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COMP530:OperatingSystems

CforJavaProgrammers&Lab0

DonPorter

PortionscourtesyKevinJeffay

1


SameBasicSyntax• DataTypes:int,char,[float]

– void- (untypedpointer)– Cancreateotherdatatypesusingtypedef

• NoStrings- onlychararrays– Lastcharacterneedstobea0

• Not‘0’,but‘\0’


struct – C’sobject• typedef struct foo{

int a;void*b;void(*op)(int c);//functionpointer

}foo_t;//<------typedeclaration• Actualcontiguousmemory• Includesdataandfunctionpointers


Pointers• Memoryplacementexplicit(heapvs.stack)

• Twosyntaxes(dot,arrow)intmain{

struct foof;struct foo*fp =&f;f.a =32;//dot:accessobjectdirectlyfp->a=33;//arrow:followapointerfp =malloc(sizeof(struct foo));fp->a=34;…

}

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Stack Heapmain:f:a=0;b=NULL;op=NULL;

struct foo:a=0;b=NULL;op=NULL;

fp:PC

f:a=32;b=NULL;op=NULL;



struct foo {int a;void*b;void(*op)(int c);

}

Ampersand:Addressoff


Functionpointerexamplefp->op=operator;fp->op(32);//Sameascalling

//operator(32);

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struct foo {int a;void*b;void(*op)(int c);

}

Codeinmemory:Main…

Operator:...

Stack Heapmain:f:a=0;b=NULL;op=NULL;

fp:




struct foo:a=34;b=NULL;op=


MoreonFunctionPointers• Callowsfunctionpointerstobeusedasmembersofastruct orpassedasargumentstoafunction

• Continuingthepreviousexample:

voidmyOp(int c){/*…*/}/*…*/foo_t *myFoo =malloc(sizeof(foo_t));myFoo->op=myOp;//setpointer/*…*/myFoo->op(5);//Actuallycallsmyop


NoConstructorsorDestructors• Mustmanuallyallocateandfreememory- NoGarbageCollection!– void*x=malloc(sizeof(foo_t));

• sizeof givesyouthenumberofbytesinafoo_t - DONOTCOUNTTHEMYOURSELF!

– free(x);• Memoryallocatorremembersthesizeofmalloc’edmemory

• Mustalsomanuallyinitializedata– Customfunction– memset(x,0,sizeof(*x))willzeroit


MemoryReferences• ‘.’ - accessamemberofastruct

– myFoo.a =5;• ‘&’ - getapointertoavariable

– foo_t *fPointer =&myFoo;• ‘->’ - accessamemberofastruct,viaapointertothestruct– fPointer->a=6;

• ‘*’- dereferenceapointer– if(5==*intPointer){…}

• Withoutthe*,youwouldbecomparing5totheaddressoftheint,notitsvalue.


Int exampleint x=5;//xisonthestackint *xp =&x;*xp =6;printf(“%d\n”,x);//prints6xp =(int *)0;*xp =7;//segmentationfault

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Stackmain:

x:5

PC

xp:xp:NULL

x:6


MemoryReferences,cont.• ‘[]’ - refertoamemberofanarray

char*str=malloc(5*sizeof(char));str[0]=‘a’;

– Note:*str=‘a’ isequivalent– str++;incrementsthepointersuchthat*str==str[1]

str

str[0] str[1] str[2] str[3] str[4]

str+1 str+2 str+3 str+4


TheChickenorTheEgg?• ManyCfunctions(printf,malloc,etc)areimplementedinlibraries

• Theselibrariesusesystemcalls• Systemcallsprovidedbykernel• Thus,kernelhasto“reimplement” basicClibraries

– Insomecases,suchasmalloc,can’tusetheselanguagefeaturesuntilmemorymanagementisimplemented


Formorehelp• manpagesareyourfriend!

– (notadatingservice)!– Ex:‘manmalloc’,or‘man3printf’

• Section3isusuallywherelibrarieslive- thereisacommand-lineutilityprintf aswell

• Use‘aproposterm’ tosearchformanentriesaboutterm

• TheCProgrammingLanguage byBrianKernighanandDennisRitchieisagreatreference.


Lab0Overview• CprogrammingonLinuxrefresher

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• WriteasimpleCcharacterstreamprocessingprogramonLinux

• Readincharactersfrom“standardinput,”write80characterlinesto“standardoutput”replacing:– Everyenter/returncharacter(newline)byaspace– Everyadjacentpairofpercents “%%” withan“*”

◆ …isoutputas:» abcdefghijklmn*pqrstuvw*%yz ab

◆ Example(fora30characteroutputline):Thestring…» abcdefghijklmn%%pqrstuvw%%%yz

abc%%%def

???

