Generalform
Intheprogram,thefunctionbodyisexecutedbycallingthefunction,andtheprocessissimilartothatofsubroutinecallsinotherlanguages.
InClanguage,thegeneralformoffunctioncallis:
Functionname(actualparameterlist)
Whencallingafunctionwithoutparameters,thereisnoactualparameterlist..Theparametersintheactualparametertablecanbeconstants,variables,orotherstructureddataandexpressions.Separateeachactualparameterwithacomma.
Userspace(usermode)andkernelspace(kernelmode)
Theprocessspaceoftheoperatingsystemcanbedividedintouserspaceandkernelspace,whichrequiredifferentexecutionpermissions.Thefunctioncallrunsinuserspace.
Includingcontent
Functionexpression
Functionsappearinexpressionsasoneitemofexpressions,andthereturnvaluesoffunctionsparticipateinthecalculationofexpressions.Thismethodrequiresthefunctiontohaveareturnvalue.Forexample:z=max(x,y)isanassignmentexpressionthatassignsthereturnvalueofmaxtothevariablez.
Functionstatement
Thegeneralformoffunctioncallplusasemicolonconstitutesafunctionstatement.Forexample:printf("%d",a);scanf("%d",&b);allcallfunctionsintheformoffunctionstatements.
FunctionArguments
Thefunctionappearsastheactualparameterofanotherfunctioncall.Inthiscase,thereturnvalueofthefunctionistransmittedasanactualparameter,sothefunctionmusthaveareturnvalue.Forexample:printf("%d",max(x,y));Thatis,thereturnvalueofthemaxcallisusedastheactualparameteroftheprintffunction.Anotherissuethatshouldbepaidattentiontoinfunctioncallsistheorderofevaluation.Theso-calledevaluationorderreferstowhetherthequantitiesintheactualparametertableareusedfromlefttorightorfromrighttoleft.Inthisregard,theregulationsofeachsystemarenotnecessarilythesame.Ithasbeenmentionedintheintroductionoftheprintffunction,andIwillemphasizeitagainfromtheperspectiveoffunctioncalls.
[Example]
main()
{inti=8;printf("%d\n%d\n%d\n%d\n",++i,--i,i++,i--);}
Ifitisevaluatedfromrighttoleft.Therunningresultshouldbe:
8
7
7
8
Forexample,intheprintfstatement++i,--i,i++,i--evaluatefromlefttoright,theresultshouldbe:
9
8
8
9
Itshouldbenotedthatwhetheritisevaluatedfromlefttorightorfromrighttoleft,theoutputorderisthesame,thatis,theoutputorderisalwaysTheorderoftheactualparametersisthesameasintheactualparameterlist.SinceTurboCiscurrentlyevaluatedfromrighttoleft,theresultis8,7,7,8.Ifyoustilldon’tunderstandtheabovequestions,youwillunderstandafteratryonthecomputer.
Declarationandfunctionprototypeofthecalledfunction
Beforecallingafunctioninthemaincallingfunction,thecalledfunctionshouldbeexplained(declared),whichisthesameasbeforeusingvariables.Thevariabledescriptionisthesame.Thepurposeofexplainingthecalledfunctioninthecallingfunctionistomakethecompilersystemknowthetypeofthereturnvalueofthecalledfunction,sothatthereturnvaluecanbeprocessedaccordinglyinthecallingfunctionaccordingtothistype.
Thegeneralformis:
Typespecifieriscalledfunctionname(typeparameter,typeparameter...);
Or:
Typespecifieriscalledfunctionname(type,type...);
Thetypeandnameoftheformalparameteraregivenintheparentheses,oronlythetypeoftheformalparameterisgiven.Thisfacilitateserrordetectionbythecompilersystemtopreventpossibleerrors.
Forexample,thedescriptionofthemaxfunctioninthemainfunctionis:
intmax(inta,intb);
Orwrittenas:
intmax(int,int);
TheClanguagealsostipulatesthatthefunctiondescriptionofthecalledfunctioninthecallingfunctioncanbeomittedinthefollowingsituations.
1)Ifthereturnvalueofthecalledfunctionisanintegerorcharactertype,youcancallthecalledfunctiondirectlywithoutexplainingit.Atthistime,thesystemwillautomaticallytreatthereturnvalueofthecalledfunctionasaninteger.ThisisthecasewhenthefunctionsisnotdescribedinthemainfunctionofExample8.2anditiscalleddirectly.
