The GCC Compilers.pdf

SoftwareDesignLectureNotes

TheGCCCompilers

Prof.StewartWeiss

TheGCCCompilers

Preface

IfallyoureallywanttoknowishowtocompileyourCorC++programusingGCC,andyoudon’thave

thetimeorinterestinunderstandingwhatyou’redoingorwhatGCCis,youcanskipmostofthesenotes

andcuttothechasebyjumpingtotheexamplesinSection6.Ithinkyouwillbebettero ifyoutakethe

timetoreadthewholething,sinceIbelievethatwhenyouunderstandwhatsomethingis,youarebetter

ableto gureouthowtouseit.

IfyouhaveneverusedGCC,orifyouhaveuseditwithoutreallyknowingwhatyoudid,(becauseyouwere

prettymuchusingitbyrote),thenyoushouldreadthis.IfyouthinkyoudounderstandGCCanddonot

useitbyrote,youmaystillbene tfromreadingthis;youmightlearnsomethinganyway.BecauseIbelieve

intheimportanceofhistoricalcontext,IbeginwithabriefhistoryofGCC.

1BriefHistory

RichardStallmanstartedtheGNUProjectin1984withthepurposeofcreatingafree,Unix-likeoperating

system.Hismotivationwastopromotefreedomandcooperationamongusersandprogrammers.Since

UnixrequiresaCcompilerandtherewerenofreeCcompilersatthetime,theGNUProjecthadtobuild

aCcompilerfromthegroundup.TheFreeSoftwareFoundationwasanon-pro torganizationcreatedto

supporttheworkoftheGNUProject.

GCCwas rstreleasedin1987.Thiswasasigni cantbreakthrough,beingthe rstportableANSIC

optimizingcompilerreleasedasfreesoftware.SincethattimeGCChasbecomeoneofthemostimportant

toolsinthedevelopmentoffreesoftware.

In1992,itwasrevisedandreleasedasGCC2.0,withtheaddedfeatureofaC++compiler.Itwasrevised

againin1997,withimprovedoptimizationandC++support.Thesefeaturesbecamewidelyavailableinthe

3.0releaseofGCCin2001.

2LanguagesSupportedbyGCC

GCCstandsfor GNUCompilerCollection .GCCisanintegratedcollectionofcompilersforseveralmajor

programminglanguages,whichasofthiswritingareC,C++,Objective-C,Java,FORTRAN,andAda.

TheGNUcompilersallgeneratemachinecode,nothigher-levellanguagecodewhichisthentranslatedvia

anothercompiler.

3LanguageStandardsSupportedbyGCC

Forthemostpart,GCCsupportsthemajorstandardsandprovidestheabilitytoturnthemonoro .GCC

itselfprovidesfeaturesforlanguageslikeCandC++thatdeviatefromcertainstandards,butturningonthe

appropriatecompileroptionswillmakeitcheckprogramsagainstthestandards.Thesectionbelowentitled

Consultthemanualfordetails.

SoftwareDesignLectureNotes

TheGCCCompilers

Prof.StewartWeiss

4ABitAbouttheCompileandLinkProcess

SupposethatyouhavewrittenaverysimpleCprogramsuchasthefollowing

1: #include<stdio.h>

3: intmain()

4: {

5: printf("Helloworld\n");

6: return0;

7: }

andplaceditina lenamedhelloworld.c.Youmayhavebeentoldthatyouneedthat rstline,whichis

the#includedirective,#include<stdio.h>,butyoumaynotreallyknowwhyitisthere.Thereasonis

thattheprintf()functionisdeclaredintheheader lestdio.handinorderforthecompilertocheckthat

youarecallingitcorrectlyinline5,itneedstocomparethedeclarationoftheprintf()functionwithits

use.Thecompilerjustneedstocheckthingssuchasthenumberofparameters,theirtypes,andthereturn

valueofthefunction.

Thewaythisisdoneisbycopyingtheentireheader leintotheprogrambeforethecompilerruns.The

#includedirectiveliterallycopiestheentire le,stdio.h,intoyourprogramstartingatline1.Thisisdone

byaprogramcalledtheCpreprocessor.Oncetheheader leisphysicallypartofyourprogram,thecompiler

willrunandwillbeabletovalidatethecalltoprintf()bycomparingittothedeclarationthatitreadin

anearlierlineinthemodi ed le.

