外文翻译 -C语言

0外文原文TheCProgrammingLanguagePrefaceThecomputingworldhasundergonearevolutionsincethepublicationofTheCProgrammingLanguagein1978.Bigcomputersaremuchbigger,andpersonalcomputershavecapabilitiesthatrivalmainframesofadecadeago.Duringthistime,Chaschangedtoo,althoughonlymodestly,andithasspreadfarbeyonditsoriginsasthelanguageoftheUNIXoperatingsystem.ThegrowingpopularityofC,thechangesinthelanguageovertheyears,andthecreationofcompilersbygroupsnotinvolvedinitsdesign,combinedtodemonstrateaneedforamorepreciseandmorecontemporarydefinitionofthelanguagethanthefirsteditionofthisbookprovided.In1983,theAmericanNationalStandardsInstitute(ANSI)establishedacommitteewhosegoalwastoproduceanunambiguousandmachine-independentdefinitionofthelanguageC,whilestillretainingitsspirit.TheresultistheANSIstandardforC.Thestandardformalizesconstructionsthatwerehintedbutnotdescribedinthefirstedition,particularlystructureassignmentandenumerations.Itprovidesanewformoffunctiondeclarationthatpermitscross-checkingofdefinitionwithuse.Itspecifiesastandardlibrary,withanextensivesetoffunctionsforperforminginputandoutput,memorymanagement,stringmanipulation,andsimilartasks.Itmakesprecisethebehavioroffeaturesthatwerenotspelledoutintheoriginaldefinition,andatthesametimestatesexplicitlywhichaspectsofthelanguageremainmachine-dependent.ThisSecondEditionofTheCProgrammingLanguagedescribesCasdefinedbytheANSIstandard.Althoughwehavenotedtheplaceswherethelanguagehasevolved,wehavechosentowriteexclusivelyinthenewform.Forthemostpart,thismakesnosignificantdifference;themostvisiblechangeisthenewformoffunctiondeclarationanddefinition.Moderncompilersalreadysupportmostfeaturesofthestandard.Wehavetriedtoretainthebrevityofthefirstedition.Cisnotabiglanguage,anditisnotwellservedbyabigbook.Wehaveimprovedtheexpositionofcriticalfeatures,suchaspointers,thatarecentraltoCprogramming.Wehaverefinedtheoriginalexamples,andhaveaddednewexamplesinseveralchapters.Forinstance,thetreatmentofcomplicateddeclarationsisaugmentedbyprogramsthatconvertdeclarationsintowordsandviceversa.Asbefore,allexampleshavebeentesteddirectlyfromthetext,whichisinmachine-readableform.1AppendixA,thereferencemanual,isnotthestandard,butourattempttoconveytheessentialsofthestandardinasmallerspace.Itismeantforeasycomprehensionbyprogrammers,butnotasadefinitionforcompilerwriters-thatroleproperlybelongstothestandarditself.AppendixBisasummaryofthefacilitiesofthestandardlibrary.Ittooismeantforreferencebyprogrammers,notimplementers.AppendixCisaconcisesummaryofthechangesfromtheoriginalversion.Aswesaidintheprefacetothefirstedition,Cwearswellasonesexperiencewithitgrows.Withadecademoreexperience,westillfeelthatway.WehopethatthisbookwillhelpyoulearnCanduseitwell.Wearedeeplyindebtedtofriendswhohelpedustoproducethissecondedition.JonBently,DougGwyn,DougMcIlroy,PeterNelson,andRobPikegaveusperceptivecommentsonalmosteverypageofdraftmanuscripts.WearegratefulforcarefulreadingbyAlAho,DennisAllison,JoeCampbell,G.R.Emlin,KarenFortgang,AllenHolub,AndrewHume,DaveKristol,JohnLinderman,DaveProsser,GeneSpafford,andChrisvanWyk.WealsoreceivedhelpfulsuggestionsfromBillCheswick,MarkKernighan,AndyKoenig,RobinLake,TomLondon,JimReeds,ClovisTondo,andPeterWeinberger.DaveProsseransweredmanydetailedquestionsabouttheANSIstandard.WeusedBjarneStroustrupsC+translatorextensivelyforlocaltestingofourprograms,andDaveKristolprovideduswithanANSICcompilerforfinaltesting.RichDrechslerhelpedgreatlywithtypesetting.Oursincerethankstoall.BrianW.KernighanDennisM.RitchiePrefacetothefirsteditionCisageneral-purposeprogramminglanguagewithfeatureseconomyofexpression,modernflowcontrolanddatastructures,andarichsetofoperators.Cisnotaveryhighlevellanguage,norabigone,andisnotspecializedtoanyparticularareaofapplication.Butitsabsenceofrestrictionsanditsgeneralitymakeitmoreconvenientandeffectiveformanytasksthansupposedlymorepowerfullanguages.CwasoriginallydesignedforandimplementedontheUNIXoperatingsystemontheDECPDP-11,byDennisRitchie.Theoperatingsystem,theCcompiler,andessentiallyallUNIXapplicationsprograms(includingallofthesoftwareusedtopreparethisbook)arewritteninC.Productioncompilersalsoexistforseveralothermachines,includingtheIBMSystem/370,theHoneywell6000,andtheInterdata8/32.Cisnottiedtoanyparticular2hardwareorsystem,however,anditiseasytowriteprogramsthatwillrunwithoutchangeonanymachinethatsupportsC.ThisbookismeanttohelpthereaderlearnhowtoprograminC.Itcontainsatutorialintroductiontogetnewusersstartedassoonaspossible,separatechaptersoneachmajorfeature,andareferencemanual.Mostofthetreatmentisbasedonreading,writingandrevisingexamples,ratherthanonmerestatementsofrules.Forthemostpart,theexamplesarecomplete,realprogramsratherthanisolatedfragments.Allexampleshavebeentesteddirectlyfromthetext,whichisinmachine-readableform.Besidesshowinghowtomakeeffectiveuseofthelanguage,wehavealsotriedwherepossibletoillustrateusefulalgorithmsandprinciplesofgoodstyleandsounddesign.Thebookisnotanintroductoryprogrammingmanual;itassumessomefamiliaritywithbasicprogrammingconceptslikevariables,assignmentstatements,loops,andfunctions.Nonetheless,anoviceprogrammershouldbeabletoreadalongandpickupthelanguage,althoughaccesstomoreknowledgeablecolleaguewillhelp.Inourexperience,Chasproventobeapleasant,expressiveandversatilelanguageforawidevarietyofprograms.Itiseasytolearn,anditwearswellasonsexperiencewithitgrows.Wehopethatthisbookwillhelpyoutouseitwell.Thethoughtfulcriticismsandsuggestionsofmanyfriendsandcolleagueshaveaddedgreatlytothisbookandtoourpleasureinwritingit.Inparticular,MikeBianchi,JimBlue,StuFeldman,DougMcIlroyBillRoome,BobRosinandLarryRoslerallreadmultiplevolumeswithcare.WearealsoindebtedtoAlAho,SteveBourne,DanDvorak,ChuckHaley,DebbieHaley,MarionHarris,RickHolt,SteveJohnson,JohnMashey,BobMitze,RalphMuha,PeterNelson,ElliotPinson,BillPlauger,JerrySpivack,KenThompson,andPeterWeinbergerforhelpfulcommentsatvariousstages,andtoMileLeskandJoeOssannaforinvaluableassistancewithtypesetting.IntroductionCisageneral-purposeprogramminglanguage.IthasbeencloselyassociatedwiththeUNIXoperatingsystemwhereitwasdeveloped,sinceboththesystemandmostoftheprogramsthatrunonitarewritteninC.Thelanguage,however,isnottiedtoanyoneoperatingsystemormachine;andalthoughithasbeencalledasystemprogramminglanguagebecauseitisusefulforwritingcompilersandoperatingsystems,ithasbeenusedequallywelltowritemajorprogramsinmanydifferentdomains.3ManyoftheimportantideasofCstemfromthelanguageBCPL,developedbyMartinRichards.TheinfluenceofBCPLonCproceededindirectlythroughthelanguageB,whichwaswrittenbyKenThompsonin1970forthefirstUNIXsystemontheDECPDP-7.BCPLandBaretypelesslanguages.Bycontrast,Cprovidesavarietyofdatatypes.Thefundamentaltypesarecharacters,andintegersandfloatingpointnumbersofseveralsizes.Inaddition,thereisahierarchyofderiveddatatypescreatedwithpointers,arrays,structuresandunions.Expressionsareformedfromoperatorsandoperands;anyexpression,includinganassignmentorafunctioncall,canbeastatement.Pointersprovideformachine-independentaddressarithmetic.Cprovidesthefundamentalcontrol-flowconstructionsrequiredforwell-structuredprograms:statementgrouping,decisionmaking(if-else),selectingoneofasetofpossiblevalues(switch),loopingwiththeterminationtestatthetop(while,for