filesSpecifyingandexaminingfilesinternalsMaintenancecommandsobscur - PDF document

files--Specifyingandexaminingfilesinternals--Maintenancecommandsobscure--Obscurefeaturesrunning--Runningtheprogramstack--Examiningthestackstatus--Statusinquiriessupport--Supportfacilitiestracepoints--Tracingofprogramexecutionwithoutstoppingtheprogramuser-defined--User-definedcommandsType"help"followedbyaclassnameforalistofcommandsinthatclass.Type"help"followedbycommandnameforfulldocumentation.Commandnameabbreviationsareallowedifunambiguous.Atthispoint,youarereadytoexecutetheprogram,setbreakpoints,andprintvariables,etc.Todebugaprogram,youmustrstrunit.The\run"commandallowsyoutodojustthis.Beforeyouu

seit,youmightwanttoknowmoreabout\run"usingthe\help"facility:(gdb)helprunStartdebuggedprogram.Youmayspecifyargumentstogiveit.Argsmayinclude"*",or"[...]";theyareexpandedusing"sh".Inputandoutputredirectionwith�"","or&#x",-5;─&#x",-5;─""arealsoallowed.%Withnoarguments,usesargumentslastspecified(with"run"or"setargs").Tocancelpreviousargumentsandrunwithnoarguments,use"setargs"withoutarguments.(gdb)Asthehelptextindicates,runningtheprogramisthesameasexecutingintheshellandyoucanusethesameC-shellredirectionoperationsaswell.Inthecaseofbug.C,thereisnoinput,sowejusttype\run"withnoarguments:(gdb)runStart

ingprogram:/nfs/home/nsa/kena/csci3308/tmp/bugFirstcallsumis6Secondcallsumis6Programexitednormally.(gdb)Here,weseethattheprogramisnotexecutingcorrectly(theresultofeachcalltosumshouldbe15).1Wearenowreadytostartdebugging.SettingBreakpoints.Tounderstandwhatyourprogramisdoing,youwillwanttostopitatdierentpointsinitsexecution.Breakpointsallowyoutodo 1Ifyougetwarningsfromgdbaboutbeingunabletondasymbol,youcanignorethem.2 Herewehavestoppedattherstbreakpoint,inthemainroutine.Whenex-ecutionstops,wecanexaminethecontentsofthestackoranylocalorglobalvariables.Forexample,the\backtrace"commandprintsthec

urrentfunctionsonthestack.Inthiscase,sincewejustenteredmain,thereisonlyone(youcanignorethestackframecausedbylibclibrary):(gdb)backtrace#0main()atbug.C:25#10x420156a4in__libc_start_main()from/lib/tls/libc.so.6(gdb)Ifwewishtocontinuefromabreakpoint,weusethe\continue"command.Notethatthisisdierentfrom\run"whichstartstheprogramfromthebeginningagain.(gdb)continueContinuing.Breakpoint3,sum(int*,int)(array_arg=0x8049a40,size=5)atbug.C:1515for(i=1;isize;i+=1){(gdb)Hereweseethatwehavestoppedatthesecondbreakpoint,insidethefunctionsumthatwascalledfrommain.Ifwenowask\backtrace",weseebothmainandsumonthecallsta

ck:(gdb)bt#0sum(int*,int)(array_arg=0x8049a40,size=5)atbug.C:15#10x0804879ainmain()atbug.C:25#20x420156a4in__libc_start_main()from/lib/tls/libc.so.6(gdb)Notethatthebacktracecommandalsoprintstheargumentstothefunctionsonthecallstack.Tondoutmoreaboutexaminingthestack,usethe\helpstack"command:Examiningthestack.Thestackismadeupofstackframes.Gdbassignsnumberstostackframescountingfromzerofortheinnermost(currentlyexecuting)frame.Atanytimegdbidentifiesoneframeasthe"selected"frame.Variablelookupsaredonewithrespecttotheselectedframe.Whentheprogrambeingdebuggedstops,gdbselectstheinnermostframe.Thecommandsbel

