OS - Process Concepts

Gives an overview about Process, PCB, Process States, Process Operations, Scheduling, Schedulers, Interprocess communication, shared memory and message passing systems

1. Process Concept

2. Process Concept
•Anoperatingsystemexecutesavarietyof
programs:
–Batchsystem–jobs
–Time-sharedsystems–userprogramsortasks
•Manymodernprocessconceptsarestillexpressed
intermsofjobs,(e.g.jobscheduling),andthe
twotermsareoftenusedinterchangeably.
2

3. Incomputing,aprocessisaninstanceofacomputerprogramthat
isbeingexecuted.Itcontainstheprogramcodeanditscurrent
activity.
Processmemoryisdividedintofoursections:
Thetextsectioncomprisesthecompiledprogramcode,readin
fromnon-volatilestoragewhentheprogramislaunched.
Thedatasectionstoresglobalandstaticvariables,allocatedand
initializedpriortoexecutingmain.
Theheapisusedfordynamicmemoryallocation,andismanagedvia
callstonew,delete,malloc,free,etc.
Thestackisusedforlocalvariables.Spaceonthestackisreserved
forlocalvariableswhentheyaredeclared(atfunctionentranceor
elsewhere,dependingonthelanguage),andthespaceisfreedup
whenthevariablesgooutofscope.

4. Aprogramisapassiveentity,suchasafilecontainingalistof
instructionsstoredondisk(oftencalledanexecutablefile).
Incontrast,aprocessisanactiveentity,withaprogramcounter
specifyingthenextinstructiontoexecuteandasetofassociated
resources.
Althoughtwoprocessesmaybeassociatedwiththesameprogram,
theyareneverthelessconsideredtwoseparateexecution
sequences.

5. Whenaprocessexecutes,itpassesthroughdifferentstates.The
stateofaprocessisdefinedinpartbythecurrentactivityofthat
process.
Thesestagesmaydifferindifferentoperatingsystems,andthe
namesofthesestatesarealsonotstandardized.
AProcessmaybeinoneothefollowingstates:
-Theprocessisbeingcreated.
-Theprocessiswaitingtobeassignedtoaprocessor.
-Instructionsarebeingexecuted.
-Theprocessiswaitingforsomeeventtooccur(such
asanI/Ocompletionorreceptionofasignal).
-Theprocesshasfinishedexecution

6. New-Theprocessisinthestageofbeingcreated.
Ready-Theprocesshasalltheresourcesavailablethatitneedstorun,but
theCPUisnotcurrentlyworkingonthisprocess'sinstructions.
Running-TheCPUisworkingonthisprocess'sinstructions.
Waiting-Theprocesscannotrunatthemoment,becauseitiswaitingfor
someresourcetobecomeavailableorforsomeeventtooccur.
Terminated-Theprocesshascompleted.

7. Eachprocessisrepresentedintheoperatingsystembyaprocesscontrol
block(PCB)—alsocalledataskcontrolblock.Itcontainsmanypiecesof
informationassociatedwithaspecificprocess,includingthese:
►Processstate:Thestatemaybenew,ready,runningandsoon
►Programcounter:Itindicatestheaddressofthenextinstructiontobeexecuted
forthisprogram.
►CPUregisters:Thesevaryinnumberandtypebasedonarchitecture.They
includeaccumulators,stackpointers,generalpurposeregistersetc.Alongwith
►CPUschedulinginformation:Thisincludesprocesspriority,pointersto
schedulingqueuesandanyschedulingparameters.
►Memory-managementinformation:Thisincludesthevalueofbaseandlimit
registers(protection)andpagetables,segmenttablesdependingonmemory.
►Accountinginformation:ItincludesamountofCPUandrealtimeused,
accountnumbers,processnumbersetc
►I/Ostatusinformation:ItincludeslistofI/Odevicesallocatedtothisprocess,
alistofopenfilesetc

8. ThearchitectureofaPCBiscompletelydependenton
OperatingSystemandmaycontaindifferentinformation
indifferentoperatingsystems.
ThePCBismaintainedforaprocessthroughoutits
lifetime,andisdeletedoncetheprocessterminates.