2)Whenthefunctiondefinitionofthecalledfunctionappearsbeforethecallingfunction,thecallingfunctioncanalsobecalleddirectlywithoutfurtherexplainingthecalledfunction.Forexample,inExample8.1,thedefinitionofthefunctionmaxisplacedbeforethemainfunction,sothefunctiondescriptionofthemaxfunctioncanbeomittedinthemainfunctionintmax(inta,intb).
3)Ifthetypeofeachfunctionisspecifiedoutsidethefunctionbeforethedefinitionofallthefunctions,theninthesubsequentcallingfunctions,thecalledfunctioncannolongerbedescribed.Forexample:
charstr(inta);
floatf(floatb);
main()
{……}
charstr(inta)
{……)
floatf(floatb)
{……}
Thefirstandsecondlineshavepre-explainedthestrfunctionandtheffunction.Therefore,thestrandffunctionscanbecalleddirectlyinthefollowingfunctionswithoutfurtherexplanation.
4)Thecallofthelibraryfunctiondoesnotneedtobeexplained,buttheheaderfileofthefunctionmustbeincludedinthefrontofthesourcefilewiththeincludecommand.
Nestedcall
NestedfunctiondefinitionsarenotallowedinClanguage.Therefore,thefunctionsareparallel,andthereisnoproblemoftheupperlevelfunctionandthenextlevelfunction.ButtheClanguageallowsacalltoanotherfunctiontoappearinthedefinitionofafunction.Inthisway,nestedcallsoffunctionsappear.Thatis,otherfunctionsarecalledinthecalledfunction.Thisissimilartothenestingofsubroutinesinotherlanguages.Therelationshipcanberepresentedasshowninthefigure.
Thefigureshowsatwo-levelnestingsituation.Theexecutionprocessis:whenthestatementthatcallsfunctionainthemainfunctionisexecuted,functionaisswitchedto,whenfunctionbiscalledinfunctiona,functionbisswitchedtoexecution,andfunctionbisexecutedtoreturntothebreakpointoffunctionatocontinueExecute,theexecutionoftheafunctioniscompleteandreturntothebreakpointofthemainfunctiontocontinueexecution.
[Example]Calculates=2∧2!+3∧2!
Thisquestioncanwritetwofunctions,oneisthefunctionf1usedtocalculatethesquarevalue,andtheotherisThefunctionf2usedtocalculatethefactorialvalue.Themainfunctionfirstadjustsf1tocalculatethesquarevalue,andthentakesthesquarevalueinf1astheactualparameter,callsf2tocalculatethefactorialvalue,andthenreturnstof1,thenreturnstothemainfunction,andcalculatesthecumulativesumintheloopprogram.
longf1(intp)
{intk;
longr;
longf2(int);
k=p*p;
r=f2(k);
returnr;}
longf2(intq)
p>{longc=1;
inti;
for(i=1;i<=q;i++)
c=c*i;
returnc;}
main()
{inti;
longs=0;
p>for(i=2;i<=3;i++)
s=s+f1(i);
printf("\ns=%ld\n",s);}
Intheprogram,thefunctionsf1andf2arebothlongintegers,whicharedefinedbeforethemainfunction,sothereisnoneedtoexplainf1andf2inthemainfunction.Inthemainprogram,theexecutionloopprogramsequentiallycallsthevalueofiastheactualparametertocallthefunctionf1toobtainthevalueofi2.Inf1,acalltofunctionf2occursagain.Atthistime,thevalueofi2isusedasanactualparametertoadjustf2,andthecalculationofi2!iscompletedinf2.Afterf2isexecuted,theCvalue(iei2!)isreturnedtof1,andthenf1returnstothemainfunctiontoachieveaccumulation.Sofar,therequirementoftheproblemisrealizedbythenestedcallofthefunction.Becausethevalueisverylarge,thetypesoffunctionsandsomevariablesaredeclaredaslongintegers,otherwiseitwillcausecalculationerrors.
