CITS2002SystemsProgramming

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WelcometoCITS2002SystemsProgrammingThisunit,firstpresentedin2012asCITS1002,isoneofthecoreunitsineachofUWA'sComputerScience,andEngineeringScience(SE)majors.

Theunitexplorestheroleofcontemporaryoperatingsystemsandtheirsupportforhigh-levelprogramminglanguages,howtheymanageefficientaccesstocomputerhardware,andhowacomputer'sresourcesmaybeaccessedandcontrolledbytheCprogramminglanguage.

TheunitwillbepresentedbyChrisMcDonald-whichispronounced'Chris',not'Dr',not'Professor'.

OurUWAHandbookentry

UnderstandingtherelationshipbetweenaprogramminglanguageandthecontemporaryoperatingsystemsonwhichitexecutesiscentraltodevelopingmanyskillsinComputerScience.ThisunitintroducesthestandardCprogramminglanguage,onwhichmanyotherprogramminglanguagesandsystemsarebased,throughastudyofcoreoperatingsystemservicesincludingprocesses,inputandoutput,memorymanagement,andfilesystems.

TheClanguageisintroducedthroughdiscussionsonbasictopicslikedatatypes,variables,expressions,controlstructures,scopingrules,functionsandparameterpassing.MoreadvancedtopicslikeC'srun-timeenvironment,systemcalls,dynamicmemoryallocation,andpointersarepresentedinthecontextofoperatingsystemservicesrelatedtoprocessexecution,memorymanagementandfilesystems.Theimportanceofprocessscheduling,memorymanagementandinterprocesscommunicationinmodernoperatingsystemsisdiscussedinthecontextofoperatingsystemsupportformultiprogramming.Laboratoryandtutorialworkplaceastrongfocusonthepracticalapplicationoffundamentalprogrammingconcepts,withexamplesdesignedtocompareandcontrastmanykeyfeaturesofcontemporaryoperatingsystems.

TheUWAHandbookentryforthisunitstronglyrecommendsthatyoutakeoneofthreeunitsofAdvisablePriorStudybeforetakingthisunit-CITS1001,CITS1401,orCITS2401.Studentswhotookthisunitinrecentyears,andhadchosentonottakeoneoftheunitsofAdvisablePriorStudy,foundthematerialinthisunitdifficult.Thisunitisnotsuitableforfirst-timeprogrammers.

CITS2002SystemsProgramming,Lecture1,p1,27thJuly2020.

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TopicstobecoveredinCITS2002SystemsProgrammingIt'simportanttoknowwherewe'reheading,sohere'salistoftopicsthatwe'llbecovering:

AnintroductiontotheISO-C99programminglanguageThestructureofaCprogram,basicdatatypesandvariables,compilingandlinking.

AnintroductiontoOperatingSystemsAbriefhistoryofoperatingsystems,theroleofcontemporaryoperatingsystems,therelationshipbetweenprogramminglanguages,programs,andoperatingsystems.

AnoverviewofcomputerhardwarecomponentsTheprocessoranditsregisters,thememoryhierarchy,inputandoutput(I/O)andstoragecomponents.

CprogramsingreaterdetailArraysandcharacterstrings,user-definedtypesandstructures,howthecomputerhardwarerepresentsdata,functions,parameterpassingandreturnvalues.

OperatingsystemservicesCreatingandterminatingprocesses,aprogram'sruntimeenvironment,command-linearguments,accessingoperatingsystemservicesfromC.

ManagingmemoryAllocatingphysicalmemorytoprocesses,sharingmemorybetweenmultipleprocesses,allocatingandmanagingmemoryinCprograms.

FilesandtheiruseinprogramsThefilemanagementsystem,fileallocationmethods,fileanddirectoryoperationsandattributes,fileinputandoutput(I/O),rawandformattedI/O,unbufferedandbufferedI/Ofunctions.

Bytheendofthisunityou'llhavethisknowledge-itjustwon'tallbepresentedstrictlyinthisorder.

Hereisourunit'sschedule.

CITS2002SystemsProgramming,Lecture1,p2,27thJuly2020.

