08_posix threads programming class

8/6/2019 08_POSIX Threads Programming Class

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POSIXThreadsProgramming

AtutorialbyBlaiseBarney

LivermoreCompu>ng(LawrenceLivermoreNa>onalLabs)

hDps://compu>ng.llnl.gov/tutorials/pthreads/

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Thread

Tothesowaredeveloper,theconceptof a"procedure"thatrunsindependently fromitsmain

programmaybestdescribeathread.

Consideramainprogramthatcontainsanumberofprocedures.Alloftheseprocedurescanbe

scheduledtorunsimultaneouslyand/or

independentlybytheopera>ngsystem.Thisisa

"mul>-threaded"program.

Athreadcanbedefinedasanindependentstreamofinstruc>onsthatcanbescheduledtorunassuchby

theopera>ngsystem.2

Aprocessiscreatedbytheopera>ngsystem,andrequiresafairamountof"overhead".Processescontaininforma>onaboutprogramresourcesandprogramexecu>onstate,including: ProcessI,processgroupI,userI,andgroupI EnvironmentWorkingdirectory. Programinstruc>ons Registers StackHeap Filedescriptors Signalac>ons Sharedlibraries Inter-processcommunica>ontools(suchasmessagequeues,pipes,semaphores,orsharedmemory).

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Threadsuseandexistwithinprocessresources,yetareabletobescheduledbytheopera>ng

systemandrunasindependenten>>eslargely

becausetheyduplicateonlythebareessen>al

resourcesthatenablethemtoexistasexecutable

code.

Thisindependentflowofcontrolisaccomplishedbecauseathreadmaintainsitsown:

StackpointerRegistersSchedulingproper>es(suchaspolicyorpriority)SetofpendingandblockedsignalsThreadspecificdata.

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Insummary,intheUNIXenvironmentathread:ExistswithinaprocessandusestheprocessresourcesHasitsownindependentflowofcontrolaslongasitsparentprocessexistsandtheOSsupportsit

uplicatesonlytheessen>alresourcesitneedstobeindependentlyschedulable

Maysharetheprocessresourceswithotherthreadsthatactequallyindependently(anddependently)

iesiftheparentprocessdies-orsomethingsimilar Is"lightweight"becausemostoftheoverheadhasalreadybeenaccomplishedthroughthecrea>onofits

process.

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Because threads within the same process shareresources:

Changes made by one thread to shared systemresources (such as closing a file) will be seen by all

other threads.

Two pointers having the same value point to thesame data.

Reading and writing to the same memory locationsis possible, and therefore requires explicit

synchronization by the programmer.7

PthreadsOverview PthreadsaredefinedasasetofClanguageprogrammingtypesandprocedurecalls,implementedwithapthread.hheader/includefileandathreadlibrary-thoughthethislibrarymaybepartofanotherlibrary,suchaslibc.

Whencomparedtothecostofcrea>ngandmanagingaprocess,athreadcanbecreatedwithmuchlessopera>ngsystemoverhead.Managingthreadsrequiresfewersystemresourcesthanmanagingprocesses.

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Thefollowingtablecompares>mingresultsforthefork()subrou>neandthe

pthreads_create()subrou>ne.Timingsreflect

50,000process/threadcrea>ons,were

performedwiththe>meu>lity,andunitsare

inseconds,noop>miza>onflags.

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Tempoparaacriaode50.000

processosouthreads(emseg)

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All threads within a process share the sameaddress space. Inter-thread communication is

more efficient and in many cases, easier to use

than inter-process communication.

In particular, if an application is using MPI foron-node communications, there is a potential

that performance could be greatly improved by

using Pthreads for on-node data transfer

instead.

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SMP-SymmetricMul+processing,acomputerarchitecturethatprovidesfastperformancebymakingmul>pleCPUsavailabletocompleteindividualprocessessimultaneously(mul>processing).Unlikeasymmetricalprocessing,anyidleprocessorcanbeassignedanytask,andaddi>onalCPUscanbeaddedtoimproveperformanceandhandleincreased

loads.Avarietyofspecializedopera>ngsystemsandhardwarearrangementsareavailabletosupportSMP.Specificapplica>onscanbenefitfromSMPifthecodeallowsmul>threading.(www.webopedia.com)

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MPI-TheMessagePassingInterface(MPI)isa

libraryspecifica>onformessage-passing.Itis

astandardAPI(Applica>onProgramming

Interface)thatcanbeusedtocreateparallel

applica>ons.(www.webopedia.com)

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Threaded applications offerpotential performancegains and practical advantages over non-threadedapplications in several other ways:

Overlapping CPU work with I/O: For example, aprogram may have sections where it is performing along I/O operation. While one thread is waiting for anI/O system call to complete, CPU intensive work canbe performed by other threads.