Theheader ledoesnotcontainthede nitionoftheprintf()function,i.e.,itsimplementation.Thatis

containedintheCStandardI/OLibrary.Thecompilerisnotabletocreatethemachineinstructionsthat

willcausetheprintf()functiontorun,becauseitdoesnotknow where theprintf()implementationis;

itcannotcreatea call tothisfunction.Instead,thecompilerplacesanotationintheexecutable lethat

says,moreorless, thecalltoprintf()mustberesolvedbythelinker.

Thelinkerisaseparateprogramthatrunsafterthecompiler.Itlooksatalloftheunresolvedsymbolsinthe

program,suchasprintf(),andtriestoresolvethembylookinguptheirlocationsinthesoftwarelibraries

thattheprogramneeds.Inthiscase,thelinkerneedstolookupintheCStandardI/OLibrarythelocation

oftheprintf()function,sothatitcanpatchthecodetomakeacalltoit.TheCStandardI/OLibrary

isspecialbecauseitisusedinalmosteveryCprogram,andthereforemanyCimplementationsincludeit

withintheCruntimelibrary.Thismakesitpossibleforthelinkerto nditeasily.

ThesamediscussionwouldapplyifyouwrotetheaboveprograminC++asin

1: #include<iostream>

3: intmain()

4: {

5: std::cout<<"Helloworld\n";

6: return0;

7: }

onlyinsteadofusingthestdio.hheader le,itwouldusetheiostreamheader le,andtheiostreamlibrary

inC++insteadoftheCStandardI/OLibrary.

Insummary,aheader lecontainsdeclarationsthatthecompilerneeds,butnotimplementations.The

correspondinglibrary lehasthose.Thecompilerneedstheheader lesbutnotthelibraries;thelinker

needsthelibraries,nottheheader les.

SoftwareDesignLectureNotes

TheGCCCompilers

Prof.StewartWeiss

5CommandOptionsandControl

Fromthispointforward,lowercasegccwillrefertotheexecutableprogramname,i.e.,whatyoutypetorun

theGCCcompiler.

5.1OptionsandFileExtensionsControllingtheKindofOutput

Whenyourungcc,itusuallyperformspreprocessing,compiling,assemblyandlinking.Thereareoptionsto

controlwhichofthesestepsareperformed.gcccanalsolookatthe leextensionforguidanceastowhich

compilertouseandwhatkindofoutputtogenerate.Forexample,a leendingin.cisassumedtobeC

sourcecode,and lesendingineither.cc,.cpp,.c++,.C,and.cxxaretakentobeC++sourcecode.

(ThereareotherextensionsthatarealsotakentoimplyC++sourcecode.)Youshouldconsultthemanual

forotherextensionsandlanguages.Thefollowingoptionsareeitherveryusefulorenlightening.

-c Compileorassemblethesource les,butdonotlink.Thelinkingstagesimplyisnotdone.

Theultimateoutputisintheformofanobject leforeachsource le.Bydefault,the

object lenameforasource leismadebyreplacingthesu x‘.c’,‘.i’,‘.s’,etc.,with

‘.o’.E.g.,

gcc cmyprog.c

producesmyprog.o.

-s Stopafterthestageofcompilationproper;donotassemble.Theoutputisintheformof

anassemblercode leforeachnon-assemblerinput lespeci ed.Bydefault,theassembler

lenameforasource leismadebyreplacingthesu x‘.c’,‘.i’,etc.,with‘.s’.Itis

unlikelythatyouwillneedtodothis,butitiseducationaltolookattheoutputofthe

compiler,whichisinassemblylanguage.

-E Stopafterthepreprocessingstage;donotrunthecompilerproper.Theoutputisinthe

formofpreprocessedsourcecode,whichissenttothestandardoutput.E.g.,

gcc Emyprog.c>myprog.i

Thisisalsoaneducationalexercise youcanseeforyourselfwhatthepreprocessordoes

toyoursourcecode,togetabetterunderstandingofhowtouseit.

-ofile Placeoutputin lefile.Thisappliesregardlessofwhateversortofoutputisbeing

produced,whetheritisanexecutable le,anobject le,anassembler leorpreprocessed

Ccode.Usuallyyouusethistonameyourexecutable.E.g.,

gcc omyprogmyprog.c

-v Print(onstandarderroroutput)thecommandsexecutedtorunthestagesofcompilation.

Alsoprinttheversionnumberofthecompilerdriverprogramandofthepreprocessorand

thecompilerproper.

--help Print(onthestandardoutput)adescriptionofthecommandlineoptionsunderstoodby

gcc.

SoftwareDesignLectureNotes

TheGCCCompilers

Prof.StewartWeiss

5.2CompilingC++Programs

gcccomeswithacompilernamedg++thatspeci callycompilesC++programs,regardlessofthe le

extension.SometimesyouneedtousetheC++compilereventhoughthe leextensionisnotaC++

extension;inthiscaseyouneedtouseg++.

