &, unsigned type,
         arrayElementCount count, const void * pData ) = 0;
 // we should probably have a different vf for each type of exception ????
-    virtual void exception ( 
-        epicsGuard < epicsMutex > &, int status, 
-        const char * pContext, unsigned type, 
+    virtual void exception (
+        epicsGuard < epicsMutex > &, int status,
+        const char * pContext, unsigned type,
         arrayElementCount count ) = 0;
 };
 
@@ -100,21 +100,21 @@ public:
 class epicsShareClass cacStateNotify {
 public:
     virtual ~cacStateNotify () = 0;
-    virtual void current ( 
-        epicsGuard < epicsMutex > &, unsigned type, 
+    virtual void current (
+        epicsGuard < epicsMutex > &, unsigned type,
         arrayElementCount count, const void * pData ) = 0;
 // we should probably have a different vf for each type of exception ????
-    virtual void exception ( 
-        epicsGuard < epicsMutex > &, int status, 
-        const char *pContext, unsigned type, 
+    virtual void exception (
+        epicsGuard < epicsMutex > &, int status,
+        const char *pContext, unsigned type,
         arrayElementCount count ) = 0;
 };
 
 class caAccessRights {
 public:
-    caAccessRights ( 
-        bool readPermit = false, 
-        bool writePermit = false, 
+    caAccessRights (
+        bool readPermit = false,
+        bool writePermit = false,
         bool operatorConfirmationRequest = false);
     void setReadPermit ();
     void setWritePermit ();
@@ -138,20 +138,30 @@ public:
     virtual void disconnectNotify ( epicsGuard < epicsMutex > & ) = 0;
     virtual void serviceShutdownNotify (
         epicsGuard < epicsMutex > & mutualExclusionGuard ) = 0;
-    virtual void accessRightsNotify ( 
+    virtual void accessRightsNotify (
         epicsGuard < epicsMutex > &, const caAccessRights & ) = 0;
-    virtual void exception ( 
+    virtual void exception (
         epicsGuard < epicsMutex > &, int status, const char *pContext ) = 0;
 // we should probably have a different vf for each type of exception ????
-    virtual void readException ( 
+    virtual void readException (
         epicsGuard < epicsMutex > &, int status, const char *pContext,
         unsigned type, arrayElementCount count, void *pValue ) = 0;
 // we should probably have a different vf for each type of exception ????
-    virtual void writeException ( 
+    virtual void writeException (
         epicsGuard < epicsMutex > &, int status, const char * pContext,
         unsigned type, arrayElementCount count ) = 0;
 };
 
+class CallbackGuard :
+    public epicsGuard < epicsMutex > {
+public:
+    CallbackGuard ( epicsMutex & mutex ) :
+        epicsGuard < epicsMutex > ( mutex ) {}
+private:
+    CallbackGuard ( const CallbackGuard & );
+    CallbackGuard & operator = ( const CallbackGuard & );
+};
+
 //
 // Notes
 // 1) This interface assumes that when a channel is deleted then all
@@ -174,43 +184,52 @@ public:
 
     cacChannel ( cacChannelNotify & );
     virtual void destroy (
+        CallbackGuard & callbackGuard,
         epicsGuard < epicsMutex > & mutualExclusionGuard ) = 0;
     cacChannelNotify & notify () const; // required ?????
     virtual unsigned getName (
-        epicsGuard < epicsMutex > &, 
+        epicsGuard < epicsMutex > &,
         char * pBuf, unsigned bufLen ) const throw () = 0;
     // !! deprecated, avoid use  !!
     virtual const char * pName (
         epicsGuard < epicsMutex > & guard ) const throw () = 0;
-    virtual void show ( 
+    virtual void show (
         epicsGuard < epicsMutex > &,
         unsigned level ) const = 0;
     virtual void initiateConnect (
         epicsGuard < epicsMutex > & ) = 0;
-    virtual unsigned requestMessageBytesPending ( 
+    virtual unsigned requestMessageBytesPending (
         epicsGuard < epicsMutex > & mutualExclusionGuard ) = 0;
-    virtual void flush ( 
+    virtual void flush (
         epicsGuard < epicsMutex > & mutualExclusionGuard ) = 0;
-    virtual ioStatus read ( 
+    virtual ioStatus read (
         epicsGuard < epicsMutex > &,
-        unsigned type, arrayElementCount count, 
+        unsigned type, arrayElementCount count,
         cacReadNotify &, ioid * = 0 ) = 0;
-    virtual void write ( 
+    virtual void write (
         epicsGuard < epicsMutex > &,
-        unsigned type, arrayElementCount count, 
+        unsigned type, arrayElementCount count,
         const void *pValue ) = 0;
-    virtual ioStatus write ( 
+    virtual ioStatus write (
         epicsGuard < epicsMutex > &,
-        unsigned type, arrayElementCount count, 
+        unsigned type, arrayElementCount count,
         const void *pValue, cacWriteNotify &, ioid * = 0 ) = 0;
-    virtual void subscribe ( 
-        epicsGuard < epicsMutex > &, unsigned type, 
-        arrayElementCount count, unsigned mask, cacStateNotify &, 
+    virtual void subscribe (
+        epicsGuard < epicsMutex > &, unsigned type,
+        arrayElementCount count, unsigned mask, cacStateNotify &,
         ioid * = 0 ) = 0;
-    virtual void ioCancel ( 
+    // The primary mutex must be released when calling the user's
+    // callback, and therefore a finite interval exists when we are
+    // moving forward with the intent to call the users callback
+    // but the users IO could be deleted during this interval.
+    // To prevent the user's callback from being called after
+    // destroying his IO we must past a guard for the callback
+    // mutex here.
+    virtual void ioCancel (
+        CallbackGuard & callbackGuard,
         epicsGuard < epicsMutex > & mutualExclusionGuard,
         const ioid & ) = 0;
-    virtual void ioShow ( 
+    virtual void ioShow (
         epicsGuard < epicsMutex > &,
         const ioid &, unsigned level ) const = 0;
     virtual short nativeType (
@@ -226,9 +245,9 @@ public:
     virtual double receiveWatchdogDelay (
         epicsGuard < epicsMutex > & ) const; // negative DBL_MAX if UKN
     virtual bool ca_v42_ok (
-        epicsGuard < epicsMutex > & ) const; 
+        epicsGuard < epicsMutex > & ) const;
     virtual bool connected (
-        epicsGuard < epicsMutex > & ) const; 
+        epicsGuard < epicsMutex > & ) const;
     virtual unsigned getHostName (
         epicsGuard < epicsMutex > &,
         char * pBuf, unsigned bufLength ) const throw ();
@@ -261,11 +280,11 @@ private:
 class epicsShareClass cacContext {
 public:
     virtual ~cacContext ();
-    virtual cacChannel & createChannel ( 
+    virtual cacChannel & createChannel (
         epicsGuard < epicsMutex > &,
-        const char * pChannelName, cacChannelNotify &, 
+        const char * pChannelName, cacChannelNotify &,
         cacChannel::priLev = cacChannel::priorityDefault ) = 0;
-    virtual void flush ( 
+    virtual void flush (
         epicsGuard < epicsMutex > & ) = 0;
     virtual unsigned circuitCount (
         epicsGuard < epicsMutex > & ) const = 0;
@@ -273,18 +292,18 @@ public:
         epicsGuard < epicsMutex > & ) const = 0;
     virtual unsigned beaconAnomaliesSinceProgramStart (
         epicsGuard < epicsMutex > & ) const = 0;
-    virtual void show ( 
+    virtual void show (
         epicsGuard < epicsMutex > &, unsigned level ) const = 0;
 };
 
 class epicsShareClass cacContextNotify {
 public:
     virtual ~cacContextNotify () = 0;
-    virtual cacContext & createNetworkContext ( 
+    virtual cacContext & createNetworkContext (
         epicsMutex & mutualExclusion, epicsMutex & callbackControl ) = 0;
 // we should probably have a different vf for each type of exception ????
-    virtual void exception ( 
-        epicsGuard < epicsMutex > &, int status, const char * pContext, 
+    virtual void exception (
+        epicsGuard < epicsMutex > &, int status, const char * pContext,
         const char * pFileName, unsigned lineNo ) = 0;
 // perhaps this should be phased out in deference to the exception mechanism
     virtual int varArgsPrintFormated ( const char * pformat, va_list args ) const = 0;
@@ -300,9 +319,9 @@ public:
 class epicsShareClass cacService {
 public:
     virtual ~cacService () = 0;
-    virtual cacContext & contextCreate ( 
-        epicsMutex & mutualExclusion, 
-        epicsMutex & callbackControl, 
+    virtual cacContext & contextCreate (
+        epicsMutex & mutualExclusion,
+        epicsMutex & callbackControl,
         cacContextNotify & ) = 0;
 };
 
@@ -320,9 +339,9 @@ inline cacChannelNotify & cacChannel::notify () const
     return this->callback;
 }
 
-inline caAccessRights::caAccessRights ( 
+inline caAccessRights::caAccessRights (
     bool readPermit, bool writePermit, bool operatorConfirmationRequest) :
-    f_readPermit ( readPermit ), f_writePermit ( writePermit ), 
+    f_readPermit ( readPermit ), f_writePermit ( writePermit ),
         f_operatorConfirmationRequest ( operatorConfirmationRequest ) {}
 
 inline void caAccessRights::setReadPermit ()
diff --git a/src/ca/client/nciu.cpp b/src/ca/client/nciu.cpp
index 4efe6f5ab..8eb89c5c3 100644
--- a/src/ca/client/nciu.cpp
+++ b/src/ca/client/nciu.cpp
@@ -5,7 +5,7 @@
 *     Operator of Los Alamos National Laboratory.
 * EPICS BASE Versions 3.13.7
 * and higher are distributed subject to a Software License Agreement found
-* in file LICENSE that is included with this distribution. 
+* in file LICENSE that is included with this distribution.
 \*************************************************************************/
 /*
  *
@@ -39,9 +39,9 @@
 #include "db_access.h" // for INVALID_DB_REQ
 #include "noopiiu.h"
 
-nciu::nciu ( cac & cacIn, netiiu & iiuIn, cacChannelNotify & chanIn, 
+nciu::nciu ( cac & cacIn, netiiu & iiuIn, cacChannelNotify & chanIn,
             const char *pNameIn, cacChannel::priLev pri ) :
-    cacChannel ( chanIn ), 
+    cacChannel ( chanIn ),
     cacCtx ( cacIn ),
     piiu ( & iiuIn ),
     sid ( UINT_MAX ),
@@ -57,7 +57,7 @@ nciu::nciu ( cac & cacIn, netiiu & iiuIn, cacChannelNotify & chanIn,
     if ( nameLengthTmp > MAX_UDP_SEND - sizeof ( caHdr ) || nameLengthTmp > USHRT_MAX ) {
         throw cacChannel::badString ();
     }
-    
+
     if ( pri > 0xff ) {
         throw cacChannel::badPriority ();
     }
@@ -76,26 +76,28 @@ nciu::~nciu ()
 // channels are created by the user, and only destroyed by the user
 // using this routine
 void nciu::destroy (
-    epicsGuard < epicsMutex > & guard )
-{    
+    CallbackGuard & callbackGuard,
+    epicsGuard < epicsMutex > & mutualExcusionGuard )
+{
     while ( baseNMIU * pNetIO = this->eventq.first () ) {
-        bool success = this->cacCtx.destroyIO ( guard, pNetIO->getId (), *this );
+        bool success = this->cacCtx.destroyIO ( callbackGuard, mutualExcusionGuard,
+                                                    pNetIO->getId (), *this );
         assert ( success );
     }
-    
+
     // if the claim reply has not returned yet then we will issue
     // the clear channel request to the server when the claim reply
     // arrives and there is no matching nciu in the client
-    if ( this->channelNode::isInstalledInServer ( guard ) ) {
-        this->getPIIU(guard)->clearChannelRequest ( 
-            guard, this->sid, this->id );
+    if ( this->channelNode::isInstalledInServer ( mutualExcusionGuard ) ) {
+        this->getPIIU(mutualExcusionGuard)->clearChannelRequest (
+                        mutualExcusionGuard, this->sid, this->id );
     }
-    this->piiu->uninstallChan ( guard, *this );
-    this->cacCtx.destroyChannel ( guard, *this );
+    this->piiu->uninstallChan ( mutualExcusionGuard, *this );
+    this->cacCtx.destroyChannel ( mutualExcusionGuard, *this );
 }
 
 void nciu::operator delete ( void * )
-{ 
+{
     // Visual C++ .net appears to require operator delete if
     // placement operator delete is defined? I smell a ms rat
     // because if I declare placement new and delete, but
@@ -105,15 +107,15 @@ void nciu::operator delete ( void * )
         __FILE__, __LINE__ );
 }
 
-void nciu::initiateConnect ( 
+void nciu::initiateConnect (
     epicsGuard < epicsMutex > & guard )
-{   
+{
     this->cacCtx.initiateConnect ( guard, *this, this->piiu );
 }
 
-void nciu::connect ( unsigned nativeType, 
-	unsigned nativeCount, unsigned sidIn, 
-    epicsGuard < epicsMutex > & /* cbGuard */, 
+void nciu::connect ( unsigned nativeType,
+	unsigned nativeCount, unsigned sidIn,
+    epicsGuard < epicsMutex > & /* cbGuard */,
     epicsGuard < epicsMutex > & guard )
 {
     guard.assertIdenticalMutex ( this->cacCtx.mutexRef () );
@@ -127,7 +129,7 @@ void nciu::connect ( unsigned nativeType,
 
     /*
      * if less than v4.1 then the server will never
-     * send access rights and there will always be access 
+     * send access rights and there will always be access
      */
     bool v41Ok = this->piiu->ca_v41_ok ( guard );
     if ( ! v41Ok ) {
@@ -138,49 +140,49 @@ void nciu::connect ( unsigned nativeType,
     /*
         * if less than v4.1 then the server will never
         * send access rights and we know that there
-        * will always be access and also need to call 
+        * will always be access and also need to call
         * their call back here
         */
     if ( ! v41Ok ) {
-        this->notify().accessRightsNotify ( 
+        this->notify().accessRightsNotify (
             guard, this->accessRightState );
     }
 
     // channel uninstal routine grabs the callback lock so
-    // a channel will not be deleted while a call back is 
+    // a channel will not be deleted while a call back is
     // in progress
     //
-    // the callback lock is also taken when a channel 
-    // disconnects to prevent a race condition with the 
+    // the callback lock is also taken when a channel
+    // disconnects to prevent a race condition with the
     // code below - ie we hold the callback lock here
     // so a chanel cant be destroyed out from under us.
     this->notify().connectNotify ( guard );
 }
 
-void nciu::unresponsiveCircuitNotify ( 
-    epicsGuard < epicsMutex > & cbGuard, 
+void nciu::unresponsiveCircuitNotify (
+    epicsGuard < epicsMutex > & cbGuard,
     epicsGuard < epicsMutex > & guard )
 {
     ioid tmpId = this->getId ();
     cac & caRefTmp = this->cacCtx;
     guard.assertIdenticalMutex ( this->cacCtx.mutexRef () );
-    this->cacCtx.disconnectAllIO ( cbGuard, guard, 
+    this->cacCtx.disconnectAllIO ( cbGuard, guard,
         *this, this->eventq );
     this->notify().disconnectNotify ( guard );
-    // if they destroy the channel in their disconnect 
+    // if they destroy the channel in their disconnect
     // handler then we have to be very careful to not
     // touch this object if it has been destroyed
     nciu * pChan = caRefTmp.lookupChannel ( guard, tmpId );
     if ( pChan ) {
         caAccessRights noRights;
         pChan->notify().accessRightsNotify ( guard, noRights );
-        // likewise, they might destroy the channel in their access rights 
-        // handler so we have to be very careful to not touch this 
+        // likewise, they might destroy the channel in their access rights
+        // handler so we have to be very careful to not touch this
         // object from here on down
     }
 }
 
-void nciu::setServerAddressUnknown ( netiiu & newiiu, 
+void nciu::setServerAddressUnknown ( netiiu & newiiu,
                                 epicsGuard < epicsMutex > & guard )
 {
     guard.assertIdenticalMutex ( this->cacCtx.mutexRef () );
@@ -193,8 +195,8 @@ void nciu::setServerAddressUnknown ( netiiu & newiiu,
     this->accessRightState.clrWritePermit();
 }
 
-void nciu::accessRightsStateChange ( 
-    const caAccessRights & arIn, epicsGuard < epicsMutex > & /* cbGuard */, 
+void nciu::accessRightsStateChange (
+    const caAccessRights & arIn, epicsGuard < epicsMutex > & /* cbGuard */,
     epicsGuard < epicsMutex > & guard )
 {
     guard.assertIdenticalMutex ( this->cacCtx.mutexRef () );
@@ -204,7 +206,7 @@ void nciu::accessRightsStateChange (
     // the channel delete routine takes the call back lock so
     // that this will not be called when the channel is being
     // deleted.
-    //       
+    //
     this->notify().accessRightsNotify ( guard, this->accessRightState );
 }
 
