The structure of C programs CITS2002 Systems Programming

CITS2002SystemsProgramming

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ThestructureofCprogramsLet'slooksatthehigh-levelstructureofashortCprogram,rotate.c(usingellipsistoomitsomestatementsfornow).Atthisstageit'snotimportantwhattheprogramissupposedtodo.

#include <stdio.h>#include <stdlib.h>#include <string.h>#include <ctype.h>

/* Compile this program with: cc -std=c99 -Wall -Werror -pedantic -o rotate rotate.c */

#define ROT 13

static char rotate(char c){ c = c + ROT; ..... return c;}

int main(int argcount, char *argvalue[]){ // check the number of arguments if(argcount != 2) { .... exit(EXIT_FAILURE); } else { .... exit(EXIT_SUCCESS); } return 0;}

Ofnoteinthisexample:

Characterssuchasaspace,tab,ornewline,mayappearalmostanywhere-theyarestrippedoutandignoredbytheCcompiler.

Weusesuchwhitespacecharacterstoprovidealayouttoourprograms.Whiletheexactlayoutisnotimportant,usingaconsistentlayoutisverygoodpractice.

Keywords,inbold,meanveryspecificthingstotheCcompiler.

Linescommencingwitha'#'inblueareprocessedbyaseparateprogram,namedtheCpreprocessor.

Inpractice,ourprogramisprovidedasinputtothepreprocessor,

http://teaching.csse.uwa.edu.au/units/CITS2002/index.php

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preprocessor,andthepreprocessor'soutputisgiventotheCcompiler.

Linesingreenarecomments.TheyareignoredbytheCcompiler,andmaycontain(almost)anycharacters.

C99providestwotypesofcomments-

/*blockcomments*/and//commentstotheendofaline

CITS2002SystemsProgramming,Lecture2,p1,27thJuly2021.


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ThestructureofCprograms,continued

#include <stdio.h>#include <stdlib.h>#include <string.h>#include <ctype.h>

/* Compile this program with: cc -std=c99 -Wall -Werror -pedantic -o rotate rotate.c */

#define ROT 13

static char rotate(char c){ c = c + ROT; ..... return c;}

int main(int argcount, char *argvalue[]){ // check the number of arguments if(argcount != 2) { .... exit(EXIT_FAILURE); } else { .... exit(EXIT_SUCCESS); } return 0;}

Sameprogram,butmoretonote:

Avarietyofbracketsareemployed,inpairs,togrouptogetheritemstobeconsideredinthesameway.Here:

anglebracketsencloseafilenameina#includedirective,roundbracketsgroupitemsinarithmeticexpressionsandfunctioncalls,squarebracketsenclosetheindexwhenaccessarrays(vectorsandmatrices...)ofdata,andcurlybracketsgrouptogethersequencesofoneormorestatementsinC.Wetermagroupofstatementsablockof



ablockofstatements.

FunctionsinC,maybethoughtofasablockofstatementstowhichwegiveaname.Inourexample,wehavetwofunctions-rotate()andmain().

Whenourprogramsarerunbytheoperatingsystem,theoperatingsystemalwaysstartsourprogramfrommain().Thus,everycompleteCprogramrequiresamain()function.

Theoperatingsystempassessomespecialinformationtoourmain()function,command-linearguments,andmain()needsaspecialsyntaxtoreceivethese.

MostCprogramsyoureadwillnamemain()'sparametersasargcandargv.

Whenourprogramfinishesitsexecution,itreturnssomeinformationtotheoperatingsystem.Ourexamplehereexitsbyannouncingeitheritsfailureorsuccess.




CompilingandlinkingourCprogramsCprogramsarehuman-readabletextfiles,thatwetermsource-codefiles.

Thismakesthemveryeasytocopy,read,andeditondifferentcomputersanddifferentoperatingsystems.Cisoftendescribedasbeingportableatthesource-codelevel.

Beforewecanrun(execute)ourCprograms,wemusttranslate,orcompile,theirsource-codefilestofilesthattheoperatingsystemcanbettermanage.Aprogramknownasacompilertranslates(compiles)source-codefilesintoobject-codefiles.

Finally,wetranslateorlinkoneormoreobject-codefilestoproduceanexecutableprogram,oftentermeda'binary',an'executable',oran'exe'file.Aprogramknownasalinkerperformsthistranslation,alsolinkingourobject-codefile(s)withstandardlibrariesand(optionally)3rd-partylibraries.