Priority/real-time scheduling: tasks which are moreimportant can be scheduled to supersede or interruptlower priority tasks.

Asynchronous event handling: tasks which serviceevents of indeterminate frequency and duration can beinterleaved. For example, a web server can bothtransfer data from previous requests and manage thearrival of new requests.

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MPI libraries usually implement on-node taskcommunication via shared memory, which

involves at least one memory copy operation

(process to process).

For Pthreads there is no intermediate memory copyrequired because threads share the same address

space within a single process. There is no data

transfer, per se. It becomes more of a cache-to-

CPU ormemory-to-CPUbandwidth (worst case)situation. These speeds are much higher.

Some local comparisons are shown below:

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DesigningThreadedPrograms Onmul>-cpumachines,pthreadsaresuitedforparallelprogramming,andwhateverappliestoparallelprogrammingingeneral,appliestoparallelpthreadsprograms.

TotakeadvantageofPthreads,aprogrammustbeorganizedintodiscrete,independenttaskswhichcanexecuteconcurrently.For

example,ifrou>ne1androu>ne2canbeinterchanged,interleavedand/oroverlappedinreal>me,theyarecandidatesforthreading.

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interchanged

interleaved

overlaped

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Pthreadscanalsobeusedforserialapplica>ons,toemulateparallelexecu>on.A

perfectexampleisthetypicalwebbrowser,

whichformostpeople,runsonasinglecpu

desktop/laptopmachine.Manythingscan

"appear"tobehappeningatthesame>me.

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Commonmodelsforthreadedprograms:Manager/worker: asinglethread,themanagerassignsworktootherthreads,theworkers.Typically,themanagerhandlesallinputandparcelsoutworktotheothertasks.

Pipeline:ataskisbrokenintoaseriesofsubopera>ons,eachofwhichishandledinseries,butconcurrently,byadifferentthread .Anautomobileassemblylinebestdescribesthismodel.

Peer:similartothemanager/workermodel,butaerthemainthreadcreatesotherthreads,itpar>cipatesinthework.

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SharedMemoryModel:

Allthreadshaveaccesstothesameglobal,sharedmemory

Threadsalsohavetheirownprivatedata Programmersareresponsibleforsynchronizingaccess,protec>ngglobally

shareddata.

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Thread-safeness :anapplica>on'sabilitytoexecutemul>plethreadssimultaneouslywithout"clobbering"shareddataorcrea>ng"race"condi>ons.

Forexample,supposeanapplica>oncreatesseveralthreads,eachofwhichmakesacalltothesamelibraryrou>ne:

Thislibraryrou>neaccesses/modifiesaglobalstructureorloca>oninmemory.

Aseachthreadcallsthisrou>neitispossiblethattheymaytrytomodifythisglobalstructure/memoryloca>onatthesame>me.

Iftherou>nedoesnotemploysomesortofsynchroniza>onconstructstopreventdatacorrup>on,thenitisnotthread-safe.

sobrepor

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Theimplica>ontousersofexternallibraryrou>nesisthatifyouaren't100%certaintherou>neisthread-safe,thenyoutakeyourchanceswithproblemsthatcouldarise.

Recommenda>on:Becarefulifyourapplica>onuseslibrariesorotherobjectsthatdon'texplicitlyguaranteethread-safeness.Whenindoubt,assumethattheyarenot

thread-safeun>lprovenotherwise.Thiscanbedoneby"serializing"thecallstotheuncertainrou>ne,etc.

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ThePthreadsAPI

ThePthreadsAPIisdefinedintheANSI/IEEE

POSIX1003.1-1995standard.