@@ -213,7 +215,7 @@ void nciu::accessRightsStateChange (
  */
 bool nciu::searchMsg ( epicsGuard < epicsMutex > & guard )
 {
-   bool success = this->piiu->searchMsg ( 
+   bool success = this->piiu->searchMsg (
         guard, this->getId (), this->pNameStr, this->nameLength );
    if ( success ) {
         if ( this->retry < UINT_MAX ) {
@@ -256,21 +258,21 @@ unsigned nciu::nameLen (
     return this->nameLength;
 }
 
-unsigned nciu::requestMessageBytesPending ( 
+unsigned nciu::requestMessageBytesPending (
     epicsGuard < epicsMutex > & guard )
 {
     return piiu->requestMessageBytesPending ( guard );
 }
 
-void nciu::flush ( 
+void nciu::flush (
     epicsGuard < epicsMutex > & guard )
 {
     piiu->flush ( guard );
 }
 
-cacChannel::ioStatus nciu::read ( 
+cacChannel::ioStatus nciu::read (
     epicsGuard < epicsMutex > & guard,
-    unsigned type, arrayElementCount countIn, 
+    unsigned type, arrayElementCount countIn,
     cacReadNotify &notify, ioid *pId )
 {
     guard.assertIdenticalMutex ( this->cacCtx.mutexRef () );
@@ -292,7 +294,7 @@ cacChannel::ioStatus nciu::read (
         throw cacChannel::badType ();
     }
 
-    netReadNotifyIO & io = this->cacCtx.readNotifyRequest ( 
+    netReadNotifyIO & io = this->cacCtx.readNotifyRequest (
         guard, *this, *this, type, countIn, notify );
     if ( pId ) {
         *pId = io.getId ();
@@ -314,7 +316,7 @@ void nciu::stringVerify ( const char *pStr, const unsigned count )
     }
 }
 
-void nciu::write ( 
+void nciu::write (
     epicsGuard < epicsMutex > & guard,
     unsigned type, arrayElementCount countIn, const void * pValue )
 {
@@ -337,8 +339,8 @@ void nciu::write (
     this->piiu->writeRequest ( guard, *this, type, countIn, pValue );
 }
 
-cacChannel::ioStatus nciu::write ( 
-    epicsGuard < epicsMutex > & guard, unsigned type, arrayElementCount countIn, 
+cacChannel::ioStatus nciu::write (
+    epicsGuard < epicsMutex > & guard, unsigned type, arrayElementCount countIn,
     const void * pValue, cacWriteNotify & notify, ioid * pId )
 {
     // make sure that they get this and not "no write access"
@@ -356,7 +358,7 @@ cacChannel::ioStatus nciu::write (
         nciu::stringVerify ( (char *) pValue, countIn );
     }
 
-    netWriteNotifyIO & io = this->cacCtx.writeNotifyRequest ( 
+    netWriteNotifyIO & io = this->cacCtx.writeNotifyRequest (
         guard, *this, *this, type, countIn, pValue, notify );
     if ( pId ) {
         *pId = io.getId ();
@@ -365,13 +367,13 @@ cacChannel::ioStatus nciu::write (
     return cacChannel::iosAsynch;
 }
 
-void nciu::subscribe ( 
-    epicsGuard < epicsMutex > & guard, unsigned type, 
-    arrayElementCount nElem, unsigned mask, 
+void nciu::subscribe (
+    epicsGuard < epicsMutex > & guard, unsigned type,
+    arrayElementCount nElem, unsigned mask,
     cacStateNotify & notify, ioid *pId )
 {
-    netSubscription & io = this->cacCtx.subscriptionRequest ( 
-                guard, *this, *this, type, nElem, mask, notify, 
+    netSubscription & io = this->cacCtx.subscriptionRequest (
+                guard, *this, *this, type, nElem, mask, notify,
                 this->channelNode::isInstalledInServer ( guard ) );
     this->eventq.add ( io );
     if ( pId ) {
@@ -379,24 +381,27 @@ void nciu::subscribe (
     }
 }
 
-void nciu::ioCancel ( 
-    epicsGuard < epicsMutex > & guard, const ioid & idIn )
+void nciu::ioCancel (
+    CallbackGuard & callbackGuard,
+    epicsGuard < epicsMutex > & mutualExclusionGuard,
+    const ioid & idIn )
 {
-    this->cacCtx.destroyIO ( guard, idIn, *this );
+    this->cacCtx.destroyIO ( callbackGuard,
+          mutualExclusionGuard, idIn, *this );
 }
 
-void nciu::ioShow ( 
-    epicsGuard < epicsMutex > & guard, 
+void nciu::ioShow (
+    epicsGuard < epicsMutex > & guard,
     const ioid &idIn, unsigned level ) const
 {
     this->cacCtx.ioShow ( guard, idIn, level );
 }
 
-unsigned nciu::getHostName ( 
+unsigned nciu::getHostName (
     epicsGuard < epicsMutex > & guard,
     char *pBuf, unsigned bufLength ) const throw ()
-{   
-    return this->piiu->getHostName ( 
+{
+    return this->piiu->getHostName (
         guard, pBuf, bufLength );
 }
 
@@ -424,7 +429,7 @@ short nciu::nativeType (
     return type;
 }
 
-arrayElementCount nciu::nativeElementCount ( 
+arrayElementCount nciu::nativeElementCount (
     epicsGuard < epicsMutex > & guard ) const
 {
     arrayElementCount countOut = 0ul;
@@ -472,20 +477,20 @@ void nciu::show ( unsigned level ) const
     this->show ( guard, level );
 }
 
-void nciu::show ( 
+void nciu::show (
     epicsGuard < epicsMutex > & guard, unsigned level ) const
 {
     if ( this->connected ( guard ) ) {
         char hostNameTmp [256];
         this->getHostName ( guard, hostNameTmp, sizeof ( hostNameTmp ) );
-        ::printf ( "Channel \"%s\", connected to server %s", 
+        ::printf ( "Channel \"%s\", connected to server %s",
             this->pNameStr, hostNameTmp );
         if ( level > 1u ) {
             int tmpTypeCode = static_cast < int > ( this->typeCode );
             ::printf ( ", native type %s, native element count %u",
                 dbf_type_to_text ( tmpTypeCode ), this->count );
-            ::printf ( ", %sread access, %swrite access", 
-                this->accessRightState.readPermit() ? "" : "no ", 
+            ::printf ( ", %sread access, %swrite access",
+                this->accessRightState.readPermit() ? "" : "no ",
                 this->accessRightState.writePermit() ? "" : "no ");
         }
         ::printf ( "\n" );
@@ -495,7 +500,7 @@ void nciu::show (
     }
 
     if ( level > 2u ) {
-        ::printf ( "\tnetwork IO pointer = %p\n", 
+        ::printf ( "\tnetwork IO pointer = %p\n",
             static_cast <void *> ( this->piiu ) );
         ::printf ( "\tserver identifier %u\n", this->sid );
         ::printf ( "\tsearch retry number=%u\n", this->retry );
@@ -503,7 +508,7 @@ void nciu::show (
     }
 }
 
-void nciu::ioCompletionNotify ( 
+void nciu::ioCompletionNotify (
     epicsGuard < epicsMutex > &, class baseNMIU & io )
 {
     this->eventq.remove ( io );
@@ -544,7 +549,7 @@ void nciu::sendSubscriptionUpdateRequests ( epicsGuard < epicsMutex > & guard )
             pNetIO->forceSubscriptionUpdate ( guard, *this );
         }
         catch ( ... ) {
-            errlogPrintf ( 
+            errlogPrintf (
                 "CAC: failed to send subscription update request "
                 "during channel connect\n" );
         }
@@ -552,68 +557,68 @@ void nciu::sendSubscriptionUpdateRequests ( epicsGuard < epicsMutex > & guard )
     }
 }
 
-void nciu::disconnectAllIO ( 
-    epicsGuard < epicsMutex > & cbGuard, 
+void nciu::disconnectAllIO (
+    epicsGuard < epicsMutex > & cbGuard,
     epicsGuard < epicsMutex > & guard )
 {
-    this->cacCtx.disconnectAllIO ( cbGuard, guard, 
+    this->cacCtx.disconnectAllIO ( cbGuard, guard,
         *this, this->eventq );
 }
 
 void nciu::serviceShutdownNotify (
-    epicsGuard < epicsMutex > & callbackControlGuard, 
+    epicsGuard < epicsMutex > & callbackControlGuard,
     epicsGuard < epicsMutex > & mutualExclusionGuard )
 {
     this->setServerAddressUnknown ( noopIIU, mutualExclusionGuard );
     this->notify().serviceShutdownNotify ( mutualExclusionGuard );
 }
 
-void channelNode::setRespPendingState ( 
-    epicsGuard < epicsMutex > &, unsigned index ) 
+void channelNode::setRespPendingState (
+    epicsGuard < epicsMutex > &, unsigned index )
 {
-    this->listMember = 
+    this->listMember =
         static_cast < channelNode::channelState >
         ( channelNode::cs_searchRespPending0 + index );
     if ( this->listMember > cs_searchRespPending17 ) {
-        throw std::runtime_error ( 
+        throw std::runtime_error (
             "resp search timer index out of bounds" );
     }
 }
 
-void channelNode::setReqPendingState ( 
-    epicsGuard < epicsMutex > &, unsigned index ) 
+void channelNode::setReqPendingState (
+    epicsGuard < epicsMutex > &, unsigned index )
 {
-    this->listMember = 
+    this->listMember =
         static_cast < channelNode::channelState >
         ( channelNode::cs_searchReqPending0 + index );
     if ( this->listMember > cs_searchReqPending17 ) {
-        throw std::runtime_error ( 
+        throw std::runtime_error (
             "req search timer index out of bounds" );
     }
 }
 
-unsigned channelNode::getMaxSearchTimerCount () 
+unsigned channelNode::getMaxSearchTimerCount ()
 {
-    return epicsMin ( 
+    return epicsMin (
         cs_searchReqPending17 - cs_searchReqPending0,
         cs_searchRespPending17 - cs_searchRespPending0 ) + 1u;
 }
 
-unsigned channelNode::getSearchTimerIndex ( 
+unsigned channelNode::getSearchTimerIndex (
     epicsGuard < epicsMutex > & )
 {
     channelNode::channelState chanState = this->listMember;
     unsigned index = 0u;
-    if ( chanState >= cs_searchReqPending0 && 
+    if ( chanState >= cs_searchReqPending0 &&
             chanState <= cs_searchReqPending17 ) {
         index = chanState - cs_searchReqPending0;
     }
-    else if ( chanState >= cs_searchRespPending0 && 
+    else if ( chanState >= cs_searchRespPending0 &&
             chanState <= cs_searchRespPending17 ) {
         index = chanState - cs_searchRespPending0;
     }
     else {
-        throw std::runtime_error ( 
+        throw std::runtime_error (
             "channel was expected to be in a search timer, but wasnt" );;
     }
     return index;
diff --git a/src/ca/client/nciu.h b/src/ca/client/nciu.h
index 0e83e9f96..4994465a8 100644
--- a/src/ca/client/nciu.h
+++ b/src/ca/client/nciu.h
@@ -5,24 +5,24 @@
 *     Operator of Los Alamos National Laboratory.
 * EPICS BASE Versions 3.13.7
 * and higher are distributed subject to a Software License Agreement found
-* in file LICENSE that is included with this distribution. 
+* in file LICENSE that is included with this distribution.
 \*************************************************************************/
-/*  
+/*
+ *
  *
- *                              
  *                    L O S  A L A M O S
  *              Los Alamos National Laboratory
  *               Los Alamos, New Mexico 87545
- *                                  
+ *
  *  Copyright, 1986, The Regents of the University of California.
- *                                  
- *           
+ *
+ *
  *	Author Jeffrey O. Hill
  *	johill@lanl.gov
  *	505 665 1831
  */
 
-#ifndef nciuh  
+#ifndef nciuh
 #define nciuh
 
 #ifdef epicsExportSharedSymbols
@@ -123,45 +123,45 @@ private:
 
 class privateInterfaceForIO {
 public:
-    virtual void ioCompletionNotify ( 
+    virtual void ioCompletionNotify (
         epicsGuard < epicsMutex > &, class baseNMIU & ) = 0;
-    virtual arrayElementCount nativeElementCount ( 
+    virtual arrayElementCount nativeElementCount (
         epicsGuard < epicsMutex > & ) const = 0;
     virtual bool connected ( epicsGuard < epicsMutex > & ) const = 0;
 protected:
     virtual ~privateInterfaceForIO() {}
 };
 
-class nciu : 
+class nciu :
     public cacChannel,
-    public chronIntIdRes < nciu >, 
+    public chronIntIdRes < nciu >,
     public channelNode,
     private privateInterfaceForIO {
 public:
-    nciu ( cac &, netiiu &, cacChannelNotify &, 
+    nciu ( cac &, netiiu &, cacChannelNotify &,
         const char * pNameIn, cacChannel::priLev );
     ~nciu ();
-    void connect ( unsigned nativeType, 
-        unsigned nativeCount, unsigned sid, 
-        epicsGuard < epicsMutex > & cbGuard, 
+    void connect ( unsigned nativeType,
+        unsigned nativeCount, unsigned sid,
+        epicsGuard < epicsMutex > & cbGuard,
         epicsGuard < epicsMutex > & guard );
-    void connect ( epicsGuard < epicsMutex > & cbGuard, 
+    void connect ( epicsGuard < epicsMutex > & cbGuard,
         epicsGuard < epicsMutex > & guard );
-    void unresponsiveCircuitNotify ( 
-        epicsGuard < epicsMutex > & cbGuard, 
+    void unresponsiveCircuitNotify (
+        epicsGuard < epicsMutex > & cbGuard,
         epicsGuard < epicsMutex > & guard );
-    void circuitHangupNotify ( class udpiiu &, 
-        epicsGuard < epicsMutex > & cbGuard, 
+    void circuitHangupNotify ( class udpiiu &,
+        epicsGuard < epicsMutex > & cbGuard,
         epicsGuard < epicsMutex > & guard );
-    void setServerAddressUnknown ( 
+    void setServerAddressUnknown (
         netiiu & newiiu, epicsGuard < epicsMutex > & guard );
-    bool searchMsg ( 
+    bool searchMsg (
         epicsGuard < epicsMutex > & );
     void serviceShutdownNotify (
-        epicsGuard < epicsMutex > & callbackControlGuard, 
+        epicsGuard < epicsMutex > & callbackControlGuard,
         epicsGuard < epicsMutex > & mutualExclusionGuard );
-    void accessRightsStateChange ( const caAccessRights &, 
-        epicsGuard < epicsMutex > & cbGuard, 
+    void accessRightsStateChange ( const caAccessRights &,
+        epicsGuard < epicsMutex > & cbGuard,
         epicsGuard < epicsMutex > & guard );
     ca_uint32_t getSID (
         epicsGuard < epicsMutex > & ) const;
@@ -172,12 +172,12 @@ public:
     const netiiu * getConstPIIU (
         epicsGuard < epicsMutex > & ) const;
     cac & getClient ();
-    void searchReplySetUp ( netiiu &iiu, unsigned sidIn, 
+    void searchReplySetUp ( netiiu &iiu, unsigned sidIn,
         ca_uint16_t typeIn, arrayElementCount countIn,
         epicsGuard < epicsMutex > & );
-    void show ( 
+    void show (
         unsigned level ) const;
-    void show ( 
+    void show (
         epicsGuard < epicsMutex > &,
         unsigned level ) const;
     unsigned getName (
@@ -189,22 +189,22 @@ public:
         epicsGuard < epicsMutex > & ) const;
     unsigned getHostName (
         epicsGuard < epicsMutex > &,
-        char * pBuf, unsigned bufLen ) const throw (); 
-    void writeException ( 
+        char * pBuf, unsigned bufLen ) const throw ();
+    void writeException (
         epicsGuard < epicsMutex > &, epicsGuard < epicsMutex > &,
         int status, const char *pContext, unsigned type, arrayElementCount count );
     cacChannel::priLev getPriority (
         epicsGuard < epicsMutex > & ) const;
-    void * operator new ( 
+    void * operator new (
         size_t size, tsFreeList < class nciu, 1024, epicsMutexNOOP > & );
     epicsPlacementDeleteOperator (
         ( void *, tsFreeList < class nciu, 1024, epicsMutexNOOP > & ))
     //arrayElementCount nativeElementCount ( epicsGuard < epicsMutex > & ) const;
     void resubscribe ( epicsGuard < epicsMutex > & );
     void sendSubscriptionUpdateRequests ( epicsGuard < epicsMutex > & );
-    void disconnectAllIO ( 
+    void disconnectAllIO (
         epicsGuard < epicsMutex > &, epicsGuard < epicsMutex > & );
-    bool connected ( epicsGuard < epicsMutex > & ) const; 
+    bool connected ( epicsGuard < epicsMutex > & ) const;
     unsigned getcount() const { return count; }
 