Dependingonhowweinvokethecompiler,sometimeswecan'move'straightfromthesource-codefilestotheexecutableprogram,allinonestep.Inrealitythecompileris'silently'executingthelinkerprogramforus,andthenremovinganyunwantedobject-files.






VariablesVariablesarelocationsinacomputer'smemory.Atypicaldesktoporlaptopcomputerwillhave4-16GBofmemory,orfour-sixteenbillionaddressablememorylocations,

AtypicalCprogramwilluse4bytestoholdasingleintegervalue,or8bytestoholdasinglefloating-pointvalue.

Anyvariablecanonlyholdasinglevalueatanytime-theydonotmaintainahistoryofpastvaluestheyoncehad.

NamingourvariablesTomakeprogramsmorereadable,weprovidevariableswithsimplenames.Weshouldcarefullychoosenamestoreflecttheroleofthevariableinourprograms.

Whilevariablenamescanbealmostanything(butnotthesameasthekeywordsinC)there'sasimplerestrictiononthepermittedcharactersinaname-

theymustcommencewithanalphabeticortheunderscorecharacter(_A-Za-z),andbefollowedbyzeroormorealphabetic,underscoreordigitcharacters(_A-Za-z0-9).

Cvariablenamesarecasesensitive,thus:

MYLIMIT,mylimit,MylimitandMyLimit

arefourdifferentvariablenames.

Whilenotrequired,it'spreferredthatvariablenamesdonotconsistentirelyofuppercasecharacters.We'llconsistentlyuseuppercase-onlynamesforconstantsprovidedbytheCpreprocessor,oruser-definedtypenames:

MAXLENGTH, AVATAR, BUFSIZ, and ROT

OlderCcompilerslimitedvariablenamesto,say,8uniquecharacters.Thus,forthem,

turn_nuclear_reactor_coolant_onandturn_nuclear_reactor_coolant_off

arethesamevariable!Keepthisinmindifeverdevelopingportablecodeforoldenvironments.






BasicdatatypesVariablesaredeclaredtobeofacertaindatatype,orjusttype.

Weusedifferenttypestorepresentthepermissiblevaluesthataprogram'svariablehas.

Forexample,ifwe'reusingavariabletojustcountthings,we'lluseanintegervariabletoholdthecount;ifperformingtrigonometryonanglesexpressedinradians,we'llusefloating-pointvariablestoholdvalueswithbothanintegralandafractionalpart.

Cprovidesanumberofstandard,orbasetypestoholdcommonlyrequiredvalues,andlaterwe'llseehowwecanalsodefineourownuser-definedtypestomeetourneeds.

Let'slookquicklyatsomeofC'sbasedatatypes:

typename description,andanexampleofvariableinitialization

bool Boolean(truthvalues),whichmayonlyholdthevaluesofeithertrueorfalsee.g.boolfinished=false;

charcharactervalues,toeachholdasinglevaluessuchasanalphabeticcharacter,adigitcharacter,aspace,atab...e.g.charinitial='C';

int integervalues,negative,positive,andzeroe.g.intyear=2006;

floatfloatingpointvalues,withatypicalprecisionof10decimaldigits(onourlabmachines)e.g.floatinflation=4.1;

double"bigger"floatingpointvalues,withatypicalprecisionof17decimaldigits(onourlabmachines)e.g.doublepi=3.1415926535897932;

Sometextbookswill(tooquickly)focusontheactualstoragesizeofthesebasictypes,andemphasisetherangesofpermissiblevalues.Whenwritingtrulyportableprograms-thatcanexecuteconsistentlyacrossdifferenthardwarearchitecturesandoperatingsystems-it'simportanttobeawareof,andavoid,theirdifferences.We'llexaminethisissuelater,butfornowwe'llfocusonusingthesebasictypesintheirmostobviousways.

Fromwheredoesthebooldatatypegetitsname?-the19thcenturymathematicianandphilosopher,GeorgeBoole.




http://en.wikipedia.org/wiki/George_Boole



TheSignificanceofIntegersinCThroughoutthe1950s,60s,and70s,thereweremanymorecomputerhardwaremanufacturersthantherearetoday.Eachcompanyneededtopromoteitsownproductsbydistinguishingthemfromtheircompetitors.