Thesubrou>neswhichcomprisethePthreads

APIcanbeinformallygroupedintothree

majorclasses:

1. Threadmanagement2. Mutexes3. Condi>onVariables

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1.Threadmanagement:Thefirstclassof

func>onsworkdirectlyonthreads-

crea>ng,detaching,joining,etc.They

includefunc>onstoset/querythread

aDributes(joinable,schedulingetc.)

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2.Mutexes:Thesecondclassoffunc>ons

dealwithsynchroniza>on,calleda"mutex",

whichisanabbrevia>onfor"mutual

exclusion".Mutexfunc>onsprovidefor

crea>ng,destroying,lockingandunlocking

mutexes.Theyarealsosupplementedby

mutexaDributefunc>onsthatsetormodifyaDributesassociatedwithmutexes.

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3.Condi@onvariables:Thethirdclassof

func>onsaddresscommunica>onsbetween

threadsthatshareamutex.Theyarebased

uponprogrammerspecifiedcondi>ons.This

classincludesfunc>onstocreate,destroy,

waitandsignalbaseduponspecifiedvariable

values.Func>onstoset/querycondi>on

variableaDributesarealsoincluded.

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Rou;nePrefix Func;onalGroup

pthread_Threadsthemselvesand

miscellaneoussubrou>nes

pthread_aAr_ ThreadaDributesobjects

pthread_mutex_ Mutexes

pthread_mutexaAr_ MutexaDributesobjects.

pthread_cond_ Condi>onvariables

pthread_condaAr_ Condi>onaDributesobjects

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ThePthreadsAPIcontainsaround100subrou>nes.Thistutorialwillfocusona

subsetofthese-specifically,thosewhichare

mostlikelytobeimmediatelyusefultothe

beginningPthreadsprogrammer.

Forportability,thepthread.hheaderfileshouldbeincludedineachsourcefileusingthePthreadslibrary.

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1.ThreadManagement

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ThreadManagement

pthread_create(thread,attr,start_routine,arg)

pthread_exit (status)

pthread_attr_init (attr)pthread_attr_destroy (attr)

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Crea>ngThreads: Ini>ally,yourmain()programcomprisesasingle,defaultthread.Allotherthreadsmust

beexplicitlycreatedbytheprogrammer.

pthread_create createsanewthreadandmakesitexecutable.Thisrou>necanbe

calledanynumberof>mesfromanywhere

withinyourcode.

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pthread_create

(thread,attr,start_routine,arg)

pthread_createarguments: thread:Anopaque,uniqueiden>fierforthenewthreadreturnedbythesubrou>ne.

aDr:AnopaqueaDributeobjectthatmaybeusedtosetthreadaDributes.YoucanspecifyathreadaDributes

object,orNULLforthedefaultvalues.

start_rou>ne:theCrou>nethatthethreadwillexecuteonceitiscreated.

arg:Asingleargumentthatmaybepassedtostart_rou>ne.Itmustbepassedbyreferenceasapointer

castoftypevoid.NULLmaybeusedifnoargumentisto

bepassed.35

Themaximumnumberofthreadsthatmaybecreatedbyaprocessisimplementa>ondependent.

Oncecreated,threadsarepeers,andmaycreateotherthreads.Thereisnoimpliedhierarchy ordependencybetweenthreads.

Ques;on:Aerathreadhasbeencreated,howdoyouknowwhenitwillbe

scheduledtorunbytheopera>ngsystem?

NSWER:UnlessyouareusingthePthreadsschedulingmechanism, itisupto

theimplementa>onand/oropera>ngsystemtodecidewhereandwhen

threadswillexecute .Robustprogramsshouldnotdependuponthreads

execu>nginaspecificorder.

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ThreadADributes: Bydefault,athreadiscreatedwithcertainaDributes.SomeoftheseaDributescanbe

changedbytheprogrammerviathethread

aDributeobject.

pthread_attr_init andpthread_attr_destroy areusedto

ini>alize/destroythethreadaDributeobject.

Otherrou>nesarethenusedtoquery/setspecificaDributesinthethreadaDributeobject. SomeoftheseaDributeswillbediscussedlater.

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Termina>ngThreads: ThereareseveralwaysinwhichaPthreadmaybeterminated:

Thethreadreturnsfromitsstar>ngrou>ne(themainrou>nefortheini>althread).