10. Aprocessisaprogramthatperformsasinglethreadof
execution.Thissinglethreadofcontrolallowstheprocessto
performonlyonetaskatatime.
Mostmodernoperatingsystemshaveextendedtheprocess
concepttoallowaprocesstohavemultiplethreadsof
executionandthustoperformmorethanonetaskatatime.
Thisfeatureisespeciallybeneficialonmulticoresystems,
wheremultiplethreadscanruninparallel.
Onasystemthatsupportsthreads,thePCBisexpandedto
includeinformationforeachthread.Otherchangesthroughout
thesystemarealsoneededtosupportthreads.

11. Process Scheduling
•Thetwomainobjectivesoftheprocessscheduling
systemaretokeeptheCPUbusyatalltimesandto
deliver"acceptable"responsetimesforall
programs,particularlyforinteractiveones.
•Theprocessschedulermustmeettheseobjectives
byimplementingsuitablepoliciesforswapping
processesinandoutoftheCPU.

12. Asprocessesenterthesystem,theyareputintoa ,which
consistsofallprocessesinthesystem.
Theprocessesthatareresidinginmainmemoryandarereadyand
waitingtoexecutearekeptonalistcalledthe .This
queueisgenerallystoredasalinkedlist.
Aready-queueheadercontainspointerstothefirstandfinalPCBs
inthelist.EachPCBincludesapointerfieldthatpointstothenext
PCBinthereadyqueue.
Thesystemalsoincludesotherqueues.Whenaprocessisallocated
theCPU,itexecutesforawhileandeventuallyquits,isinterrupted,
orwaitsfortheoccurrenceofaparticularevent,suchasthe
completionofanI/Orequest.
ThelistofprocesseswaitingforaparticularI/Odeviceiscalleda
.Eachdevicehasitsowndevicequeue.

13. Ready Queue And Various I/O Device Queues

14. Acommonrepresentationofprocessschedulingisa
.Eachrectangularboxrepresentsaqueue.
Twotypesofqueuesarepresent:the andasetof
.Thecirclesrepresenttheresourcesthatservethequeues,
andthearrowsindicatetheflowofprocessesinthesystem.

15. Inthefirsttwocases,theprocesseventuallyswitchesfromthe
waitingstatetothereadystateandisthenputbackintheready
queue.
Aprocesscontinuesthiscycleuntilitterminates,atwhichtimeitis
removedfromallqueuesandhasitsPCBandresourcesdeallocated.
Anewprocessisinitiallyputinthereadyqueue.Itwaitsthereuntilitis
selectedforexecution,ordispatched.
OncetheprocessisallocatedtheCPUandisexecuting,oneofseveral
eventscouldoccur:
TheprocesscouldissueanI/OrequestandthenbeplacedinanI/Oqueue.
Theprocesscouldcreateanewchildprocessandwaitforthechild’s
termination.
TheprocesscouldberemovedforciblyfromtheCPU,asaresultofan
interrupt,andbeputbackinthereadyqueue.

16. Aprocessmigratesamongthevariousschedulingqueues
throughoutitslifetime.Theoperatingsystemmustselect,for
schedulingpurposes,processesfromthesequeuesinsome
fashion.Theselectionprocessiscarriedoutbytheappropriate
scheduler.
A istypicalofabatchsystemoraveryheavily
loadedsystem.Whenmoreprocessesaresubmittedthanthatcan
beexecutedimmediately,theyarespooledtoamass-storage
deviceandarekeptthereforlaterexecution.Thelong-term
scheduler,orjobscheduler,selectsprocessesfromthispooland
loadsthemintomemoryforexecution.
The ,or ,selectsfromamong
theprocessesthatarereadytoexecuteandallocatestheCPUto
oneofthem.