)oratthebottom(do),andearlyloopexit(break).Functionsmayreturnvaluesofbasictypes,structures,unions,orpointers.Anyfunctionmaybecalledrecursively.Localvariablesaretypicallyautomatic,orcreatedanewwitheachinvocation.Functiondefinitionsmaynotbenestedbutvariablesmaybedeclaredinablock-structuredfashion.ThefunctionsofaCprogrammayexistinseparatesourcefilesthatarecompiledseparately.Variablesmaybeinternaltoafunction,externalbutknownonlywithinasinglesourcefile,orvisibletotheentireprogram.Apreprocessingstepperformsmacrosubstitutiononprogramtext,inclusionofothersourcefiles,andconditionalcompilation.Cisarelativelylow-levellanguage.Thischaracterizationisnotpejorative;itsimplymeansthatCdealswiththesamesortofobjectsthatmostcomputersdo,namelycharacters,numbers,andaddresses.Thesemaybecombinedandmovedaboutwiththearithmeticandlogicaloperatorsimplementedbyrealmachines.Cprovidesnooperationstodealdirectlywithcompositeobjectssuchascharacterstrings,sets,listsorarrays.Therearenooperationsthatmanipulateanentirearrayorstring,althoughstructuresmaybecopiedasaunit.Thelanguagedoesnotdefineanystorageallocationfacilityotherthanstaticdefinitionandthestackdisciplineprovidedbythelocalvariablesoffunctions;thereisnoheaporgarbagecollection.Finally,Citselfprovidesnoinput/outputfacilities;therearenoREADorWRITEstatements,andnobuilt-infileaccessmethods.Allofthesehigher-levelmechanismsmustbeprovidedbyexplicitlycalledfunctions.MostCimplementationshaveincludedareasonablystandardcollectionofsuchfunctions.4Similarly,Coffersonlystraightforward,single-threadcontrolflow:tests,loops,grouping,andsubprograms,butnotmultiprogramming,paralleloperations,synchronization,orcoroutines.Althoughtheabsenceofsomeofthesefeaturesmayseemlikeagravedeficiency,(YoumeanIhavetocallafunctiontocomparetwocharacterstrings?),keepingthelanguagedowntomodestsizehasrealbenefits.SinceCisrelativelysmall,itcanbedescribedinsmallspace,andlearnedquickly.Aprogrammercanreasonablyexpecttoknowandunderstandandindeedregularlyusetheentirelanguage.Formanyyears,thedefinitionofCwasthereferencemanualinthefirsteditionofTheCProgrammingLanguage.In1983,theAmericanNationalStandardsInstitute(ANSI)establishedacommitteetoprovideamodern,comprehensivedefinitionofC.Theresultingdefinition,theANSIstandard,orANSIC,wascompletedinlate1988.Mostofthefeaturesofthestandardarealreadysupportedbymoderncompilers.Thestandardisbasedontheoriginalreferencemanual.Thelanguageisrelativelylittlechanged;oneofthegoalsofthestandardwastomakesurethatmostexistingprogramswouldremainvalid,or,failingthat,thatcompilerscouldproducewarningsofnewbehavior.Formostprogrammers,themostimportantchangeisthenewsyntaxfordeclaringanddefiningfunctions.Afunctiondeclarationcannowincludeadescriptionoftheargumentsofthefunction;thedefinitionsyntaxchangestomatch.Thisextrainformationmakesitmucheasierforcompilerstodetecterrorscausedbymismatchedarguments;inourexperience,itisaveryusefuladditiontothelanguage.Thereareothersmall-scalelanguagechanges.Structureassignmentandenumerations,whichhadbeenwidelyavailable,arenowofficiallypartofthelanguage.Floating-pointcomputationsmaynowbedoneinsingleprecision.Thepropertiesofarithmetic,especiallyforunsignedtypes,areclarified.Thepreprocessorismoreelaborate.Mostofthesechangeswillhaveonlyminoreffectsonmostprogrammers.AsecondsignificantcontributionofthestandardisthedefinitionofalibrarytoaccompanyC.Itspecifiesfunctionsforaccessingtheoperatingsystem(forinstance,toreadandwritefiles),formattedinputandoutput,memoryallocation,stringmanipulation,andthelike.Acollectionofstandardheadersprovidesuniformaccesstodeclarationsoffunctionsindatatypes.Programsthatusethislibrarytointeractwithahostsystemareassuredofcompatiblebehavior.MostofthelibraryiscloselymodeledonthestandardI/OlibraryoftheUNIXsystem.Thislibrarywasdescribedinthefirstedition,andhasbeenwidelyusedonothersystemsaswell.Again,mostprogrammerswillnotseemuchchange.5BecausethedatatypesandcontrolstructuresprovidedbyCaresupporteddirectlybymostcomputers,therun-timelibraryrequiredtoimplementself-containedprogramsistiny.Thestandardlibraryfunctionsareonlycalledexplicitly,sotheycanbeavoidediftheyarenotneeded.MostcanbewritteninC,andexceptfortheoperatingsystemdetailstheyconceal,arethemselvesportable.AlthoughCmatchesthecapabilitiesofmanycomputers,itisindependentofanyparticularmachinearchitecture.Withalittlecareitiseasytowriteportableprograms,thatis,programsthatcanberunwithoutchangeonavarietyofhardware.Thestandardmakesportabilityissuesexplicit,andprescribesasetofconstantsthatcharacterizethemachineonwhichtheprogramisrun.Cisnotastrongly-typedlanguage,butasithasevolved,itstype-checkinghasbeenstrengthened.TheoriginaldefinitionofCfrownedon,butpermitted,theinterchangeofpointersandintegers;thishaslongsincebeeneliminated,andthestandardnowrequirestheproperdeclarationsandexplicitconversionsthathadalreadybeenenforcedbygoodcompilers.Thenewfunctiondeclarationsareanotherstepinthisdirection.Compilerswillwarnofmosttypeerrors,andthereisnoautomaticconversionofincompatibledatatypes.Nevertheless,Cretainsthebasicphilosophythatprogrammersknowwhattheyaredoing;itonlyrequiresthattheystatetheirintentionsexplicitly.C,likeanyotherlanguage,hasitsblemishes.Someoftheoperatorshavethewrongprecedence;somepartsofthesyntaxcouldbebetter.Nonetheless,Chasproventobenanextremelyeffectiveandexpressivelanguageforawidevarietyofprogrammingapplications.Thebookisorganizedasfollows.Chapter1isatutorialonthecentralpartofC.Thepurposeistogetthereaderstartedasquicklyaspossible,sincewebelievestronglythatthewaytolearnanewlanguageistowriteprogramsinit.Thetutorialdoesassumeaworkingknowledgeofthebasicelementsofprogramming;thereisnoexplanationofcomputers,ofcompilation,norofthemeaningofanexpressionliken=n+1.Althoughwehavetriedwherepossibletoshowusefulprogrammingtechniques,thebookisnotintendedtobeareferenceworkondatastructuresandalgorithms;whenforcedtomakeachoice,wehaveconcentratedonthelanguage.Chapters2through6discussvariousaspectsofCinmoredetail,andrathermoreformally,thandoesChapter1,althoughtheemphasisisstillonexamplesofcompleteprograms,ratherthanisolatedfragments.Chapter2dealswiththebasicdatatypes,operatorsandexpressions.Chapter3threatscontrolflow:if-else,switch,while,for,etc.Chapter4coversfunctionsandprogramstructure-externalvariables,scoperules,multiplesourcefiles,andsoon-andalsotouchesonthepreprocessor.Chapter5discussespointersandaddressarithmetic.Chapter6coversstructuresandunions.6Chapter7describesthestandardlibrary,whichprovidesacommoninterfacetotheoperatingsystem.ThislibraryisdefinedbytheANSIstandardandismeanttobesupportedonallmachinesthatsupportC,soprogramsthatuseitforinput,output,andotheroperatingsystemaccesscanbemovedfromonesystemtoanotherwithoutchange.Chapter8describesaninterfacebetweenCprogramsandtheUNIXoperatingsystem,concentratingoninput/output,thefilesystem,andstorageallocation.AlthoughsomeofthischapterisspecifictoUNIXsystems,programmerswhouseothersystemsshouldstillfindusefulmaterialhere,includingsomeinsightintohowoneversionofthestandardlibraryisimplemented,andsuggestionsonportability.AppendixAcontainsalanguagereferencemanual.TheofficialstatementofthesyntaxandsemanticsoftheClanguageistheANSIstandarditself.Thatdocument,however,isintendedforemostforcompilerwriters.Thereferencemanualhereconveysthedefinitionofthelanguagemoreconciselyandwithoutthesamelegalisticstyle.AppendixBisasummaryofthestandardlibrary,againforusersratherthanimplementers.AppendixCisashortsummaryofchangesfromtheoriginallanguage.Incasesofdoubt,however,thestandardandonesowncompilerremainthefinalauthoritiesonthelanguage.Chapter1-ATutorialIntroductionLetusbeginwithaquickintroductioninC.Ouraimistoshowtheessentialelementsofthelanguageinrealprograms,butwithoutgettingboggeddownindetails,rules,andexceptions.Atthispoint,wearenottryingtobecompleteorevenprecise(savethattheexamplesaremeanttobecorrect).Wewanttogetyouasquicklyaspossibletothepointwhereyoucanwriteusefulprograms,andtodothatwehavetoconcentrateonthebasics:variablesandconstants,arithmetic,controlflow,functions,andtherudimentsofinputandoutput.WeareintentionallyleavingoutofthischapterfeaturesofCthatareimportantforwritingbiggerprograms.Theseincludepointers,structures,mostofCsrichsetofoperators,severalcontrol-flowstatements,andthestandardlibrary.Thisapproachanditsdrawbacks.Mostnotableisthatthecompletestoryonanyparticularfeatureisnotfoundhere,andthetutorial,bybeingbrief,mayalsobemisleading.AndbecausetheexamplesdonotusethefullpowerofC,theyarenotasconciseandelegantastheymightbe.Wehavetriedtominimizetheseeffects,butbewarned.Anotherdrawbackisthatlaterchapterswillnecessarilyrepeatsomeofthischapter.Wehopethattherepetitionwillhelpyoumorethanitannoys.Inanycase,experiencedprogrammersshouldbeabletoextrapolatefromthematerialinthischaptertotheirownprogrammingneeds.Beginnersshouldsupplementitbywritingsmall,similarprogramsoftheirown.BothgroupscanuseitasaframeworkonwhichtohangthemoredetaileddescriptionsthatbegininChapter2.7GettingStartedTheonlywaytolearnanewprogramminglanguageisbywritingprogramsinit.Thefirstprogramtowriteisthesameforalllanguages:Printthewordshello,worldThisisabighurdle;toleapoverityouhavetobeabletocreatetheprogramtextsomewhere,compileitsuccessfully,loadit,runit,andfindoutwhereyouroutputwent.Withthesemechanicaldetailsmastered,everythingelseiscomparativelyeasy.InC,theprogramtoprinthello,worldis#includemain()printf(hello,worldn);Justhowtorunthisprogramdependsonthesystemyouareusing.Asaspecificexample,ontheUNIXoperatingsystemyoumustcreatetheprograminafilewhosenameendsin.c,suchashello.c,thencompileitwiththecommandcchello.cIfyouhaventbotchedanything,suchasomittingacharacterormisspellingsomething,thecompilationwillproceedsilently,andmakeanexecutablefilecalleda.out.Ifyouruna.outbytypingthecommanda.outitwillprinthello,worldOnothersystems,theruleswillbedifferent;checkwithalocalexpert.Now,forsomeexplanationsabouttheprogramitself.ACprogram,whateveritssize,consistsoffunctionsandvariables.Afunctioncontainsstatementsthatspecifythecomputingoperationstobedone,andvariablesstorevaluesusedduringthecomputation.CfunctionsarelikethesubroutinesandfunctionsinFortranortheproceduresandfunctionsofPascal.Ourexampleisafunctionnamedmain.Normallyyouareatlibertytogivefunctionswhatevernamesyoulike,butmainisspecial-yourprogrambeginsexecutingatthebeginningofmain.Thismeansthateveryprogrammusthaveamainsomewhere.mainwillusuallycallotherfunctionstohelpperformitsjob,somethatyouwrote,andothersfromlibrariesthatareprovidedforyou.Thefirstlineoftheprogram,8#includetellsthecompilertoincludeinformationaboutthestandardinput/outputlibrary;thelineappearsatthebeginningofmanyCsourcefiles.ThestandardlibraryisdescribedinChapter7andAppendixB.Onemethodofcommunicatingdatabetweenfunctionsisforthecallingfunctiontoprovidealistofvalues,calledarguments,tothefunctionitcalls.Theparenthesesafterthefunctionnamesurroundth