Actualimplementation
Pointerregister
EBP
EBPistheso-calledframepointer,pointingtothecurrentactivityAbovetherecord(thebottomofthepreviousactivityrecord)
ESP
ESPistheso-calledstackpointer,whichpointstothebottomofthecurrentactivityrecord(bottomAnactivityrecordtobeinsertedatthetop)
Thevalueofthesetwopointersspecifiesthepositionofthecurrentactivityrecord
Parameterpassing
PressfunctionparametersStack:moveax,dwordptr[n];(nisaparameterargument)
pusheax
Operation
Thefunctioncallwillperformthefollowingoperations:
⒈Pushtheframepointerontothestack:pushebp
⒉maketheframepointerequaltothestackpointer:movebp,esp
⒊makethestackpointerdecrement,Thememoryaddressobtainedbysubtractionshouldbeableto(enough)beusedtostorethelocalstateofthecalledfunction:subesp,0CCh
Note:0CChis0xCC,whichvarieswiththespecificfunction.
Passinsavestate
pushebx;savethevalueofebxregister
pushesi;savethevalueofesiregister
pushedi;Savethevalueoftheediregister
Loadedi
leaedi,[ebp-0CCh];0cchisthesizeofthecurrentactiverecord.
EDIisthedestinationindexregister.
Restoretheincomingsavestate
00411417popedi
00411418popesi
popebx
stackMovethepointeruptorestorespace
addesp,0CCh
Functionreturnstoreleasespace
Whenthefunctionreturns,thecompilerandhardwarewillperformthefollowingoperations:
⒈Makethestackpointerequaltotheframepointer:movesp,ebp
⒉Poptheoldframepointerfromthestack:popebp
⒊Return:ret
Example1
;voidfunction(intn);{pushebp
movebp,esp
subesp,0CCh
pushebx
pushesi
pushedi
leaedi,[ebp-0CCh]
movecx,33h
moveax,0CCCCCCCCh
repstosdwordptres:[edi]
;chara=1;
movbyteptr[a],1
;if(n==0)return;
cmpdwordptr[n],0
jnefunction+2Ah(4113CAh)
jmpfunction+77h(411417h)
;printf("%d\t(0x%08x)\n",n,&n);
movesi,esp
leaeax,[n]
pusheax
movecx,dwordptr[n]
pushecx
pushoffsetstring"%d\t(0x%08x)\n"(415750h)
calldwordptr[__imp__printf(4182B8h)]
addesp,0Ch
cmpesi,esp
call@ILT+305(__RTC_CheckEsp)(411136h)
;function(n-1);
moveax,dwordptr[n]
subeax,1
pusheax
callfunction(411041h)
addesp,4
;printf("----%d\t(0x%08x)\n",n,&n);
movesi,esp
leaeax,[n]
pusheax
movecx,dwordptr[n]
pushecx
pushoffsetstring"----%d\t(0x%08x)\n"(41573Ch)
calldwordptr[__imp__printf(4182B8h)]
addesp,0Ch
cmpesi,esp
call@ILT+305(__RTC_CheckEsp)(411136h);}
popedi
popesi
popebx
addesp,0CCh
cmpebp,esp
call@ILT+305(__RTC_CheckEsp)(411136h)
movesp,ebp
popebp
ret
Example2
117:bR=t1(p);
Theassemblycodeisasfollows:
00401FB8movecx,dwordptr[ebp-8];puttheparameterintotheecxregister
00401FBBpushecx;theparameterintothestack
00401FBCcall@ILT+10(t1)(0040100f);Functioncall,thenextlineaddress00401FC1ispushedintothestack
00401FC1addesp,4;Thefunctionreturns,thestackpointerisincreasedby4,anditisrestoredtothevalueat00401FB8
00401FC4movdwordptr[ebp-10h],eax;Takethefunctionreturnvalueinthehigh-levellanguagefromeaxandputitintothebRvariable
Thet1functionisasfollows:
125:BOOLt1(void*p)
126:{
00402030pushebp;ebpintothestack
00402031movebp,esp;ebppointstothetopofthestackatthistime
00402033subesp,44h;espdecreasesbyonevalueandfreesupastoragearea
p>00402036pushebx;Putthevaluesofthethreeregistersontothestacksothatyoucanuseitinthefunction
00402037pushesi;
00402038pushedi;
00402039leaedi,[ebp-44h];
0040203Cmovecx,11h;
00402041moveax,0CCCCCCCCh;
00402046repstosdwordptr[edi];
127:int*q=(int*)p;;
00402048moveax,dwordptr[ebp+8];ebp+8pointstofunctioninputThelowestbitaddressoftheparameter;
;ifitisebp+4,itpointstothelowestbitofthefunctionreturnaddress00401FC1,thevalueisC1
0040204Bmovdwordptr[ebp-4],eax;
128:return0;
0040204Exoreax,eax;Thereturnvalueisplacedintheeaxregister
129:}
00402050popedi;Threeregisterspopoutofthestack
00402051popesi;
00402052popebx;
00402053movesp,ebp;esprestore
00402055popebp;ebppopsoutofthestack,anditsvalueisrestored
00402056ret;Returntothecodeaddressstoredatthetopofthestackatthistime:00401FC1
;soifyouareunluckyIfthereturnaddressismodified,theprogramwillbeunexpected
TheaboveassemblycodeiscompiledbyVC++6.0.