17. 
Theprimarydistinctionbetweenthesetwoschedulersliesin
frequencyofexecution.Theshort-termschedulermustselecta
newprocessfortheCPUfrequently(intheorderofmilliseconds).
Thelong-termschedulerexecutesmuchlessfrequently;minutes
mayseparatethecreationofonenewprocessandthenext.

Thelong-termschedulercontrolsthedegreeof
multiprogramming(thenumberofprocessesinmemory).
Anefficientschedulingsystemwillselectagoodprocessmix
ofCPU-boundprocessesandI/Oboundprocesses.

18. 
Someoperatingsystems,suchastime-sharingsystems,mayintroducean
additional,intermediatelevelofschedulingcalled
.

Thekeyideabehindamedium-termscheduleristhatsometimesitcan
beadvantageoustoremoveaprocessfrommemory(andfromactive
contentionfortheCPU)andthusreducethedegreeof
multiprogramming.

Swappingisamechanisminwhichaprocesscanbeswapped/moved
temporarilyoutofmainmemorytoabackingstore,andthenbrought
backintomemoryforcontinuedexecution.

Theprocessisswappedout,andislaterswappedin,bythemedium-
termscheduler.

20. Whenaninterruptoccurs,thesystemneedstosavethecurrent
contextoftheprocessrunningontheCPUsothatitcanrestorethat
contextwhenitsprocessingisdone,essentiallysuspendingthe
processandthenresumingit.ThecontextisrepresentedinthePCB
oftheprocess.
.
Context-switchtimeispureoverhead,becausethesystemdoesno
usefulworkwhileswitching.Switchingspeedvariesfrommachine
tomachine,dependingonthememoryspeed,thenumberof
registersthatmustbecopied,andtheexistenceofspecial
instructions.
Context-switchtimesarehighlydependentonhardwaresupport.

21. Operations on Processes
•Theprocessesinthesystemcanexecute
concurrently,andtheymustbecreatedand
deleteddynamically.Thus,theoperatingsystem
mustprovideamechanism(orfacility)forprocess
creationandtermination.
•Otherkindsofoperationsonprocessesinclude
Deletion,Suspension,Resumption,Cloning,Inter-
ProcessCommunicationandSynchronization

22. Process Creation
Duringthecourseofexecution,aprocessmaycreate
severalnewprocesses.
Thecreatingprocessiscalledaparentprocess,andthe
newprocessesarecalledthechildrenofthatprocess.Each
ofthesenewprocessesmayinturncreateotherprocesses,
formingatreeofprocesses.
Mostoperatingsystems(includingUNIX,Linux,and
Windows)identifyprocessesaccordingtoauniqueprocess
identifier(orpid),whichistypicallyanintegernumber.
Thepidprovidesauniquevalueforeachprocessinthe
system,anditcanbeusedasanindextoaccessvarious
attributesofaprocesswithinthekernel.

23. OntypicalUNIXsystemstheprocessscheduleristermedsched,and
isgivenPID0.Thefirstthingitdoesatsystemstartuptimeisto
launchinit,whichgivesthatprocessPID1.
Initthenlaunchesallsystemdaemonsanduserlogins,andbecomes
theultimateparentofallotherprocesses.
A typical process tree for a Linux system

24. 
Ingeneral,whenaprocesscreatesachildprocess,thatchildprocess
willneedcertainresources(CPUtime,memory,files,I/Odevices)to
accomplishitstask.

Achildprocessmaybeabletoobtainitsresourcesdirectlyfromthe
operatingsystem,oritmaybeconstrainedtoasubsetofthe
resourcesoftheparentprocess.

Theparentmayhavetopartitionitsresourcesamongitschildren,or
itmaybeabletosharesomeresources(suchasmemoryorfiles)
amongseveralofitschildren.