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WhyteachC?Sinceitsbeginningsinearly1973,theCprogramminglanguagehasevolvedtobecomeoneoftheworld'smostpopular,andwidelydeployedprogramminglanguages.ThelanguagehasundergoneextensiveformalstandardizationtoproducetheANSI-Cstandardin1989,theISO-C99standardin1999,ISO-C11(revision)inDec2011,andISO-C18inJune2018(whichintroducesnonewlanguagefeatures,onlytechnicalcorrectionsandclarificationstodefectsinC11).

Cistheprogramminglanguageofchoiceformostsystems-level,engineering,andscientificprogramming:

mostoftheworld'spopularoperatingsystems,Linux,WindowsandmacOS,theirinterfacesandfile-systems,arewritteninC,theinfrastructureoftheInternet,includingmostofitsnetworkingprotocols,webservers,andemailsystems,arewritteninC,softwarelibrariesprovidinggraphicalinterfacesandtools,andefficientnumerical,statistical,encryption,andcompressionalgorithms,arewritteninC,thesoftwareformostembeddeddevices,includingthoseincars,aircraft,robots,smartappliances,sensors,mobilephones,andgameconsoles,iswritteninC,thesoftwareontheMarsPhoenixLanderiswritteninC,muchofthesafety-criticalsoftwareontheF-35jointstrikefighter,iswritteninC,butCwasnotusedontheApollo-11mission!

IsCstillrelevant?[ref:Tiobesurvey](TheTiobesurveyisbasedonsearch-enginequeries-isnotaboutthebestprogramminglanguageorthelanguageinwhichmostlinesofcode havebeenwritten.)

Though,ofcourse,popularityisapoormeasureofquality-otherwise,McDonald'sRestaurantswouldreceiveMichelinstars.

Otherinterestingsurveys:

Stackoverflow'sDeveloperSurveyResults2020Jetbrains'TheStateofDeveloperEcosystemin2020HackerRank's2020DeveloperSkillsReport

CITS2002SystemsProgramming,Lecture1,p3,27thJuly2020.

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SowhatisC?

Inonebreath,Cisoftendescribedasagoodgeneralpurposelanguage,anexcellentsystemsprogramminglanguage,andjustaglorifiedassemblylanguage.Sohowcanitbeallthree?

Ccanbecorrectlydescribedasageneralpurposeprogramminglanguage-adescriptionalsogiventoJava,Python,Visual-Basic,C++,andC#.

Cisaproceduralprogramminglanguage,notanobject-orientedlanguagelikeJava,(partsof)Python,Objective-C,orC#.

Cprogramscanbe"good"programs,iftheyare:

welldesigned,clearlywritten,writtenforportability,welldocumented,usehighlevelprogrammingpractices,andwelltested.

Ofcourse,theabovepropertiesareindependentofC,andareofferedbymanyhighlevellanguages.

Chasprogrammingfeaturesprovidedbymostproceduralprogramminglanguages-stronglytypedvariables,constants,standard(orbase)datatypes,enumeratedtypes,user-definedtypes,aggregatestructures,standardcontrolflow,recursion,andprogrammodularization.Cdoesnotoffertuplesorsets,Java'sconceptofclassesorobjects,nestedfunctions,subrangetypes,andhasonlyrecentlyaddedaBooleandatatype.Cdoeshave,however,separatecompilation,conditionalcompilation,bitwiseoperators,pointerarithmetic,andlanguageindependentinputandoutput.

CITS2002SystemsProgramming,Lecture1,p4,27thJuly2020.

Aprogramminglanguagethatdoesn'taffectthewayyouthinkaboutprogrammingisn'tworthknowing.— AlanPerlis,1stTuringAwardwinner

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ASystemsProgrammingLanguageCisfrequently,andcorrectly,describedasanexcellentsystemsprogramminglanguage.

Calsoprovidesanexcellentoperatingsysteminterfacethroughitswelldefined,hardwareandoperatingsystemindependent,standardlibrary.

TheClanguagebeganitsdevelopmentin1972,asaprogramminglanguageinwhichtore-writesignificantportionsontheUnixoperatingsystem:

UnixwasfirstwritteninassemblylanguagesforPDP-7andPDP-11computers.

In1973DennisRitchiewasworkingonaprogramminglanguageforoperatingsystemdevelopment.BasinghisideasuponBCPL,hedevelopedBandfinallycreatedonecalledC.(Yes,thereisalanguagenamed'D',butit'snotadescendantofC)

Bytheendof1973,theUNIXkernelwas85%writteninCwhichenabledittobeportedtoothermachinesforwhichaCcompilercouldbefashioned.

