Hermes: A Tutorial and Reference Manual - Researcher - IBM
Hermes: A Tutorial and Reference Manual - Researcher - IBM
Hermes: A Tutorial and Reference Manual - Researcher - IBM
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ceptionInterfaceMismatch,whichcanberaisedbythecreatestatement. InterfaceMismatchisoneofthefewtypecheckingerrorsthatcannotbe detectedatcompile-time,butonlyatrun-time.Forexample,thesecond createstatementinstantiatesachildprocess,<strong>and</strong>returnsanoutputport boundtotheinitializationport.Itisknownstatically(thatis,atcompile time)thatthetypeofthisoutputportmustbecompatiblewiththeoutput 1.Introductionto<strong>Hermes</strong>17<br />
exceptionwillberaised. anarbitraryprogram.Ifthisprogramdoesn'thavetheproper3-pluginterface(as,forexample,Filterdoesnot),thentheInterfaceMismatch<br />
portClient.Butitisnotknownstaticallywhethertheinitializationinput portoftheprogramisofamatchingtype,sinceGetProgrammayreturn<br />
akernelhackerorasuperusertowriteaccesscontrolpolicies.Youcannot Anytypeofcallcanbereboundprovideditistoaserviceofmatchingtype. ThinkoftheexibilityofUNIXpipesextendedtoanykindofcalls. tionofports.Thetechniqueisveryexible.ProgramslikeCompoundcan containarbitrarypoliciesforbindingtheportsoftheprogramsitcreates, therebycontrollingtheiraccesstootherprocesses.Youdon'tneedtobe systemsprogrammingondynamicprocesscreation<strong>and</strong>dynamicconnec-<br />
Becausemodulesaren'thard-wiredtospecicservices,theycanbereused.<br />
accidentallyorintentionallycircumventsomeoneelse'saccesscontrol.You cannotaccessanotheruser'sglobalvariables,sincetherearenoglobalvariables.Youcannotaccessaservicebyguessingitsname,orbyusingsome<br />
Althoughthisexampleisverysimple,itillustratestheeleganceofbasing<br />
undenedvalueasaname.YoucanonlygetabindingtoaninputportP if(1)youownPyourself<strong>and</strong>connecttoit,(2)Pistheinitializationport primitivesintothelanguage.Type-checkingguaranteesthatyoucan'tcopy aninteger,string,orotherinappropriatedatatypeintoacapability(output ofaprocessyoucreate,or(3)someoneelsegivesyouabindingtoP. port)variable.Typestate-checkingguaranteesthatyoucan'tuseanuninitialized<strong>and</strong>possiblygarbagevalueofacapability.Butyoumaycreate<br />
oldideainoperatingsystems.Itisnotwidespread,becauseitspremise| thatcapabilities(accessrights)areeasytostore<strong>and</strong>passaround,but impossibletoforge|isoftendicult<strong>and</strong>expensivetoimplement.Her-<br />
Thisapproachtosystemcongurationiscalledcapability-based.Itisan messolvesthisprobleminexpensivelybyintegratingtheoperatingsystems<br />
processes,<strong>and</strong>settingupportconnectionsbetweenprocesses.Youhave learnedthat<strong>Hermes</strong>isacapability-basedoperatingsystemencapsulated aserveristypicallydividedintoaninitializationpartwhichimports<strong>and</strong> Youhavelearnedhowtoimplementthebasicfunctionsofashell|creating aprogramwhichwasbothaclient<strong>and</strong>aserver.Youhavelearnedthat exportsports,<strong>and</strong>aserviceloopwhichh<strong>and</strong>lesrequestsfromitsclients. sagesasmuchasyouplease. capabilitieswithconnect,<strong>and</strong>copythemorpassthemaroundinmes-<br />
withinaprogramminglanguage.Youhavelearnedthefollowing<strong>Hermes</strong> Summary:WehavestudiedtheprogramFilter|ourrstexampleof
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