Thethreadmakesacalltothepthread_exitsubrou>ne(coveredbelow).

Thethreadiscanceledbyanotherthreadviathepthread_cancel rou>ne(notcoveredhere).

Theen>reprocessisterminatedduetoacalltoeithertheexecorexit subrou>nes.

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pthread_exitisusedtoexplicitlyexitathread.Typically,thepthread_exit()rou>neiscalledaerathreadhascompleteditsworkandisnolongerrequiredtoexist.

Ifmain()finishesbeforethethreadsithascreated,andexitswithpthread_exit(),theotherthreadswillcon>nuetoexecute.Otherwise,theywillbeautoma>callyterminatedwhenmain()finishes.

Theprogrammermayop>onallyspecifyatermina>onstatus,whichisstoredasavoidpointerforanythreadthatmayjointhecallingthread.

Cleanup:thepthread_exit()rou>nedoesnotclosefiles;anyfilesopenedinsidethethreadwillremainopenaerthethreadisterminated.

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iscussion:Insubrou>nesthatexecuteto

comple>onnormally,youcanoendispense

withcallingpthread_exit()-unless,ofcourse,

youwanttopassareturncodeback.However,

thereisadefiniteproblemifmain()completes

beforethethreadsitspawned.

Ifyoudon'tcallpthread_exit()explicitly,when

main()completes,theprocess(andallthreads)willbeterminated.Bycalling

pthread_exit()inmain(),theprocessandallof

itsthreadswillbekeptaliveeventhoughallof

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A"helloworld"Pthreadsprogramemonstratesthreadcrea>onandtermina>on

#include

#include #include

#define NUM_THREADS 5void *PrintHello(void *threadid) {

long tid;

tid = (long)threadid;printf("Hello World! It's me, thread #%ld!\n", tid);

pthread_exit(NULL);}

int main(int argc, char *argv[]) {pthread_t threads[NUM_THREADS];

int rc; long t;for(t=0;tngthread1

HelloWorld!It'sme,thread#0!

Inmain:crea>ngthread2







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PassingrgumentstoThreads

Thepthread_create()rou>nepermitstheprogrammertopassoneargumenttothe

threadstartrou>ne.Forcaseswheremul>ple

argumentsmustbepassed,thislimita>onis

easilyovercomebycrea>ngastructurewhich

containsallofthearguments,andthen

passingapointertothatstructureinthepthread_create()rou>ne.

Allargumentsmustbepassedbyreferenceandcastto(void*).

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ThreadrgumentPassing(Incorrect)

Thisexampleperformsargumentpassingincorrectly.Itpassestheaddressofvariablet,whichissharedmemoryspaceand

visibletoallthreads.Astheloopiterates,thevalueofthis

memoryloca;onchanges,possiblybeforethecreated

threadscanaccessit.

int rc; long t;

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ThreadManagement

Crea>ngTermina>ng Joining etaching Managingstack

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JoiningandDetachingThreads

pthread_join (threadid,status)pthread_detach

(threadid,status)

pthread_attr_setdetachstate(attr,detachstate)

pthread_attr_getdetachstate(attr,detachstate)

"Joining"isonewaytoaccomplishsynchroniza>onbetweenthreads.For

example:46

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Thepthread_join() subrou>neblocksthecallingthreadun>lthespecifiedthreadidthreadterminates.

Theprogrammerisabletoobtainthetargetthread'stermina>onreturnstatusifitwasspecifiedinthetargetthread'scalltopthread_exit().

Ajoiningthreadcanmatchonepthread_join()call.ItisalogicalerrortoaDemptmul>plejoinsonthesamethread.

Twoothersynchroniza>onmethods,mutexesandcondi>onvariables,willbediscussedlater.

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Whenathreadiscreated,oneofitsaDributesdefineswhetheritisjoinableordetached.

Onlythreadsthatarecreatedasjoinablecan

bejoined.Ifathreadiscreatedasdetached,it

canneverbejoined.

ThefinaldraofthePOSIXstandardspecifiesthatthreadsshouldbecreatedasjoinable.

However,notallimplementa>onsmayfollowthis.