Restrictingachildprocesstoasubsetoftheparent’sresources
preventsanyprocessfromoverloadingthesystembycreatingtoo
manychildprocesses.
Inadditiontosupplyingvariousphysicalandlogicalresources,the
parentprocessmaypassalonginitializationdata(input)tothechild
process.

25. 
Therearetwooptionsfortheparentprocessaftercreatingthechild:
•
Waitforthechildprocesstoterminatebeforeproceeding.Theparentmakesa
wait()systemcall,foreitheraspecificchildorforanychild,whichcausesthe
parentprocesstoblockuntilthewait()returns.UNIXshellsnormallywaitfor
theirchildrentocompletebeforeissuinganewprompt.
•
Runconcurrentlywiththechild,continuingtoprocesswithoutwaiting.Thisis
theoperationseenwhenaUNIXshellrunsaprocessasabackgroundtask.It
isalsopossiblefortheparenttorunforawhile,andthenwaitforthechild
later,whichmightoccurinasortofaparallelprocessingoperation.(E.g.the
parentmayforkoffanumberofchildrenwithoutwaitingforanyofthem,
thendoalittleworkofitsown,andthenwaitforthechildren.
)

Twopossibilitiesfortheaddressspaceofthechildrelativetotheparent:
•
Thechildmaybeanexactduplicateoftheparent,sharingthesameprogram
anddatasegmentsinmemory.EachwillhavetheirownPCB,including
programcounter,registers,andPID.ThisisthebehavioroftheforkinUNIX.
•
Thechildprocessmayhaveanewprogramloadedintoitsaddressspace,with
allnewcodeanddatasegments.Thisisthebehaviorofthespawnsystemcalls
inWindows.UNIXsystemsimplementthisasasecondstep,usingtheexec
systemcall.

26. Thefigureshowstheforkandexec
processonaUNIXsystem.
TheforksystemcallreturnsthePID
oftheprocesses.
Itreturnsazerotothechildprocess
andanon-zerochildPIDtothe
parent,sothereturnvalueindicates
whichprocessiswhich.
ProcessIDscanbelookedupany
timeforthecurrentprocessorits
directparentusingthegetpid()and
getppid()systemcallsrespectively.

27. Creating a Separate Process via Windows API
Thefigureshowsthemorecomplicated
processforWindows,whichmust
providealloftheparameterinformation
forthenewprocessaspartoftheforking
process.

28. Process Termination
Aprocessterminateswhenitfinishesexecutingitsfinalstatement
andaskstheoperatingsystemtodeleteitbyusingtheexit()
systemcall.
Alltheresourcesoftheprocess—includingphysicalandvirtual
memory,openfiles,andI/Obuffers—aredeallocatedbythe
operatingsystem.
Aprocesscancausetheterminationofanotherprocessviaan
appropriatesystemcall.Suchsystemcallsareusuallyinvoked
onlybytheparentoftheprocessthatistobeterminated.Aparent
needstoknowtheidentitiesofitschildrenifitistoterminate
them.

29. 
AParentmayterminatetheexecutionofoneofitschildrenfora
varietyofreasons,suchasthese:
1.Thechildhasexceededitsusageofsomeoftheresourcesthatit
hasbeenallocated.(Todeterminewhetherthishasoccurred,
theparentmusthaveamechanismtoinspectthestateofits
children.)
2.Thetaskassignedtothechildisnolongerrequired.
3.Theparentisexiting,andtheoperatingsystemdoesnotallowa
childtocontinueifitsparentterminates.
Somesystemsdonotallowachildtoexistifitsparenthas
terminated.Insuchsystems,ifaprocessterminates(either
normallyorabnormally),thenallitschildrenmustalsobe
terminated.Thisphenomenon,referredtoascascading
termination,isnormallyinitiatedbytheoperatingsystem.

30. Whenaprocessterminates,allofitssystemresourcesarefreedup,
openfilesflushedandclosed,etc.

Theprocessterminationstatusandexecutiontimesarereturnedto
theparentiftheparentiswaitingforthechildtoterminate,or
eventuallyreturnedtoinitiftheprocessbecomesanorphan.