ThesituationofthestackaftertheEBPisputintothestack:
Lowandhigh
↓↓
Memoryaddressstack
┆┆
0012F600├───────┤←edi=0012F600
││
0012F604├─┄┄┄┄─┤
││
││
┆44hspace┆
┆┆
││
││
0012F640├─┄┄┄┄─┤
││
0012F644├───────┤←Ebpisassignedtopointtothisunit,atthistimeebp=0012F644
│ACF61200│ebpisassignedtothevaluebeforeesp
0012F648├────────┤
│C11F4000│Returnaddress
0012F64C├─────────┤←ebp+8
│A0F61200│Thevalueofthefunctionparameterp;
0012F650├─────────┤
││
├────────┤
┆┆
Note:Thememorystoragespacestackisarrangedfromhightolow,andtheaddressmarkedontheleftisthelowestaddressofthestorageunitatthebottomright.Forexample,0012F644pointstotheACbyteof0012F6AC,andACisatthetopofthestack.Thecontentinthememoryinthefigureiswrittenfromlowtohigh,"ACF61200"=0x0012F6AC
Explanation
(1)Acprogramiscomposedofoneormoreprogrammodules,Eachprogrammoduleasasourceprogramfile.Forlargerprograms,yougenerallydon'twanttoputallthecontentinonefile,butputtheminseveralsourcefiles,andacprogramiscomposedofseveralsourceprogramfiles.Thismakesiteasytowriteandcompileseparately,andimprovedebuggingefficiency.Asourceprogramfilecanbesharedbymultiplecprograms.
(2)Asourceprogramfileiscomposedofoneormorefunctionsandotherrelatedcontent(suchasinstructions,datadeclarationsanddefinitions,etc.).Asourceprogramfileisacompilationunit,andthesubprogramiscompiledintheunitofthesourceprogramfile,notintheunitofthefunction.
(3)Theexecutionofthecprogramstartsfromthemainfunction.Ifyoucallotherfunctionsinthemainfunction,theprocessreturnstothemainfunctionafterthecall,andthewholeprogramendsinthemainfunction.
(4)Allfunctionsareparallel,thatis,whendefiningfunctions,theyareperformedseparatelyandareindependentofeachother.Afunctionisnotsubordinatetoanotherfunction,thatis,functionscannotbenesteddefinitions.Functionscancalleachother,butcannotcallthemainfunction.Themainfunctioniscalledbytheoperatingsystem.
(5)Fromtheuser'spointofview,therearetwotypesoffunctions
a:libraryfunctions,whichareprovidedbythesystem,andusersdonotneedtodefinethembythemselves,butcanusethemdirectly.Itshouldbenotedthatthenumberandfunctionsoflibraryfunctionsprovidedbydifferentclanguagecompilationsystemswillbesomewhatdifferent,ofcourse,manybasicfunctionsarecommon.
b:User-definedfunction.Itisafunctionthataddressesthespecificneedsofusers.
(6)Fromtheformoffunctions,functionsaredividedintotwocategories.
a:Functionwithoutparameters.No-parameterfunctionscanbringbackordonotbringbackfunctionvalues,butgenerallytherearemorefunctionvaluesthatdonotbringback.
b:Functionwithparameters.Whencallingafunction,thecallingfunctionpassesdatatothecalledfunctionthroughparameterswhencallingthecalledfunction.Undernormalcircumstances,afunctionvaluewillbeobtainedwhenthecallingfunctionisexecuted,whichcanbeusedbythecallingfunction.