 private:
@@ -218,38 +218,47 @@ private:
     unsigned retry; // search retry number
     unsigned short nameLength; // channel name length
     ca_uint16_t typeCode;
-    ca_uint8_t priority; 
+    ca_uint8_t priority;
     virtual void destroy (
+        CallbackGuard & callbackGuard,
         epicsGuard < epicsMutex > & mutualExclusionGuard );
     void initiateConnect (
         epicsGuard < epicsMutex > & );
-    unsigned requestMessageBytesPending ( 
+    unsigned requestMessageBytesPending (
         epicsGuard < epicsMutex > & mutualExclusionGuard );
-    void flush ( 
+    void flush (
         epicsGuard < epicsMutex > & mutualExclusionGuard );
-    ioStatus read ( 
+    ioStatus read (
         epicsGuard < epicsMutex > &,
-        unsigned type, arrayElementCount count, 
+        unsigned type, arrayElementCount count,
         cacReadNotify &, ioid * );
-    void write ( 
+    void write (
         epicsGuard < epicsMutex > &,
-        unsigned type, arrayElementCount count, 
+        unsigned type, arrayElementCount count,
         const void *pValue );
-    ioStatus write ( 
+    ioStatus write (
         epicsGuard < epicsMutex > &,
-        unsigned type, arrayElementCount count, 
+        unsigned type, arrayElementCount count,
         const void *pValue, cacWriteNotify &, ioid * );
-    void subscribe ( 
-        epicsGuard < epicsMutex > & guard, 
-        unsigned type, arrayElementCount nElem, 
+    void subscribe (
+        epicsGuard < epicsMutex > & guard,
+        unsigned type, arrayElementCount nElem,
         unsigned mask, cacStateNotify &notify, ioid * );
-    virtual void ioCancel ( 
+    // The primary mutex must be released when calling the user's
+    // callback, and therefore a finite interval exists when we are
+    // moving forward with the intent to call the users callback
+    // but the users IO could be deleted during this interval.
+    // To prevent the user's callback from being called after
+    // destroying his IO we must past a guard for the callback
+    // mutex here.
+    virtual void ioCancel (
+        CallbackGuard & callbackGuard,
         epicsGuard < epicsMutex > & mutualExclusionGuard,
         const ioid & );
-    void ioShow ( 
+    void ioShow (
         epicsGuard < epicsMutex > &,
         const ioid &, unsigned level ) const;
-    short nativeType ( 
+    short nativeType (
         epicsGuard < epicsMutex > & ) const;
     caAccessRights accessRights (
         epicsGuard < epicsMutex > & ) const;
@@ -264,7 +273,7 @@ private:
     arrayElementCount nativeElementCount (
         epicsGuard < epicsMutex > & ) const;
     static void stringVerify ( const char *pStr, const unsigned count );
-    void ioCompletionNotify ( 
+    void ioCompletionNotify (
         epicsGuard < epicsMutex > &, class baseNMIU & );
     const char * pHostName (
         epicsGuard < epicsMutex > & guard ) const throw ();
@@ -273,16 +282,16 @@ private:
     void operator delete ( void * );
 };
 
-inline void * nciu::operator new ( size_t size, 
+inline void * nciu::operator new ( size_t size,
     tsFreeList < class nciu, 1024, epicsMutexNOOP > & freeList )
-{ 
+{
     return freeList.allocate ( size );
 }
 
 #ifdef CXX_PLACEMENT_DELETE
 inline void nciu::operator delete ( void * pCadaver,
     tsFreeList < class nciu, 1024, epicsMutexNOOP > & freeList )
-{ 
+{
     freeList.release ( pCadaver, sizeof ( nciu ) );
 }
 #endif
@@ -300,19 +309,19 @@ inline ca_uint32_t nciu::getCID (
 }
 
 // this is to only be used by early protocol revisions
-inline void nciu::connect ( epicsGuard < epicsMutex > & cbGuard, 
+inline void nciu::connect ( epicsGuard < epicsMutex > & cbGuard,
                 epicsGuard < epicsMutex > & guard )
 {
-    this->connect ( this->typeCode, this->count, 
+    this->connect ( this->typeCode, this->count,
         this->sid, cbGuard, guard );
 }
 
-inline void nciu::searchReplySetUp ( netiiu &iiu, unsigned sidIn, 
+inline void nciu::searchReplySetUp ( netiiu &iiu, unsigned sidIn,
     ca_uint16_t typeIn, arrayElementCount countIn,
     epicsGuard < epicsMutex > & )
 {
     this->piiu = & iiu;
-    this->typeCode = typeIn;      
+    this->typeCode = typeIn;
     this->count = countIn;
     this->sid = sidIn;
 }
@@ -323,13 +332,13 @@ inline netiiu * nciu::getPIIU (
     return this->piiu;
 }
 
-inline void nciu::writeException ( 
-    epicsGuard < epicsMutex > & /* cbGuard */, 
-    epicsGuard < epicsMutex > & guard, 
-    int status, const char * pContext, 
+inline void nciu::writeException (
+    epicsGuard < epicsMutex > & /* cbGuard */,
+    epicsGuard < epicsMutex > & guard,
+    int status, const char * pContext,
     unsigned typeIn, arrayElementCount countIn )
 {
-    this->notify().writeException ( guard, 
+    this->notify().writeException ( guard,
         status, pContext, typeIn, countIn );
 }
 
@@ -357,15 +366,15 @@ inline channelNode::channelNode () :
 
 inline bool channelNode::isConnected ( epicsGuard < epicsMutex > & ) const
 {
-    return 
-        this->listMember == cs_connected || 
+    return
+        this->listMember == cs_connected ||
         this->listMember == cs_subscripReqPend ||
         this->listMember == cs_subscripUpdateReqPend;
 }
 
 inline bool channelNode::isInstalledInServer ( epicsGuard < epicsMutex > & ) const
 {
-    return 
+    return
         this->listMember == cs_connected ||
         this->listMember == cs_subscripReqPend ||
         this->listMember == cs_unrespCircuit ||
diff --git a/src/ca/client/oldAccess.h b/src/ca/client/oldAccess.h
index ff4f36e4a..ee5020513 100644
--- a/src/ca/client/oldAccess.h
+++ b/src/ca/client/oldAccess.h
@@ -5,19 +5,19 @@
 *     Operator of Los Alamos National Laboratory.
 * EPICS BASE Versions 3.13.7
 * and higher are distributed subject to a Software License Agreement found
-* in file LICENSE that is included with this distribution. 
+* in file LICENSE that is included with this distribution.
 \*************************************************************************/
 
-/*  
+/*
+ *
  *
- *                              
  *                    L O S  A L A M O S
  *              Los Alamos National Laboratory
  *               Los Alamos, New Mexico 87545
- *                                  
+ *
  *  Copyright, The Regents of the University of California.
- *                                  
- *           
+ *
+ *
  *	Author Jeffrey O. Hill
  *	johill@lanl.gov
  *	505 665 1831
@@ -48,92 +48,94 @@
 
 struct oldChannelNotify : private cacChannelNotify {
 public:
-    oldChannelNotify ( 
-        epicsGuard < epicsMutex > &, struct ca_client_context &, 
-        const char * pName, caCh * pConnCallBackIn, 
+    oldChannelNotify (
+        epicsGuard < epicsMutex > &, struct ca_client_context &,
+        const char * pName, caCh * pConnCallBackIn,
         void * pPrivateIn, capri priority );
-    void destructor ( 
-        epicsGuard < epicsMutex > & guard );
+    void destructor (
+        CallbackGuard & cbGuard,
+        epicsGuard < epicsMutex > & mutexGuard );
 
     // legacy C API
-    friend unsigned epicsShareAPI ca_get_host_name ( 
+    friend unsigned epicsShareAPI ca_get_host_name (
         chid pChan, char * pBuf, unsigned bufLength );
-    friend const char * epicsShareAPI ca_host_name ( 
+    friend const char * epicsShareAPI ca_host_name (
         chid pChan );
-    friend const char * epicsShareAPI ca_name ( 
+    friend const char * epicsShareAPI ca_name (
         chid pChan );
-    friend void epicsShareAPI ca_set_puser ( 
+    friend void epicsShareAPI ca_set_puser (
         chid pChan, void * puser );
-    friend void * epicsShareAPI ca_puser ( 
+    friend void * epicsShareAPI ca_puser (
         chid pChan );
-    friend int epicsShareAPI ca_change_connection_event ( 
+    friend int epicsShareAPI ca_change_connection_event (
         chid pChan, caCh * pfunc );
-    friend int epicsShareAPI ca_replace_access_rights_event ( 
+    friend int epicsShareAPI ca_replace_access_rights_event (
         chid pChan, caArh *pfunc );
-    friend int epicsShareAPI ca_array_get ( chtype type, 
+    friend int epicsShareAPI ca_array_get ( chtype type,
         arrayElementCount count, chid pChan, void * pValue );
-    friend int epicsShareAPI ca_array_get_callback ( chtype type, 
+    friend int epicsShareAPI ca_array_get_callback ( chtype type,
         arrayElementCount count, chid pChan,
         caEventCallBackFunc *pfunc, void *arg );
-    friend int epicsShareAPI ca_array_put ( 
-        chtype type, arrayElementCount count, 
+    friend int epicsShareAPI ca_array_put (
+        chtype type, arrayElementCount count,
         chid pChan, const void * pValue );
-    friend int epicsShareAPI ca_array_put_callback ( 
-        chtype type, arrayElementCount count, 
-        chid pChan, const void *pValue, 
+    friend int epicsShareAPI ca_array_put_callback (
+        chtype type, arrayElementCount count,
+        chid pChan, const void *pValue,
         caEventCallBackFunc *pfunc, void *usrarg );
-    friend double epicsShareAPI ca_beacon_period ( 
+    friend double epicsShareAPI ca_beacon_period (
         chid pChan );
-    friend unsigned epicsShareAPI ca_search_attempts ( 
+    friend unsigned epicsShareAPI ca_search_attempts (
         chid pChan );
-    friend unsigned epicsShareAPI ca_write_access ( 
+    friend unsigned epicsShareAPI ca_write_access (
         chid pChan );
-    friend unsigned epicsShareAPI ca_read_access ( 
+    friend unsigned epicsShareAPI ca_read_access (
         chid pChan );
-    friend short epicsShareAPI ca_field_type ( 
+    friend short epicsShareAPI ca_field_type (
         chid pChan );
-    friend arrayElementCount epicsShareAPI ca_element_count ( 
+    friend arrayElementCount epicsShareAPI ca_element_count (
         chid pChan );
-    friend int epicsShareAPI ca_v42_ok ( 
+    friend int epicsShareAPI ca_v42_ok (
         chid pChan );
-    friend int epicsShareAPI ca_create_subscription ( 
-        chtype type, arrayElementCount count, chid pChan, 
-        long mask, caEventCallBackFunc * pCallBack, 
+    friend int epicsShareAPI ca_create_subscription (
+        chtype type, arrayElementCount count, chid pChan,
+        long mask, caEventCallBackFunc * pCallBack,
         void * pCallBackArg, evid * monixptr );
-    friend enum channel_state epicsShareAPI ca_state ( 
-        chid pChan ); 
-    friend double epicsShareAPI ca_receive_watchdog_delay ( 
+    friend enum channel_state epicsShareAPI ca_state (
+        chid pChan );
+    friend double epicsShareAPI ca_receive_watchdog_delay (
         chid pChan );
 
     unsigned getName (
         epicsGuard < epicsMutex > &,
         char * pBuf, unsigned bufLen ) const throw ();
-    void show ( 
+    void show (
         epicsGuard < epicsMutex > &,
         unsigned level ) const;
     void initiateConnect (
         epicsGuard < epicsMutex > & );
-    void read ( 
+    void read (
         epicsGuard < epicsMutex > &,
-        unsigned type, arrayElementCount count, 
+        unsigned type, arrayElementCount count,
         cacReadNotify &notify, cacChannel::ioid *pId = 0 );
-    void write ( 
+    void write (
         epicsGuard < epicsMutex > &,
-        unsigned type, arrayElementCount count, const void *pValue, 
+        unsigned type, arrayElementCount count, const void *pValue,
         cacWriteNotify &, cacChannel::ioid *pId = 0 );
-    void ioCancel ( 
-        epicsGuard < epicsMutex > & mutualExclusionGuard, 
+    void ioCancel (
+        CallbackGuard & callbackGuard,
+        epicsGuard < epicsMutex > & mutualExclusionGuard,
         const cacChannel::ioid & );
-    void ioShow ( 
+    void ioShow (
         epicsGuard < epicsMutex > & guard,
         const cacChannel::ioid &, unsigned level ) const;
     ca_client_context & getClientCtx ();
-    void eliminateExcessiveSendBacklog ( 
+    void eliminateExcessiveSendBacklog (
         epicsGuard < epicsMutex > & );
 
-    void * operator new ( size_t size, 
+    void * operator new ( size_t size,
         tsFreeList < struct oldChannelNotify, 1024, epicsMutexNOOP > & );
-    epicsPlacementDeleteOperator (( void * , 
+    epicsPlacementDeleteOperator (( void * ,
         tsFreeList < struct oldChannelNotify, 1024, epicsMutexNOOP > & ))
 protected:
     ~oldChannelNotify ();
@@ -150,7 +152,7 @@ private:
     void disconnectNotify ( epicsGuard < epicsMutex > & );
     void serviceShutdownNotify (
         epicsGuard < epicsMutex > & mutualExclusionGuard );
-    void accessRightsNotify (  
+    void accessRightsNotify (
         epicsGuard < epicsMutex > &, const caAccessRights & );
     void exception ( epicsGuard < epicsMutex > &,
         int status, const char * pContext );
@@ -160,24 +162,24 @@ private:
     void writeException ( epicsGuard < epicsMutex > &,
         int status, const char * pContext,
         unsigned type, arrayElementCount count );
-	oldChannelNotify ( const oldChannelNotify & );
-	oldChannelNotify & operator = ( const oldChannelNotify & );
+    oldChannelNotify ( const oldChannelNotify & );
+    oldChannelNotify & operator = ( const oldChannelNotify & );
     void operator delete ( void * );
 };
 
 class getCopy : public cacReadNotify {
 public:
-    getCopy ( 
-        epicsGuard < epicsMutex > & guard, 
-        ca_client_context & cacCtx, 
-        oldChannelNotify &, unsigned type, 
+    getCopy (
+        epicsGuard < epicsMutex > & guard,
+        ca_client_context & cacCtx,
+        oldChannelNotify &, unsigned type,
         arrayElementCount count, void *pValue );
     ~getCopy ();
     void show ( unsigned level ) const;
     void cancel ();
-    void * operator new ( size_t size, 
+    void * operator new ( size_t size,
         tsFreeList < class getCopy, 1024, epicsMutexNOOP > & );
-    epicsPlacementDeleteOperator (( void *, 
+    epicsPlacementDeleteOperator (( void *,
         tsFreeList < class getCopy, 1024, epicsMutexNOOP > & ))
 private:
     arrayElementCount count;
@@ -187,10 +189,10 @@ private:
     unsigned ioSeqNo;
     unsigned type;
     void completion (
-        epicsGuard < epicsMutex > &, unsigned type, 
+        epicsGuard < epicsMutex > &, unsigned type,
         arrayElementCount count, const void *pData );
-    void exception ( 
-        epicsGuard < epicsMutex > &, int status, 
+    void exception (
+        epicsGuard < epicsMutex > &, int status,
         const char *pContext, unsigned type, arrayElementCount count );
 	getCopy ( const getCopy & );
 	getCopy & operator = ( const getCopy & );
@@ -199,23 +201,23 @@ private:
 
 class getCallback : public cacReadNotify {
 public:
-    getCallback ( 
-        oldChannelNotify & chanIn, 
+    getCallback (
+        oldChannelNotify & chanIn,
         caEventCallBackFunc *pFunc, void *pPrivate );
-    ~getCallback (); 
-    void * operator new ( size_t size, 
+    ~getCallback ();
+    void * operator new ( size_t size,
         tsFreeList < class getCallback, 1024, epicsMutexNOOP > & );
-    epicsPlacementDeleteOperator (( void *, 
+    epicsPlacementDeleteOperator (( void *,
         tsFreeList < class getCallback, 1024, epicsMutexNOOP > & ))
 private:
     oldChannelNotify & chan;
     caEventCallBackFunc * pFunc;
     void * pPrivate;
     void completion (
-        epicsGuard < epicsMutex > &, unsigned type, 
+        epicsGuard < epicsMutex > &, unsigned type,
         arrayElementCount count, const void *pData);
-    void exception ( 
-        epicsGuard < epicsMutex > &, int status, 
+    void exception (
+        epicsGuard < epicsMutex > &, int status,
         const char * pContext, unsigned type, arrayElementCount count );
 	getCallback ( const getCallback & );
 	getCallback & operator = ( const getCallback & );
@@ -224,21 +226,21 @@ private:
 