Aprocessthathasterminated,butwhoseparenthasnotyetcalled
wait(),isknownasa .

Ifaparentdidnotinvokewait()andinsteadterminated,thereby
leavingitschildprocessesas .LinuxandUNIXaddressthis
scenariobyassigningtheinitprocessasthenewparenttoorphan
processes.
Theinitprocessperiodicallyinvokeswait(),therebyallowingtheexit
statusofanyorphanedprocesstobecollectedandreleasingthe
orphan’sprocessidentifierandprocess-tableentry.

31. InterprocessCommunication
•Processesexecutingconcurrentlyintheoperating
systemmaybeeitherindependentprocessesor
cooperatingprocesses.
Aprocessis ifitcannotaffectorbe
affectedbytheotherprocessesexecutinginthesystem.
Anyprocessthatdoesnotsharedatawithanyother
processisindependent.
Aprocessis ifitcanaffectorbeaffected
bytheotherprocessesexecutinginthesystem.Clearly,
anyprocessthatsharesdatawithotherprocessesisa
cooperatingprocess.

32. Thereareseveralreasonsforprovidinganenvironmentthatallows
processcooperation:

InformationSharing-Theremaybeseveralprocesseswhich
needaccesstothesamefile.(e.g.pipelines.)

Computationspeedup-Oftenasolutiontoaproblemcanbe
solvedfasteriftheproblemcanbebrokendownintosub-tasksto
besolvedsimultaneously(particularlywhenmultipleprocessors
areinvolved.)

Modularity-Themostefficientarchitecturemaybetobreaka
systemdownintocooperatingmodules.(E.g.databaseswitha
client-serverarchitecture.)
Convenience-Evenasingleusermaybemulti-tasking,suchas
editing,compiling,printing,andrunningthesamecodein
differentwindows.

33. Cooperatingprocessesrequiresometypeofinter-processcommunication,
whichismostcommonlyoneoftwotypes:MessagePassingsystems(a)or
SharedMemorysystems(b)
MessagePassingrequires
systemcallsforeverymessage
transfer,andisthereforeslower,
butitissimplertosetupand
workswellacrossmultiple
computers.
Messagepassingisgenerally
preferablewhentheamount
and/orfrequencyofdata
transfersissmall,orwhen
multiplecomputersare
involved.
SharedMemoryisfasteronceitissetup,becausenosystemcallsare
requiredandaccessoccursatnormalmemoryspeeds.Howeveritismore
complicatedtosetup,anddoesn'tworkaswellacrossmultiplecomputers.
Sharedmemoryisgenerallypreferablewhenlargeamountsofinformation
mustbesharedquicklyonthesamecomputer.

34. Interprocesscommunicationusingsharedmemoryrequires
communicatingprocessestoestablisharegionofsharedmemory.
Typically,ashared-memoryregionresidesintheaddressspaceofthe
processcreatingtheshared-memorysegment.
Otherprocessesthatwishtocommunicateusingthisshared-memory
segmentmustattachittotheiraddressspace.
Sharedmemoryrequiresthattwoormoreprocessesagreetoremove
therestrictionofpreventingoneprocessaccessinganotherprocesses
memory.
Theycanthenexchangeinformationbyreadingandwritingdatain
thesharedareas.Theformofthedataandthelocationare
determinedbytheseprocessesandarenotundertheoperating
system’scontrol.Theprocessesarealsoresponsibleforensuringthat
theyarenotwritingtothesamelocationsimultaneously.

