Jump Tables via Function Pointer Arrays in C/C++

Here's a look at the use of arrays of function pointers in C/C++ as jump ... the function that uses it–and thus keeping it off the stack. Home Home Bloggers MichaelBarr NigelJones MiroSamek PostsbyCategory NewsletterSignup AboutUs ContactUs Login    Searchfor: «InPraiseoftheCPreprocessor’s#errorDirective RS-485TransmitEnableSignalControl» JumpTablesviaFunctionPointerArraysinC/C++ Thursday,December17th,2009byNigelJones AlsoavailableinPDFversion. Jumptables,alsocalledbranchtables,areanefficientmeansofhandlingsimilareventsinsoftware.Here’salookattheuseofarraysoffunctionpointersinC/C++asjumptables. Examinationofassemblylanguagecodethathasbeencraftedbyanexpertwillusuallyrevealextensiveuseoffunction“branchtables.”Branchtables(a.k.a.,jumptables)areusedbecausetheyofferauniqueblendofcompactnessandexecutionspeed,particularlyonmicroprocessorsthatsupportindexedaddressing.WhenoneexaminestypicalC/C++code,however,thebranchtable(i.e.,anarrayoffuntionpointers)isamuchrarerbeast.ThepurposeofthisarticleistoexaminewhybranchtablesarenotusedbyC/C++programmersandtomakethecasefortheirextensiveuse.Realworldexamplesoftheiruseareincluded. Functionpointers IntalkingtoC/C++programmersaboutthistopic,threereasonsareusuallycitedfornotusingfunctionpointers.Theyare: Theyaredangerous Agoodoptimizingcompilerwillgenerateajumptablefromaswitchstatement,soletthecompilerdothework Theyaretoodifficulttocodeandmaintain Arefunctionpointersdangerous? Thisschoolofthoughtcomesabout,becausecodethatindexesintoatableandthencallsafunctionbasedontheindexhasthecapabilitytoendupjustaboutanywhere.Forinstance,considerthefollowingcodefragment: void(*pf[])(void)={fna,fnb,fnc,…,fnz}; voidtest(constINTjump_index) { /*Callthefunctionspecifiedbyjump_index*/ pf[jump_index](); } Theabovecodedeclarespf[]tobeanarrayofpointerstofunctions,eachofwhichtakesnoargumentsandreturnsvoid.Thetest()functionsimplycallsthespecifiedfunctionviathearray.Asitstands,thiscodeisdangerousforthefollowingreasons. pf[]isaccessiblebyanyone Intest(),thereisnoboundschecking,suchthatanerroneousjump_indexwouldspelldisaster Amuchbetterwaytocodethisthatavoidstheseproblemsisasfollows voidtest(uint8_tconstump_index) { staticvoid(*pf[])(void)={fna,fnb,fnc,…,fnz}; if(jump_indexinterval!=0;ptr++) { if(!(time%ptr->interval)) { /*Timetocallthefunction*/ (ptr->proc)(); } } } } } Inthiscase,wedefineourowndatatype(TIMED_TASK)thatconsistssimplyofanintervalandapointertoafunction.WethendefineanarrayofTIMED_TASK,andinitializeitwiththelistoffunctionsthataretobecalledandtheircallinginterval.Inmain(),wehavethestartupcodewhichmustenableaperiodictimerinterruptthatincrementsthevolatilevariabletickatafixedinterval.Wethenentertheinfiniteloop. Theinfiniteloopchecksforanon-zerotickvalue,decrementsthetickvariableandcomputestheelapsedtimesincetheprogramstartedrunning.Thecodethensimplystepsthrougheachofthetasks,toseewhetheritistimeforthatonetobeexecutedand,ifso,callsitviathefunctionpointer. Ifyourapplicationonlyconsistsoftwoorthreetasks,thenthisapproachisprobablyoverkill.However,ifyourprojecthasalargenumberoftimedtasks,oritislikelythatyouwillhavetoaddtasksinthefuture,thenthisapproachisratherpalatable.Notethataddingtasksand/orchangingintervalssimplyrequireseditingofthetimed_task[]array.Nocode,perse,hastobechanged. Interruptvectortables