 class putCallback : public cacWriteNotify {
 public:
-    putCallback ( 
-        oldChannelNotify &, 
+    putCallback (
+        oldChannelNotify &,
         caEventCallBackFunc *pFunc, void *pPrivate );
     ~putCallback ();
-    void * operator new ( size_t size, 
+    void * operator new ( size_t size,
         tsFreeList < class putCallback, 1024, epicsMutexNOOP > & );
-    epicsPlacementDeleteOperator (( void *, 
+    epicsPlacementDeleteOperator (( void *,
         tsFreeList < class putCallback, 1024, epicsMutexNOOP > & ))
 private:
     oldChannelNotify & chan;
     caEventCallBackFunc * pFunc;
     void *pPrivate;
     void completion ( epicsGuard < epicsMutex > & );
-    void exception ( 
-        epicsGuard < epicsMutex > &, int status, const char *pContext, 
+    void exception (
+        epicsGuard < epicsMutex > &, int status, const char *pContext,
         unsigned type, arrayElementCount count );
 	putCallback ( const putCallback & );
 	putCallback & operator = ( const putCallback & );
@@ -249,17 +251,25 @@ struct oldSubscription : private cacStateNotify {
 public:
     oldSubscription (
         epicsGuard < epicsMutex > & guard,
-        oldChannelNotify & chanIn, cacChannel & io, 
+        oldChannelNotify & chanIn, cacChannel & io,
         unsigned type, arrayElementCount nElem, unsigned mask,
         caEventCallBackFunc * pFuncIn, void * pPrivateIn,
         evid * );
     ~oldSubscription ();
     oldChannelNotify & channel () const;
-    void cancel ( 
-        epicsGuard < epicsMutex > & guard );
-    void * operator new ( size_t size, 
+    // The primary mutex must be released when calling the user's
+    // callback, and therefore a finite interval exists when we are
+    // moving forward with the intent to call the users callback
+    // but the users IO could be deleted during this interval.
+    // To prevent the user's callback from being called after
+    // destroying his IO we must past a guard for the callback
+    // mutex here.
+    void cancel (
+        CallbackGuard & callbackGuard,
+        epicsGuard < epicsMutex > & mutualExclusionGuard );
+    void * operator new ( size_t size,
         tsFreeList < struct oldSubscription, 1024, epicsMutexNOOP > & );
-    epicsPlacementDeleteOperator (( void *, 
+    epicsPlacementDeleteOperator (( void *,
         tsFreeList < struct oldSubscription, 1024, epicsMutexNOOP > & ))
 private:
     oldChannelNotify & chan;
@@ -267,10 +277,10 @@ private:
     caEventCallBackFunc * pFunc;
     void * pPrivate;
     void current (
-        epicsGuard < epicsMutex > &, unsigned type, 
+        epicsGuard < epicsMutex > &, unsigned type,
         arrayElementCount count, const void *pData );
-    void exception ( 
-        epicsGuard < epicsMutex > &, int status, 
+    void exception (
+        epicsGuard < epicsMutex > &, int status,
         const char *pContext, unsigned type, arrayElementCount count );
 	oldSubscription ( const oldSubscription & );
 	oldSubscription & operator = ( const oldSubscription & );
@@ -285,16 +295,16 @@ struct ca_client_context : public cacContextNotify
 public:
     ca_client_context ( bool enablePreemptiveCallback = false );
     virtual ~ca_client_context ();
-    void changeExceptionEvent ( 
+    void changeExceptionEvent (
         caExceptionHandler * pfunc, void * arg );
-    void registerForFileDescriptorCallBack ( 
+    void registerForFileDescriptorCallBack (
         CAFDHANDLER * pFunc, void * pArg );
     void replaceErrLogHandler ( caPrintfFunc * ca_printf_func );
-    cacChannel & createChannel ( 
-        epicsGuard < epicsMutex > &, const char * pChannelName, 
+    cacChannel & createChannel (
+        epicsGuard < epicsMutex > &, const char * pChannelName,
         cacChannelNotify &, cacChannel::priLev pri );
     void flush ( epicsGuard < epicsMutex > & );
-    void eliminateExcessiveSendBacklog ( 
+    void eliminateExcessiveSendBacklog (
         epicsGuard < epicsMutex > &, cacChannel & );
     int pendIO ( const double & timeout );
     int pendEvent ( const double & timeout );
@@ -304,18 +314,18 @@ public:
     unsigned sequenceNumberOfOutstandingIO (
         epicsGuard < epicsMutex > & ) const;
     unsigned beaconAnomaliesSinceProgramStart () const;
-    void incrementOutstandingIO ( 
+    void incrementOutstandingIO (
         epicsGuard < epicsMutex > &, unsigned ioSeqNo );
-    void decrementOutstandingIO ( 
+    void decrementOutstandingIO (
         epicsGuard < epicsMutex > &, unsigned ioSeqNo );
-    void exception ( 
-        epicsGuard < epicsMutex > &, int status, const char * pContext, 
-        const char * pFileName, unsigned lineNo );
-    void exception ( 
+    void exception (
         epicsGuard < epicsMutex > &, int status, const char * pContext,
-        const char * pFileName, unsigned lineNo, oldChannelNotify & chan, 
+        const char * pFileName, unsigned lineNo );
+    void exception (
+        epicsGuard < epicsMutex > &, int status, const char * pContext,
+        const char * pFileName, unsigned lineNo, oldChannelNotify & chan,
         unsigned type, arrayElementCount count, unsigned op );
-    void blockForEventAndEnableCallbacks ( 
+    void blockForEventAndEnableCallbacks (
         epicsEvent & event, const double & timeout );
     CASG * lookupCASG ( epicsGuard < epicsMutex > &, unsigned id );
     static void installDefaultService ( cacService & );
@@ -325,9 +335,9 @@ public:
 // perhaps these should be eliminated in deference to the exception mechanism
     int printFormated ( const char * pformat, ... ) const;
     int varArgsPrintFormated ( const char * pformat, va_list args ) const;
-    void signal ( int ca_status, const char * pfilenm, 
+    void signal ( int ca_status, const char * pfilenm,
                      int lineno, const char * pFormat, ... );
-    void vSignal ( int ca_status, const char * pfilenm, 
+    void vSignal ( int ca_status, const char * pfilenm,
                      int lineno, const char  *pFormat, va_list args );
     bool preemptiveCallbakIsEnabled () const;
     void destroyGetCopy ( epicsGuard < epicsMutex > &, getCopy & );
@@ -336,25 +346,27 @@ public:
     void destroySubscription ( epicsGuard < epicsMutex > &, oldSubscription & );
     epicsMutex & mutexRef () const;
 
-
-    // legacy C API    
+    template < class T >
+    void whenThereIsAnExceptionDestroySyncGroupIO ( epicsGuard < epicsMutex > &, T & );
+   
+    // legacy C API
     friend int epicsShareAPI ca_create_channel (
         const char * name_str, caCh * conn_func, void * puser,
         capri priority, chid * chanptr );
     friend int epicsShareAPI ca_clear_channel ( chid pChan );
-    friend int epicsShareAPI ca_array_get ( chtype type, 
+    friend int epicsShareAPI ca_array_get ( chtype type,
         arrayElementCount count, chid pChan, void * pValue );
-    friend int epicsShareAPI ca_array_get_callback ( chtype type, 
+    friend int epicsShareAPI ca_array_get_callback ( chtype type,
         arrayElementCount count, chid pChan,
         caEventCallBackFunc *pfunc, void *arg );
-    friend int epicsShareAPI ca_array_put ( chtype type, 
+    friend int epicsShareAPI ca_array_put ( chtype type,
         arrayElementCount count, chid pChan, const void * pValue );
-    friend int epicsShareAPI ca_array_put_callback ( chtype type, 
-        arrayElementCount count, chid pChan, const void * pValue, 
+    friend int epicsShareAPI ca_array_put_callback ( chtype type,
+        arrayElementCount count, chid pChan, const void * pValue,
         caEventCallBackFunc *pfunc, void *usrarg );
-    friend int epicsShareAPI ca_create_subscription ( 
-        chtype type, arrayElementCount count, chid pChan, 
-        long mask, caEventCallBackFunc * pCallBack, void * pCallBackArg, 
+    friend int epicsShareAPI ca_create_subscription (
+        chtype type, arrayElementCount count, chid pChan,
+        long mask, caEventCallBackFunc * pCallBack, void * pCallBackArg,
         evid *monixptr );
     friend int epicsShareAPI ca_flush_io ();
     friend int epicsShareAPI ca_clear_subscription ( evid pMon );
@@ -363,6 +375,17 @@ public:
     friend int epicsShareAPI ca_sg_block ( const CA_SYNC_GID gid, ca_real timeout );
     friend int epicsShareAPI ca_sg_reset ( const CA_SYNC_GID gid );
     friend int epicsShareAPI ca_sg_test ( const CA_SYNC_GID gid );
+    friend int epicsShareAPI ca_sg_array_get ( const CA_SYNC_GID gid,
+                              chtype type, arrayElementCount count,
+                              chid pChan, void *pValue );
+    friend int epicsShareAPI ca_sg_array_put ( const CA_SYNC_GID gid,
+                              chtype type, arrayElementCount count,
+                              chid pChan, const void *pValue );
+    friend int ca_sync_group_destroy ( CallbackGuard & cbGuard,
+                                 epicsGuard < epicsMutex > & guard,
+                                ca_client_context & cac, const CA_SYNC_GID gid );
+    friend void sync_group_reset ( ca_client_context & client, 
+                                                  CASG & sg );
 
     // exceptions
     class noSocket {};
@@ -375,11 +398,11 @@ private:
     tsFreeList < struct oldSubscription, 1024, epicsMutexNOOP > subscriptionFreeList;
     tsFreeList < struct CASG, 128, epicsMutexNOOP > casgFreeList;
     mutable epicsMutex mutex;
-    mutable epicsMutex cbMutex; 
+    mutable epicsMutex cbMutex;
     epicsEvent ioDone;
     epicsEvent callbackThreadActivityComplete;
     epicsThreadId createdByThread;
-    epics_auto_ptr < epicsGuard < epicsMutex > > pCallbackGuard;
+    epics_auto_ptr < CallbackGuard > pCallbackGuard;
     epics_auto_ptr < cacContext > pServiceContext;
     caExceptionHandler * ca_exception_func;
     void * ca_exception_arg;
@@ -397,15 +420,15 @@ private:
     void attachToClientCtx ();
     void callbackProcessingInitiateNotify ();
     void callbackProcessingCompleteNotify ();
-    cacContext & createNetworkContext ( 
+    cacContext & createNetworkContext (
         epicsMutex & mutualExclusion, epicsMutex & callbackControl );
     void _sendWakeupMsg ();
 
-	ca_client_context ( const ca_client_context & );
-	ca_client_context & operator = ( const ca_client_context & );
-	
-	friend void cacOnceFunc ( void * );
-	friend void cacExitHandler ( void *);
+    ca_client_context ( const ca_client_context & );
+    ca_client_context & operator = ( const ca_client_context & );
+
+    friend void cacOnceFunc ( void * );
+    friend void cacExitHandler ( void *);
     static cacService * pDefaultService;
     static epicsMutex * pDefaultServiceInstallMutex;
     static const unsigned flushBlockThreshold;
@@ -425,7 +448,7 @@ inline unsigned oldChannelNotify::getName (
     return this->io.getName ( guard, pBuf, bufLen );
 }
 
-inline void oldChannelNotify::show ( 
+inline void oldChannelNotify::show (
     epicsGuard < epicsMutex > & guard,
     unsigned level ) const
 {
@@ -438,58 +461,60 @@ inline void oldChannelNotify::initiateConnect (
     this->io.initiateConnect ( guard );
 }
 
-inline void oldChannelNotify::ioCancel (    
-    epicsGuard < epicsMutex > & guard, 
+inline void oldChannelNotify::ioCancel (
+    CallbackGuard & callbackGuard,
+    epicsGuard < epicsMutex > & mutualExclusionGuard,
     const cacChannel::ioid & id )
 {
-    this->io.ioCancel ( guard, id );
+    this->io.ioCancel ( callbackGuard, mutualExclusionGuard, id );
 }
 
-inline void oldChannelNotify::ioShow ( 
+inline void oldChannelNotify::ioShow (
     epicsGuard < epicsMutex > & guard,
     const cacChannel::ioid & id, unsigned level ) const
 {
     this->io.ioShow ( guard, id, level );
 }
 
-inline void oldChannelNotify::eliminateExcessiveSendBacklog ( 
+inline void oldChannelNotify::eliminateExcessiveSendBacklog (
     epicsGuard < epicsMutex > & guard )
 {
     this->cacCtx.eliminateExcessiveSendBacklog ( guard, this->io );
 }
 
-inline void * oldChannelNotify::operator new ( size_t size, 
+inline void * oldChannelNotify::operator new ( size_t size,
     tsFreeList < struct oldChannelNotify, 1024, epicsMutexNOOP > & freeList )
 {
     return freeList.allocate ( size );
 }
 
 #ifdef CXX_PLACEMENT_DELETE
-inline void oldChannelNotify::operator delete ( void *pCadaver, 
-    tsFreeList < struct oldChannelNotify, 1024, epicsMutexNOOP > & freeList ) 
+inline void oldChannelNotify::operator delete ( void *pCadaver,
+    tsFreeList < struct oldChannelNotify, 1024, epicsMutexNOOP > & freeList )
 {
     freeList.release ( pCadaver );
 }
 #endif
 
-inline void * oldSubscription::operator new ( size_t size, 
+inline void * oldSubscription::operator new ( size_t size,
     tsFreeList < struct oldSubscription, 1024, epicsMutexNOOP > & freeList )
 {
     return freeList.allocate ( size );
 }
 
 #ifdef CXX_PLACEMENT_DELETE
-inline void oldSubscription::operator delete ( void *pCadaver, 
-    tsFreeList < struct oldSubscription, 1024, epicsMutexNOOP > & freeList ) 
+inline void oldSubscription::operator delete ( void *pCadaver,
+    tsFreeList < struct oldSubscription, 1024, epicsMutexNOOP > & freeList )
 {
     freeList.release ( pCadaver );
 }
 #endif
 
-inline void oldSubscription::cancel ( 
-    epicsGuard < epicsMutex > & guard ) 
+inline void oldSubscription::cancel (
+    CallbackGuard & callbackGuard,
+    epicsGuard < epicsMutex > & mutualExclusionGuard )
 {
-    this->chan.ioCancel ( guard, this->id );
+    this->chan.ioCancel ( callbackGuard, mutualExclusionGuard, this->id );
 }
 
 inline oldChannelNotify & oldSubscription::channel () const
@@ -497,43 +522,43 @@ inline oldChannelNotify & oldSubscription::channel () const
     return this->chan;
 }
 
-inline void * getCopy::operator new ( size_t size, 
+inline void * getCopy::operator new ( size_t size,
     tsFreeList < class getCopy, 1024, epicsMutexNOOP > & freeList )
 {
     return freeList.allocate ( size );
 }
 
 #ifdef CXX_PLACEMENT_DELETE
-inline void getCopy::operator delete ( void *pCadaver, 
-    tsFreeList < class getCopy, 1024, epicsMutexNOOP > & freeList ) 
+inline void getCopy::operator delete ( void *pCadaver,
+    tsFreeList < class getCopy, 1024, epicsMutexNOOP > & freeList )
 {
     freeList.release ( pCadaver );
 }
 #endif
 
-inline void * putCallback::operator new ( size_t size, 
+inline void * putCallback::operator new ( size_t size,
     tsFreeList < class putCallback, 1024, epicsMutexNOOP > & freeList )
 {
     return freeList.allocate ( size );
 }
 
 #ifdef CXX_PLACEMENT_DELETE
-inline void putCallback::operator delete ( void * pCadaver, 
-    tsFreeList < class putCallback, 1024, epicsMutexNOOP > & freeList ) 
+inline void putCallback::operator delete ( void * pCadaver,
+    tsFreeList < class putCallback, 1024, epicsMutexNOOP > & freeList )
 {
     freeList.release ( pCadaver );
 }
 #endif
 
-inline void * getCallback::operator new ( size_t size, 
+inline void * getCallback::operator new ( size_t size,
     tsFreeList < class getCallback, 1024, epicsMutexNOOP > & freeList )
 {
     return freeList.allocate ( size );
 }
 