35. Producer-Consumerproblemisacommonparadigmforcooperating
processesinwhichoneprocessisproducingdataandanotherprocess
isconsumingthedata.
Theproducer–consumerproblemprovidesausefulmetaphorforthe
client–serverparadigm.Aserveristhoughtasaproducerandaclient
asaconsumer.
Onesolutiontotheproducer–consumerproblemusesshared
memory.Toallowproducerandconsumerprocessestorun
concurrently,wemusthaveavailableabufferofitemsthatcanbe
filledbytheproducerandemptiedbytheconsumer.
Thisbufferwillresideinaregionofmemorythatissharedbythe
producerandconsumerprocesses.Aproducercanproduceoneitem
whiletheconsumerisconsuminganotheritem.Theproducerand
consumermustbesynchronized,sothattheconsumerdoesnottryto
consumeanitemthathasnotyetbeenproduced.

36. Twotypesofbufferscanbeused.Theunboundedbufferplaces
nopracticallimitonthesizeofthebuffer.Theconsumermay
havetowaitfornewitems,buttheproducercanalways
producenewitems.
Theboundedbufferassumesafixedbuffersize.Inthiscase,the
consumermustwaitifthebufferisempty,andtheproducer
mustwaitifthebufferisfull.
Aproducertriestoinsertdatainto
anemptyslotofthebuffer.A
consumertriestoremovedata
fromafilledslotinthebuffer.As
youmighthaveguessedbynow,
thosetwoprocesseswon'tproduce
theexpectedoutputiftheyare
beingexecutedconcurrently.

37. Theproducerprocesshasalocalvariablenextproducedinwhich
thenewitemtobeproducedisstored.Theconsumerprocesshasa
localvariablenextconsumedinwhichtheitemtobeconsumedis
stored.

38. Messagepassingprovidesamechanismtoallowprocessesto
communicateandtosynchronizetheiractionswithoutsharingthe
sameaddressspace.
Itisparticularlyusefulinadistributedenvironment,wherethe
communicatingprocessesmayresideondifferentcomputers
connectedbyanetwork.
Amessage-passingfacilityprovidesatleasttwooperations:
send(message) receive(message)
Messagessentbyaprocesscanbeeitherfixedorvariableinsize.
Ifonlyfixed-sizedmessagescanbesent,thesystem-level
implementationisstraightforward.However,makesthetaskof
programmingmoredifficult.
Ifitisvariable-sizedmessagesthenitrequireamorecomplexsystem-
levelimplementation,buttheprogrammingtaskbecomessimpler.

39. IfprocessesPandQwanttocommunicate,theymustsend
messagestoandreceivemessagesfromeachother:a
communicationlinkmustexistbetweenthem.Thislinkcanbe
implementedinavarietyofways.
Hereareseveralmethodsforlogicallyimplementingalinkand
thesend()/receive()operations:
Directorindirectcommunication(Naming)
Synchronousorasynchronouscommunication
Automaticorexplicitbuffering