Lab0- Overview


◆ Thisistheonly outputyourprogramshouldgenerate» Thereshould benoprompts, debugging messages,statusmessages,...

%classroom> a.outAbcdefghijklmn%%pqrstuvw%%%yzabc%%%defAbcdefghijklmn*pqrstuvw*%yz ab1234567890123456789012345c*%def 12345678901234567890123

%classroom>

◆ Notethatyouroutputwillbeinterleavedwithyourinputontheconsole(indicatedinpurpleabove)» Thisisfine!» (Youcaneliminatethisifyouuse“I/Oredirection”)


◆ Whenexecutingyourprogram,terminatestdin witha<enter/return><control-D>sequence» This(non-printable) charactersequenceisreferredtoas

“end-of-file”or“EOF”» IfyouuseI/Oredirectionandreadfromafileyouneednotaddthecontrol-D

characterattheend(Linuxdoesthisforyou)

control-D

%classroom> a.outAbcdefghijklmn%%pqrstuvw%%%yzabc%%%defAbcdefghijklmn*pqrstuvw*%yz ab1234567890123456789012345c*%def 12345678901234567890123

%classroom>


• YoushouldallhaveLinuxaccountsintheDepartment– Ifyoudon’t,[email protected] knowASAP!– Ifyouneedtohaveyourpasswordresetvisit

https://www.cs.unc.edu/webpass/onyen/• Logintoclassroom.cs.unc.edu todotheassignments• Createthedirectorystructurecomp530 inyourLinuxhomedirectory

• Executethemagicincantationstoprotectyourhomework:

fs sa ~/comp530 system:anyuser none

WorkingonHomeworkAssignments

Executetheseinstructionsbefore thenextsteps!


Checkingoutthestartercode• Onceyouhaveagithub accountregistered

– Makesureyouaccepttheinvite:• Clickhttps://github.com/comp530-f18

• Clickthelinkinthehomeworktocreateaprivaterepo

• Then,onyourmachineorclassroom(substitutingyourteamfor‘team-don’– seethegreenclonebutton):git [email protected]:comp530-f18/lab0-team-don.git

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• Commityourpendingchanges– Seetheoutputof:‘git status’– Committhechangesyouwishtosubmit:

• git addex2.c– Andanyotherfilesthatchanged

• git commit–m“Finishedlab0”• makehandin

– Youmayneedtoaddfilestoyour.gitignore file(andcommit)thatshouldnotbehandedin

– 2xcheckongithub:yourchangesarethere,andtagged‘handin’

Ifyoudon’tfollowtheseinstructionsexactly,yourHWwillnotbegraded!

Submittinghomework


• Themachinesyoushoulduseforprogrammingare:– classroom.cs.unc.edu (primary)– snapper.cs.unc.edu (secondary)Accesseithermachineviaasecureshell(securetelnet)applicationonyourPC

• Youcandevelopyourcodeanywhereyoulikebut…

• Yourprogramswillbetestedonclassroom andcorrectnesswillbeassessedbasedontheirperformanceonclassroom– Alwaysmakesureyourprogramworksonclassroom!

Lab0ProgrammingNotes


• Programs should be neatly formatted (i.e., easy to read) andwell documented

• In general, 75% of your grade for a program will be forcorrectness, 25% for programming style– For this assignment, correctness & style will each count for 50% of

your grade• Style refers to…

– Appropriate use of language features, including variable/procedure names, and

– Documentation (descriptions of functions, general comments, use of invariants, pre- and post conditions where appropriate)

– Simple test: Can I understand what you’ve done in 3 minutes?• Correctness will be assessed comprehensively!

– You’ve got to learn to test for “edge” and “corner cases”

Grading


(“Hard But that is fine.Some of the grading scales for programming assignments were weird and not straightforward.Tended to place little emphasis on implementing what the assignment actually intended and emphasizedhow hard did you try to break your own program”)

• Programs that “mostly work” don’t cut it in a senior-level course!

Dr.Jeffay’s Experience


• Working in teams on programming assignments is OK– But you can only collaborate with other students in the course– Every line of code handed in must be written exclusively by team

members themselves, and – All collaborators must be acknowledged in writing (and part of the

team)• Use of the Internet

– Using code from the Internet in any form is not allowed– Websites may be consulted for reference (e.g., to learn how a system

call works)– But all such websites used or relied on must be listed as a reference

in a header comment in your program– Warning: Sample code found on the Internet rarely helps the student

HonorCode:AcceptableandUnacceptableCollaboration

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