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Toexplicitlycreateathreadasjoinableordetached,theaDrargumentinthe

pthread_create()rou>neisused.Thetypical4

stepprocessis:

eclareapthreadaDributevariableofthepthread_aDr_tdatatype

Ini>alizetheaDributevariablewithpthread_aDr_init()

SettheaDributedetachedstatuswithpthread_aDr_setdetachstate()

Whendone,freelibraryresourcesusedbytheaDributewithpthread_aDr_destroy()

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Thepthread_detach()rou>necanbeusedtoexplicitlydetachathreadeventhoughit

wascreatedasjoinable.

Thereisnoconverserou>ne.

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Ifathreadrequiresjoining,considerexplicitlycrea>ngitasjoinable.Thisprovidesportability

asnotallimplementa>onsmaycreatethreads

asjoinablebydefault.

Ifyouknowinadvancethatathreadwillneverneedtojoinwithanotherthread,considercrea>ngitinadetachedstate.Some

systemresourcesmaybeabletobefreed.

hDps://compu>ng.llnl.gov/tutorials/pthreads/samples/join1.c

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ThreadManagement

Crea>ngTermina>ngJoiningetaching Managingstack

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StackManagement

pthread_attr_getstacksize (attr, stacksize) pthread_attr_setstacksize (attr, stacksize) pthread_attr_getstackaddr (attr, stackaddr) pthread_attr_setstackaddr (attr, stackaddr) Exceedingthedefaultstacklimitisoenveryeasytodo,withtheusualresults:programtermina>onand/

orcorrupteddata.

Safeandportableprogramsdonotdependuponthedefaultstacklimit,butinstead,explicitlyallocateenoughstackforeachthreadbyusingthe

pthread_aDr_setstacksizerou>ne.

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2.Mutexes

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MutexVariables Mutexisanabbrevia>onfor"mutualexclusion".Mutexvariablesareoneoftheprimarymeansofimplemen>ngthreadsynchroniza>onandforprotec>ngshareddatawhenmul>plewritesoccur.

Amutexvariableactslikea"lock"protec>ngaccesstoashareddataresource.ThebasicconceptofamutexasusedinPthreadsisthatonlyonethreadcanlock(orown)amutexvariableatanygiven>me.Thus,evenifseveralthreadstrytolockamutexonlyonethreadwill

besuccessful.Nootherthreadcanownthatmutexun>ltheowningthreadunlocksthatmutex.Threadsmust"taketurns"accessingprotecteddata.

Mutexescanbeusedtoprevent"race"condi>ons.Anexampleofaracecondi>oninvolvingabanktransac>onisshownbelow:

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Intheexamplebelow,amutexshouldbeusedtolockthe"Balance"whileathreadisusing

thisshareddataresource.

Veryoentheac>onperformedbyathreadowningamutexistheupda>ngofglobal

variables.Thisisasafewaytoensurethat

whenseveralthreadsupdatethesame

variable,thefinalvalueisthesameaswhatit

wouldbeifonlyonethreadperformedtheupdate.Thevariablesbeingupdatedbelongto

a"cri>calsec>on".

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Thread1 Thread2 Balance

Readbalance:

$1000$1000

Readbalance:

$1000$1000

eposit$200 $1000

eposit$200 $1000

Updatebalance$1000+$200

$1200

Updatebalance

$1000+$200$1200

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Atypicalsequenceintheuseofamutexisasfollows:

Createandini>alizeamutexvariableSeveralthreadsaDempttolockthemutexOnlyonesucceedsandthatthreadownsthemutex

Theownerthreadperformssomesetofac>onsTheownerunlocksthemutexAnotherthreadacquiresthemutexandrepeatstheprocess

Finallythemutexisdestroyed59

Whenseveralthreadscompeteforamutex,thelosersblockatthatcall-anunblockingcall

isavailablewith"trylock"insteadofthe"lock"

call.

Whenprotec>ngshareddata,itistheprogrammer'sresponsibilitytomakesure

everythreadthatneedstouseamutexdoes

so.Forexample,if4threadsareupda>ngthe

samedata,butonlyoneusesamutex,the

datacans>llbecorrupted.