40. Processesthatwanttocommunicatemusthaveawaytoreferto
eachother.Theycanuseeitherdirectorindirectcommunication.
Under ,eachprocessthatwantsto
communicatemustexplicitlynametherecipientorsenderofthe
communication.Inthisscheme,thesend()andreceive()primitives
aredefinedas:
•send(P,message)—SendamessagetoprocessP.
•receive(Q,message)—ReceiveamessagefromprocessQ.Acommunicationlinkinthisschemehasthefollowingproperties:
Alinkisestablishedautomaticallybetweeneverypairof
processesthatwanttocommunicate.Theprocessesneedto
knowonlyeachother’sidentitytocommunicate.
Alinkisassociatedwithexactlytwoprocesses.
Betweeneachpairofprocesses,thereexistsexactlyonelink.
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Naming
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41. Thepreviousschemeexhibitssymmetryinaddressing;thatis,boththe
senderprocessandthereceiverprocessmustnametheotherto
communicate.
Avariantofthisschemeemploysasymmetryinaddressing.Here,only
thesendernamestherecipient;therecipientisnotrequiredtonamethe
sender.Inthisscheme,thesend()andreceive()primitivesaredefinedas
follows:
•send(P,message)—SendamessagetoprocessP.
•receive(id,message)—Receiveamessagefromanyprocess.
Thevariableidissettothenameoftheprocesswithwhich
communicationhastakenplace.
Thedisadvantageinbothoftheseschemes(symmetricand
asymmetric)isthelimitedmodularityoftheresultingprocess
definitions.
Anysuchhard-codingtechniques,whereidentifiersmustbeexplicitly
stated,arelessdesirablethantechniquesinvolvingindirection.
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Naming
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42. Withindirectcommunication,themessagesaresenttoandreceived
from .
Amailboxcanbeviewedabstractlyasanobjectintowhichmessages
canbeplacedbyprocessesandfromwhichmessagescanberemoved.
Eachmailboxhasauniqueidentification.
Aprocesscancommunicatewithanotherprocessviaanumberof
differentmailboxes,buttwoprocessescancommunicateonlyifthey
haveasharedmailbox.Thesend()andreceive()primitivesaredefined
asfollows:
send(A,message)—SendamessagetomailboxA.
receive(A,message)—ReceiveamessagefrommailboxA.
Inthisscheme,acommunicationlinkhasthefollowingproperties:
Alinkisestablishedbetweenapairofprocessesonlyifbothmembersofthe
pairhaveasharedmailbox.
Alinkmaybeassociatedwithmorethantwoprocesses.
Betweeneachpairofcommunicatingprocesses,anumberofdifferentlinks
mayexist,witheachlinkcorrespondingtoonemailbox
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Naming –Indirect Communication
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43. AmailboxmaybeownedeitherbyaprocessorbytheOS.
Ifthemailboxisownedbyaprocess(thatis,themailboxispartofthe
addressspaceoftheprocess),thenwedistinguishbetweentheowner
(whichcanonlyreceivemessagesthroughthismailbox)andtheuser
(whichcanonlysendmessagestothemailbox).
Sinceeachmailboxhasauniqueowner,therecanbenoconfusion
aboutwhichprocessshouldreceiveamessagesenttothismailbox.
Whenaprocessthatownsamailboxterminates,themailbox
disappears.Anyprocessthatsubsequentlysendsamessagetothis
mailboxmustbenotifiedthatthemailboxnolongerexists.
AmailboxownedbyanOSisindependentandisnotattachedtoany
process.ThentheOSneedstoprovideamechanismtodothe
following:
Createanewmailbox.
Sendandreceivemessagesthroughthemailbox.
Deleteamailbox.
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Naming –Indirect Communication
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44. 44

45. Communicationbetweenprocessestakesplacethroughcallsto
send()andreceive()primitives.Therearedifferentdesignoptions
forimplementingeachprimitive.
Messagepassingmaybeeitherblockingornonblocking—also
knownassynchronousandasynchronous.
Thesendingprocessisblockeduntilthe
messageisreceivedbythereceivingprocessorbythemailbox.
.Thesendingprocesssendsthemessageand
resumesoperation.
.Thereceiverblocksuntilamessageis
available.
Thereceiverretrieveseitheravalid
messageoranull.
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Synchronization
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46. Whethercommunicationisdirectorindirect,messagesexchanged
bycommunicatingprocessesresideinatemporaryqueue.
Basically,suchqueuescanbeimplementedinthreeways:
.Thequeuehasamaximumlengthofzero;thus,the
linkcannothaveanymessageswaitinginit.Inthiscase,thesender
mustblockuntiltherecipientreceivesthemessage.
Thequeuehasfinitelengthn;thus,atmostn
messagescanresideinit.Ifthequeueisnotfullwhenanew
messageissent,themessageisplacedinthequeue,andthesender
cancontinueexecutionwithoutwaiting.Ifthelinkisfull,the
sendermustblockuntilspaceisavailableinthequeue.
Thequeue’slengthispotentiallyinfinite;
thus,anynumberofmessagescanwaitinit.Thesendernever
blocks
.
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Buffering
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