Atalowlevel,differentmanufacturersemployeddifferentmemorysizesforabasiccharacter-somejust6bits,some8bits,9,and10.Theunfortunateoutcomewastheincompatabilityofcomputerprogramsanddatastorageformats.

TheCprogramminglanguage,developedintheearly1970s,addressedthisissuebynotdefiningtherequiredsizeofitsdatatypes.Thus,Cprogramsareportableattheleveloftheirsourcecode-portingaprogramtoadifferentcomputerarchitectureispossible,providedthattheprogramsarecompiledon(orfor)eacharchitecture.Theonlyrequirementwasthat:

sizeof(char)≤sizeof(short)≤sizeof(int)≤sizeof(long)

Sincethe1980s,fortunately,theindustryhasagreedon8-bitcharactersorbytes.But(compilingand)runningtheCprogramondifferentarchitectures:

#include <stdio.h>

int main(void){ printf("char %lu\n", sizeof(char)); printf("short %lu\n", sizeof(short)); printf("int %lu\n", sizeof(int)); printf("long %lu\n", sizeof(long)); return 0;}

mayproducedifferent(thoughstillcorrect)results:

char 1short 2int 4long 8

It'spermissiblefordifferentCcompilersondifferentarchitecturestoemploydifferentsizedintegers.

Whydoesthismatter?Differentsizedintegerscanstoredifferentmaximumvalues-theabovedatatypesaresigned(supportingpositiveandnegativevalues)soa4-byteintegercanonlyrepresentthevalues-2,147,483,648to2,147,483,647.

Ifemployingintegersfor'simple'counting,orloopingoveraknownrangeofvalues,there'srarelyaproblem.Butifusingintegerstocountmany(small)values,suchasmilli-ormicro-seconds,itmatters:

Funfact:GPSuses10bitstostoretheweek.Thatmeansitrunsout...ohheck?April6,2019AirlinesHaveToRebootTheirAirbusA350PlanesAfterEvery149HoursTokeepaBoeingDreamlinerflying,rebootonceevery248days




https://www.theregister.co.uk/2019/02/12/current_gps_epoch_ends/

https://www.ubergizmo.com/2019/07/airlines-have-to-reboot-their-airbus-a350-planes-after-every-149-hours/

https://www.engadget.com/2015/05/01/boeing-787-dreamliner-software-bug/



ThescopeofvariablesThescopeofavariabledescribestherangeoflinesinwhichthevariablemaybeused.Sometextbooksmayalsotermthisthevisibilityorlexicalrangeofavariable.

Chasonly2primarytypesofscope:

globalscope(sometimestermedfilescope)inwhichvariablesaredeclaredoutsideofallfunctionsandstatementblocks,and

blockscopeinwhichvariablesaredeclaredwithinafunctionorstatementblock.

01 #include <stdio.h>02 #include <stdlib.h>03 #include <string.h>04 #include <ctype.h>0506 static int count = 0;0708 int main(int argcount, char *argvalue[])09 {10 int nfound = 0;1112 // check the number of arguments13 if(argcount != 2) {14 int nerrors = 1;1516 ....17 exit(EXIT_FAILURE);18 }19 else {20 int ntimes = 100;2122 ....23 exit(EXIT_SUCCESS);24 }25 return 0;26 }

Thevariablecounthasglobalscope.

Itisdefinedonline06,andmaybeusedanywherefromline06untiltheendofthefile(line26).

Thevariablecountisalsoprecededbythekeywordstatic,whichpreventsitfrombeing'seen'(readorwritten)fromoutsideofthisfilerotate.c

Thevariablenfoundhasblockscope.

Itisdefinedonline10,andmaybeusedanywherefromline10untiltheendoftheblockinwhichitwasdefined(untilline26).

Thevariablenerrorshasblockscope.

Itisdefinedonline14,andmaybeusedanywherefromline14untilline18.

Thevariablentimeshasblockscope.

Itisdefinedonline20,andmaybeusedanywherefromline20untilline24.



Wecoulddefineadifferentvariablenamednerrorsintheblockoflines20-24-withoutproblems.

Wecoulddefineadifferentvariablenamednfoundintheblockoflines20-24-butthiswouldbeaverybadpractice!