 #ifdef CXX_PLACEMENT_DELETE
-inline void getCallback::operator delete ( void * pCadaver, 
-    tsFreeList < class getCallback, 1024, epicsMutexNOOP > & freeList ) 
+inline void getCallback::operator delete ( void * pCadaver,
+    tsFreeList < class getCallback, 1024, epicsMutexNOOP > & freeList )
 {
     freeList.release ( pCadaver );
 }
@@ -555,5 +580,32 @@ inline unsigned ca_client_context::sequenceNumberOfOutstandingIO (
     // perhaps on SMP systems THERE should be lock/unlock around this
     return this->ioSeqNo;
 }
-        
+
+template < class T >
+void ca_client_context :: whenThereIsAnExceptionDestroySyncGroupIO ( 
+                            epicsGuard < epicsMutex > & guard, T & io )
+{
+    if ( this->pCallbackGuard.get() &&
+        this->createdByThread == epicsThreadGetIdSelf () ) {
+        io.destroy ( *this->pCallbackGuard.get(), guard );
+    }
+    else {
+        // dont reverse the lock hierarchy
+        epicsGuardRelease < epicsMutex > guardRelease ();
+        {
+            //
+            // we will definately stall out here if all of the
+            // following are true
+            //
+            // o user creates non-preemtive mode client library context
+            // o user doesnt periodically call a ca function
+            // o user calls this function from an auxiillary thread
+            //
+            CallbackGuard cbGuard ( this->cbMutex );
+            epicsGuard < epicsMutex > guard ( this->mutex );
+            io.destroy ( cbGuard, guard );
+        }
+    }
+}
+
 #endif // ifndef oldAccessh
diff --git a/src/ca/client/oldChannelNotify.cpp b/src/ca/client/oldChannelNotify.cpp
index 508792b2e..d8c259040 100644
--- a/src/ca/client/oldChannelNotify.cpp
+++ b/src/ca/client/oldChannelNotify.cpp
@@ -5,18 +5,18 @@
 *     Operator of Los Alamos National Laboratory.
 * EPICS BASE Versions 3.13.7
 * and higher are distributed subject to a Software License Agreement found
-* in file LICENSE that is included with this distribution. 
+* in file LICENSE that is included with this distribution.
 \*************************************************************************/
-/*  
+/*
+ *
  *
- *                              
  *                    L O S  A L A M O S
  *              Los Alamos National Laboratory
  *               Los Alamos, New Mexico 87545
- *                                  
+ *
  *  Copyright, 1986, The Regents of the University of California.
- *                                  
- *           
+ *
+ *
  *	Author Jeffrey O. Hill
  *	johill@lanl.gov
  *	505 665 1831
@@ -43,13 +43,13 @@ extern "C" void cacNoopAccesRightsHandler ( struct access_rights_handler_args )
 {
 }
 
-oldChannelNotify::oldChannelNotify ( 
-        epicsGuard < epicsMutex > & guard, ca_client_context & cacIn, 
-        const char *pName, caCh * pConnCallBackIn, 
+oldChannelNotify::oldChannelNotify (
+        epicsGuard < epicsMutex > & guard, ca_client_context & cacIn,
+        const char *pName, caCh * pConnCallBackIn,
         void * pPrivateIn, capri priority ) :
-    cacCtx ( cacIn ), 
-    io ( cacIn.createChannel ( guard, pName, *this, priority ) ), 
-    pConnCallBack ( pConnCallBackIn ), 
+    cacCtx ( cacIn ),
+    io ( cacIn.createChannel ( guard, pName, *this, priority ) ),
+    pConnCallBack ( pConnCallBackIn ),
     pPrivate ( pPrivateIn ), pAccessRightsFunc ( cacNoopAccesRightsHandler ),
     ioSeqNo ( 0 ), currentlyConnected ( false ), prevConnected ( false )
 {
@@ -65,19 +65,20 @@ oldChannelNotify::~oldChannelNotify ()
 }
 
 void  oldChannelNotify::destructor (
-    epicsGuard < epicsMutex > & guard )
+    CallbackGuard & cbGuard,
+    epicsGuard < epicsMutex > & mutexGuard )
 {
-    guard.assertIdenticalMutex ( this->cacCtx.mutexRef () );
-    this->io.destroy ( guard );
+    mutexGuard.assertIdenticalMutex ( this->cacCtx.mutexRef () );
+    this->io.destroy ( cbGuard, mutexGuard );
     // no need to worry about a connect preempting here because
     // the io (the nciu) has been destroyed above
     if ( this->pConnCallBack == 0 && ! this->currentlyConnected ) {
-        this->cacCtx.decrementOutstandingIO ( guard, this->ioSeqNo );
+        this->cacCtx.decrementOutstandingIO ( mutexGuard, this->ioSeqNo );
     }
     this->~oldChannelNotify ();
 }
 
-void oldChannelNotify::connectNotify ( 
+void oldChannelNotify::connectNotify (
     epicsGuard < epicsMutex > & guard )
 {
     this->currentlyConnected = true;
@@ -97,7 +98,7 @@ void oldChannelNotify::connectNotify (
     }
 }
 
-void oldChannelNotify::disconnectNotify ( 
+void oldChannelNotify::disconnectNotify (
     epicsGuard < epicsMutex > & guard )
 {
     this->currentlyConnected = false;
@@ -112,7 +113,7 @@ void oldChannelNotify::disconnectNotify (
         }
     }
     else {
-        this->cacCtx.incrementOutstandingIO ( 
+        this->cacCtx.incrementOutstandingIO (
             guard, this->ioSeqNo );
     }
 }
@@ -123,7 +124,7 @@ void oldChannelNotify::serviceShutdownNotify (
     this->disconnectNotify ( guard );
 }
 
-void oldChannelNotify::accessRightsNotify ( 
+void oldChannelNotify::accessRightsNotify (
     epicsGuard < epicsMutex > & guard, const caAccessRights & ar )
 {
     struct access_rights_handler_args args;
@@ -137,25 +138,25 @@ void oldChannelNotify::accessRightsNotify (
     }
 }
 
-void oldChannelNotify::exception ( 
+void oldChannelNotify::exception (
     epicsGuard < epicsMutex > & guard, int status, const char * pContext )
 {
     this->cacCtx.exception ( guard, status, pContext, __FILE__, __LINE__ );
 }
 
-void oldChannelNotify::readException ( 
+void oldChannelNotify::readException (
     epicsGuard < epicsMutex > & guard, int status, const char *pContext,
     unsigned type, arrayElementCount count, void * /* pValue */ )
 {
-    this->cacCtx.exception ( guard, status, pContext, 
+    this->cacCtx.exception ( guard, status, pContext,
         __FILE__, __LINE__, *this, type, count, CA_OP_GET );
 }
 
-void oldChannelNotify::writeException ( 
+void oldChannelNotify::writeException (
     epicsGuard < epicsMutex > & guard, int status, const char *pContext,
     unsigned type, arrayElementCount count )
 {
-    this->cacCtx.exception ( guard, status, pContext, 
+    this->cacCtx.exception ( guard, status, pContext,
         __FILE__, __LINE__, *this, type, count, CA_OP_PUT );
 }
 
@@ -173,7 +174,7 @@ void oldChannelNotify::operator delete ( void * )
 /*
  * ca_get_host_name ()
  */
-unsigned epicsShareAPI ca_get_host_name ( 
+unsigned epicsShareAPI ca_get_host_name (
     chid pChan, char * pBuf, unsigned bufLength )
 {
     epicsGuard < epicsMutex > guard ( pChan->cacCtx.mutexRef() );
@@ -186,7 +187,7 @@ unsigned epicsShareAPI ca_get_host_name (
  * !!!! not thread safe !!!!
  *
  */
-const char * epicsShareAPI ca_host_name ( 
+const char * epicsShareAPI ca_host_name (
     chid pChan )
 {
     epicsGuard < epicsMutex > guard ( pChan->cacCtx.mutexRef () );
@@ -196,7 +197,7 @@ const char * epicsShareAPI ca_host_name (
 /*
  * ca_set_puser ()
  */
-void epicsShareAPI ca_set_puser ( 
+void epicsShareAPI ca_set_puser (
     chid pChan, void * puser )
 {
     epicsGuard < epicsMutex > guard ( pChan->cacCtx.mutexRef () );
@@ -206,7 +207,7 @@ void epicsShareAPI ca_set_puser (
 /*
  * ca_get_puser ()
  */
-void * epicsShareAPI ca_puser ( 
+void * epicsShareAPI ca_puser (
     chid pChan )
 {
     epicsGuard < epicsMutex > guard ( pChan->cacCtx.mutexRef () );
@@ -220,7 +221,7 @@ int epicsShareAPI ca_change_connection_event ( chid pChan, caCh * pfunc )
 {
     epicsGuard < epicsMutex > guard ( pChan->cacCtx.mutexRef () );
     if ( ! pChan->currentlyConnected ) {
-         if ( pfunc ) { 
+         if ( pfunc ) {
             if ( ! pChan->pConnCallBack ) {
                 pChan->cacCtx.decrementOutstandingIO ( guard, pChan->ioSeqNo );
             }
@@ -238,7 +239,7 @@ int epicsShareAPI ca_change_connection_event ( chid pChan, caCh * pfunc )
 /*
  * ca_replace_access_rights_event
  */
-int epicsShareAPI ca_replace_access_rights_event ( 
+int epicsShareAPI ca_replace_access_rights_event (
     chid pChan, caArh *pfunc )
 {
     epicsGuard < epicsMutex > guard ( pChan->cacCtx.mutexRef () );
@@ -246,11 +247,11 @@ int epicsShareAPI ca_replace_access_rights_event (
     // The order of the following is significant to guarantee that the
     // access rights handler is always gets called even if the channel connects
     // while this is running. There is some very small chance that the
-    // handler could be called twice here with the same access rights state, but 
+    // handler could be called twice here with the same access rights state, but
     // that will not upset the application.
     pChan->pAccessRightsFunc = pfunc ? pfunc : cacNoopAccesRightsHandler;
     caAccessRights tmp = pChan->io.accessRights ( guard );
-      
+
     if ( pChan->currentlyConnected ) {
         struct access_rights_handler_args args;
         args.chid = pChan;
@@ -265,7 +266,7 @@ int epicsShareAPI ca_replace_access_rights_event (
 /*
  * ca_array_get ()
  */
-int epicsShareAPI ca_array_get ( chtype type, 
+int epicsShareAPI ca_array_get ( chtype type,
             arrayElementCount count, chid pChan, void *pValue )
 {
     int caStatus;
@@ -279,10 +280,10 @@ int epicsShareAPI ca_array_get ( chtype type,
         unsigned tmpType = static_cast < unsigned > ( type );
         epicsGuard < epicsMutex > guard ( pChan->cacCtx.mutexRef () );
         pChan->eliminateExcessiveSendBacklog ( guard );
-        autoPtrFreeList < getCopy, 0x400, epicsMutexNOOP > pNotify 
+        autoPtrFreeList < getCopy, 0x400, epicsMutexNOOP > pNotify
             ( pChan->getClientCtx().getCopyFreeList,
-                new ( pChan->getClientCtx().getCopyFreeList ) 
-                    getCopy ( guard, pChan->getClientCtx(), *pChan, 
+                new ( pChan->getClientCtx().getCopyFreeList )
+                    getCopy ( guard, pChan->getClientCtx(), *pChan,
                                 tmpType, count, pValue ) );
         pChan->io.read ( guard, type, count, *pNotify, 0 );
         pNotify.release ();
@@ -333,7 +334,7 @@ int epicsShareAPI ca_array_get ( chtype type,
 /*
  * ca_array_get_callback ()
  */
-int epicsShareAPI ca_array_get_callback ( chtype type, 
+int epicsShareAPI ca_array_get_callback ( chtype type,
             arrayElementCount count, chid pChan,
             caEventCallBackFunc *pfunc, void *arg )
 {
@@ -346,7 +347,7 @@ int epicsShareAPI ca_array_get_callback ( chtype type,
 
         epicsGuard < epicsMutex > guard ( pChan->cacCtx.mutexRef () );
         pChan->eliminateExcessiveSendBacklog ( guard );
-        autoPtrFreeList < getCallback, 0x400, epicsMutexNOOP > pNotify 
+        autoPtrFreeList < getCallback, 0x400, epicsMutexNOOP > pNotify
             ( pChan->getClientCtx().getCallbackFreeList,
             new ( pChan->getClientCtx().getCallbackFreeList )
                 getCallback ( *pChan, pfunc, arg ) );
@@ -396,9 +397,9 @@ int epicsShareAPI ca_array_get_callback ( chtype type,
     return caStatus;
 }
 
-void oldChannelNotify::read ( 
+void oldChannelNotify::read (
     epicsGuard < epicsMutex > & guard,
-    unsigned type, arrayElementCount count, 
+    unsigned type, arrayElementCount count,
     cacReadNotify & notify, cacChannel::ioid * pId )
 {
     this->io.read ( guard, type, count, notify, pId );
@@ -407,7 +408,7 @@ void oldChannelNotify::read (
 /*
  *  ca_array_put_callback ()
  */
-int epicsShareAPI ca_array_put_callback ( chtype type, arrayElementCount count, 
+int epicsShareAPI ca_array_put_callback ( chtype type, arrayElementCount count,
     chid pChan, const void *pValue, caEventCallBackFunc *pfunc, void *usrarg )
 {
     int caStatus;
@@ -468,7 +469,7 @@ int epicsShareAPI ca_array_put_callback ( chtype type, arrayElementCount count,
 /*
  *  ca_array_put ()
  */
-int epicsShareAPI ca_array_put ( chtype type, arrayElementCount count, 
+int epicsShareAPI ca_array_put ( chtype type, arrayElementCount count,
                                 chid pChan, const void * pValue )
 {
     if ( type < 0 ) {
@@ -522,9 +523,9 @@ int epicsShareAPI ca_array_put ( chtype type, arrayElementCount count,
     return caStatus;
 }
 
-int epicsShareAPI ca_create_subscription ( 
-        chtype type, arrayElementCount count, chid pChan, 
-        long mask, caEventCallBackFunc * pCallBack, void * pCallBackArg, 
+int epicsShareAPI ca_create_subscription (
+        chtype type, arrayElementCount count, chid pChan,
+        long mask, caEventCallBackFunc * pCallBack, void * pCallBackArg,
         evid * monixptr )
 {
     if ( type < 0 ) {
@@ -552,7 +553,7 @@ int epicsShareAPI ca_create_subscription (
     try {
         epicsGuard < epicsMutex > guard ( pChan->cacCtx.mutexRef () );
         try {
-            // if this stalls out on a live circuit then an exception 
+            // if this stalls out on a live circuit then an exception
             // can be forthcoming which we must ignore (this is a
             // special case preserving legacy ca_create_subscription
             // behavior)
@@ -562,7 +563,7 @@ int epicsShareAPI ca_create_subscription (
             // intentionally ignored (its ok to subscribe when not connected)
         }
         new ( pChan->getClientCtx().subscriptionFreeList )
-            oldSubscription  ( 
+            oldSubscription  (
                 guard, *pChan, pChan->io, tmpType, count, mask,
                 pCallBack, pCallBackArg, monixptr );
         // dont touch object created after above new because
@@ -603,8 +604,8 @@ int epicsShareAPI ca_create_subscription (
     }
 }
 
-void oldChannelNotify::write ( 
-    epicsGuard < epicsMutex > & guard, unsigned type, arrayElementCount count, 
+void oldChannelNotify::write (
+    epicsGuard < epicsMutex > & guard, unsigned type, arrayElementCount count,
     const void * pValue, cacWriteNotify & notify, cacChannel::ioid * pId )
 {
     this->io.write ( guard, type, count, pValue, notify, pId );
@@ -613,7 +614,7 @@ void oldChannelNotify::write (
 /*
  * ca_field_type()
  */
-short epicsShareAPI ca_field_type ( chid pChan ) 
+short epicsShareAPI ca_field_type ( chid pChan )
 {
     epicsGuard < epicsMutex > guard ( pChan->cacCtx.mutexRef () );
     return pChan->io.nativeType ( guard );
@@ -622,7 +623,7 @@ short epicsShareAPI ca_field_type ( chid pChan )
 /*
  * ca_element_count ()
  */
-arrayElementCount epicsShareAPI ca_element_count ( chid pChan ) 
+arrayElementCount epicsShareAPI ca_element_count ( chid pChan )
 {
     epicsGuard < epicsMutex > guard ( pChan->cacCtx.mutexRef () );
     return pChan->io.nativeElementCount ( guard );
@@ -674,7 +675,7 @@ const char * epicsShareAPI ca_name ( chid pChan )
 
 unsigned epicsShareAPI ca_search_attempts ( chid pChan )
 {
-    epicsGuard < epicsMutex > guard ( pChan->cacCtx.mutexRef () ); 
+    epicsGuard < epicsMutex > guard ( pChan->cacCtx.mutexRef () );
     return pChan->io.searchAttempts ( guard );
 }
 
diff --git a/src/ca/client/sgAutoPtr.h b/src/ca/client/sgAutoPtr.h
index b3753bb44..e2899468c 100644
--- a/src/ca/client/sgAutoPtr.h
+++ b/src/ca/client/sgAutoPtr.h
@@ -29,8 +29,7 @@
 template < class T >
 class sgAutoPtr {
 public:
-    sgAutoPtr ( epicsGuard < epicsMutex > &, 
-        struct CASG &, tsDLList < syncGroupNotify > & );
+    sgAutoPtr ( epicsGuard < epicsMutex > &, struct CASG & );
     ~sgAutoPtr ();
     sgAutoPtr < T > & operator = ( T * );
     T * operator -> ();
@@ -38,18 +37,16 @@ public:
     T * get ();
     T * release ();
 private:
-    tsDLList < syncGroupNotify > & list;
     T * pNotify;
     struct CASG & sg;
     epicsGuard < epicsMutex > & guard;
-	sgAutoPtr & operator = ( const sgAutoPtr & );
+	  sgAutoPtr & operator = ( const sgAutoPtr & );
 };
 
 template < class T >
 inline sgAutoPtr < T > :: sgAutoPtr ( 
-    epicsGuard < epicsMutex > & guardIn, 
-    struct CASG & sgIn, tsDLList < syncGroupNotify > & listIn  ) : 
-    list ( listIn ), pNotify ( 0 ), sg ( sgIn ), guard ( guardIn )
+    epicsGuard < epicsMutex > & guardIn, struct CASG & sgIn ) : 
+    pNotify ( 0 ), sg ( sgIn ), guard ( guardIn )
 {
 }
 
