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MultithreadedProgrammingWiththeWin32APIAndrewTuckerDebuggerDevelopmentLeadMarch13,1998WhatWeWillCover

IntrotoMultithreadedConceptsStartingandStoppingThreadsSynchronizationDebuggingandTestingIssuesInterprocessCommunicationAdvancedTopicsandAdditionalResourcesCaveatsMultithreadedfeaturesetsdifferbetweenNT,Win95andCEandversionsofthesameOSIntrotoMultithreadedConceptsWhatisathread?“pathofexecutioninaprocess”ownedbyasingleprocessallprocesseshavemainthread,somehavemorehasfullaccesstoprocessaddressspaceProcess1Process2ProcessNMainT1T2T3MainMainT1OperatingSystemIntrotoMultithreadedConceptsScheduling-cooperativevspreemptivePreemptive-allowathreadtoexecuteforaspecifiedamountoftimeandthenautomaticallyperformsa“contextswitch”tochangetoanewthread(e.G.NT,win95,WCE)Cooperative-performscontextswitchonlywhentheuserspecifies(“manuallyscheduled”)Win16isneither:multitasking,butnotmultithreadStartingandStoppingThreadsCreateThreadAPIHANDLECreateThread(LPSECURITY_ATTRIBUTESlpsa, //pointertothreadsecurityattributes DWORDdwStackSize, //initialthreadstacksize,inbytes LPTHREAD_START_ROUTINElpStartAddress, //pointertothreadfunction LPVOIDlpParameter, //argumentfornewthread DWORDdwCreationFlags, //creationflags LPDWORDlpThreadId //pointertoreturnedthreadidentifier );_beginthreadexCRTfunctionunsignedlong_beginthreadex(void*security, unsignedstack_size, unsigned(__stdcall*start_address)(void*), void*arglist, unsignedinitflag, unsigned*thrdaddr);So,what’sthedifference?StartingandStoppingThreadsDifferenceistheinitializationoftheCRTlibraryLinkingwithmultithreadedCRTisnotenoughDWORDThreadFunc(PVOIDpv){char*psz=strtok((char*)pv,“;”);while(psz){….processdata….psz=strtok(NULL.“;”);}}intmain(){//BUG-use_beginthreadextoensurethreadsafeCRTHANDLEhthrd1=CreateThread(…ThreadFunc…);HANDLEhthrd2=CreateThread(…ThreadFunc…);}_beginthreadexcreatesastructuretoensureglobalandstaticCRTvariablesarethread-specificStartingandStoppingThreadsThreadfunctionshavethefollowingprototype:

DWORDWINAPIThreadFunc(PVOIDpv);Itisveryusefultousepvasapointertoauser-definedstructuretopassextradataStartingandStoppingThreadsAreturnwillautomaticallycalltherespective_endthreadexorEndThreadAPIAreturndoesnotclosethehandlefromthecreationroutine(usermustcallCloseHandletoavoidresourceleak)Threadsshouldbeself-terminating(avoidtheTerminateThreadAPI)StartingandStoppingThreadsReasonstoavoidtheTerminateThreadAPI:Ifthetargetthreadownsacriticalsection,itwillnotbereleasedIfthetargetthreadisexecutingcertainkernelcalls,thekernelstateforthethread’sprocesscouldbeinconsistentIfthetargetthreadismanipulatingtheglobalstateofasharedDLL,thestateoftheDLLcouldbedestroyed,affectingotherusersoftheDLLStartingandStoppingThreadsUsingaC++memberasathreadfunction(fixingthe‘this’problem):classThreadedClass{public:ThreadedClass();BOOLStart();voidStop();private:HANDLEm_hThread;BOOLm_bRunning;staticUINTWINAPIStaticThreadFunc(LPVOIDlpv);DWORDMemberThreadFunc();};UINTWINAPIThreadedClass::StaticThreadFunc( LPVOIDlpv){ThreadedClass*pThis=(ThreadedClass*)lpv;returnpThis->MemberThreadFunc();}DWORDThreadedClass::MemberThreadFunc(){while(m_bRunning){…doprocessing... }}BOOLThreadedClass::Start(DWORDdwStart){UINTnTID;m_hThread=(HANDLE)_beginthreadex(NULL,0, StaticThreadFunc,this,0,&nTID)returnTRUE;}voidThreadedClass::Stop(){m_bRunning=FALSE;//waitforthreadtofinishDWORDdwExitCode;GetExitCodeThread(m_hThread,&dwExitCode);while(dwExitCode==STILL_ACTIVE){GetExitCodeThread(m_hThread,&dwExitCode);}m_hThread=0;}intmain(){

ThreadedClasstc1,tc2;tc1.Start(5);tc2.Start(5000);Sleep(3000);tc1.Stop();tc2.Stop();return0;}StartingandStoppingThreadsSuspendThreadandResumeThreadallowyoutopauseandrestartanythreadSuspensionstateisacountnotaboolean-callsshouldbebalancedExample:hittingabpinadebuggercausesallcurrentthreadstobesuspendedandresumedonsteporgoStartingandStoppingThreadsGetCurrentThreadandGetCurrentThreadIdareusefulforidentifyingcurrentthreadGetExitCodeThreadisusefulfordeterminingifathreadisstillaliveGetThreadTimesisusefulforperformanceanalysisandmeasurementSynchronizationUsedtocoordinatetheactivitiesofconcurrentlyrunningthreadsAlwaysavoidcoordinatingwithapollloopwhenpossibleforefficiencyreasonsSynchronizationInterlockedfunctionsCriticalSectionsWaitfunctionsMutexesSemaphoresEventsWaitableTimersSynchronizationInterlockedfunctions:PVOIDInterlockedCompareExchange(PVOID*destination,PVOIDExchange,PVOIDComperand) if(*Destination==Comperand) *Destination=Exchange;LONGInterlockedExchange(LPLONGTarget,LONGValue) *Target=Value;LONGInterlockedExchangeAdd(LPLONGAddend,LONGIncrement) *Addend+=Increment;LONGInterlockedDecrement(LPLONGAddend) *Addend-=1;LONGInterlockedIncrement(LPLONGAddend) *Addend+=1;Alloperationsareguaranteedtobe“atomic”-theentireroutinewillexecutew/oacontextswitchSynchronizationWhymustsimpleoperationslikeincrementinganintegerbe“atomic”?MultipleCPUinstructionsarerequiredfortheactualimplementation.Ifweretrievedavariableandwerethenpreemptedbyathreadthatchangedthatvariable,wewouldbeusingthewrongvalue.SynchronizationAcriticalsectionisatoolforguaranteeingthatonlyonethreadisexecutingasectionofcodeatanytimevoidInitializeCriticalSection(LPCRITICAL_SECTIONlpCritSec)voidDeleteCriticalSection(LPCRITICAL_SECTIONlpCritSec)voidEnterCriticalSection(LPCRITICAL_SECTIONlpCritSec)voidLeaveCriticalSection(LPCRITICAL_SECTIONlpCritSec)BOOLTryEnterCriticalSection(LPCRITICAL_SECTIONlpCritSec)Synchronization EnterCriticalSectionwillnotblockonnestedcallsaslongasthecallsareinthesamethread.CallstoLeaveCriticalSectionmuststillbebalancedSynchronization Criticalsectionexample:countingsourcelinesinmultiplefilesDWORDg_dwTotalLineCount=0;DWORDCountLinesThread(PVOIDpv){PSTRpszFileName=(PSTR)pv;DWORDdwCount;dwCount=CountSourceLines(pszFileName);EnterCriticalSection(&cs);g_dwTotalLineCount+=dwCount;LeaveCriticalSection(&cs);return0;}voidUpdateSourceLineCount(){FileNameListfnl;HANDLE*pHandleList;CRITICAL_SECTIONcs;GetFileNameList(&fnl);InitializeCriticalSection(&cs);pHandleList=malloc(sizeof(HANDLE)*fnl.Size());for(inti=0;i<FileNameList.Size();i++)pHandleList[i]=_beginthreadex(…CountLinesThread,fnl[i]…);//we’llcoverthisshortly…WaitForMultipleObjects(fnl.Size(),pHandleList,TRUE,INFINITE);DeleteCriticalSection(&cs);…processg_dwTotalLineCount...