Introductory4.4BSDIPCPSD:20-1AnIntroductory4.4BSDInterprocessCommunicationTutorialStuartSechrestComputerScienceResearchGroupComputerScienceDivisionDepartmentofElectricalEngineeringandComputerScienceUniversityofCalifornia,BerkeleyABSTRACTBerkeleyUNIX†4.4BSDoffersseveralchoicesforinterprocesscommunication.Toaidthepro-grammerindevelopingprogramswhicharecomprisedofcooperatingprocesses,thedifferentchoicesarediscussedandaseriesofexampleprogramsarepresented.Theseprogramsdemonstrateinasimplewaytheuseofpipes,socketpairs,socketsandtheuseofdatagramandstreamcommunication.Theintentofthisdocumentistopresentafewsimpleexampleprograms,nottodescribethenetworkingsysteminfull.1.GoalsFacilitiesforinterprocesscommunication(IPC)andnetworkingwereamajoradditiontoUNIXintheBerkeleyUNIX4.2BSDrelease.Thesefacilitiesrequiredmajoradditionsandsomechangestothesys-teminterface.ThebasicideaofthisinterfaceistomakeIPCsimilartofileI/O.InUNIXaprocesshasasetofI/Odescriptors,fromwhichonereadsandtowhichonewrites.Descriptorsmayrefertonormalfiles,todevices(includingterminals),ortocommunicationchannels.Theuseofadescriptorhasthreephases:itscreation,itsuseforreadingandwriting,anditsdestruction.Byusingdescriptorstowritefiles,ratherthansimplynamingthetargetfileinthewritecall,onegainsasurprisingamountofflexibility.Often,theprogramthatcreatesadescriptorwillbedifferentfromtheprogramthatusesthedescriptor.Forexampletheshellcancreateadescriptorfortheoutputofthe‘ls’commandthatwillcausethelistingtoappearinafileratherthanonaterminal.PipesareanotherformofdescriptorthathavebeenusedinUNIXforsometime.Pipesallowone-waydatatransmissionfromoneprocesstoanother;thetwoprocessesandthepipemustbesetupbyacommonancestor.TheuseofdescriptorsisnottheonlycommunicationinterfaceprovidedbyUNIX.Thesignalmech-anismsendsatinyamountofinformationfromoneprocesstoanother.Thesignaledprocessreceivesonlythesignaltype,nottheidentityofthesender,andthenumberofpossiblesignalsissmall.Thesignalsemanticslimittheflexibilityofthesign