C language callback function details and examples

A "callback" is any function that is called by another function which takes the first function as a parameter. That is, when function F1 calls ... Clanguagecallbackfunctiondetailsandexamples Keywords: C 1.Whatisacallbackfunction?Callbackfunction,justlistentothenameishigherthanordinaryfunctions.Whatisacallbackfunction?I'msorryIhaven'treadmuch.Ihaven'tseenthedefinitionofcallbackfunctioninanybook.IsearchedBaiduandfoundthattherearedifferentopinions.Alargepartofthemuseasimilarscenetoexplain:whenaJungoestoBJun'sstoretobuysomething,ithappenstobeoutofstock.AJunleavesanumbertoBJunandnotifiesaJunwhenitisavailable.Itseemsthatasynchronousoperationiseasiertothinkofthancallback.Inaddition,therearetwoEnglishsentencesthatimpressme:1)Ifyoucallme,Iwillcallyouback;2)Don'tcallme,Iwillcallyou?Therefore,Idon'tthinksuchstatementsareveryappropriate,becauseIdon'tthinkthesestatementsexpressthecharacteristicsofcallbackfunctions,thatis,theycan'tseethedifferencebetweencallbackfunctionsandordinaryfunctions.However,Ithinkit'sprettygoodtomakecomplaintsaboutBaiduEncyclopedia(thoughIoftensearchBaidusearch):callbackfunctionisafunctioncalledbyfunctionpointer.Ifyoupassthepointer(address)ofafunctionasaparametertoanotherfunction,whenthepointerisusedtocallthefunctionitpointsto,wesayitisacallbackfunction. Let'stalkaboutmyopinionfirst.Wecanliterallydecomposeitfirst.For"callbackfunction",Chinesecanactuallyunderstandtwomeanings:1)thecallbackfunction;2)Gobacktothefunctionthatcalledtheaction.Whatthehellisthiscallback? Incomputerprogramming,acallbackisanyexecutablecodethatispassedasanargumenttoothercode,whichisexpectedtocallback(execute)theargumentatagiventime.Thisexecutionmaybeimmediateasinasynchronouscallback,oritmighthappenatalatertimeasinanasynchronouscallback.Ifthecodeisexecutedimmediately,itiscalledasynchronousCallback.Ifitisexecutedlater,itiscalledanasynchronousCallback.Synchronousandasynchronousarenotdiscussedhere.Pleaserefertorelevantmaterials. A"callback"isanyfunctionthatiscalledbyanotherfunctionwhichtakesthefirstfunctionasaparameter.Thatis,whenfunctionF1callsfunctionF2,functionF1passesthepointerofanotherfunctionF3tofunctionF2throughparameters.DuringtheexecutionoffunctionF2,functionF2callsfunctionF3.Thisactioniscalledcallback,andthefunctionF3firstpassedasapointerandthencalledbackisthecallbackfunction.Nowyoushouldunderstandthedefinitionofcallbackfunction? 2.Whyusecallbackfunction?Manyfriendsmaythink,whynotwritethenameofthefunctiondirectlyinthecallbackplacelikeordinaryfunctioncalls?Isn'tthatok?WhydoIhavetousecallbackfunctions?It'sagoodideabecausemanyexamplesofparsingcallbackfunctionscanbecalledwithordinaryfunctions.Toanswerthisquestion,let'sfirstunderstandthebenefitsandfunctionsofreturningtofunctions,thatis,decoupling.Yes,it'ssuchasimpleanswer.Becauseofthisfeature,ordinaryfunctionscan'treplacecallbackfunctions.Therefore,inmyeyes,thisisthebiggestfeatureofcallbackfunctions.Let'stakealookatapictureonWikipediathatIthinkiswelldrawn.ThefollowingisanincompleteClanguagecodetoshowthemeaningoftheabovefigure: example #include #Include//containstheheaderfileofthereadSoftwarelibrarywheretheLibraryFunctionislocated intCallback()//CallbackFunction { //TODO return0; } intmain()//Mainprogram { //TODO Library(Callback); //TODO