Briefnotesonmemoryoperatinsinfork(),exec()andvfork()byDr.Beck
Background:Theaddressspaceofaprocessismadeupofanumberofsegments,eachof
whichisacontiguousareaofmemoryusedforaparticularpurpose.Inthisdiscussion,we
considerprocessedwiththesesegments:code,staticdata,heapandstack.
Fork():Thefork()callcreatesanewprocess(thechild)byallocatinganunusedtask
controlblock(TCB)andcopyingmanyofthefieldsfromthecallingprocess’(theparent’s)
TCB.Thus,whilethechildisnotidenticaltotheparentinallrespects,theysharemany
attributesincommon.Forexample,theyhavethesameuserID(uid)butdifferentprocess
IDs(pid).
Fork()isabletoshareread-onlysegmentsbetweentheparentandchild,allowingthe
child’saddressspacetobecreatedusingthesamememoryastheparent.Inourexample,
theonlyready-onlysegmentisthecodesegment.Sinceneitherparentnorchildcanmodify
thissegment,theycanuseitsimultaneouslywithnopossibilityofinterfering.
Theothersegmentsinourexample(staticdata,heapandstack)areallwritable,andso
sharingthembetweenaparentandchildexecutingsimultaneouslycouldresultin
interferencebetweenthosetwoprocesses(raceconditions).Thus,theforksystemcall
allocatesnewmemoryforthosesegmentsinthechild(thesamesizeasthesegmentsinthe
parent)andcopiesthecontentsofeachsegmentfromtheparenttothecorresponding
segmentinthechild.Theparentandchildtheneachhaveaprivatecopyofthecontentsof
eachofthesesegmentsstoredinitsownprivatememory,andsoparentandchildcanboth
executesimultaneouslywithoutthepossibilityofinterference.
Exec(): Exec()isasystemcallthatreplacestheaddressspaceofarunningprocess
withthecontentsofanexecutablefile.Thecodeandstaticdatasegmentsarereadfromthe
fileintonewlyallocatedmemory,thestackandheapareinitializedtobeempty.
Itisverycommonforacommandlineinterpreter(shell)orGUItoinvokeanexecutablefile
(commandoruserprogram)bycallfork()tocreateanewchildprocess,andthechild
processthenalmostimmediatelycallingexec()tostarttheprogramstoredinthefile.In
thisscenarioalltheworkthatdoesintocopyingtheaddressspaceofthechild(creatingthe
staticdata,heapandstacksegments)isalmostimmediatelyundonebythecalltoexec().
Vfork(): Vfork()isameansforaparenttoavoidtheoverheadofcopyingtheparent’s
addressspacewhenacalltoexec()willbemadealmostimmediatelyafterprocess
creation.Thecalltovfork()issimilartofork()butitdoesnotallocatenewmemoryfor
thewritablesegments,insteadallowingthechildtoexecuteusingthememorysegmentsof
theparent.Inordertoavoidinterferencebetweenparentandchild,theparentisnot
allowedtoexecuteafterthecalltovfork()untilthechildmakesacalltoexec().
Vfork()avoidstheneedtoallocateandcopymemoryforthechild’saddressspace,but
becausethechilddoessharetheparent’saddressspaceitmustbeverycarefulaboutwhat
variablesitmodifiesbeforecallingexec().Ifitchangesanythingthatwillcauseproblems
intheparentonceitstartsrunningagain,theparentmightexperienceerrors.Thuswecan
saythatvfork()isnotcompletelysafe(becauseitcancauseproblemsifthechildmakes
error-causingmodifications)butitcanbemuchmoreefficientthanfork().