ThiswasagreatstepbecauseitnolongertiedtheoperatingsystemtothePDP-7asitwouldhavebeenifitremainedinassemblylanguage.In1976DennisRitchieandStephenJohnstonportedUnixtoanInterdata8/32machine.Sincethen,UnixandLinuxhavebeenportedtoover260differentprocessorarchitectures.

Today,wellinexcessof95%oftheUnix,Linux,macOS,andWindowsoperatingsystemkernelsandtheirstandardlibraryroutinesareallwrittenintheCprogramminglanguage-it'sextremelydifficulttofindanoperatingsystemnotwrittenineitherCoritsdescendantsC++orObjective-C.

CITS2002SystemsProgramming,Lecture1,p5,27thJuly2020.

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PortabilityondifferentarchitecturesCcompilershavebeenbothdevelopedandportedtoalargenumberandtypeofcomputerarchitectures:

from4-bitand8-bitmicrocontrollers,throughtraditional16-,32-,and64-bitvirtualmemoryarchitecturesinmostPCsandworkstations,tolarger64-and128-bitsupercomputers.

Compilershavebeendevelopedfor:

traditionallargeinstructionsetarchitectures,suchasIntelx86,AMD,ARM,Motorola680x0,SunSPARCs,andDEC-Alpha,newerreducedinstructionsetarchitectures(RISC),suchasSGIMIPS,IBM/MotorolaPowerPC,mobilephones,hometheatreequipment,routersandaccess-points,andparallelandpipelinedarchitectures.

CITS2002SystemsProgramming,Lecture1,p6,27thJuly2020.

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AllitrequiresisaportedCcompilerOnceaCcompilerhasbeendevelopedforanewarchitecture,theterabytesofCprogramsandlibrariesavailableonotherC-basedplatformscanalsobeportedtothenewarchitecture.

Whataboutassemblylanguages?ItisoftenquotedthatacompiledCprogramwillrunonly1-2%slowerthanthesameprogramhand-codedinthenativeassemblylanguageforthemachine.

Buttheobviousadvantageofhavingtheprogramcodedinareadable,highlevellanguage,providestheoverwhelmingadvantagesofmaintainabilityandportability.

Verylittleofanoperatingsystem,suchasWindows,macOS,orLinux,iswritteninanassemblylanguage-inmostcasesthemajorityiswritteninC.

Evenanoperatingsystem'sdevicedrivers,oftenconsideredthemosttime-criticalcodeinanoperatingsystemkernel,todaycontainassemblylanguagenumberedinonlythehundredsoflines.

CITS2002SystemsProgramming,Lecture1,p7,27thJuly2020.

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TheunreadabilityofCprogramsCisdescribedasnothingmorethanaglorifiedassemblylanguage,meaningthatCprogramscanbewritteninsuchanunreadablefashionthattheylooklikeyourmonitorissetatthewrongspeed.

(infactthere'sahumorouscontestheldeachyear,TheInternationalObfuscatedCCodeContesttodesignfullyworkingbutindecipherablecode,andtheUnderhandedCContestwhosegoalistowritecodethatisasreadable,clear,innocentandstraightforwardaspossible,andyetitmustfailtoperformatitsapparentfunction).

PerhapsC'sbiggestproblemisthatthelanguagewasdesignedbyprogrammerswho,folkloresays,werenotveryproficienttypists.

Cmakesextensiveuseofpunctuationcharactersinthesyntaxofitsoperatorsandcontrolflow.Infact,onlythepunctuationcharacters

@`and$

arenotusedinC'ssyntax!(andDEC-Conceusedthe$character,andObjective-Cnowusesthe@).

Itisnotsurprising,then,thatifCprogramsarenotformattedbothconsistentlyandwithsufficientwhitespacebetweenoperators,andifveryshortidentifiernamesareused,aCprogramwillbeverydifficulttoread.

Topartiallyaddresstheseproblems,anumberoftext-editors,integrateddevelopmentenvironments(IDEs),andbeautificationprograms(suchasindent)canautomaticallyreformatourCcodeaccordingtoconsistentspecifications.