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Crea;ngandDestroyingMutexes

pthread_mutex_init(mutex,aAr)pthread_mutex_destroy(mutex)

pthread_mutexaAr_init(aAr) pthread_mutexaAr_destroy(aAr)

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Mutexvariablesmustbedeclaredwithtypepthread_mutex_t,andmustbeini>alized

beforetheycanbeused.Therearetwoways

toini>alizeamutexvariable:

Sta>cally,whenitisdeclared.Forexample:pthread_mutex_tmymutex=

PTHREA_MUTEX_INITIALIZER;

ynamically,withthepthread_mutex_init()rou>ne.Thismethodpermitssengmutexobject

aDributes,a5r.

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Thea5robjectisusedtoestablishproper>esforthemutexobject,andmustbeoftype

pthread_mutexaDr_tifused(maybespecified

asNULLtoacceptdefaults).ThePthreads

standarddefinesthreeop>onalmutex

aDributes:

Protocol:Specifiestheprotocolusedtopreventpriorityinversionsforamutex.

Prioceiling:Specifiesthepriorityceilingofamutex.

Process-shared:Specifiestheprocesssharingofamutex.

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Notethatnotallimplementa>onsmayprovidethethreeop>onalmutexaDributes.

Thepthread_mutexaDr_init()andpthread_mutexaDr_destroy()rou>nesare

usedtocreateanddestroymutexaDribute

objectsrespec>vely.

pthread_mutex_destroy()shouldbeusedtofreeamutexobjectwhichisnolonger

needed.

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LockingandUnlockingMutexes

pthread_mutex_lock(mutex)pthread_mutex_trylock(mutex)

pthread_mutex_unlock(mutex) Thepthread_mutex_lock()rou>neisusedbyathreadtoacquirealockonthespecified

mutexvariable.Ifthemutexisalreadylockedbyanotherthread,thiscallwillblockthecallingthreadun>lthemutexisunlocked.

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pthread_mutex_trylock()willaDempttolockamutex.However,ifthemutexisalready

locked,therou+newillreturnimmediately

witha"busy"errorcode.Thisrou>nemaybe

usefulinpreven+ngdeadlockcondi+ons,asin

apriority-inversionsitua>on.

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pthread_mutex_unlock()willunlockamutexifcalledbytheowningthread.Callingthis

rou>neisrequiredaerathreadhas

completeditsuseofprotecteddataifother

threadsaretoacquirethemutexfortheir

workwiththeprotecteddata.Anerrorwillbe

returnedif:IfthemutexwasalreadyunlockedIfthemutexisownedbyanotherthread

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Thereisnothing"magical"aboutmutexes...infacttheyareakintoa"gentlemen'sagreement"betweenpar>cipa>ngthreads.Itisuptothecodewritertoinsurethatthenecessarythreadsallmakethemutexlockandunlockcallscorrectly.Thefollowingscenariodemonstratesalogicalerror:

Thread1Thread2 Thread3

Lock Lock

A=2 A=A+1 A=A*B

Unlock Unlock

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Ques>on:Whenmorethanonethreadiswai>ngforalockedmutex,whichthreadwill

begrantedthelockfirstaeritisreleased?

ANSWER:Unlessthreadpriorityscheduling(notcovered)isused,theassignmentwillbe

letothena>vesystemschedulerandmay

appeartobemoreorlessrandom.

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Example:UsingMutexes

Serialversion:


samples/dotprod_serial.c

Pthreadsversion:

hDps://compu>ng.llnl.gov/tutorials/pthreads/samples/dotprod_mutex.c

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3.Condi>onVariables

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Overview

Condi>onvariablesprovideyetanotherwayforthreadstosynchronize.Whilemutexesimplementsynchroniza>onby

controllingthreadaccesstodata,condi>onvariablesallow

threadstosynchronizebasedupontheactualvalueofdata.

Withoutcondi>onvariables,theprogrammerwouldneedtohavethreadscon>nuallypolling(possiblyinacri>calsec>on),

tocheckifthecondi>onismet.Thiscanbeveryresourceconsumingsincethethreadwouldbecon>nuouslybusyin

thisac>vity.Acondi>onvariableisawaytoachievethesame

goalwithoutpolling.

Acondi>onvariableisalwaysusedinconjunc>onwithamutexlock.