@@ -57,8 +54,9 @@ template < class T >
 inline sgAutoPtr < T > :: ~sgAutoPtr () 
 {
     if ( this->pNotify ) {
-        list.remove ( *this->pNotify );
-        pNotify->destroy ( this->guard, this->sg );
+        this->sg.ioPendingList.remove ( *this->pNotify );
+        this->sg.client.
+            whenThereIsAnExceptionDestroySyncGroupIO ( this->guard, *this->pNotify );
     }
 }
 
@@ -66,11 +64,12 @@ template < class T >
 inline sgAutoPtr < T > & sgAutoPtr < T > :: operator = ( T * pNotifyIn )
 {
     if ( this->pNotify ) {
-        list.remove ( *this->pNotify );
-        pNotify->destroy ( this->guard, this->sg );
+        this->sg.ioPendingList.remove ( *this->pNotify );
+        this->sg.client.
+            whenThereIsAnExceptionDestroySyncGroupIO ( this->guard, *this->pNotify );
     }
     this->pNotify = pNotifyIn;
-    list.add ( *this->pNotify );
+    this->sg.ioPendingList.add ( *this->pNotify );
     return *this;
 }
 
diff --git a/src/ca/client/syncGroup.h b/src/ca/client/syncGroup.h
index ab4d7b6df..3b9c3cd4f 100644
--- a/src/ca/client/syncGroup.h
+++ b/src/ca/client/syncGroup.h
@@ -5,24 +5,24 @@
 *     Operator of Los Alamos National Laboratory.
 * EPICS BASE Versions 3.13.7
 * and higher are distributed subject to a Software License Agreement found
-* in file LICENSE that is included with this distribution. 
+* in file LICENSE that is included with this distribution.
 \*************************************************************************/
-/*  
+/*
+ *
  *
- *                              
  *                    L O S  A L A M O S
  *              Los Alamos National Laboratory
  *               Los Alamos, New Mexico 87545
- *                                  
+ *
  *  Copyright, 1986, The Regents of the University of California.
- *                                  
- *           
+ *
+ *
  *	Author Jeffrey O. Hill
  *	johill@lanl.gov
  *	505 665 1831
  */
 
-#ifndef syncGrouph  
+#ifndef syncGrouph
 #define syncGrouph
 
 #ifdef epicsExportSharedSymbols
@@ -46,152 +46,153 @@
 
 static const unsigned CASG_MAGIC = 0xFAB4CAFE;
 
-// used to control access to CASG's recycle routines which
-// should only be indirectly invoked by CASG when its lock
-// is applied
-class casgRecycle {
-public:
-    virtual void recycleSyncGroupWriteNotify ( 
-        epicsGuard < epicsMutex > &, class syncGroupWriteNotify & io ) = 0;
-    virtual void recycleSyncGroupReadNotify ( 
-        epicsGuard < epicsMutex > &, class syncGroupReadNotify & io ) = 0;
-protected:
-    virtual ~casgRecycle ();
-};
-
 class syncGroupNotify : public tsDLNode < syncGroupNotify > {
 public:
     syncGroupNotify ();
-    virtual void destroy (  
-        epicsGuard < epicsMutex > & guard, 
-        casgRecycle & ) = 0;
-    virtual bool ioPending (  
+    virtual void destroy (
+        CallbackGuard & cbGuard,
+        epicsGuard < epicsMutex > & guard ) = 0;
+    virtual bool ioPending (
         epicsGuard < epicsMutex > & guard ) = 0;
     virtual void cancel (
-        epicsGuard < epicsMutex > & guard ) = 0;
-    virtual void show ( 
-        epicsGuard < epicsMutex > &, 
+        CallbackGuard & cbGuard,
+        epicsGuard < epicsMutex > & mutualExclusionGuard ) = 0;
+    virtual void show (
+        epicsGuard < epicsMutex > &,
         unsigned level ) const = 0;
 protected:
-    virtual ~syncGroupNotify (); 
-	syncGroupNotify ( const syncGroupNotify & );
-	syncGroupNotify & operator = ( const syncGroupNotify & );
+    virtual ~syncGroupNotify ();
+    syncGroupNotify ( const syncGroupNotify & );
+    syncGroupNotify & operator = ( const syncGroupNotify & );
 };
 
+struct CASG;
+
 class syncGroupReadNotify : public syncGroupNotify, public cacReadNotify {
 public:
-    static syncGroupReadNotify * factory ( 
-        tsFreeList < class syncGroupReadNotify, 128, epicsMutexNOOP > &, 
-        struct CASG &, chid, void *pValueIn );
-    void destroy (  
-        epicsGuard < epicsMutex > & guard, 
-        casgRecycle & );
-    bool ioPending (  
+    typedef void ( CASG :: * PRecycleFunc ) 
+        ( epicsGuard < epicsMutex > &, syncGroupReadNotify & );
+    static syncGroupReadNotify * factory (
+        tsFreeList < class syncGroupReadNotify, 128, epicsMutexNOOP > &,
+        CASG &, PRecycleFunc, chid, void *pValueIn );
+    void destroy (
+        CallbackGuard & cbGuard,
         epicsGuard < epicsMutex > & guard );
-    void begin (  epicsGuard < epicsMutex > &, 
+    bool ioPending (
+        epicsGuard < epicsMutex > & guard );
+    void begin (  epicsGuard < epicsMutex > &,
         unsigned type, arrayElementCount count );
     void cancel (
+        CallbackGuard & cbGuard,
         epicsGuard < epicsMutex > & guard );
     void show (  epicsGuard < epicsMutex > &, unsigned level ) const;
 protected:
-    syncGroupReadNotify ( struct CASG & sgIn, chid, void * pValueIn );
+    syncGroupReadNotify ( CASG & sgIn, PRecycleFunc, chid, void * pValueIn );
     virtual ~syncGroupReadNotify ();
 private:
     chid chan;
-    struct CASG & sg;
+    PRecycleFunc pRecycleFunc;
+    CASG & sg;
     void * pValue;
     const unsigned magic;
     cacChannel::ioid id;
     bool idIsValid;
     bool ioComplete;
     void operator delete ( void * );
-    void * operator new ( size_t, 
+    void * operator new ( size_t,
         tsFreeList < class syncGroupReadNotify, 128, epicsMutexNOOP > & );
-    epicsPlacementDeleteOperator (( void *, 
+    epicsPlacementDeleteOperator (( void *,
         tsFreeList < class syncGroupReadNotify, 128, epicsMutexNOOP > & ))
     void completion (
-        epicsGuard < epicsMutex > &, unsigned type, 
+        epicsGuard < epicsMutex > &, unsigned type,
         arrayElementCount count, const void * pData );
     void exception (
-        epicsGuard < epicsMutex > &, int status, 
+        epicsGuard < epicsMutex > &, int status,
         const char * pContext, unsigned type, arrayElementCount count );
-	syncGroupReadNotify ( const syncGroupReadNotify & );
-	syncGroupReadNotify & operator = ( const syncGroupReadNotify & );
+    syncGroupReadNotify ( const syncGroupReadNotify & );
+    syncGroupReadNotify & operator = ( const syncGroupReadNotify & );
 };
 
 class syncGroupWriteNotify : public syncGroupNotify, public cacWriteNotify {
 public:
-    static syncGroupWriteNotify * factory ( 
-        tsFreeList < class syncGroupWriteNotify, 128, epicsMutexNOOP > &, 
-        struct CASG &, chid );
-    void destroy (  
-        epicsGuard < epicsMutex > & guard, 
-        casgRecycle & );
-    bool ioPending (  
+    typedef void ( CASG :: * PRecycleFunc ) 
+        ( epicsGuard < epicsMutex > &, syncGroupWriteNotify & );
+    static syncGroupWriteNotify * factory (
+        tsFreeList < class syncGroupWriteNotify, 128, epicsMutexNOOP > &,
+        CASG &, PRecycleFunc, chid );
+    void destroy (
+        CallbackGuard & cbGuard,
         epicsGuard < epicsMutex > & guard );
-    void begin (  epicsGuard < epicsMutex > &, unsigned type, 
+    bool ioPending (
+        epicsGuard < epicsMutex > & guard );
+    void begin (  epicsGuard < epicsMutex > &, unsigned type,
         arrayElementCount count, const void * pValueIn );
     void cancel (
+        CallbackGuard & cbGuard,
         epicsGuard < epicsMutex > & guard );
     void show (  epicsGuard < epicsMutex > &, unsigned level ) const;
 protected:
-    syncGroupWriteNotify  ( struct CASG &, chid );
+    syncGroupWriteNotify  ( struct CASG &, PRecycleFunc, chid );
     virtual ~syncGroupWriteNotify (); // allocate only from pool
 private:
     chid chan;
-    struct CASG & sg;
+    PRecycleFunc pRecycleFunc;
+    CASG & sg;
     const unsigned magic;
     cacChannel::ioid id;
     bool idIsValid;
     bool ioComplete;
     void operator delete ( void * );
-    void * operator new ( size_t, 
+    void * operator new ( size_t,
         tsFreeList < class syncGroupWriteNotify, 128, epicsMutexNOOP > & );
-    epicsPlacementDeleteOperator (( void *, 
+    epicsPlacementDeleteOperator (( void *,
         tsFreeList < class syncGroupWriteNotify, 128, epicsMutexNOOP > & ))
     void completion ( epicsGuard < epicsMutex > & );
-    void exception ( 
-        epicsGuard < epicsMutex > &, int status, const char *pContext, 
+    void exception (
+        epicsGuard < epicsMutex > &, int status, const char *pContext,
 		unsigned type, arrayElementCount count );
-	syncGroupWriteNotify ( const syncGroupWriteNotify & );
-	syncGroupWriteNotify & operator = ( const syncGroupWriteNotify & );
+    syncGroupWriteNotify ( const syncGroupWriteNotify & );
+    syncGroupWriteNotify & operator = ( const syncGroupWriteNotify & );
 };
 
 struct ca_client_context;
 
 template < class T > class sgAutoPtr;
 
-struct CASG : public chronIntIdRes < CASG >, private casgRecycle {
+struct CASG : public chronIntIdRes < CASG > {
 public:
     CASG ( epicsGuard < epicsMutex > &, ca_client_context & cacIn );
-    void destructor ( 
+    void destructor (
+        CallbackGuard &,
         epicsGuard < epicsMutex > & guard );
-    bool ioComplete ( 
+    bool ioComplete (
+        CallbackGuard &,
         epicsGuard < epicsMutex > & guard );
     bool verify ( epicsGuard < epicsMutex > & ) const;
-    int block ( epicsGuard < epicsMutex > * pcbGuard, 
+    int block ( epicsGuard < epicsMutex > * pcbGuard,
         epicsGuard < epicsMutex > & guard, double timeout );
-    void reset ( epicsGuard < epicsMutex > & guard );
+    void reset ( CallbackGuard &, epicsGuard < epicsMutex > & );
     void show ( epicsGuard < epicsMutex > &, unsigned level ) const;
     void show ( unsigned level ) const;
     void get ( epicsGuard < epicsMutex > &, chid pChan, 
         unsigned type, arrayElementCount count, void * pValue );
     void put ( epicsGuard < epicsMutex > &, chid pChan, 
         unsigned type, arrayElementCount count, const void * pValue );
-    void completionNotify ( 
+    void completionNotify (
         epicsGuard < epicsMutex > &, syncGroupNotify & );
     int printFormated ( const char * pFormat, ... );
-    void exception ( 
-         epicsGuard < epicsMutex > &, int status, const char * pContext, 
-        const char * pFileName, unsigned lineNo );
-    void exception ( 
+    void exception (
          epicsGuard < epicsMutex > &, int status, const char * pContext,
-        const char * pFileName, unsigned lineNo, oldChannelNotify & chan, 
+        const char * pFileName, unsigned lineNo );
+    void exception (
+         epicsGuard < epicsMutex > &, int status, const char * pContext,
+        const char * pFileName, unsigned lineNo, oldChannelNotify & chan,
         unsigned type, arrayElementCount count, unsigned op );
-    void * operator new ( size_t size, 
+    void * operator new ( size_t size,
         tsFreeList < struct CASG, 128, epicsMutexNOOP  > & );
-    epicsPlacementDeleteOperator (( void *, 
+    epicsPlacementDeleteOperator (( void *,
         tsFreeList < struct CASG, 128, epicsMutexNOOP > & ))
+
 private:
     tsDLList < syncGroupNotify > ioPendingList;
     tsDLList < syncGroupNotify > ioCompletedList;
@@ -200,18 +201,20 @@ private:
     unsigned magic;
     tsFreeList < class syncGroupReadNotify, 128, epicsMutexNOOP > freeListReadOP;
     tsFreeList < class syncGroupWriteNotify, 128, epicsMutexNOOP > freeListWriteOP;
-    void recycleSyncGroupWriteNotify (  
-        epicsGuard < epicsMutex > &, syncGroupWriteNotify & io );
-    void recycleSyncGroupReadNotify (  
-        epicsGuard < epicsMutex > &, syncGroupReadNotify & io );
 
-    void destroyPendingIO ( 
+    void destroyPendingIO (
+        CallbackGuard & cbGuard,
         epicsGuard < epicsMutex > & guard );
-    void destroyCompletedIO ( 
+    void destroyCompletedIO (
+        CallbackGuard & cbGuard,
         epicsGuard < epicsMutex > & guard );
+    void recycleReadNotifyIO ( epicsGuard < epicsMutex > &, 
+                                syncGroupReadNotify & );
+    void recycleWriteNotifyIO ( epicsGuard < epicsMutex > &, 
+                                syncGroupWriteNotify & );
 
-	CASG ( const CASG & );
-	CASG & operator = ( const CASG & );
+    CASG ( const CASG & );
+    CASG & operator = ( const CASG & );
 
     void operator delete ( void * );
 
@@ -248,27 +251,27 @@ inline void boolFlagManager::release ()
     this->pBool = 0;
 }
 
-inline void * CASG::operator new ( size_t size, 
+inline void * CASG::operator new ( size_t size,
     tsFreeList < struct CASG, 128, epicsMutexNOOP > & freeList )
 {
     return freeList.allocate ( size );
 }
 
 #if defined ( CXX_PLACEMENT_DELETE )
-inline void CASG::operator delete ( void * pCadaver, 
-    tsFreeList < struct CASG, 128, epicsMutexNOOP > & freeList ) 
+inline void CASG::operator delete ( void * pCadaver,
+    tsFreeList < struct CASG, 128, epicsMutexNOOP > & freeList )
 {
     freeList.release ( pCadaver );
 }
 #endif
 
-inline bool syncGroupWriteNotify::ioPending (  
+inline bool syncGroupWriteNotify::ioPending (
     epicsGuard < epicsMutex > & /* guard */ )
 {
     return ! this->ioComplete;
 }
 
-inline bool syncGroupReadNotify::ioPending (  
+inline bool syncGroupReadNotify::ioPending (
     epicsGuard < epicsMutex > & /* guard */ )
 {
     return ! this->ioComplete;
diff --git a/src/ca/client/syncGroupReadNotify.cpp b/src/ca/client/syncGroupReadNotify.cpp
index 5d2be25c6..711a2fae4 100644
--- a/src/ca/client/syncGroupReadNotify.cpp
+++ b/src/ca/client/syncGroupReadNotify.cpp
@@ -5,7 +5,7 @@
 *     Operator of Los Alamos National Laboratory.
 * EPICS BASE Versions 3.13.7
 * and higher are distributed subject to a Software License Agreement found
-* in file LICENSE that is included with this distribution. 
+* in file LICENSE that is included with this distribution.
 \*************************************************************************/
 
 /*
@@ -26,16 +26,18 @@
 #include "syncGroup.h"
 #include "oldAccess.h"
 
-syncGroupReadNotify::syncGroupReadNotify ( 
-    CASG & sgIn, chid pChan, void * pValueIn ) :
-    chan ( pChan ), sg ( sgIn ), pValue ( pValueIn ), 
-    magic ( CASG_MAGIC ), id ( 0u ), 
+syncGroupReadNotify::syncGroupReadNotify (
+    CASG & sgIn, PRecycleFunc pRecycleFuncIn,
+    chid pChan, void * pValueIn ) :
+    chan ( pChan ), pRecycleFunc ( pRecycleFuncIn ), 
+    sg ( sgIn ), pValue ( pValueIn ),
+    magic ( CASG_MAGIC ), id ( 0u ),
     idIsValid ( false ), ioComplete ( false )
 {
 }
 