CITS2002SystemsProgramming,Lecture1,p8,27thJuly2020.

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CriticismsofC'sexecutionmodelCiscriticizedforbeingtooforgivinginitstype-checkingatcompiletime.

Itispossibletocastaninstanceofsometypesintoothertypes,evenifthetwoinstanceshaveconsiderablydifferenttypes.

Apointertoaninstanceofonetypemaybecoercedintoapointertoaninstanceofanothertype,therebypermittingtheitem'scontentstobeinterpreteddifferently.

BadlywrittenCprogramsmakeincorrectassumptionsaboutthesizeofitemstheyaremanaging.Integersof8-,16-,and32-bitscanholddifferentrangesofvalues.Poorchoices,orunderspecificationcaneasilyleadtoerrors.

Cprovidesnoruntimeprotectionagainstarithmeticerrors.

Thereisnoexceptionhandlingmechanism,anderrorssuchasdivision-by-zeroandarithmeticoverflowandunderflow,arenotcaughtandreportedatrun-time.

Coffersnoruntimecheckingofpopularandpowerfulconstructslikepointervariablesandarrayindices.

Subjecttoconstraintsimposedbytheoperatingsystem'smemorymanagementroutines,apointermaypointalmostanywhereinaprocess'addressspaceandseeminglyrandomaddressesmaybereadorwrittento.

AlthoughallarrayindicesinCbeginat0,itispossibletoaccessanarray'selementswithnegativeindicesorindicesbeyondthedeclaredendofthearray.

Thereareoccasionswheneachoftheseoperationsmakesense,buttheyarerare.

Cdoesnotholdthehandoflazyprogrammers.

Weavoidallofthesepotentialproblemsbylearningthelanguagewell,andemployingsafeprogrammingpractices.

CITS2002SystemsProgramming,Lecture1,p9,27thJuly2020.

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Whatisthebestprogramminglanguage?Thequestion,evenarguments,ofwhetherC,Java,Visual-Basic,C++,orC#isthebestgeneralpurposeprogramminglanguageispointless.

Theimportantquestionis:"whichlanguageismostsuitedforthetaskathand?"

Thisunitwillanswerthequestions:

"whenisCthebestlanguagetouse?"and"howdowebestuseC'sfeaturesforsystemsprogramming?"

ThroughasequenceofunitsofferedbyComputerScience&SoftwareEngineeringyoucanbecomeproficientinawidevarietyofprogramminglanguages-procedural,object-oriented,functional,logic,set-based,andformal-andknowthemostappropriateonetoselectforanyproject.

CITS2002SystemsProgramming,Lecture1,p10,27thJuly2020.

CandC++areonlythefoundationbecausetheyhappenedtobecomepopularduetoabunchofmiscellaneousfactors,notbecausetheyareinherentlygreatinventionsinthemselves.Also,they(andtheirstandardlibraries)evolvedovertimetotheircurrentstate.

It'slikesayingEnglishandSpanisharethemostimportantlanguagesbecausetheyarefundamentallythe"best-invented"ones,notbecauseoftheaccidentsoffatethatwerecolonialexpansion,WWII,andtheInternet.— Anon

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TheStandardizationofC-K&RC

DespiteC'slonghistory,beingfirstdesignedintheearly1970s,itunderwentconsiderablylittlechangeuntilthelate1980s.

Thisisaverylengthyperiodoftimewhentalkingaboutaprogramminglanguage'sevolution.

TheoriginalClanguagewasmostlydesignedbyDennisRitchieandthendescribedbyBrianKernighanandDennisRitchieintheirimaginativelytitledbookTheCProgrammingLanguage.

Thelanguagedescribedinthisseminalbook,describedasthe"K&R"book,isnowdescribedas"K&R"or"old"C.

228pages.

CITS2002SystemsProgramming,Lecture1,p11,27thJuly2020.

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TheStandardizationofC-ANSI-C(K&R-2)

Inthelate1980s,anumberofstandardsformingbodies,andinparticulartheAmericanNationalStandardsAssociationX3J11Committee,commencedworkonrigorouslydefiningboththeClanguageandthecommonlyprovidedstandardClibraryroutines.TheresultsoftheirlengthymeetingsaretermedtheANSI-X3J11standard,orinformallyasANSI-C,C89,orC90.