}SynchronizationWaitfunctions-allowyoutopauseuntiloneormoreobjectsbecomesignaledAtalltimes,anobjectisinoneoftwostates:signaledornonsignaledPicturesignaledasaflagbeingraisedandnonsignaledasaflagbeinglowered-thewaitfunctionsarewatchingforaflagtoberaisedSynchronizationTypesofobjectsthatcanbe“waitedon“:ProcessesThreadsConsoleInputFileChangeNotificationsMutexes*Semaphores*Events*WaitableTimers*SynchronizationProcessesandthreadsarenon-signaledatcreationandbecomesignaledwhentheyterminateSynchronizationDWORDWaitForSingleObject(HANDLEhHandle,DWORDdwMilliseconds)ReturnsWAIT_OBJECT_0ifhHandlehasbecomesignaledorWAIT_TIMEOUTifdwMillisecondselapsedandtheobjectisstillnon-signaledDWORDWaitForMultipleObjects(DWORDnCount,HANDLE*pHandles,BOOLbWaitAll,DWORDdwMilliseconds)IfbWaitAllisFALSEandoneoftheobjecthandleswassignaled,thereturnvalueminusWAIT_OBJECT_0isthearrayindexofthathandle.IfbWaitAllisTRUEandalloftheobjectsbecomesignaledthereturnvalueminusWAIT_OBJECT_0isavalidindexintothehandlearray.IfdwMillisecondselapsedandnoobjectwassignaled,WAIT_TIMEOUTisreturned.nCountcanbenomorethanMAXIMUM_WAIT_OBJECTS(currentlydefinedas64)INFINITEcanbeusedasatimeoutvalueSynchronizationMutexesprovidemutuallyexclusiveaccesstoanobject(hencethename)HANDLECreateMutex(LPSECURITY)ATTRIBUTESlpsa,BOOLbInitialOwner,LPCTSTRlpName)Ownershipisequivalenttothenonsignaledstate-ifbInitialOwnerisTRUEthecreationstateofthemutexisnonsignaledlpNameisoptionalReleaseMutexisusedtoendownershipSynchronizationWhat’sthedifferencebetweenacriticalsectionandamutex?AmutexisaOSkernelobject,andcanthusbeusedacrossprocessboundaries.AcriticalsectionislimitedtotheprocessinwhichitwascreatedSynchronizationTwomethodstogetahandletoanamedmutexcreatedbyanotherprocess:OpenMutex-returnshandletoanexistingmutexCreateMutex-createsorreturnshandletoanexistingmutex.GetLastErrorwillreturnERROR_ALREADY_EXISTSforthelattercaseSynchronizationComparingmutexandcriticalsectionperformanceSynchronizationAmutexwillnotblockonnestedcallsaslongastheyareinthesamethread.ReleaseMutexcallsmuststillbebalancedExamplesofwhentouseamutex:ErrorloggingsystemthatcanbeusedfromanyprocessDetectingmultipleinstancesofanapplicationSynchronizationSemaphoresallowaccesstoaresourcetobelimitedtoafixednumberHANDLECreateSemaphore(LPSECURITY_ATTRIBUTElpsa,LONGcSemInitial,LONGcSemMax,LPCTSTRlpName)SemaphoresareinthesignaledstatewhentheiravailablecountisgreaterthanzeroReleaseSemaphoreisusedtodecrementusageConceptually,amutexisabinarysemaphoreSynchronizationNamedsemaphorescanbeusedacrossprocessboundarieswithOpenSemaphoreandCreateSemaphoreCanbeusedtosolvetheclassic“singlewriter/multiplereaders”problemSynchronizationExample:limitingnumberofentriesinaqueueconstintQUEUE_SIZE=5;HANDLEg_hSem=NULL;longg_iCurSize=0;UINTWINAPIPrintJob(PVOIDpv){WaitForSingleObject(g_hSem,INFINITE);InterlockedIncrement(&g_iCurSize);printf("%08lX-enteredqueue:size=%d\n",GetCurrentThreadId(),g_iCurSize);Sleep(500);//printjob....InterlockedDecrement(&g_iCurSize);longlPrev;ReleaseSemaphore(g_hSem,1,&lPrev);return0;}intmain(){constintMAX_THREADS=64;HANDLEhThreads[MAX_THREADS];g_hSem=CreateSemaphore(NULL,QUEUE_SIZE,QUEUE_SIZE,NULL);UINTdwTID;for(inti=0;i<MAX_THREADS;i++)hThreads[i]=(HANDLE)_beginthreadex(NULL,0,PrintJob,NULL,0,&dwTID);WaitForMultipleObjects(MAX_THREADS,hThreads,TRUE,INFINITE);return0;}Synchronization EventsprovidenotificationwhensomeconditionhasbeenmetHANDLECreateEvent(LPSECURITY_ATTRIBUTESlpsa,BOOLbManualReset,BOOLbInitialState,LPCTSTRlpName)IfbInitialStateisTRUE,objectiscreatedinthesignaledstatebManualResetspecifiesthetypeofeventrequestedSynchronizationTwokindsofeventobjects:Autoreset-whensignaleditisautomaticallychangedtoanonsignaledstateafterasinglewaitingthreadhasbeenreleasedManualreset-whensignaleditremainsinthesignaledstateuntilitismanuallychangedtothenonsignaledstateSynchronizationNamedeventobjectscanbeusedacrossprocessboundarieswithOpenEventandCreateEventSetEventsetstheobjectstatetosignaledResetEventsetstheobjectstatetononsignaledPulseEventconceptuallycallsSetEvent/ResetEventsequentially,but...Synchronization PulseEventvsSetEventSynchronizationExample:displayingOutputDebugStringtextwithoutadebuggerintmain(){HANDLEhAckEvent,hReadyEvent;PSTRpszBuffer;hAckEvent=CreateEvent(NULL,FALSE,FALSE,“DBWIN_BUFFER_READY”);if(GetLastError()==ERROR_ALREADY_EXISTS){//handlemultipleinstancecase}hReadyEvent=CreateEvent(NULL,FALSE,FALSE,“DBWIN_DATA_READY”);pszBuffer=/*getpointertodatainmemorymappedfile*/;

SetEvent(hAckEvent);while(TRUE){intret=WaitForSingleObject(hReadyEvent,INFINITE);if(ret!=WAIT_OBJECT_0){//handleerror}else{printf(pszBuffer);SetEvent(hAckEvent);}}}SynchronizationWaitabletimersarekernelobjectsthatprovideasignalataspecifiedtimeintervalHANDLECreateWaitableTimer(LPSECURITY_ATTRIBUTESlpsa,BOOLbManualReset,LPCTSTRlpName)Manual/autoresetbehaviorisidenticaltoeventsTimeintervalisspecifiedwithSetWaitableTimerSynchronizationBOOLSetWaitableTimer(HANDLEhTimer,LARGE_INTEGER*pDueTime,LONGlPeriod,PTIMERACPROUTINEpfnCompletion,PVOIDpArg,BOOLfResume)pDueTimespecifieswhenthetimershouldgooffforthefirsttime(positiveisabsolute,negativeisrelative)lPeriodspecifieshowfrequentlytogooffaftertheinitialtimefResumecontrolswhetherthesystemisawakenedwhentimerissignaledSynchronizationConsecutivecallstoSetWaitableTimeroverwriteeachotherCancelWaitabletimerstopsthetimersothatitwillnotgooffagain(unlessSetWaitableTimeriscalled)SynchronizationExample:firinganeventeveryNsecondsconstintMAX_TIMES=3;constintN=10;DWORDWINAPIThreadFunc(PVOIDpv){HANDLEhTimer=(HANDLE)pv;intiCount=0;DWORDdwErr=0;while(TRUE){if(WaitForSingleObject(hTimer,INFINITE)==WAIT_OBJECT_0){…handletimerevent...if(++iCount>=MAX_TIMES)break;