FlowofcontrolinaCprogramAprogram'scontrolflowdescribeshowsequencesofstatementsareexecuted.FlowcontrolinaCprogramisverysimilartomostotherimperativeandobject-orientedlanguages.

Cprogramscommencetheirexecutionattheirmain()function,executetheirstatements,andexit(returntheflowofcontrol)totheoperatingsystem.

It'sfairlyobviousthatstatementsneedtobeexecutedinawell-definedorder,asweexpectprogramstoalwaysbehavethesameway(unlesssomerandomdatadirectstheexecutionpath,asincomputergames,simulations,andheuristicalgorithms).

Defaultflowofcontrolexecuteseachstatementinorder,top-to-bottom.

Programsthatonlyexecutefromtop-to-bottomareprettyboring,andweneedtocontroltheirflowwithavarietyofconditionalstatements,loops,andfunction-calls.






ConditionalexecutionConditionalstatementsfirstevaluateaBooleanconditionandthen,basedonwhetherit'strueorfalse,executeotherstatements.

Themostcommonformis: Sometimes,theelseclauseisomitted: Often,theelseclauseprovidesfurtherifstatements:

if(condition) { // more statements; .....}else{ // more statements; .....}

if(condition) { // more statements; .....}

if(condition1) { // more statements; .....}else if(condition2){ // more statements; .....}else{ // more statements; .....}

Notethatintheexamples,above,eachblockofstatementstobeexecutedhasbeenwrittenwithincurly-brackets.

Thecurly-bracketsarenotrequired(wecouldjustwriteasinglestatementforeitheriforelseclause).However,addingcurly-bracketsisconsideredagoodpractice.Theyprovideasafeguardforwhenadditionalstatementsareaddedlater.






BooleanvaluesOfsignificance,andaverycommoncauseoferrorsinCprograms,isthatCstandards,priortoISO-C99,hadnoBooleandatatype.

Historically,anintegervalueofzeroevaluatedequivalenttoaBooleanvalueoffalse;anynon-zerointegervalueevaluatedastrue.

YoumayreadsomeolderCcode: whichmaybebadlyandaccidentlycodedas: so,employdefensiveprogramming:

int initialised = 0; // set to false....

if(! initialised) { // initialisation statements; ..... initialised = 1; // set to true}


if(initialised = 0) { // initialisation statements; ..... initialised = 1; // set to true}


if(0 = initialised) // invalid syntax!{ // initialisation statements; ..... initialised = 1; // set to true}

Inthesecondexample,theconditionaltestalwaysevaluatestofalse,asthesingleequalscharacterrequestsanassignment,notacomparison.

Itispossible(andoccassionallyreasonable)toperformanassignmentaspartofaBooleancondition-you'lloftensee:

while((nextch=getc(file))!=EOF){....

Wheneverrequiringthetrueandfalseconstants(introducedinC99),weneedtoprovidetheline:

#include <stdbool.h>






SwitchstatementsWhenthesame(integer)expressioniscomparedagainstanumberofdistinctvalues,it'spreferredtoevaluatetheexpressiononce,andcompareitwithpossiblevalues:

Cascadingif..else..if..statements: Theequivalentswitchstatement:

Less-commonfeaturesoftheswitchstatement:

if(expression == value1){ // more statements; .....}else if(expression == value2){ // more statements; .....}else{ // more statements; .....}

switch(expression){ case value1 : { // more statements; ..... break; } case value2 : { // more statements; ..... break; } default : { // more statements; ..... break; }}

switch(expression){ case value1 : case value2 : { // handle either value1 or value2 ..... break; } case value3 : { // more statements; ..... // no 'break' statement, drop through } default : { // more statements; ..... break; }}

Typicallythe'expression'issimplyanidentifier,butitmaybearbitrarilycomplex-suchasanarithmeticexpression,orafunctioncall.Thedatatypeofthe'expression'mustbeaninteger(whichincludescharacters,Booleans,andenumeratedtypes),butitcannotbearealorfloating-pointdatatype.Thebreakstatementattheendofeachcaseindicatesthatwehavefinishedwiththe'current'value,andcontrol-flowleavestheswitchstatement.Withoutabreakstatement,control-flowcontinues"downwards",flowingintothenextcasebranch(eventhoughtheexpressiondoesnothavethatcase'svalue!).switchstatementswith'dense'values(none,orfewintegermissing)providegoodopportunitiesforoptimisedcode.






