Lecture12

ProgramOrganizationCProgrammingLanguageOutlineOrganizationofLargeProgramsFunctionScopeRulesVariableScopeRulesVariableLifetimeandInitializationsPreprocessorCommandsOrganizationofLargeProgramAlargeprogrammaycontainhundredsandthousandsoffunctions.Itmustbedividedintosmallpartstocomplete.Project,sourcefile,functionSomerulesmustbesettomakesureallofpartsworkproperly.namingconflictobjectssharingFunctionScopeRulesScopedescribesthevisibilityofanobject.Bothvariablesandfunctionshavetheirscopes.Bydefault,thescopesoffunctionsareexternal.Thatmeansifyoudefineafunction,itcanbecalledbyotherseitherwithinthesamesourcecodefile,orinothersourcecodefiles.FunctionScopeRulesIfthedefinitionofafunctionappearsbelowitscaller,youhavetodeclareitsprototypebeforecallingit.Itisthesameforcallingafunctiondefinedinanothersourcecodefile;declarationisrequiredbeforecallingthefunction.Example1FunctionScopeRulesIfyouwanttokeepafunctioninprivate,whichmeanstomakethescopeofafunctionwithinthecurrentsourcecodefile,youcanusekeywordstatic.Thesyntaxistoputstaticaheadofthefunctiondefinitionsuchas

staticintfactoria(intn)VariableScopeRulesTodistinguishscopesofvariables,wedivideallvariablesintotwocategories:localvariablesandexternalvariables.LocalVariables(AutomaticVariables/InternalVariables):Variablesthatdeclaredinsideafunction.ExternalVariables:Variablesthatdefinedoutsideofallfunctions.VariableScopeRulesAlocalvariableisonlyvisiblewithinthefunctionitstays,itsscopeisfromitsdefinitiontotheendofthefunction.Itispossibletodefinelocalvariableswiththesamenameindifferentfunctions.Therewillbenoconflictasthescopeofalocalvariableisonlyinsidethefunction.VariableScopeRulesWithinafunction,youcandefinetwolocalvariableswiththesamenameindifferentlevelsofblocks.Inthiscasethescopeofeachvariableisonlylimitedwithintheblockitstays.Example2VariableScopeRulesExternalvariablesalsoneedtobedefinedbeforeyoucanusethem.Thescopeofanexternalvariableisfromitsdefinitiontotheendofthecurrentsourcecodefile.Anexternalvariablecanbevisiblebyallfunctionswithinitsscope.Thatmeansanexternalvariablecouldbesharedandoperatedbyseveralfunctions.VariableScopeRulesThenameofanexternalvariablecanbethesameasalocalvariable.Ifthishappens,thelocalvariableisvisibleforthatfunctionandtheexternalvariablewillbeinvisibleforthatfunction.Example3,4VariableScopeRulesIfyoudefineanexternalvariableinasourcecodefile,youcanuseitinanothersourcecodefilebydeclaringitwithkeywordextern.Todeclareavariableissimilartodefiningavariable.Butdeclarationanddefinitionaretwodifferentconcepts.Example5,6VariableScopeRulesIfyouwanttokeepyourexternalvariablesinprivate,anddonotwanttheyareusedbyothers,usethekeywordstaticwhendefiningit.ThisissimilartothestaticfunctionsVariableScopeRulesKeywordregisterisanothermodifier.Definingavariableasregisteristotellthecompilertotrytoputthisvariableinsideoneoftheregistersinsteadofputtingitonmemory.Example7VariableScopeRulesPleasenoteitisnotguaranteedtheregisterwillbeusedevenifyoudefinethevariableasregister.Inthiscasearegistervariablewillbethesameasalocalvariable.Aregistervariablecanonlybedefinedastypeintorchar.VariableLifetimeandInitializationVariablelifetimemeansthedurationavariablewillexist.Therulesare:Localvariablescomeintoexistencewhenthefunctioniscalled,anddisappearwhenthefunctionisreturned.Externalvariablesaresharedbyseveralfunctions,theyareexistentafterdefiningandwillstaypermanently.VariableLifetimeandInitializationIfyoudonotinitializealocalvariablewhiledefiningit,thevalueofitwillbeuncertainExternalvariableswillbeautomaticallyinitializedbydefaultvaluesevenifyoudonotexplicitlyinitializethem.Thedefaultvaluewillbe0ifthevariableisnumeric,andthedefaultvaluewillbe‘\0’ifthevariableisacharacter.VariableLifetimeandInitializationKeywordstaticcanbealsousedasamodifiertodefinelocalvariables.Definingalocalvariableasstaticwillnotchangeitsscope;itchangesthevariable’slifetimeandinitialization.VariableLifetimeandInitializationThelifetimeofastaticlocalvariablebeginsfromitsdefinitionandwillkeeppermanently.Eachtimeafunctioncallisend,itsstaticlocalvariablewillkeepalive,whileotherordinarylocalvariableswilldisappear.VariableLifetimeandInitializationThestaticlocalvariableswillbeinitializedbydefaultvaluesautomaticallyeveniftherearenoexplicitlyinitializations.Astaticlocalvariableisinitializedonlyonceatthefirsttimethefunctionitbelongstoiscalled.Whenthefirstfunctioncallisend,thestaticlocalvariablewillkeepitsvalue.VariableLifetimeandInitializationWheneverthefunctioniscalledafterthefirstfunctioncall,thestaticvariableswillnotbeinitializedagain.Aftereachfunctioncall,thestaticvariableswillkeeptheirvaluestilltheendoftheprogram.Example8SummaryKeywordsrelativetoscopeandlifetimestaticexternregisterautoPleasebeawaretheaffectmaycausedbyabovekeywordsonvariablesandfunctionsPreprocessorProcessTherearesomeworktodobeforethecompilertocompilethesourcecodefileFileInclusionMacroSubstitutionConditionalInclusionFileInclusionCommand#includeisusedtoincludeaspecificfileintothecurrentfile.Format#include<File_Name> forsystemfiles,especiallysystemheadfile#include“File_Name” foruser-definedfilesMacroDefinitionCommand#defineisusedtodefinemacrosAmacrohasanameanditscontent,whichisasequenceofcharacters.Syntax

#defineMacro_NameMacro_ContentMacroDefinitionAfterdefiningamacro,thepreprocessorwillreplaceallthemacrosthroughoutthecodesbythedefinedcontent.Thisiscalledmacroexpansion,ormacrosubstitution.Exception:Themacronameappearsduringthe“”willnotbereplaced.MacroDefinitionNestedMacrodefinitionispermitted.#definePI3.14

#defineAreaPI*r*rints=Area;Thesubstitutionsofthenestedmacrosarealsoinanestedway.MacroDefinitionScopeoftheMacro:startfromthedefinitiontilltheendofthefile.Oryoucanexplicitlyenditbyusingcommand#undef#defineand#undefcanbeonlyputoutsideofthefunctiondefinitions.Example9,10MacroDefinitionwithArgumentLikethefunctiondefinition,itisabletodefineamacrowithsomearguments.Correspondingly,youneedtogivethevalueoftheargumentsonmacrosubstitution.Example11ConditionalInclusionItisbeabletocontrolthepartsofthecodetobecompiledbycertainconditions.Commands #ifdef #ifndef #ifConditionalInclusion