《unix操作系统设计》英文版习题答案1

《unix操作系统设计》英文版习题答案1S.1

UnixInternals(April/May-2024,Set-1)JNTU-Anantapur

B.Tech.III-YearII-Sem.(JNTU-Anantapur)

CodeNo.:9A05602/R09

B.Tech.IIIYearIISemesterRegularExaminations

April/May-2024UNIXINTERNALS

(ComputerScienceandEngineering)

Time:3Hours

Max.Marks:70

AnsweranyFIVEQuestionsAllQuestionscarryequalmarks

---1.

(a)DrawandexplainthearchitectureofUNIXsystems.(Unit-I,TopicNo.1.5.1)(b)Discussindetailthesleepandwakeupprocedures.(Unit-V,TopicNo.5.6)

2.

(a)

Ifseveralprocessescontendforabuffer,thekernelguaranteesthatnoneofthemsleepforever,butitdoesnotguaranteethataprocesswillnotbestarvedoutfromuseofabuffer.Redesigngetblksothataprocessisguaranteedeventualuseofabuffer.(Unit-II,TopicNo.2.2)(b)Whataretheadvantagesofbuffercache?(Unit-II,TopicNo.2.5)

3.

(a)Givebriefdescriptionabouttheaccessingofinodes.(Unit-III,TopicNo.3.1)(b)Explainindetailthecontinuousfileallocationsystem.(Unit-III,TopicNo.3.7)

4.

(a)Whatisthepurposeoflinksystemcall?Writeandexplainthealgorithmforthesame.(Unit-IV,TopicNo.4.12)(b)Writeshortnotesonmaintenanceofthefilesystem.(Unit-IV,TopicNo.4.15)5.

(a)Whatareregions?Explaintheirroleinthecreationofprocesses.(Unit-V,TopicNo.5.2)(b)Describeindetailaboutthecontextofaprocess.(Unit-V,TopicNo.5.3)

6.

(a)Explainthesecurityproblemsthatexistsifasetuidprogramisnotwriteprotected.(Unit-VI,TopicNo.6.6)(b)Writeaprogramthataparentandchildsharesafileaccess.(Unit-VI,TopicNo.6.1)7.

(a)Discussindetailabouttheschedulingparameters.(Unit-VII,TopicNo.7.1)(b)Whatisprofiling?Explainitindetail.(Unit-VII,TopicNo.7.3)8.

(a)Explainclistsindetail.(Unit-VIII,TopicNo.8.3)

(b)

Writeaprogramforreadingdatafromriskbyusingblockandrawinterfaces.

(Unit-VIII,TopicNo.8.3)

S.2

SpectrumALL-IN-ONEJournalforEngineeringStudents,2024

B.Tech.III-YearII-Sem.(JNTU-Anantapur)

(a)

DrawandexplainthearchitectureofUNIXsystems.

Answer:April/May-12,Set-1,Q1(a)

ForanswerreferUnit-I,Q10.

(b)Discussindetailthesleepandwakeup

procedures.

Answer:April/May-12,Set-1,Q1(b)

ForanswerreferUnit-V,Q17.

(a)Ifseveralprocessescontendforabuffer,

thekernelguaranteesthatnoneofthemsleepforever,butitdoesnotguaranteethataprocesswillnotbestarvedoutfromuseofabuffer.Redesigngetblksothataprocessisguaranteedeventualuseofabuffer.

Answer:April/May-12,Set-1,Q2(a)

Algorithmgetblk

Input:filesystemnumber

blocknumber

Output:lockedbufferthatcannowbeusedforblock{

While(buffernotfound)

{

If(blockisnotinhashqueue)

If(therearenobuffersonfreelist){

sleep(eventanybufferbeesfree);continue;

}}}}

(b)Whataretheadvantagesofbuffer

cache?

Answer:April/May-12,Set-1,Q2(b)

ForanswerreferUnit-II,Q12,Topic:AdvantagesofBufferCache.

(a)Givebriefdescriptionaboutthe

accessingofinodes.

Answer:April/May-12,Set-1,Q3(a)

Accessingofinodescanbeexplainedwellthroughthealgorithmthatallocatesin-coreinodes.

(b)Explainindetailthecontinuousfileallocationsystem.

Answer:

April/May-12,Set-1,Q3(b)

Note:Ingivenquestion,thewordcontiguousismisprintedascontinous.