Thefourthapplicationoffunctionjumptablesisthearrayofinterruptvectors.Onmostprocessors,theinterruptvectorsareincontiguouslocations,witheachvectorrepresentingapointertoaninterruptserviceroutinefunction.Dependinguponthecompiler,theworkmaybedoneforyouimplicitly,oryoumaybeforcedtogeneratethefunctiontable.Inthelattercase,implementingthevectorsviaaswitchstatementwillnotwork! Hereisthevectortablefromtheindustrialpowersupplyprojectmentionedabove.ThisprojectwasimplementedusingaWhitesmiths’compileranda68HC11microncontroller. IMPORTVOID_stext();/*68HC11-specificstartuproutine*/ staticVOID(*const_vectab[])()= { SCI_Interrupt, /*SCI*/ badSPI_Interrupt, /*SPI*/ badPAI_Interrupt, /*Pulseaccinput*/ badPAO_Interrupt, /*Pulseaccoverf*/ badTO_Interrupt, /*Timeroverf*/ badOC5_Interrupt, /*Outputcompare5*/ badOC4_Interrupt, /*Outputcompare4*/ badOC3_Interrupt, /*Outputcompare3*/ badOC2_Interrupt, /*Outputcompare2*/ badOC1_Interrupt, /*Outputcompare1*/ badIC3_Interrupt, /*Inputcapture3*/ badIC2_Interrupt, /*Inputcapture2*/ badIC1_Interrupt, /*Inputcapture1*/ RTI_Interrupt, /*Realtime*/ Uart_Interrupt, /*IRQ*/ PFI_Interrupt, /*XIRQ*/ badSWI_Interrupt, /*SWI*/ IlOpC_Interrupt, /*illegal*/ _stext, /*copfail*/ _stext, /*copclockfail*/ _stext, /*RESET*/ }; Acoupleofpointsareworthmaking: Theaboveisinsufficienttolocatethetablecorrectlyinmemory.Thishastobedonevialinkerdirectives. Notethatunusedinterruptsstillhaveanentryinthetable.Doingsoensuresthatthetableiscorrectlyalignedandtrapscanbeplacedonunexpectedinterrupts. Ifanyoftheseexampleshaswhetyourappetiteforusingarraysoffunctionpointers,butyouarestilluncomfortablewiththedeclarationcomplexity,thenfearnot!Youwillfindavarietyofdeclarations,rangingfromthestraightforwardtothedownrightappallingbelow.Theexamplesareallreasonablypracticalinthesensethatthedesiredfunctionalityisnotoutlandish(thatis,therearenodeclarationsforarraysofpointerstofunctionsthattakepointerstoarraysoffunctionpointersandsoon). Declarationandusehints AlloftheexamplesbelowadheretoconventionsthatIhavefoundtobeusefulovertheyears,specifically: 1.Alloftheexamplesareprecededbystatic.Thisisdoneontheassumptionthatthescopeofafunctiontableshouldbehighlylocalized,ideallywithinanenclosingfunction. 2.Ineveryexamplethearraypf[]isalsoprecededwithconst.Thisdeclaresthatthepointersinthearraycannotbemodifiedafterinitialization.Thisisthenormal(andsafe)usagescenario. 