-void syncGroupReadNotify::begin ( 
-    epicsGuard < epicsMutex > & guard, 
+void syncGroupReadNotify::begin (
+    epicsGuard < epicsMutex > & guard,
     unsigned type, arrayElementCount count )
 {
     this->chan->eliminateExcessiveSendBacklog ( guard );
@@ -45,28 +47,31 @@ void syncGroupReadNotify::begin (
     mgr.release ();
 }
 
-void syncGroupReadNotify::cancel ( 
-    epicsGuard < epicsMutex > & guard )
+void syncGroupReadNotify::cancel (
+    CallbackGuard & callbackGuard,
+    epicsGuard < epicsMutex > & mutualExcusionGuard )
 {
     if ( this->idIsValid ) {
-        this->chan->ioCancel ( guard, this->id );
+        this->chan->ioCancel ( callbackGuard, mutualExcusionGuard, this->id );
         this->idIsValid = false;
     }
 }
 
-syncGroupReadNotify * syncGroupReadNotify::factory ( 
-    tsFreeList < class syncGroupReadNotify, 128, epicsMutexNOOP > & freeList, 
-    struct CASG & sg, chid chan, void * pValueIn )
+syncGroupReadNotify * syncGroupReadNotify::factory (
+    tsFreeList < class syncGroupReadNotify, 128, epicsMutexNOOP > & freeList,
+    struct CASG & sg, PRecycleFunc pRecycleFunc, chid chan, void * pValueIn )
 {
     return new ( freeList )
-        syncGroupReadNotify ( sg, chan, pValueIn );
+        syncGroupReadNotify ( sg, pRecycleFunc, chan, pValueIn );
 }
 
-void syncGroupReadNotify::destroy ( 
-    epicsGuard < epicsMutex > & guard, casgRecycle & recycle )
+void syncGroupReadNotify::destroy (
+    CallbackGuard &,
+    epicsGuard < epicsMutex > & guard )
 {
+    CASG & sgRef ( this->sg );
     this->~syncGroupReadNotify ();
-    recycle.recycleSyncGroupReadNotify ( guard, *this );
+    ( sgRef.*pRecycleFunc ) ( guard, *this );
 }
 
 syncGroupReadNotify::~syncGroupReadNotify ()
@@ -75,11 +80,11 @@ syncGroupReadNotify::~syncGroupReadNotify ()
 }
 
 void syncGroupReadNotify::completion (
-    epicsGuard < epicsMutex > & guard, unsigned type, 
+    epicsGuard < epicsMutex > & guard, unsigned type,
     arrayElementCount count, const void * pData )
 {
     if ( this->magic != CASG_MAGIC ) {
-        this->sg.printFormated ( 
+        this->sg.printFormated (
             "cac: sync group io_complete(): bad sync grp op magic number?\n" );
         return;
     }
@@ -94,16 +99,16 @@ void syncGroupReadNotify::completion (
 }
 
 void syncGroupReadNotify::exception (
-    epicsGuard < epicsMutex > & guard, int status, const char * pContext, 
+    epicsGuard < epicsMutex > & guard, int status, const char * pContext,
     unsigned type, arrayElementCount count )
 {
     if ( this->magic != CASG_MAGIC ) {
-        this->sg.printFormated ( 
+        this->sg.printFormated (
             "cac: sync group io_complete(): bad sync grp op magic number?\n" );
         return;
     }
     this->idIsValid = false;
-    this->sg.exception ( guard, status, pContext, 
+    this->sg.exception ( guard, status, pContext,
         __FILE__, __LINE__, *this->chan, type, count, CA_OP_GET );
     //
     // This notify is left installed at this point as a place holder indicating that
@@ -112,7 +117,7 @@ void syncGroupReadNotify::exception (
     //
 }
 
-void syncGroupReadNotify::show ( 
+void syncGroupReadNotify::show (
     epicsGuard < epicsMutex > &, unsigned level ) const
 {
     ::printf ( "pending sg read op: pVal=%p\n", this->pValue );
@@ -133,15 +138,15 @@ void syncGroupReadNotify::operator delete ( void * )
         __FILE__, __LINE__ );
 }
 
-void * syncGroupReadNotify::operator new ( size_t size, 
+void * syncGroupReadNotify::operator new ( size_t size,
     tsFreeList < class syncGroupReadNotify, 128, epicsMutexNOOP > & freeList )
 {
     return freeList.allocate ( size );
 }
 
 #if defined ( CXX_PLACEMENT_DELETE )
-void syncGroupReadNotify::operator delete ( void *pCadaver, 
-    tsFreeList < class syncGroupReadNotify, 128, epicsMutexNOOP > &freeList ) 
+void syncGroupReadNotify::operator delete ( void *pCadaver,
+    tsFreeList < class syncGroupReadNotify, 128, epicsMutexNOOP > &freeList )
 {
     freeList.release ( pCadaver );
 }
diff --git a/src/ca/client/syncGroupWriteNotify.cpp b/src/ca/client/syncGroupWriteNotify.cpp
index 7c0c582e9..f0bf42753 100644
--- a/src/ca/client/syncGroupWriteNotify.cpp
+++ b/src/ca/client/syncGroupWriteNotify.cpp
@@ -5,7 +5,7 @@
 *     Operator of Los Alamos National Laboratory.
 * EPICS BASE Versions 3.13.7
 * and higher are distributed subject to a Software License Agreement found
-* in file LICENSE that is included with this distribution. 
+* in file LICENSE that is included with this distribution.
 \*************************************************************************/
 
 /*
@@ -26,45 +26,50 @@
 #include "syncGroup.h"
 #include "oldAccess.h"
 
-syncGroupWriteNotify::syncGroupWriteNotify ( CASG & sgIn, chid pChan ) :
-    chan ( pChan ), sg ( sgIn ), magic ( CASG_MAGIC ), 
-        id ( 0u ), idIsValid ( false ), ioComplete ( false )
+syncGroupWriteNotify::syncGroupWriteNotify ( CASG & sgIn, 
+                      PRecycleFunc pRecycleFuncIn, chid pChan ) :
+    chan ( pChan ), pRecycleFunc ( pRecycleFuncIn ),
+    sg ( sgIn ), magic ( CASG_MAGIC ),
+    id ( 0u ), idIsValid ( false ), ioComplete ( false )
 {
 }
 
-void syncGroupWriteNotify::begin ( 
-    epicsGuard < epicsMutex > & guard, unsigned type, 
+void syncGroupWriteNotify::begin (
+    epicsGuard < epicsMutex > & guard, unsigned type,
     arrayElementCount count, const void * pValueIn )
 {
     this->chan->eliminateExcessiveSendBacklog ( guard );
     this->ioComplete = false;
     boolFlagManager mgr ( this->idIsValid );
-    this->chan->write ( guard, type, count, 
+    this->chan->write ( guard, type, count,
         pValueIn, *this, &this->id );
     mgr.release ();
 }
 
-void syncGroupWriteNotify::cancel ( 
-    epicsGuard < epicsMutex > & guard )
+void syncGroupWriteNotify::cancel (
+    CallbackGuard & callbackGuard,
+    epicsGuard < epicsMutex > & mutualExcusionGuard )
 {
     if ( this->idIsValid ) {
-        this->chan->ioCancel ( guard, this->id );
+        this->chan->ioCancel ( callbackGuard, mutualExcusionGuard, this->id );
         this->idIsValid = false;
     }
 }
 
-syncGroupWriteNotify * syncGroupWriteNotify::factory ( 
-    tsFreeList < class syncGroupWriteNotify, 128, epicsMutexNOOP > &freeList, 
-    struct CASG & sg, chid chan )
+syncGroupWriteNotify * syncGroupWriteNotify::factory (
+    tsFreeList < class syncGroupWriteNotify, 128, epicsMutexNOOP > &freeList,
+    struct CASG & sg, PRecycleFunc pRecycleFunc, chid chan )
 {
-    return new ( freeList ) syncGroupWriteNotify ( sg, chan );
+    return new ( freeList ) syncGroupWriteNotify ( sg, pRecycleFunc, chan );
 }
 
-void syncGroupWriteNotify::destroy ( 
-    epicsGuard < epicsMutex > & guard, casgRecycle & recycle )
+void syncGroupWriteNotify::destroy (
+    CallbackGuard &,
+    epicsGuard < epicsMutex > & guard )
 {
+    CASG & sgRef ( this->sg );
     this->~syncGroupWriteNotify ();
-    recycle.recycleSyncGroupWriteNotify ( guard, *this );
+    ( sgRef.*pRecycleFunc ) ( guard, *this );
 }
 
 syncGroupWriteNotify::~syncGroupWriteNotify ()
@@ -72,7 +77,7 @@ syncGroupWriteNotify::~syncGroupWriteNotify ()
     assert ( ! this->idIsValid );
 }
 
-void syncGroupWriteNotify::completion ( 
+void syncGroupWriteNotify::completion (
     epicsGuard < epicsMutex > & guard )
 {
     if ( this->magic != CASG_MAGIC ) {
@@ -85,14 +90,14 @@ void syncGroupWriteNotify::completion (
 }
 
 void syncGroupWriteNotify::exception (
-    epicsGuard < epicsMutex > & guard, 
+    epicsGuard < epicsMutex > & guard,
     int status, const char *pContext, unsigned type, arrayElementCount count )
 {
     if ( this->magic != CASG_MAGIC ) {
         this->sg.printFormated ( "cac: sync group io_complete(): bad sync grp op magic number?\n" );
         return;
     }
-    this->sg.exception ( guard, status, pContext, 
+    this->sg.exception ( guard, status, pContext,
             __FILE__, __LINE__, *this->chan, type, count, CA_OP_PUT );
     this->idIsValid = false;
     //
@@ -102,7 +107,7 @@ void syncGroupWriteNotify::exception (
     //
 }
 
-void syncGroupWriteNotify::show ( 
+void syncGroupWriteNotify::show (
     epicsGuard < epicsMutex > &, unsigned level ) const
 {
     ::printf ( "pending write sg op\n" );
@@ -123,15 +128,15 @@ void syncGroupWriteNotify::operator delete ( void * )
         __FILE__, __LINE__ );
 }
 
-void * syncGroupWriteNotify::operator new ( size_t size, 
+void * syncGroupWriteNotify::operator new ( size_t size,
     tsFreeList < class syncGroupWriteNotify, 128, epicsMutexNOOP > & freeList )
 {
     return freeList.allocate ( size );
 }
 
 #if defined ( CXX_PLACEMENT_DELETE )
-void syncGroupWriteNotify::operator delete ( void *pCadaver, 
-    tsFreeList < class syncGroupWriteNotify, 128, epicsMutexNOOP > &freeList ) 
+void syncGroupWriteNotify::operator delete ( void *pCadaver,
+    tsFreeList < class syncGroupWriteNotify, 128, epicsMutexNOOP > &freeList )
 {
     freeList.release ( pCadaver );
 }
diff --git a/src/ca/client/syncgrp.cpp b/src/ca/client/syncgrp.cpp
index 9b65c4f57..f11ce3551 100644
--- a/src/ca/client/syncgrp.cpp
+++ b/src/ca/client/syncgrp.cpp
@@ -5,7 +5,7 @@
 *     Operator of Los Alamos National Laboratory.
 * EPICS BASE Versions 3.13.7
 * and higher are distributed subject to a Software License Agreement found
-* in file LICENSE that is included with this distribution. 
+* in file LICENSE that is included with this distribution.
 \*************************************************************************/
 /*
  *      Author: Jeffrey O. Hill
@@ -48,6 +48,22 @@ extern "C" int epicsShareAPI ca_sg_create ( CA_SYNC_GID * pgid )
     }
 }
 
+int ca_sync_group_destroy ( CallbackGuard & cbGuard, epicsGuard < epicsMutex > & guard,
+                                ca_client_context & cac, const CA_SYNC_GID gid )
+{
+    int caStatus;
+    CASG * pcasg = cac.lookupCASG ( guard, gid );
+    if ( pcasg ) {
+        pcasg->destructor ( cbGuard, guard );
+        cac.casgFreeList.release ( pcasg );
+        caStatus = ECA_NORMAL;
+    }
+    else {
+        caStatus = ECA_BADSYNCGRP;
+    }
+    return caStatus;
+}
+
 /*
  * ca_sg_delete()
  */
@@ -56,42 +72,80 @@ extern "C" int epicsShareAPI ca_sg_delete ( const CA_SYNC_GID gid )
     ca_client_context * pcac;
     int caStatus = fetchClientContext ( & pcac );
     if ( caStatus == ECA_NORMAL ) {
-        epicsGuard < epicsMutex > guard ( pcac->mutexRef() );
-        CASG * pcasg = pcac->lookupCASG ( guard, gid );
-        if ( pcasg ) {
-            pcasg->destructor ( guard );
-            pcac->casgFreeList.release ( pcasg );
+        if ( pcac->pCallbackGuard.get() &&
+            pcac->createdByThread == epicsThreadGetIdSelf () ) {
+          epicsGuard < epicsMutex > guard ( pcac->mutex );
+          caStatus = ca_sync_group_destroy ( *pcac->pCallbackGuard.get(),
+                                          guard, *pcac, gid );
         }
         else {
-            caStatus = ECA_BADSYNCGRP;
+          //
+          // we will definately stall out here if all of the
+          // following are true
+          //
+          // o user creates non-preemtive mode client library context
+          // o user doesnt periodically call a ca function
+          // o user calls this function from an auxiillary thread
+          //
+          CallbackGuard cbGuard ( pcac->cbMutex );
+          epicsGuard < epicsMutex > guard ( pcac->mutex );
+          caStatus = ca_sync_group_destroy ( cbGuard, guard, *pcac, gid );
         }
     }
     return caStatus;
 }
 
+void sync_group_reset ( ca_client_context & client, CASG & sg )
+{
+    if ( client.pCallbackGuard.get() &&
+        client.createdByThread == epicsThreadGetIdSelf () ) {
+        epicsGuard < epicsMutex > guard ( client.mutex );
+        sg.reset ( *client.pCallbackGuard.get(), guard );
+    }
+    else {  
+        //
+        // we will definately stall out here if all of the
+        // following are true
+        //
+        // o user creates non-preemtive mode client library context
+        // o user doesnt periodically call a ca function
+        // o user calls this function from an auxiillary thread
+        //
+        CallbackGuard cbGuard ( client.cbMutex );
+        epicsGuard < epicsMutex > guard ( client.mutex );
+        sg.reset ( cbGuard, guard );
+    }
+}
+
 //
 // ca_sg_block ()
 //
-// !!!! This routine is only visible in the old interface - or in a new ST interface. 
-// !!!! In the old interface we restrict thread attach so that calls from threads 
-// !!!! other than the initializing thread are not allowed if preemptive callback 
+// !!!! This routine is only visible in the old interface - or in a new ST interface.
+// !!!! In the old interface we restrict thread attach so that calls from threads
+// !!!! other than the initializing thread are not allowed if preemptive callback
 // !!!! is disabled. This prevents the preemptive callback lock from being released
 // !!!! by other threads than the one that locked it.
 //
-extern "C" int epicsShareAPI ca_sg_block ( 
+extern "C" int epicsShareAPI ca_sg_block (
     const CA_SYNC_GID gid, ca_real timeout )
 {
     ca_client_context *pcac;
     int status = fetchClientContext ( &pcac );
     if ( status == ECA_NORMAL ) {
-        epicsGuard < epicsMutex > guard ( pcac->mutex );
-        CASG * pcasg = pcac->lookupCASG ( guard, gid );
-        if ( ! pcasg ) {
-            status = ECA_BADSYNCGRP;
+        CASG * pcasg;
+        {
+          epicsGuard < epicsMutex > guard ( pcac->mutex );
+          pcasg = pcac->lookupCASG ( guard, gid );
+          if ( pcasg ) {
+              status = pcasg->block (
+                  pcac->pCallbackGuard.get (), guard, timeout );
+          }
+          else {
+              status = ECA_BADSYNCGRP;
+          }
         }
-        else {
-            status = pcasg->block ( 
-                pcac->pCallbackGuard.get (), guard, timeout );
+        if ( pcasg ) {
+            sync_group_reset ( *pcac, *pcasg );
         }
     }
     return status;
@@ -105,10 +159,14 @@ extern "C" int epicsShareAPI ca_sg_reset ( const CA_SYNC_GID gid )
     ca_client_context *pcac;
     int caStatus = fetchClientContext (&pcac);
     if ( caStatus == ECA_NORMAL ) {
-        epicsGuard < epicsMutex > guard ( pcac->mutex );
-        CASG * pcasg = pcac->lookupCASG ( guard, gid );
+        CASG * pcasg;
+        {
+            epicsGuard < epicsMutex > guard ( pcac->mutex );
+            pcasg = pcac->lookupCASG ( guard, gid );
+        }
         if ( pcasg ) {
-            pcasg->reset ( guard );
+            sync_group_reset ( *pcac, *pcasg );
+            caStatus = ECA_NORMAL;
         }
         else {
             caStatus = ECA_BADSYNCGRP;
@@ -151,7 +209,26 @@ extern "C" int epicsShareAPI ca_sg_test ( const CA_SYNC_GID gid )
         epicsGuard < epicsMutex > guard ( pcac->mutexRef() );
         CASG * pcasg = pcac->lookupCASG ( guard, gid );
         if ( pcasg ) {
-            if ( pcasg->ioComplete ( guard ) ) {
+            bool isComplete;
+            if ( pcac->pCallbackGuard.get() &&
+                pcac->createdByThread == epicsThreadGetIdSelf () ) {
+              epicsGuard < epicsMutex > guard ( pcac->mutex );
+              isComplete = pcasg->ioComplete ( *pcac->pCallbackGuard.get(), guard );
+            }
+            else {
+              //
+              // we will definately stall out here if all of the
+              // following are true
+              //
+              // o user creates non-preemtive mode client library context
+              // o user doesnt periodically call a ca function
+              // o user calls this function from an auxiillary thread
+              //
+              CallbackGuard cbGuard ( pcac->cbMutex );
+              epicsGuard < epicsMutex > guard ( pcac->mutex );
+              isComplete = pcasg->ioComplete ( cbGuard, guard );
+            }
+            if ( isComplete ) {
                 caStatus = ECA_IODONE;
             }
             else{
@@ -172,17 +249,14 @@ extern "C" int epicsShareAPI ca_sg_array_put ( const CA_SYNC_GID gid, chtype typ
     arrayElementCount count, chid pChan, const void *pValue )
 {
     ca_client_context *pcac;
-    CASG *pcasg;
-    int caStatus;
-    
-    caStatus = fetchClientContext ( &pcac );
+
+    int caStatus = fetchClientContext ( &pcac );
     if ( caStatus != ECA_NORMAL ) {
         return caStatus;
     }
 
     epicsGuard < epicsMutex > guard ( pcac->mutexRef() );
-
-    pcasg = pcac->lookupCASG ( guard, gid );
+    CASG * const pcasg = pcac->lookupCASG ( guard, gid );
     if ( ! pcasg ) {
         return ECA_BADSYNCGRP;
     }
@@ -237,17 +311,14 @@ extern "C" int epicsShareAPI ca_sg_array_get ( const CA_SYNC_GID gid, chtype typ
     arrayElementCount count, chid pChan, void *pValue )
 {
     ca_client_context *pcac;
-    CASG *pcasg;
-    int caStatus;
 