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Ar

epresenta>ve

sequence

forusingcondi>on

variables

MainThread

oeclareandini>alizeglobaldata/variableswhichrequiresynchroniza>on(suchas"count")

oeclareandini>alizeacondi>onvariableobjectoeclareandini>alizeanassociatedmutexoCreatethreadsAandBtodoworkThread

ooworkuptothepointwhereacertaincondi>onmustoccur(suchas"count"

mustreachaspecifiedvalue)

oLockassociatedmutexandcheckvalueofaglobalvariable

oCallpthread_cond_wait()toperformablockingwaitforsignalfromThread-B.

Notethatacalltopthread_cond_wait()

automa>callyandatomicallyunlocks

theassociatedmutexvariablesothatitcanbeusedbyThread-B.

oWhensignalled,wakeup.Mutexisautoma>callyandatomicallylocked.

oExplicitlyunlockmutexoCon>nue

ThreadB

ooworkoLockassociatedmutexoChangethevalueoftheglobalvariablethatThread-Aiswai>ngupon.

oCheckvalueoftheglobalThread-Awaitvariable.Ifitfulfillsthedesired

condi>on,signalThread-A.

oUnlockmutex.oCon>nue

MainThread

Join/Con>nue73

Crea;ngandDestroyingCondi;on

Variables

pthread_cond_init(condi;on,aAr) pthread_cond_destroy(condi;on) pthread_condaAr_init(aAr) pthread_condaAr_destroy(aAr)

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Usage

Condi>onvariablesmustbedeclaredwithtypepthread_cond_t,andmustbeini>alizedbeforethey

canbeused.Therearetwowaystoini>alizea

condi>onvariable:

Sta>cally,whenitisdeclared.Forexample:pthread_cond_tmyconvar=PTHREA_CON_INITIALIZER;

ynamically,withthepthread_cond_init()rou>ne.TheIofthecreatedcondi>onvariableisreturnedtothecalling

threadthroughthe condi+onparameter.Thismethod

permitssengcondi>onvariableobjectaDributes,a5r.

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Usage(cont.)

Theop>onala5robjectisusedtosetcondi>onvariableaDributes.ThereisonlyoneaDributedefinedforcondi>on

variables:process-shared,whichallowsthecondi>onvariable

tobeseenbythreadsinotherprocesses.TheaDributeobject,

ifused,mustbeoftypepthread_condaDr_t(maybespecified

asNULLtoacceptdefaults).Notethatnotallimplementa>ons

mayprovidetheprocess-sharedaDribute. Thepthread_condaDr_init()andpthread_condaDr_destroy()rou>nesareusedtocreateanddestroycondi>onvariable

aDributeobjects.

pthread_cond_destroy()shouldbeusedtofreeacondi>onvariablethatisnolongerneeded.

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Wai;ngandSignalingon

Condi;onVariables

pthread_cond_wait(condi;on,mutex) pthread_cond_signal(condi;on) pthread_cond_broadcast(condi;on)

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Usage

pthread_cond_wait()blocksthecallingthreadun>lthespecifiedcondi+onissignalled.Thisrou>neshouldbecalled

whilemutexislocked,anditwillautoma>callyreleasethe

mutexwhileitwaits.Aersignalisreceivedandthreadis

awakened,mutexwillbeautoma>callylockedforusebythe

thread.Theprogrammeristhenresponsibleforunlocking

mutexwhenthethreadisfinishedwithit.

Thepthread_cond_signal()rou>neisusedtosignal(orwakeup)anotherthreadwhichiswai>ngonthecondi>onvariable.

Itshouldbecalledaermutexislocked,andmustunlock

mutexinorderforpthread_cond_wait()rou>netocomplete.

Thepthread_cond_broadcast()rou>neshouldbeusedinsteadofpthread_cond_signal()ifmorethanonethreadisina

blockingwaitstate. 78

Usage(cont.)

Itisalogicalerrortocallpthread_cond_signal()beforecallingpthread_cond_wait().

Properlockingandunlockingoftheassociatedmutexvariableisessen>alwhenusingtheserou>nes.For

example:

Failingtolockthemutexbeforecallingpthread_cond_wait()maycauseitNOTtoblock.

Failingtounlockthemutexaercallingpthread_cond_signal()maynotallowamatching

pthread_cond_wait()rou>netocomplete(itwillremain

blocked).

79

Example:UsingCondi;on

Variables

hDps://compu>ng.llnl.gov/tutorials/pthreads/samples/condvar.c

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