return0; } Atfirstglance,thecallbackseemstobejustacallbetweenfunctions,whichisnodifferentfromordinaryfunctioncalls,butacloserlookshowsakeydifferencebetweenthetwo:inthecallback,themainprogrampassesthecallbackfunctionintothefunctionlikeaparameter.Inthisway,aslongaswechangetheparameterspassedintothelibraryfunction,wecanrealizedifferentfunctions.Doyouthinkitisveryflexible?Andthereisnoneedtomodifytheimplementationoflibraryfunctions,whichisdecoupling.Takeacloserlook.Themainfunctionandcallbackfunctionareonthesamelayer,whilethelibraryfunctionisonanotherlayer.Thinkaboutit.Ifthelibraryfunctionisinvisibletous,wecan'tmodifytheimplementationofthelibraryfunction,thatis,wecan'tmodifythelibraryfunctiontocallordinaryfunctions,sowecanonlypassindifferentcallbackfunctions,Thisisacommonsituationindailywork.Nowputthemain(),Library(),andCallback()functionsbackintotheF1,F2,andF3functions.Isthatmoreclear? Afterunderstandingthecharacteristicsofcallbackfunction,canyouroughlyknowunderwhatcircumstancesitshouldbeused?Yes,youcanusecallbackfunctionsinmanyplacesinsteadofordinaryfunctioncalls,butinmyopinion,callbackfunctionsshouldbeusedifyouneedtoreducethecoupling.-----------------------------------------------------------------Instance-----------------------------------------------------------------3.Howtousecallbackfunction?Ifyouknowwhatacallbackfunctionisandthecharacteristicsofacallbackfunction,howshouldyouuseacallbackfunction?Let'stakealookatasimpleexecutablesynchronouscallbackfunctioncode. example #include intCallback_1()//CallbackFunction1 { printf("Hello,thisisCallback_1"); return0; } intCallback_2()//CallbackFunction2 { printf("Hello,thisisCallback_2"); return0; } intCallback_3()//CallbackFunction3 { printf("Hello,thisisCallback_3"); return0; } intHandle(int(*Callback)()) { printf("EnteringHandleFunction."); Callback(); printf("LeavingHandleFunction."); } intmain() { printf("EnteringMainFunction."); Handle(Callback_1); Handle(Callback_2); Handle(Callback_3); printf("LeavingMainFunction."); return0; } Operationresults: EnteringMainFunction. EnteringHandleFunction. Hello,thisisCallback_1 LeavingHandleFunction. EnteringHandleFunction. Hello,thisisCallback_2 LeavingHandleFunction. EnteringHandleFunction. Hello,thisisCallback_3 LeavingHandleFunction. LeavingMainFunction. YoucanseethattheparameterintheHandle()functionisApointer.WhencallingtheHandle()functioninthemain()function,thefunctioncallbackispassedintoit_1()/Callback_2()/Callback_3(),thefunctionnameatthistimeisthepointerofthecorrespondingfunction,thatis,thecallbackfunctionisactuallyAuseoffunctionpointer.Nowreadthissentenceagain:A"callback"isanyfunctionthatiscalledbyanotherfunctionwhichtakesthefirstfunctionasaparameter? 4.Howtousecallbackfunctionwithparameters?Sharpeyedfriendsmayfindthatthecallbackfunctioninthepreviousexamplehasnoparameters,socanwecallbackthosefunctionswithparameters?Theanswerisyes.Sohowtocallit?Wecanmodifytheaboveexampleslightly: example #include intCallback_1(intx)//CallbackFunction1 { printf("Hello,thisisCallback_1:x=%d",x); return0; } intCallback_2(intx)//CallbackFunction2 { printf("Hello,thisisCallback_2:x=%d",x); return0; } intCallback_3(intx)//CallbackFunction3 { printf("Hello,thisisCallback_3:x=%d",x); return0; } intHandle(inty,int(*Callback)(int)) { printf("EnteringHandleFunction."); Callback(y); printf("LeavingHandleFunction."); } intmain() { inta=2; intb=4; intc=6; printf("EnteringMainFunction."); Handle(a,Callback_1); Handle(b,Callback_2); Handle(c,Callback_3); printf("LeavingMainFunction."); return0; } Operationresults: EnteringMainFunction. EnteringHandleFunction. Hello,thisisCallback_1:x=2 LeavingHandleFunction. EnteringHandleFunction. Hello,thisisCallback_2:x=4 LeavingHandleFunction. EnteringHandleFunction. Hello,thisisCallback_3:x=6 LeavingHandleFunction. LeavingMainFunction. Asyoucansee,insteadofdirectlychangingintHandle(int(*Callback)(int))tointHandle(int(*Callback)(int)),youcanaddanotherparametertosavetheparametervalueofthecallbackfunction,suchastheparameterYofintHandle(inty,int(*Callback)(int)).Similarly,callbackfunctionswithmultipleparameterscanbeused. PostedbywerkkrewonMon,06Dec202112:51:50-0800 HotKeywords Java-6961 Database-2683 Python-2616 Programming-2419 Attribute-2418 Javascript-2383 Spring-2111 Android-2077 xml-1972 Linux-1818 JSON-1764 network-1748 github-1720 less-1676 MySQL-1538 SQL-1332 PHP-1318 encoding-1179 Mobile-1029 Apache-925