-    caStatus = fetchClientContext ( &pcac );
+    int caStatus = fetchClientContext ( &pcac );
     if ( caStatus != ECA_NORMAL ) {
         return caStatus;
     }
 
     epicsGuard < epicsMutex > guard ( pcac->mutexRef() );
-
-    pcasg = pcac->lookupCASG ( guard, gid );
+    CASG * const pcasg = pcac->lookupCASG ( guard, gid );
     if ( ! pcasg ) {
         return ECA_BADSYNCGRP;
     }
diff --git a/src/ioc/db/dbCAC.h b/src/ioc/db/dbCAC.h
index d4949e568..1a4086b4e 100644
--- a/src/ioc/db/dbCAC.h
+++ b/src/ioc/db/dbCAC.h
@@ -87,9 +87,9 @@ public:
         epicsGuard < epicsMutex > &, epicsMutex &,
         dbContext &, dbChannelIO &, struct dbChannel *, cacStateNotify &,
         unsigned type, unsigned long count, unsigned mask, dbEventCtx );
-    void destructor ( epicsGuard < epicsMutex > & );
-    void unsubscribe ( epicsGuard < epicsMutex > & );
-    void channelDeleteException ( epicsGuard < epicsMutex > & );
+    void destructor ( CallbackGuard &, epicsGuard < epicsMutex > & );
+    void unsubscribe ( CallbackGuard &, epicsGuard < epicsMutex > & );
+    void channelDeleteException ( CallbackGuard &, epicsGuard < epicsMutex > & );
     void show ( epicsGuard < epicsMutex > &, unsigned level ) const;
     void show ( unsigned level ) const;
     void * operator new ( size_t size,
@@ -165,7 +165,7 @@ public:
     dbContext ( epicsMutex & cbMutex, epicsMutex & mutex,
         cacContextNotify & notify );
     virtual ~dbContext ();
-    void destroyChannel ( epicsGuard < epicsMutex > &, dbChannelIO & );
+    void destroyChannel ( CallbackGuard &,epicsGuard < epicsMutex > &, dbChannelIO & );
     void callReadNotify ( epicsGuard < epicsMutex > &,
             struct dbChannel * dbch, unsigned type, unsigned long count,
             cacReadNotify & notify );
@@ -182,9 +182,9 @@ public:
             cacWriteNotify & notify, cacChannel::ioid * pId );
     void show ( unsigned level ) const;
     void showAllIO ( const dbChannelIO & chan, unsigned level ) const;
-    void destroyAllIO (
-        epicsGuard < epicsMutex > &, dbChannelIO & chan );
-    void ioCancel ( epicsGuard < epicsMutex > &,
+    void destroyAllIO ( CallbackGuard & cbGuard,
+              epicsGuard < epicsMutex > &, dbChannelIO & chan );
+    void ioCancel ( CallbackGuard &, epicsGuard < epicsMutex > &,
         dbChannelIO & chan, const cacChannel::ioid &id );
     void ioShow ( epicsGuard < epicsMutex > &,
         const cacChannel::ioid & id, unsigned level ) const;
diff --git a/src/ioc/db/dbChannelIO.cpp b/src/ioc/db/dbChannelIO.cpp
index bdf7e459f..4632ffb16 100644
--- a/src/ioc/db/dbChannelIO.cpp
+++ b/src/ioc/db/dbChannelIO.cpp
@@ -58,19 +58,21 @@ dbChannelIO::~dbChannelIO ()
 {
 }
 
-void dbChannelIO::destructor ( epicsGuard < epicsMutex > & guard )
+void dbChannelIO::destructor ( CallbackGuard & cbGuard, 
+                              epicsGuard < epicsMutex > & guard )
 {
     guard.assertIdenticalMutex ( this->mutex );
-    this->serviceIO.destroyAllIO ( guard, *this );
+    this->serviceIO.destroyAllIO ( cbGuard, guard, *this );
     dbChannelDelete ( this->dbch );
     this->~dbChannelIO ();
 }
 
 void dbChannelIO::destroy (
+    CallbackGuard & cbGuard,
     epicsGuard < epicsMutex > & guard )
 {
     guard.assertIdenticalMutex ( this->mutex );
-    this->serviceIO.destroyChannel ( guard, *this );
+    this->serviceIO.destroyChannel ( cbGuard, guard, *this );
     // don't access this pointer after above call because
     // object no longer exists
 }
@@ -130,11 +132,12 @@ void dbChannelIO::subscribe (
 }
 
 void dbChannelIO::ioCancel (
+    CallbackGuard & cbGuard,
     epicsGuard < epicsMutex > & mutualExclusionGuard,
     const ioid & id )
 {
     mutualExclusionGuard.assertIdenticalMutex ( this->mutex );
-    this->serviceIO.ioCancel ( mutualExclusionGuard, *this, id );
+    this->serviceIO.ioCancel ( cbGuard, mutualExclusionGuard, *this, id );
 }
 
 void dbChannelIO::ioShow (
diff --git a/src/ioc/db/dbChannelIO.h b/src/ioc/db/dbChannelIO.h
index bdb92cc5b..372f809a3 100644
--- a/src/ioc/db/dbChannelIO.h
+++ b/src/ioc/db/dbChannelIO.h
@@ -48,8 +48,10 @@ public:
         epicsMutex &, cacChannelNotify &,
         dbChannel *, dbContext & );
     void destructor (
+        CallbackGuard &,
         epicsGuard < epicsMutex > & );
     void destroy (
+        CallbackGuard &,
         epicsGuard < epicsMutex > & mutualExclusionGuard );
     void callReadNotify (
         epicsGuard < epicsMutex > &,
@@ -99,7 +101,8 @@ private:
         unsigned type, unsigned long count,
         unsigned mask, cacStateNotify &notify, ioid * );
     void ioCancel (
-        epicsGuard < epicsMutex > & mutualExclusionGuard,
+        CallbackGuard &,
+        epicsGuard < epicsMutex > &,
         const ioid & );
     void ioShow (
         epicsGuard < epicsMutex > &,
@@ -108,8 +111,8 @@ private:
         epicsGuard < epicsMutex > & ) const;
     unsigned long nativeElementCount (
         epicsGuard < epicsMutex > & ) const;
-	dbChannelIO ( const dbChannelIO & );
-	dbChannelIO & operator = ( const dbChannelIO & );
+	  dbChannelIO ( const dbChannelIO & );
+	  dbChannelIO & operator = ( const dbChannelIO & );
     void operator delete ( void * );
 };
 
diff --git a/src/ioc/db/dbContext.cpp b/src/ioc/db/dbContext.cpp
index e337d698b..d005f084a 100644
--- a/src/ioc/db/dbContext.cpp
+++ b/src/ioc/db/dbContext.cpp
@@ -120,18 +120,20 @@ cacChannel & dbContext::createChannel (
 }
 
 void dbContext::destroyChannel (
-    epicsGuard < epicsMutex > & guard, dbChannelIO & chan )
+                  CallbackGuard & cbGuard,
+                  epicsGuard < epicsMutex > & guard, 
+                  dbChannelIO & chan )
 {
     guard.assertIdenticalMutex ( this->mutex );
 
     if ( chan.dbContextPrivateListOfIO::pBlocker ) {
         this->ioTable.remove ( *chan.dbContextPrivateListOfIO::pBlocker );
-        chan.dbContextPrivateListOfIO::pBlocker->destructor ( guard );
+        chan.dbContextPrivateListOfIO::pBlocker->destructor ( cbGuard, guard );
         this->dbPutNotifyBlockerFreeList.release ( chan.dbContextPrivateListOfIO::pBlocker );
         chan.dbContextPrivateListOfIO::pBlocker = 0;
     }
 
-    chan.destructor ( guard );
+    chan.destructor ( cbGuard, guard );
     this->dbChannelIOFreeList.release ( & chan );
 }
 
@@ -270,7 +272,9 @@ void dbContext::initiatePutNotify (
 }
 
 void dbContext::destroyAllIO (
-    epicsGuard < epicsMutex > & guard, dbChannelIO & chan )
+                  CallbackGuard & cbGuard,
+                  epicsGuard < epicsMutex > & guard, 
+                  dbChannelIO & chan )
 {
     guard.assertIdenticalMutex ( this->mutex );
     dbSubscriptionIO * pIO;
@@ -287,24 +291,24 @@ void dbContext::destroyAllIO (
     while ( ( pIO = tmp.get() ) ) {
         // This prevents a db event callback from coming
         // through after the notify IO is deleted
-        pIO->unsubscribe ( guard );
+        pIO->unsubscribe ( cbGuard, guard );
         // If they call ioCancel() here it will be ignored
         // because the IO has been unregistered above.
-        pIO->channelDeleteException ( guard );
-        pIO->destructor ( guard );
+        pIO->channelDeleteException ( cbGuard, guard );
+        pIO->destructor ( cbGuard, guard );
         this->dbSubscriptionIOFreeList.release ( pIO );
     }
 
     if ( chan.dbContextPrivateListOfIO::pBlocker ) {
-        chan.dbContextPrivateListOfIO::pBlocker->destructor ( guard );
+        chan.dbContextPrivateListOfIO::pBlocker->destructor ( cbGuard, guard );
         this->dbPutNotifyBlockerFreeList.release ( chan.dbContextPrivateListOfIO::pBlocker );
         chan.dbContextPrivateListOfIO::pBlocker = 0;
     }
 }
 
 void dbContext::ioCancel (
-    epicsGuard < epicsMutex > & guard, dbChannelIO & chan,
-    const cacChannel::ioid &id )
+    CallbackGuard & cbGuard, epicsGuard < epicsMutex > & guard, 
+    dbChannelIO & chan, const cacChannel::ioid &id )
 {
     guard.assertIdenticalMutex ( this->mutex );
     dbBaseIO * pIO = this->ioTable.remove ( id );
@@ -312,13 +316,13 @@ void dbContext::ioCancel (
         dbSubscriptionIO *pSIO = pIO->isSubscription ();
         if ( pSIO ) {
             chan.dbContextPrivateListOfIO::eventq.remove ( *pSIO );
-            pSIO->unsubscribe ( guard );
-            pSIO->channelDeleteException ( guard );
-            pSIO->destructor ( guard );
+            pSIO->unsubscribe ( cbGuard, guard );
+            pSIO->channelDeleteException ( cbGuard, guard );
+            pSIO->destructor ( cbGuard, guard );
             this->dbSubscriptionIOFreeList.release ( pSIO );
         }
         else if ( pIO == chan.dbContextPrivateListOfIO::pBlocker ) {
-            chan.dbContextPrivateListOfIO::pBlocker->cancel ( guard );
+            chan.dbContextPrivateListOfIO::pBlocker->cancel ( cbGuard, guard );
         }
         else {
             errlogPrintf ( "dbContext::ioCancel() unrecognized IO was probably leaked or not canceled\n" );
diff --git a/src/ioc/db/dbPutNotifyBlocker.cpp b/src/ioc/db/dbPutNotifyBlocker.cpp
index d2ffa2fa6..54cdec9fe 100644
--- a/src/ioc/db/dbPutNotifyBlocker.cpp
+++ b/src/ioc/db/dbPutNotifyBlocker.cpp
@@ -58,10 +58,11 @@ dbPutNotifyBlocker::~dbPutNotifyBlocker ()
 {
 }
 
-void dbPutNotifyBlocker::destructor ( epicsGuard < epicsMutex > & guard )
+void dbPutNotifyBlocker::destructor ( CallbackGuard & cbGuard, 
+                                  epicsGuard < epicsMutex > & guard )
 {
     guard.assertIdenticalMutex ( this->mutex );
-    this->cancel ( guard );
+    this->cancel ( cbGuard, guard );
     if ( this->maxValueSize > sizeof ( this->dbrScalarValue ) ) {
         char * pBuf = static_cast < char * > ( this->pbuffer );
         delete [] pBuf;
@@ -70,6 +71,7 @@ void dbPutNotifyBlocker::destructor ( epicsGuard < epicsMutex > & guard )
 }
 
 void dbPutNotifyBlocker::cancel (
+    CallbackGuard & cbGuard,
     epicsGuard < epicsMutex > & guard )
 {
     guard.assertIdenticalMutex ( this->mutex );
diff --git a/src/ioc/db/dbPutNotifyBlocker.h b/src/ioc/db/dbPutNotifyBlocker.h
index e4d8e92ed..78acc0a13 100644
--- a/src/ioc/db/dbPutNotifyBlocker.h
+++ b/src/ioc/db/dbPutNotifyBlocker.h
@@ -42,11 +42,11 @@
 class dbPutNotifyBlocker : public dbBaseIO {
 public:
     dbPutNotifyBlocker ( epicsMutex & );
-    void destructor ( epicsGuard < epicsMutex > & );
+    void destructor ( CallbackGuard &, epicsGuard < epicsMutex > & );
     void initiatePutNotify ( epicsGuard < epicsMutex > &,
             cacWriteNotify &, struct dbChannel *,
             unsigned type, unsigned long count, const void * pValue );
-    void cancel ( epicsGuard < epicsMutex > & );
+    void cancel ( CallbackGuard &, epicsGuard < epicsMutex > & );
     void show ( epicsGuard < epicsMutex > &, unsigned level ) const;
     void show ( unsigned level ) const;
     void * operator new ( size_t size,
diff --git a/src/ioc/db/dbSubscriptionIO.cpp b/src/ioc/db/dbSubscriptionIO.cpp
index cfb4251bd..02b2b0105 100644
--- a/src/ioc/db/dbSubscriptionIO.cpp
+++ b/src/ioc/db/dbSubscriptionIO.cpp
@@ -66,14 +66,15 @@ dbSubscriptionIO::~dbSubscriptionIO ()
 {
 }
 
-void dbSubscriptionIO::destructor ( epicsGuard < epicsMutex > & guard )
+void dbSubscriptionIO::destructor ( CallbackGuard & cbGuard, 
+                              epicsGuard < epicsMutex > & guard )
 {
     guard.assertIdenticalMutex ( this->mutex );
     this->~dbSubscriptionIO ();
 }
 
-void dbSubscriptionIO::unsubscribe (
-    epicsGuard < epicsMutex > & guard )
+void dbSubscriptionIO::unsubscribe ( CallbackGuard & cbGuard, 
+                                epicsGuard < epicsMutex > & guard )
 {
     guard.assertIdenticalMutex ( this->mutex );
     if ( this->es ) {
@@ -87,6 +88,7 @@ void dbSubscriptionIO::unsubscribe (
 }
 
 void dbSubscriptionIO::channelDeleteException (
+    CallbackGuard &,
     epicsGuard < epicsMutex > & guard )
 {
     guard.assertIdenticalMutex ( this->mutex );