Inacontiguousallocationmethodallthefilesarearrangedinasequentialblocksofmemory.Therefore,accordingtothistechnique,ifafilesizeiskblocksanditstartsfromablocksthenitspanstillstk-1blocknumbers.Withthisapproachfileaccessismuchfasterasallfilesoccupycontiguousblocks.Forsequentialaccessoffilesthephysicaladdressofthelastreferredblockisnoted,sothatfileaccesscanstartfromthenextblocktoavoidrepeatedaccesseofthepreviousblocks.Directaccessisalsosupportedsinceonlythestartingaddressandtheblocknumbersarerequired.

Thoughaccesstimeisminimal,contiguousallocationdoesnotmakeefficientuseofthediskspace.Thismeansthattheallocationofspaceforanewfileshouldbeefficientenoughsothatdiskspaceisnotwasted.

0123456

7

891011IMP121314

15

16171819USER2024222324252627CHAT

2829303132

33

34

35

DISK

FOODirectory

FilenameStartinglocation

SizeFooImpUserChat

0614263764

Figure:ContiguousAllocation

S.3

UnixInternals(April/May-2024,Set-1)JNTU-Anantapur

B.Tech.III-YearII-Sem.(JNTU-Anantapur)

monstrategiesusedforthispurposesarebestfit,worstfitandfirstfit.Firstfitisconsideredtousuallybefasterthanothertwostrategies.However,bestfitisalsoconsideredtobeefficientthanworsefitintermsofbothtimeanddiskspaceutilization.

Evenifanyofthesestrategiesareusedallofthemsufferwithexternalfragmentation.Externalfragmentationoccurswhenarequestforanewfilecannotbesatisfiedbecausethelargestavailablecontinouschunkofblocksisnotsufficientenoughforthefile.Therefore,eventhoughthereisenoughspaceavailableforthefile,butitisnotutilizedbecauseitisnotcontiguous.

Externalfragmentationcanberesolvedwiththehelpofaschemecalled“paction”.Compactioninvolvesrearrangingallfreememorylocationsinasequentialordersothatcontiguousdiskspacecanbeallocatedfornewfilesinsteadofspacegettingwasted.Compactioniscarriedoutinbothofflineandonlinemodes.Inofflinemode,alloperationsaresuspendedandthefilesystemisunmountedandfinallypactionisperformed.Inthisstrategyalotoftimeiswastedandhenceitisavoided.Inonlinemode,pactionisperformedalongwithothersystemoperations,butiteffectsbusinessperformanceandreducestimewastages.

However,contiguousallocationschemealsorequirestoknowthesizeofthenewfileforstorageallocation.Thisrequirementlookssimplebutitmightbedifficulttodeterminethesizeofanoutputfileafteritsexecution.Alsoifthesizeofthefileisdeterminedinadvanceandthefiletakesalotoftimetoreachitsfinalsize,thenalotofspaceiswastedwhenthefilehasnotreacheditsfinalsize.Therefore,amodifiedcontiguousallocationschemeisusedthatallocatessomefixedamountofspaceforthefirsttimeandifthefilerequiresmorespacethenanotherchunkofcontiguousblocksisallocated.Thechunksoffreeblocksallocatedafterthefirstallocationiscalled“infant”andthesetwochunksofcontiguousblocksarelinkedtooneanother.

Q4.(a)Whatisthepurposeoflinksystemcall?

Writeandexplainthealgorithmforthesame.

Answer:April/May-12,Set-1,Q4(a)Link()SystemCall

Linksystemcallcreatesanewdirectoryforaninodethatalreadyexists.Itdoesthisbylinkingafiletoanewnameinthefilesystemdirectory.Ithasthefollowingsyntax,

Link(existingfilename,newfilename)

Thefilesystemconsistsofpathnamesforeachlink.Processesusethepathnamestoaccessfiles.AllthefilenamesaregivenequalprioritiessincetheKernelisunawareoftheoriginalfilename.

Figure:FilesLinkedintheFileSystemTreeAfterexecutingthesystemcalls

Link(“/usr/foo/sys/src”,“/usr/lib/src'');

Link(“/usr/lib/file3.h”,“/usr/Foo/src/file2h”);Thepaths–“/usr/Foo/sys/file2.h”,“/usr/lib/file3.h”and“/usr/lib/src/file2-h”belongtothesamefile.

TheKernelgivestheauthorityoflinkingdirectoriesonlytothesuperuses.Thismakesthecodingparteasyfortheprogramthattraversesthefilesystemtree.Superusershavetobecarefulenoughwhilelinkingdirectories.Theabilityoflinkingdirectoriesmustbesupportedonearlyversionsofthesystem.Thisisbecause,themkdirmandusedforcreatingnewdirectoriesdependsontheabilityoflinkingdirectories.Algorithmoflinksystemcall,thefollowingalgorithmshowsthelinkingoffiles.