3.Therearetwosyntacticallydifferentwaysofinvokingafunctionviaapointer.Ifwehaveafunctionpointerwiththedeclaration: void(*fnptr)(int); /*fnptrisafunctionpointer*/ Thenitmaybeinvokedusingeitherofthesemethods: fnptr(3); /*Method1ofinvokingthefunction*/ (*fnptr)(3); /*Method2ofinvokingthefunction*/ Theadvantageofthefirstmethodisanunclutteredsyntax.However,itmakesitlookasiffnptrisafunction,asopposedtobeingafunctionpointer.Someonemaintainingthecodemayendupsearchinginvainforthefunctionfnptr().Withmethod2,itismuchclearerthatwearedereferencingapointer.However,whenthedeclarationsgetcomplex,theadded(*)canbeasignificantburden.Throughouttheexamples,eachsyntaxisshown.Inpractice,thelattersyntaxseemstobemorepopular–andyoushoulduseonlyone. 4.Ineveryexample,thesyntaxforusingatypedefisalsogiven.Itisquitepermissibletouseatypedeftodefineacomplexdeclaration,andthenusethenewtypelikeasimpletype.Ifwestaywiththeexampleabove,thenanalternativedeclarationis: typedefvoid(*PFV_I)(int); /*DeclareaPVFV_Itypedvariableandinitit*/ PFV_Ifnptr=fna; /*Callfnawithparameter3usingmethod1*/ fnptr(3); /*Callfnawithparameter3usingmethod2*/ (*fnptr)(3); ThetypedefdeclaresthetypePFV_Itobeapointertoafunctionthatreturnsvoidandispassedaninteger.Wethensimplydeclarefnptrtoavariableofthistype,anduseit.Typedefsareverygoodwhenyouregularlyuseacertainfunctionpointertype,sinceitsavesyouhavingtorememberandtypeinthedeclaration.Thedownsideofusingatypedef,isthefactthatitisnotobviousthatthevariablethathasbeendeclaredisapointertoafunction.Thus,justasforthetwoinvocationmethodsabove,youcangainsyntacticalsimplicitybyhidingtheunderlyingfunctionality. Inthetypedefs,aconsistentnamingconventionisused.EverytypestartswithPF(PointertoFunction)andisthenfollowedwiththereturntype,followedbyanunderscore,thefirstparametertype,underscore,secondparametertypeandsoon.Forvoid,boolean,char,int,long,floatanddouble,thecharactersV,B,C,I,L,S,Dareused.(NotetheuseofS(ingle)forfloat,toavoidconfusionwithF(unction)).Forapointertoadatatype,thetypeisprecededwithP.ThusPLisapointertoalong.Ifaparameterisconst,thenacappearsintheappropriateplace.Thus,cPLisaconstpointertoalong,whereasaPcLisapointertoaconstlong,andcPcLisaconstpointertoaconstlong.Forvolatilequalifiers,visused.Forunsignedtypes,auprecedesthebasetype.Foruserdefineddatatypes,youareonyourown! Anextremeexample:PFcPcI_uI_PvuC.ThisisapointertoafunctionthatreturnsaconstpointertoaconstIntegerthatispassedanunsignedintegerandapointertoavolatileunsignedchar. Functionpointertemplates Thefirstelevenexamplesaregenericinthesensethattheydonotusememoryspacequalifiersandhencemaybeusedonanytarget.Example12showshowtoaddmemoryspacequalifiers,suchthatallthecomponentsofthedeclarationendupinthecorrectmemoryspaces. Example1 pf[]isastaticarrayofpointerstofunctionsthattakeanINTasanargumentandreturnvoid. voidfna(INT); //Exampleprototypeofafunctiontobecalled //Declarationusingtypedef typedefvoid(*constPFV_I)(INT); staticPFV_Ipf[]={fna,fnb,fnc,…fnz); //Directdeclaration staticvoid(*constpf[])(INT)={fna,fnb,fnc,…fnz}; //Exampleuse INTa=6; pf[jump_index](a); //Callingmethod1 (*pf[jump_index])(a); //Callingmethod2 Example2 pf[]isastaticarrayofpointerstofunctionsthattakeapointertoanINTasanargumentandreturnvoid. voidfna(INT*); //Exampleprototypeofafunctiontobecalled //Declarationusingtypedef typedefvoid(*constPFV_PI)(INT*); staticPVF_PI[]={fna,fnb,fnc,…fnz}; //Directdeclaration staticvoid(*constpf[])(INT*)={fna,fnb,fnc,…fnz}; //Exampleuse INTa=6; pf[jump_index](&a); //Callingmethod1 (*pf[jump_index])(&a); //Callingmethod2 Example3 pf[]isastaticarrayofpointerstofunctionsthattakeanINTasanargumentandreturnaCHAR CHARfna(INT); //Exampleprototypeofafunctiontobecalled //Declarationusingtypedef typedefCHAR(*constPFC_I)(INT); staticPVC_I[]={fna,fnb,fnc,…fnz}; //Directdeclaration staticCHAR(*constpf[])(INT)={fna,fnb,fnc,…fnz}; //Exampleuse INTa=6; CHARres; res=pf[jump_index](a); //Callingmethod1 res=(*pf[jump_index])(a); //Callingmethod2 Example4 pf[]isastaticarrayofpointerstofunctionsthattakeanINTasanargumentandreturnapointertoaCHAR. CHAR*fna(INT); //Exampleprototypeofafunctiontobecalled //Declarationusingtypedef typedefCHAR*(*constPFPC_I)(INT); staticPVPC_I[]={fna,fnb,fnc,…fnz}; //Directdeclaration staticCHAR*(*constpf[])(INT)={fna,fnb,fnc,…fnz}; //Exampleuse INTa=6; CHAR*res; res=pf[jump_index](a); //Callingmethod1 res=(*pf[jump_index])(a); //Callingmethod2 Example5 pf[]isastaticarrayofpointerstofunctionsthattakeanINTasanargumentandreturnapointertoaconstCHAR(i.e.thepointermaybemodified,butwhatitpointstomaynot). constCHAR*fna(INT); //Exampleprototypeofafunctiontobecalled //Declarationusingtypedef typedefconstCHAR*(*constPFPcC_I)(INT); staticPVPcC_I[]={fna,fnb,fnc,…fnz}; //Directdeclaration staticconstCHAR*(*constpf[])(INT)={fna,fnb,fnc,…fnz}; //Exampleuse INTa=6; constCHAR*res; res=pf[jump_index](a); //Callingmethod2 res=(*pf[jump_index])(a); //Callingmethod2 Example6 pf[]isastaticarrayofpointerstofunctionsthattakeanINTasanargumentandreturnaconstpointertoaCHAR(i.e.thepointermaynotbemodified,butwhatitpointstomaybemodified). CHAR*constfna(INTi);//Exampleprototypeofafunctiontobecalled //Declarationusingtypedef typedefCHAR*const(*constPFcPC_I)(INT); staticPVcPC_I[]={fna,fnb,fnc,…fnz}; //Directdeclaration staticCHAR*const(*constpf[])(INT)={fna,fnb,fnc,…fnz}; //Exampleuse INTa=6; CHAR*constres=pf[jump_index](a); //Callingmethod1 CHAR*constres=(*pf[jump_index])(a); //Callingmethod2 Example7 pf[]isastaticarrayofpointerstofunctionsthattakeanINTasanargumentandreturnaconstpointertoaconstCHAR(i.e.thepointer,norwhatitpointstomaybemodified) constCHAR*constfna(INTi);//Examplefunctionprototype //Declarationusingtypedef typedefconstCHAR*const(*constPFcPcC_I)(INT); staticPVcPcC_I[]={fna,fnb,fnc,…fnz}; //Directdeclaration staticconstCHAR*const(*constpf[])(INT)={fna,fnb,fnc,…fnz}; //Exampleuse INTa=6; constCHAR*constres=pf[jump_index](a); //Callingmethod1 constCHAR*constres=(*pf[jump_index])(a); //Callingmethod2 Example8 pf[]isastaticarrayofpointerstofunctionsthattakeapointertoaconstINTasanargument(i.e.thepointermaybemodified,butwhatitpointstomaynot)andreturnaconstpointertoaconstCHAR(i.e.thepointer,norwhatitpointstomaybemodified) constCHAR*constfna(constINT*i); //Exampleprototype //Declarationusingtypedef typedefconstCHAR*const(*constPFcPcC_PcI)(constINT*); staticPVcPcC_PcI[]={fna,fnb,fnc,…fnz}; //Directdeclaration staticconstCHAR*const(*constpf[])(constINT*)={fna,fnb,fnc,…fnz}; //Exampleuse constINTa=6; constINT*aptr; aptr=&a; constCHAR*constres=pf[jump_index](aptr); //Callingmethod1 constCHAR*constres=(*pf[jump_index])(aptr);//Callingmethod2 Example9 pf[]isastaticarrayofpointerstofunctionsthattakeaconstpointertoanINTasanargument(i.e.thepointermaynotbemodified,butwhatitpointstomay)andreturnaconstpointertoaconstCHAR(i.e.thepointer,norwhatitpointstomaybemodified) constCHAR*constfna(INT*consti); //Exampleprototype //Declarationusingtypedef typedefconstCHAR*const(*constPFcPcC_cPI)(INT*const); staticPVcPcC_cPI[]={fna,fnb,fnc,…fnz}; //Directdeclaration staticconstCHAR*const(*constpf[])(INT*const)={fna,fnb,fnc,…fnz}; //Exampleuse INTa=6; INT*constaptr=&a; constCHAR*constres=pf[jump_index](aptr); //Method1 constCHAR*constres=(*pf[jump_index])(aptr); //Method2 Example10 pf[]isastaticarrayofpointerstofunctionsthattakeaconstpointertoaconstINTasanargument(i.e.thepointernorwhatitpointstomaybemodified)andreturnaconstpointertoaconstCHAR(i.e.thepointer,norwhatitpointstomaybemodified) constCHAR*constfna(constINT*consti); //Exampleprototype //Declarationusingtypedef typedefconstCHAR*const(*constPFcPcC_cPcI)(constINT*const); staticPVcPcC_cPcI[]={fna,fnb,fnc,…fnz}; //Directdeclaration staticconstCHAR*const(*constpf[])(constINT*const)={fna,fnb,fnc,…fnz}; //Exampleuse constINTa=6; constINT*constaptr=&a; constCHAR*constres=pf[jump_index](aptr); //Method1 constCHAR*constres=(*pf[jump_index])(aptr); //Method2 Thisexamplemanagestocombinefiveincidencesofconstandoneofstaticintoasingledeclaration.Forallofitscomplexity,however,thisisnotanartificialexample.Youcouldgoaheadandremovealltheconstandstaticdeclarationsandthecodewouldstillwork.Itwould,however,bealotlesssafe,andpotentiallylessefficient. Justtobreakupthemonotony,hereisthesamedeclaration,butwithatwist. Example11 pf[]isastaticarrayofpointerstofunctionsthattakeaconstpointertoaconstINTasanargument(i.e.thepointernorwhatitpointstomaybemodified)andreturnaconstpointertoavolatileCHAR(i.e.thepointermaynotbemodified,butwhatitpointstomaychangeunexpectedly) volatileCHAR*constfna(constINT*consti); //Exampleprototype //Declarationusingtypedef typedefvolatileCHAR*const(*constPFcPvC_cPcI)(constINT*const); staticPVcPvC_cPcI[]={fna,fnb,fnc,…fnz}; //Directdeclaration staticvolatileCHAR*const(*constpf[])(constINT*const)={fna,fnb,fnc,…fnz}; //Exampleuse constINTa=6; constINT*constaptr=&a; volatileCHAR*constres=pf[jump_index](aptr); //Method1 volatileCHAR*constres=(*pf[jump_index])(aptr); //Method2 while(*res) ; //Waitforvolatileregistertoclear