TheformaldefinitionofANSI-Cintroducedsurprisinglyfewmodificationstotheold"K&R"languageandonlyafewadditions.

MostoftheadditionsweretheresultofsimilarenhancementsthatweretypicallyprovidedbydifferentvendorsofCcompilers,andthesehadgenerallybeenconsideredasessentialextensionstooldC.TheANSI-ClanguageisextremelysimilartooldC.Thecommitteeonlyintroducedanewbasedatatype,modifiedthesyntaxoffunctionprototypes,addedfunctionalitytothepreprocessor,andformalizedtheadditionofconstructssuchasconstantsandenumeratedtypes.

272pages.

CITS2002SystemsProgramming,Lecture1,p12,27thJuly2020.

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TheStandardizationofC-ANSI/ISO-C99andISO/IEC9899:2011(C11)

AnewrevisionoftheClanguage,namedANSI/ISO-C99(knownasC99),wascompletedin1999.

Manyfeatureswere"cleanedup",includingtheadditionofBooleanandcomplexdatatypes,singlelinecomments,andvariablelengtharrays,andtheremovalofmanyunsafefeatures,andill-definedconstructs.

753pages.

ArevisionofC99,ISO/IEC9899:2011(knownasC11),wascompletedinDecember2011.

InthisunitwewillfocusexclusivelyonANSI/ISO-C99,andonlymentionotherversionsofCwhenthedifferencesaresignificant.

CITS2002SystemsProgramming,Lecture1,p13,27thJuly2020.

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WhatStandardizationProvidesThesequiteformalstandardsspecifytheformandestablishestheinterpretationofprogramswrittenintheCprogramminglanguage.Theyspecify:

therepresentationofCprograms;thesyntaxandconstraintsoftheClanguage;thesemanticrulesforinterpretingCprograms;therepresentationofinputdatatobeprocessedbyCprograms;therepresentationofoutputdataproducedbyCprograms;therestrictionsandlimitsimposedbyaconformingimplementationofC.

Theydonotspecify:

themechanismbywhichCprogramsaretransformedforusebyadata-processingsystem;themechanismbywhichCprogramsareinvokedforusebyadata-processingsystem;themechanismbywhichinputdataaretransformedforusebyaCprogram;themechanismbywhichoutputdataaretransformedafterbeingproducedbyaCprogram;thesizeorcomplexityofaprogramanditsdatathatwillexceedthecapacityofanyspecificdata-processingsystemorthecapacityofaparticularprocessor;allminimalrequirementsofadata-processingsystemthatiscapableofsupportingaconformingimplementation.

CITS2002SystemsProgramming,Lecture1,p14,27thJuly2020.

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What's(deliberately)missingfromtheClanguage?Atfirstglance,theClanguageappearstobemissingsomecommonlyrequiredfeaturesthatotherlanguages,suchasJava,provideintheirstandards.

Forexample,Cdoesnotprovidefeaturesforgraphics,networking,cryptography,ormultimedia.

Instead,Cpermits,enables,andencouragesadditional3rd-partylibraries(bothopen-sourceandcommercial)toprovidethesefacilities.Thereasonforthese"omissions"isthatCrigorouslydefineswhatitdoesprovide,andrigorouslydefineshowCmustinteractwithexternallibraries.

Herearesomewell-respected3rd-partylibraries,frequentlyemployedinlargeCprograms:

Functiondomain 3rd-partylibrariesoperatingsystemservices(files,directories,processes,inter-processcommunication)

OS-specificlibraries,e.g.glibc,Win32,Carbon

web-basedprogramming libcgi,libxml,libcurl

datastructuresandalgorithms thegenericdatastructureslibrary(GDSL)

GUIandgraphicsdevelopment OpenGL,GTK,Qt,UIKit,Win32,Tcl/Tk

imageprocessing(GIFs,JPGs,etc) gd

networking Berkeleysockets,AT&T'sTLI

security,cryptography openssl,libmp

scientificcomputing NAG,Blas3,GNUscientificlibrary(gsl)

concurrency,parallelandGPUprogramming pthreads,OpenMPI,openLinda,CUDA,OpenCL

CITS2002SystemsProgramming,Lecture1,p15,27thJuly2020.