FlowofcontrolinaCprogram-boundedloopsOneofthemostpowerfulfeaturesofcomputers,ingeneral,istoperformthousands,millions,ofrepetitivetasksquickly

(infact,oneofthemotivatingfirstusesofcomputersinthe1940swastocalculatetrigonometrictablesforthefiringofartilleryshells).

Cprovidesitsforcontrolstatementtoloopthroughasequenceofstatements,ablockofstatements,aknownnumberoftimes:

Themostcommonformappearsbelow,inwhichweintroducealoopcontrolvariable,i,tocounthowmanytimeswegothroughtheloop:

Theloopcontrolvariabledoesnotalwayshavetobeaninteger:

// here, variable i holds the values 1,2,...10

for(int i = 1 ; i <= 10 ; i = i+1){// the above introduced a loop-control variable, i ..... printf("loop number %i\n", i); .....// variable i is available down to here}

// but variable i is not available from here

// here, variable ch holds each lowercase value

for(char ch = 'a' ; ch <= 'z' ; ch = ch+1){ ..... printf("loop using character '%c'\n", ch); .....}

Noticethatinbothcases,above,wehaveintroducednewvariables,hereiandch,tospecificallycontroltheloop.

Thevariablesmaybeusedinsideeachloop,inthestatementblock,butthen"disappear"oncetheblockisfinished(afteritsbottomcurlybracket).

It'salsopossibletouseanyothervariableastheloopcontrolvariable,evenifdefinedoutsideoftheforloop.Ingeneral,we'lltrytoavoidthispractice-unlessthevalueofthevariableisrequiredoutsideoftheloop.






FlowofcontrolinaCprogram-unboundedloopsTheforloopsthatwe'vejustseenshouldbeusedwhenweknow,aheadoftime,howmanytimesweneedtoloop(i.e.10times,orovertherange'a'..'z').

Suchloopsaretermedboundedloopsand,unlesswe'vemadeanunseencodingerror,alwaysterminateafterafixednumberofiterations.

Therearealsomanyoccasionswhenwedon'tknow,aheadoftime,howmanyiterationsmayberequired.Suchoccasionsrequireunboundedloops.

Cprovidestwotypesofunboundedloop:Themostcommonisthewhileloop,wherezeroormoreiterationsaremadethroughtheloop:

Lesscommonisthedo....whileloop,whereatleastoneiterationismadethroughtheloop:

#define NLOOPS 20

int i = 1;int n = 0; .....

while(i <= NLOOPS){ printf("iteration number %i\n", i); ..... ..... i = some_calculation_setting_i; n = n + 1;}

printf("loop was traversed %i times\n", n);

#define NLOOPS 20

int i = 1;int n = 0; .....

do{ printf("iteration number %i\n", i); ..... ..... i = some_calculation_setting_i; n = n + 1;} while(i <= NLOOPS);

printf("loop was traversed %i times\n", n);

Noticethatinbothcaseswestilluseavariable,i,tocontrolthenumberofiterationsofeachloop,andthatthechangingvalueofthevariableisusedtodetermineiftheloopshould"keepgoing".

However,thestatementsusedtomodifythecontrolvariablemayappearalmostanywhereintheloops.Theyprovideflexibility,butcanalsobeconfusingwhenloopsbecomeseveralstensorhundredsoflineslong.

Noticealsothatwhile,anddo....whileloopscannotintroducenewvariablestocontroltheiriterations,andsowehavetouseexistingvariablesfromanouterlexicalscope.






WritingloopswithinloopsThere'sanumberofoccassionswhenwewishtoloopanumberoftimes(andsoweuseaforloop)andwithinthatloopwewishtoperformanotherloop.Whilealittleconfusing,thisconstructisoftenquitecommon.Itistermedanestedloop.