owcanbeusedtoselectotherframesbynumberoraddress.Listofcommands:backtrace--Printbacktraceofallstackframes4 (gdb)step16result+=array_arg[i];(gdb)Therearetwocommandstostepaheadastatement:\step"and\next".Thedierencebetweentheseisthatnextwillstepoverfunctioncalls,whilestepwillstep\into"functioncalls.Toseethedierence,supposewedeleteallbreakpoints,setabreakpointatline27inbug.C,andstarttheprogramfromthebeginning:(gdb)infobreakpointsNumTypeDispEnbAddressWhat1breakpointkeepy0x00004004inmainatbug.C:25breakpointalreadyhit1time3breakpointkeepy0x00003f8cinsum(int*,int)atbug.C:15breakpointalreadyhit1time(g

db)d13(gdb)breakbug.C:25Breakpoint4at0x8048786:filebug.C,line25.(gdb)rTheprogrambeingdebuggedhasbeenstartedalready.Startitfromthebeginning?(yorn)yStartingprogram:/nfs/home/nsa/kena/csci3308/tmp/bugBreakpoint4,main()atbug.C:2525sum1=sum(a1,5);(gdb)Ifwesay\next"wewillstepoverthecalltosumendingupatthenextline:(gdb)next26cout"Firstcallsumis"sum1"\n";(gdb)However,ifwesay\step"wewillstepintothecalltosum.Herewereruntheprogramtoillustratethepoint:(gdb)rTheprogrambeingdebuggedhasbeenstartedalready.Startitfromthebeginning?(yorn)yStartingprogram:/nfs/home/nsa/kena/csci3308/tmp/bugBreakpoint4,main()atbug.C:2525su

m1=sum(a1,5);(gdb)stepsum(array_arg=0x400010e4,size=5)atbug.C:156 16result+=array_arg[i];1:result=1073797554(gdb)Hereweseethatresultchangesasitisupdated,butbecauseitmighthaveanimproperinitialvalue,wecannotrelyonthisfunctionworking.Thisleadsustomodifyoursourcele(resultinginthelebug1.C),addinginitializationtotheresultinsum.(Goaheadandmakethischange,initializeresulttozero.Youcanexitgdbusingthequitcommand.)Notethattodisplayavariable,wemustwaituntilweareatabreakpointwherethevariableisvisible.Forexample,ifwetrytodisplayresultbeforewehaveenteredthefunctionsum,gdbgivesusanerrormessage.�nagg++

-g-obug1bug.C�naggdbbug1(gdb)displayresultNosymbol"result"incurrentcontext.(gdb)bsumBreakpoint1at0x3f8c:filebug.C,line12.(gdb)Sowemustwaittoentersumbeforeresultcanbedisplayed:(gdb)rStartingprogram:/nfs/home/nsa/kena/csci3308/tmp/bug1Breakpoint1,sum(array_arg=0x400010e4,size=5)atbug1.C:1212result=0;(gdb)displayresult1:result=1073797552(gdb)n16for(i=1;isize;i+=1){1:result=0(gdb)n17result+=array_arg[i];1:result=0(gdb)n16for(i=1;isize;i+=1){1:result=2(gdb)Nowweseethatresultgetsaninitialvalue,butaswestartaddingarrayelementstoit,therstvalueaddedis2insteadof1.Thisleadsustotheo-by-oneloopinde

xbug.Ifwechangetheboundsofthearrayfrom1tosizeto0to(size-1)wehavexedthesecondbug.Makethischangenow.FinishingtheExample.Recompiletheprogramintobug2andstartthedebuggeronthisversionoftheprogram.8 #include&#xstre; m.h;intdummy1=100;inta1[]={1,2,3,4,5};intdummy2=100;intsum(intarray_arg[],intsize){intresult,i;//Hereisthefirstbug:resultisneverinitialized.//Hereisthesecondbug:irangesfrom1tosize,not0to(size-1)for(i=1;isize;i+=1){result+=array_arg[i];}//Hereisthethirdbug:nothingisreturnedfromsum.}intmain(){intsum1,sum2;sum1=sum(a1,5);cout"Firstcallsumis"sum1"\n";sum2=sum(a1,5);cout"Secondcallsumis"sum2"\