1.Obtaintheinodeforexistingfilenameusingthe

“namei”algorithm.2.Iftherearemanylinksonfile/linkingdirectory

withoutsuperuserpermission.

(a)Releasetheinodeusing“iput”algorithm.(b)Returnerror.3.Incrementthelinkcount.4.Updatethediskcopyofinode.5.Unlockinode.6.Obtaintheparentforthenewfilenameinode

using“namei”algorithm.7.Ifthenewfilealreadyexists,

(a)Undotheupdatedoneinstep(4)(b)Returnerror.8.Makeanewdirectoryintheparentdirectoryof

thenewfilename.9.Addthenewfilenameandtheinodenumberof

theexistingfilename.

10.Releaseparentdirectoryinodeusing“iput”

algorithm.

11.Releasetheinodeoftheexistingfileusing“iput”

algorithm.

S.4SpectrumALL-IN-ONEJournalforEngineeringStudents,2024TheKerneluses“namei”algorithmtolookupfortheinodeforthesourcefile.Itincrementsthelinkcountandupdatesthediskcopyoftheinode.Thenitunlockstheinodeandsearchesforthetargetfile.The“Link”callwillfailifthefileisnotavailable.TheKerneldecrementsthelinkcountwhichwaspreviouslyincremented.Ifthefileisnotavailablethenitfindsafreeslotintheparentdirectoryofthetargetfile.Itwritesthenameofthetargetfileandinodenumberofthesourcefileintoit.Usingthe“iput”algorithm,itreleasestheinodeofthetargetfileparentdirectory.Thetargetfiledidnotexistinactual.Hencenomoreinodesaretheretobereleased.

Finally,theKernelreleasesthesourcefileinodewhichisaccessedbyanotherfileinthefilesystem.Thelinkcountincreasesby1.Itkeepstrackofthecountofdirectoryentriesbelongingtothefile.Thisdifferentiatesitfromtheinodereferencecount.Ifthefileisnotaccessedbyanyotherprocesstheninodereferencecountofthefilebees0andthelinkcountwillbeatleast2.

(b)Writeshortnotesonmaintenanceofthefilesystem.

Answer:April/May-12,Set-1,Q4(b)ForanswerreferUnit-IV,Q17.

Q5.(a)Whatareregions?Explaintheirroleinthecreationofprocesses.

Answer:April/May-12,Set-1,Q5(a)ForanswerreferUnit-V,Q3,Topic:Regions.

(b)Describeindetailaboutthecontextofaprocess.

Answer:April/May-12,Set-1,Q5(b)ForanswerreferUnit-V,Q7.

Q6.(a)Explainthesecurityproblemsthatexistsifasetuidprogramisnotwriteprotected.Answer:April/May-12,Set-1,Q6(a)Thesecurityproblemsthatexistsifasetuidprogramisnotwriteprotectedareasfollows,

1.Thefirstproblemisassociatedwithexecutingasequenceofmandsdefinedbytheattacker.

2.Thesecondproblemisassociatedwithsubstitutingdataoftheattacker’schoosingfordatacreatedbya

program.

Inthefirstproblem,anattackertakesadvantageofthesetuidprogram’srunningwithspecialprivilegestoforceittoexecutewhatevermandstheattackerneeds.Forinstance,assumethatanattackerfoundacopyoftheBourneshellthatwassetuidtoroot.Theattackercouldthenexecutetheshellsince,theshellissetuidtorootthesemandswouldbeexecutedasthoughroothadtypedthem.Hence,theattackercoulddoanythinghewants,sincerootisthemosthighlyprivilegeduseronthesystem.Eveniftheshellwerechangedtoreadfromamandfileratherthanacceptmandsinteractively,theattackercouldfilesimplycreatehisownscriptandruntheshellusingit.Thissituationmustbeavoidedanditiseasytoavoidbutitoccursfrequently.

Severalproblemsarepossiblewithworld-writablefiles.Occasionallyprogramswillusetemporaryfilesforvariouspurposes,thefunctionoftheprogramdependingonwhatisinthefile.Iftheprogramclosesthetemporaryfileatanypointandthenreopensitlater,anattackercanreplacethetemporaryfilewithafilewithotherdatathatwillcausetheprogramtoactastheattackerdesires.Ifthereplacementfilehasthesameownerandgroupasthetemporaryfile,itcanbeverydifficultfortheprogramtodetermineifitisbeingspoofed.