5.3OptionsthatControltheCDialect

By dialect ismeantaspeci ccollectionoffeaturesofC.Forexample,theANSIstandardknownasISO90

CisadialectofC.ThefullsetoffeaturessupportedbygccismuchlargerthantheANSIstandard,and

thisisalsoadialect.AnotherdialectisobtainedbyaddingGNUextensionstotheANSIISOC90standard.

YoucanselectivelyremovefeaturesfromthefullGNUsetofextensions.Thebasicoptions,however,are:

-ansi InCmode,supportallISOC90programs.InC++mode,removeGNU

extensionsthatcon ictwithISOC++.Thisturnso certainfeaturesofgccthat

areincompatiblewithISOC90(whencompilingCcode),orofstandardC++

(whencompilingC++code).

Forexample,thegetline()functionisaGNUextensiontoC.ItisnotinANSIC.

Ifyourprogrammyprog.ccontainsitsowngetline()functionthenifyoucompile

withtheline

gccmyprog.c

youwillgettheerror

temp4.c:4:error:conflictingtypesfor‘getline’

butifyouuse

gcc-ansimyprog.c

theGNUextensionswillbedisabled,andtherewillbenotypecon ict.

-std= Whenfollowedbyaspeci cdialectdesignatingstringsuchas‘c90’or‘gnu9x’,it

speci esthatdialect.

-fno-... Therearemanyoptionsthatbeginwith fno-andarefollowedbyastringthat

representssomefeaturetodisable.The’no’means’turno ’.Consultthemanual.

5.4OptionsthatControlWarnings

Warningsarediagnosticmessagesaboutconstructionsthatarenoterrors,butareoftenassociatedwith

errors.Forexample,whenavariableisdeclaredbutneverusedinaprogram,itispossiblethatthe

programmeroverlookedsomething,sothecompilercouldissueawarningwhenit ndssuchaconstruction.

gccallowsyoutosuppresscertainwarningsandrequestothers.Someofthemorecommonoptionsarelisted

below.

SoftwareDesignLectureNotes

TheGCCCompilers

Prof.StewartWeiss

-w Inhibitallwarningmessages.

-Wall Enableallwarningmessages(recommendedforalldevelopmentwork).

-Wextra Enablewarningsthatarenotcheckedby-Wall.Forexample,comparingan

unsignedintvariableagainst-1suggeststhatyouforgotthatthevariablewas

unsigned.

5.5OptionsForDebugging

-g Thisproducesdebugginginformationintheoperatingsystem’snativeformat.

GDBcanworkwiththisdebugginginformation.Onmostsystemsthatusestabs

format,‘-g’enablesuseofextradebugginginformationthatonlyGDBcanuse;

thisextrainformationmakesdebuggingworkbetterinGDBbutwillprobably

makeotherdebuggerscrashorrefusetoreadtheprogram.E.g.,

gcc-g-omyprogmyprog.c

Onceyouhavecompiledaprogramwithdebugginginformation,youcanrunitunderthecontrolofthe

debugger.Ofcourseyouhavetolearnhowtousethedebugger,gdb,fromthecommandline,orfromwithin

aGUI-basedSDKthatusesitastheunderlyingdebugger.

5.6OptionsControllingthePreprocessor

Ofcourseyouareawareofthefactthatwhenyourprogramiscompiled,thevery rststepthatthecompiler

takesistorunthepreprocessor,whichprocessesallofthepreprocessordirectives,thoselinesthatbeginwith

thepoundsign’#’.Thereareoptionsthatcontrolhowthepreprocessorbehaves,andtheseareimportant

toknowandunderstand.Themostimportantare(1)howtode nesymbolsonthecommandline,and(2)

howtotellthepreprocessorwheretolookforinclude- les.

-Dname Thisprede nesnameasamacrosymbol,withthevalue1,ortrueifyouwant

tothinkofitthatway.

-Dname=definition Thecontentsofde nitionaretokenizedandprocessedasiftheyappeared

duringtranslationphasethreeina‘#define’directive.E.g.,

gcc-Dtestvalue=6-omyprogmyprog.c

isthesameasifyouhadplaced

#definetestvalue6

inmyprog.c

Ifyouareinvokingthepreprocessorfromashellorshell-likeprogramyouneedtousetheshell’squoting

syntaxtoprotectcharacterssuchasspacesthathaveameaningintheshellsyntax.Forexample,

gcc-D’introduction=AArgh$#^*!!’-omyprogmyprog.c

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