Withmemoryspacequalifiers,thingscangetevenmorehairy.Formostvendors,thememoryspacequalifieristreatedsyntacticallyasatypequalifier(suchasconstorvolatile)andthusfollowsthesameplacementrules.Forconsistency,Iplacetypequalifierstotheleftofthe“thing”beingqualified.Wheretherearemultipletypequalifiers,alphabeticorderingisused.Sincememoryspacequalifiersaretypicallycompilerextensions,theyarenormallyprecededbyanunderscore,andhencecomefirstalphabetically.Thus,anastydeclarationmaylooklikethis: _ramconstvolatileUCHARstatus_register; Todemonstratememoryspacequalifieruse,hereisexample11again,exceptthistimememoryspacequalifiershavebeenadded.Thequalifiersarenamed_m1…_m5. Example12 pf[]isastaticarrayofpointerstofunctionsthattakeaconstpointertoaconstINTasanargument(i.e.thepointernorwhatitpointstomaybemodified)andreturnaconstpointertoavolatileCHAR(i.e.thepointermaybemodified,butwhatitpointstomaychangeunexpectedly).Eachelementofthedeclarationliesinadifferentmemoryspace.Inthisparticularcase,itisassumedthatyoucanevendeclarethememoryspaceinwhichparameterspassedbyvalueappear.Thisisextreme,butisjustifiedonpedagogicalgrounds. /*Anexampleprototype.Thisdeclarationreadsasfollows. *Functionfnaispassedaconstpointerin_m5spacethatpointstoa *constintegerin_m4space.Itreturnsaconstpointerin_m2spaceto *avolatilecharacterin_m1space. */ _m1volatileCHAR*_m2constfna(_m4constINT*_m5consti); /*Declarationusingtypedef.Thisdeclarationreadsasfollows. *PFcPvC_cPcIisapointertofunctiondatatype,variablesbased *uponwhichliein_m3space.EachFunctionispassedaconst *pointerin_m5spacethatpointstoaconstintegerin_m4space. *Itreturnsaconstpointerin_m2spacetoavolatilecharacter *in_m1space. */ typedef_m1volatileCHAR*_m2const(*_m3constPFcPvC_cPcI)(_m4constINT*_m5const); staticPVcPvC_cPcI[]={fna,fnb,fnc,…fnz}; /*Directdeclaration.Thisdeclarationreadsasfollows.pf[]is *astaticallyallocatedconstantarrayin_m3spaceofpointerstofunctions. *EachFunctionispassedaconstpointerin_m5spacethatpointsto *aconstintegerin_m4space.Itreturnsaconstpointerin_m2space *toavolatilecharacterin_m1space. */ static_m1volatileCHAR*_m2const(*_m3constpf[])(_m4constINT*_m5const)={fna,fnb,fnc,…fnz}; //Declareaconstvariablethatliesin_m4space _m4constINTa=6; //Nowdeclareaconstpointerin_m5spacethatpointstoaconst //variablethatisin_m4space _m4constINT*_m5constaptr=&a; //Makethefunctioncall,andgetbackthepointer volatileCHAR*constres=pf[jump_index](&a); //Method1 volatileCHAR*constres=(*pf[jump_index])(&a); //Method2 while(*res) ; //Waitforvolatileregistertoclear Acknowledgments MythankstoMikeStevensnotonlyforreadingoverthismanuscriptandmakingsomeexcellentsuggestionsbutalsoforovertheyearsshowingmemorewaystousefunctionpointersthatIeverdreamedwaspossible. ThisarticlewaspublishedintheMay1999issueofEmbeddedSystemsProgramming.Ifyouwishtocitethearticleinyourownwork,youmayfindthefollowingMLA-styleinformationhelpful: Jones,Nigel.“ArraysofPointerstoFunctions”EmbeddedSystemsProgramming,May1999. Thisentrywasposted onThursday,December17th,2009at4:43pm andisfiledunderPublications. YoucanfollowanyresponsestothisentrythroughtheRSS2.0feed. Bothcommentsandpingsarecurrentlyclosed. Commentsareclosed. 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