how to write shared libraries.pdf

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1、HowToWriteSharedLibrariesUlrichDrepperdrepper@gmail.comDecember10,2011AbstractToday,sharedlibrariesareubiquitous.Developersusethemformultiplereasonsandcreatethemjustastheywouldcreateapplicationcode.Thisisaproblem,though,sinceonmanyplatformssomeadditionalte

2、chniquesmustbeappliedeventogeneratedecentcode.Evenmoreknowledgeisneededtogenerateoptimizedcode.Thispaperintroducestherequiredrulesandtechniques.Inaddition,itintroducestheconceptofABI(ApplicationBinaryInterface)stabilityandshowshowtomanageit.1Preface1.1ALit

3、tleBitofHistoryThebinaryformatusedinitiallyforLinuxwasana.outForalongtime,programmerscollectedcommonlyusedvariant.Whenintroducingsharedlibrariescertaindesigncodeinlibrariessothatcodecouldbereused.Thissavesdecisionshadtobemadetoworkinthelimitationsofdevelop

4、menttimeandreduceserrorssincereusedcodea.out.Themainacceptedlimitationwasthatnoreloca-onlyhastobedebuggedonce.Withsystemsrunningtionsareperformedatthetimeofloadingandafterward.dozensorhundredsofprocessesatthesametimereuseThesharedlibrarieshavetoexistinthef

5、ormtheyareofthecodeatlink-timesolvesonlypartoftheproblem.usedatrun-timeondisk.Thisimposesamajorrestric-Manyprocesseswillusethesamepiecesofcodewhichtiononthewaysharedlibrariesarebuiltandused:everytheyimportforlibraries.Withthememorymanagementsharedlibrarymu

6、sthaveafixedloadaddress;otherwiseitsystemsinmodernoperatingsystemsitisalsopossiblewouldnotbepossibletogeneratesharedlibrarieswhichtosharethecodeatrun-time.Thisisdonebyloadingthedonothavetoberelocated.codeintophysicalmemoryonlyonceandreusingitinmultipleproce

7、ssesviavirtualmemory.LibrariesofthisThefixedloadaddresseshadtobeassignedandthishaskindarecalledsharedlibraries.tohappenwithoutoverlapsandconflictsandwithsomefuturesafetybyallowinggrowthofthesharedlibrary.Theconceptisnotverynew.OperatingsystemdesignersItisthe

8、reforenecessarytohaveacentralauthorityforimplementedextensionstotheirsystemusingtheinfras-theassignmentofaddressrangeswhichinitselfisama-tructuretheyusedbefore.TheextensiontotheOScouldjorproblem.Butitgetswors