#define NROWS 6#define NCOLS 4

for(int row = 1 ; row <= NROWS ; row = row+1) // the 'outer' loop{ for(int col = 1 ; col <= NCOLS ; col = col+1) // the 'inner' loop { printf("(%i,%i) ", row, col); // print row and col as if "coordinates" } printf("\n"); // finish printing on this line}

Theresultingoutputwillbe:

(1,1) (1,2) (1,3) (1,4) (2,1) (2,2) (2,3) (2,4) (3,1) (3,2) (3,3) (3,4) (4,1) (4,2) (4,3) (4,4) (5,1) (5,2) (5,3) (5,4) (6,1) (6,2) (6,3) (6,4)

Noticethatwehavetwodistinctloop-controlvariables,rowandcol.

Eachtimethattheinnerloop(col'sloop)starts,col'svalueisinitializedto1,andadvancesto4(NCOLS).

Asprogramsbecomemorecomplex,wewillseetheneedfor,andwrite,allcombinationsof:

forloopswithinforloops,whileloopswithinwhileloops,forloopswithinwhileloops,andsoon....






ChangingtheregularflowofcontrolwithinloopsTherearemanyoccasionswhenthedefaultflowofcontrolinloopsneedstobemodified.

Sometimesweneedtoleavealoopearly,usingthebreakstatement,possiblyskippingsomeiterationsandsomestatements:

Sometimesweneedtostartthenextiterationofaloop,evenbeforeexecutingallstatementsintheloop:

for(int i = 1 ; i <= 10 ; i = i+1){ // Read an input character from the keyboard ..... if(input_char == 'Q') // Should we quit? break; ..... .....}// Come here after the 'break'. i is unavailable

for(char ch = 'a' ; ch <= 'z' ; ch = ch+1){ if(ch == 'm') // skip over the character 'm' continue; ..... ..... statements that will never see ch == 'm' ..... .....}

Inthefirstexample,weiteratethroughtheloopatmost10times,eachtimereadingalineofinputfromthekeyboard.Iftheuserindicatestheywishtoquit,webreakoutoftheboundedloop.

Inthesecondexample,wewishtoperformsomeworkforalllowercasecharacters,except'm'.Weusecontinuetoignorethefollowingstatements,andtostartthenextloop(withch=='n').






TheequivalenceofboundedandunboundedloopsWeshouldnowbeabletoseethatthefor,while,anddo...whilecontrolflowstatementsareeachcloselyrelated.

Tofullyunderstandthis,however,weneedtoaccept(fornow),thatthethree"pieces"oftheforconstruct,arenotalwaysinitialization,condition,modification.

Moregenerally,thethreepiecesmaybeCexpressions-forthemomentwe'llconsidertheseasCstatementswhich,iftheyproduceavalue,thevalueisoftenignored.

Thefollowingloopsareactuallyequivalent:

for( expression1 ; expression2 ; expression3 ){ statement1; ....}

expression1; while(expression2){ statement1; .... expression3;}

Inbothcases,we'reexpectingexpression2toproduceaBooleanvalue,eithertrueorfalse,asweneedthattruthvaluetodetermineifourloopsshould"keepgoing".

Youshouldthinkaboutthesecarefully,perhapsperformsomeexperiments,todeterminewherecontrolflowreallygoeswhenweintroducebreakandcontinuestatements.





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SomeunusualloopsyouwillencounterAsyoureadmoreCprogramswrittenbyothers,you'llseesomestatementsthatlooklikefororwhileloops,butappeartohavesomethingmissing.Infact,any(orall!)ofthe3"parts"ofaforloopmaybeomitted.

Forexample,thefollowingloopinitiallysetsito1,andincrementsiteachiteration,butitdoesn'thavea"middle"conditionaltesttoseeiftheloophasfinished.Themissingconditionconstantlyevaluatestotrue:

for(int i = 1 ; /* condition is missing */ ; i = i+1){ ..... .....}

Someloopsdon'tevenhavealoop-controlvariable,anddon'ttestfortheirtermination.Thisloopwillrunforever,untilweinterruptorterminatetheoperatingsystemprocessrunningtheCprogram.Wetermtheseinfiniteloops:

// cryptic - avoid this mechanismfor( ; ; ){ ..... .....}

#include <stdbool.h>

// clearer - use this mechanismwhile( true ){ ..... .....}

Whileweoftenseeandwritesuchloops,wedon'tusuallywantthemtorunforever!

Wewilltypicallyuseanenclosedconditionandabreakstatementtoterminatetheloop,eitherbasedonsomeuserinput,orthestateofsomecalculation.




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The structure of C programs CITS2002 Systems Programming