翻译文献-11 程序和信号

外文原文11PROCESSESANDSIGNALSPROCESSESANDSIGNALSFORMAFUNDAMENTALPARTOFTHELINUXOPERATINGENVIRONMENTTHEYCONTROLALMOSTALLACTIVITIESPERFORMEDBYLINUXANDALLOTHERUNIXLIKECOMPUTERSYSTEMSANUNDERSTANDINGOFHOWLINUXANDUNIXMANAGEPROCESSESWILLHOLDANYSYSTEMSPROGRAMMER,APPLICATIONSPROGRAMMER,ORSYSTEMADMINISTRATORINGOODSTEADINTHISCHAPTER,WELLLOOKATHOWPROCESSESAREHANDLEDINTHELINUXENVIRONMENTANDHOWTOFINDOUTEXACTLYWHATTHECOMPUTERISDOINGATANYGIVENTIMEWELLALSOSEEHOWTOSTARTANDSTOPOTHERPROCESSESFROMWITHINOUROWNPROGRAMS,HOWTOMAKEPROCESSESSENDANDRECEIVEMESSAGES,ANDHOWTOAVOIDZOMBIESINPARTICULAR,WELLLEARNABOUTPROCESSSTRUCTURE,TYPE,ANDSCHEDULINGSTARTINGNEWPROCESSESINDIFFERENTWAYSPARENT,CHILD,ANDZOMBIEPROCESSESWHATSIGNALSAREANDHOWTOUSETHEMWHATISAPROCESSTHESINGLEUNIXSPECIFICATION,VERSION2UNIX98ANDITSPREDECESSORVERSION1UNIX95,DEFINESAPROCESSAS“ANADDRESSSPACEWITHONEORMORETHREADS”WEWILLLOOKATTHREADSINCHAPTER12FORNOW,WEWILLREGARDAPROCESSASJUSTAPROGRAMTHATISRUNNINGAMULTITASKINGOPERATINGSYSTEMSUCHASLINUXLETSMANYPROGRAMSRUNATONCEEACHINSTANCEOFARUNNINGPROGRAMCONSTITUTESAPROCESSTHISISESPECIALLYEVIDENTWITHAWINDOWINGSYSTEMSUCHASTHEXWINDOWSYSTEMOFTENSIMPLYCALLEDXLIKEWINDOWS,XPROVIDESAGRAPHICALUSERINTERFACETHATALLOWSMANYAPPLICATIONSTOBERUNATONCEEACHAPPLICATIONCANDISPLAYONEORMOREWINDOWSASAMULTIUSERSYSTEM,LINUXALLOWSMANYUSERSTOACCESSTHESYSTEMATTHESAMETIMEEACHUSERCANRUNMANYPROGRAMS,OREVENMANYINSTANCESOFTHESAMEPROGRAM,ATTHESAMETIMETHESYSTEMITSELFRUNSOTHERPROGRAMSTOMANAGESYSTEMRESOURCESANDCONTROLUSERACCESSASWESAWINCHAPTER4,APROGRAMORPROCESSTHATISRUNNINGCONSISTSOFPROGRAMCODE,DATA,VARIABLESOCCUPYINGSYSTEMMEMORY,OPENFILEFILEDESCRIPTORS,ANDANENVIRONMENTTYPICALLY,ALINUXSYSTEMWILLSHARECODEANDSYSTEMLIBRARIESAMONGPROCESSESSOTHATTHERESONLYONECOPYOFTHECODEINMEMORYATANYONETIMEPROCESSSTRUCTURELETSHAVEALOOKATHOWACOUPLEOFPROCESSESMIGHTBEARRANGEDWITHINTHEOPERATINGSYSTEMIFTWOUSERS,NEILANDRICK,BOTHRUNTHEGREPPROGRAMATTHESAMETIMETOLOOKFORDIFFERENTSTRINGSINDIFFERENTFILES,THEPROCESSEDBEINGUSEDMIGHTLOOKLIKEFIGURE111IFWERUNTHEPSCOMMAND,DESCRIBED,ASFOLLOWS,THEOUTPUTWILLLOOKSOMETHINGLIKETHISEACHPROCESSISALLOCATEDAUNIQUENUMBER,CALLEDAPROCESSIDENTIFIERORPIDTHISISUSUALLYAPOSITIVEINTEGERBETWEEN2AND32768WHENAPROCESSISSTARTED,THENEXTUNUSEDNUMBERINSEQUENCEISCHOSENANDTHENUMBERSRESTARTAT2SOTHATTHEYWRAPAROUNDTHENUMBER1ISTYPICALLYRESERVEDFORTHESPECIALINITPROCESS,WHICHMANAGESOTHERPROCESSESWEWILLCOMEBACKTOINITSHORTLYHEREWESEETHATTHETWOPROCESSESSTARTEDBYNEILANDRICKHAVEBEENALLOCATEDTHEIDENTIFIERS101AND102THEPROGRAMCODETHATWILLBEEXECUTEDBYTHEGREPCOMMANDISSTOREDINADISKFILENORMALY,ALINUXPROCESSCANTWRITETOTHEMEMORYAREAUSEDTOHOLETHEPROGRAMCODE,SOTHECODEISLOADEDINTOMEMORYASREADONLYWESAWINFIGURE111THAT,ALTHOUGHTHISAREACANTBEWRITTENTO,ITCANSAFELYBESHAREDTHESYSTEMLIBRARIESCANALSOBESHAREDTHUS,THERENEEDBEONLYONECOPYOFPRINTF,FOREXAMPLEINMEMORY,EVENIFMANYRUNNINGPROGRAMSCALLITTHISISAMORESOPHISTICATED,BUTSIMILAR,SCHEMETOTHEWAYDYNAMICLINKLIBRARIESDLLSWORKINWINDOWSASWECANSEEINTHEPRECEDINGDIAGRAM,ANADDITIONALBENEFITISTHATTHEDISKFILECONTAININGTHEEXECUTABLEPROGRAMGREPISSMALLERBECAUSEITDOESNTCONTAINSHAREDLIBRARYCODETHISMIGHTNOTSEEMMUCHFORASINGLEPROGRAM,BUTEXTRACTINGTHECOMMONROUTINESFORSAYTHESTANDARDCLIBRARYSAVESASIGNIFICANTAMOUNTOFSPACEOVERAWHOLEOPERATINGSYSTEMOFCOURSE,NOTEVERYTHINGTHATAPROGRAMNEEDSTORUNCANBESHAREDFOREXAMPLE,THEVARIABLESTHATITUSESAREDISTINCTFOREACHPROCESSINTHISEXAMPLE,WESEETHATTHESEARCHSTRINGPASSEDTOTHEGREPCOMMANDAPPEARSASAVARIABLE,S,INTHEDATASPACEOFEACHPROCESSTHESEARESEPARATEANDUSUALLYCANTBEREADBYOTHERPROCESSESTHEFILESTHATAREBEINGUSEDINTHETWOGREPCOMMANDSAREALSODIFFERENTTHEPROCESSESHAVETHEIROWNSETOFFILEDESCRIPTORSUSEDFORFILEACCESSADDITIONALLY,APROCESSHASITSOWNSTACKSPACE,USEDFORLOCALVARIABLESINFUNCTIONSANDFORCONTROLLINGFUNCTIONCALLSANDRETURNSITALSOHASITSOWNENVIRONMENTSPACE,CONTAININGENVIRONMENTVARIABLESTHATMAYBEESTABLISHEDSOLELYFORTHISPROCESSTOUSE,ASWESAWWHENWELOOKEDATPUTENVANDGETENVINCHAPTER4APROCESSMUSTALSOMAINTAINITSOWNPROGRAMCOUNTER,ARECORDOFWHEREITHASGOTTOINITSEXECUTION,WHICHISTHEEXECUTIONTHREADINTHENEXTCHAPTERWEWILLSEETHATWHENWEUSETHREADS,PROCESSESCANHAVEMORETHANONETHREADOFEXECUTIONONMANYLINUXSYSTEMS,ANDSOMEUNIXSYSTEMS,THEREISASPECIALSETOF“FILES”INADIRECTORYCALLED/PROCTHESEARESPECIALINTHATRATHERTHANBEINGTRUEFILESTHEYALLOWYOUTO“LOOKINSIDE”PROCESSESWHILETHEYARERUNNINGASIFTHEYWEREFILESINDIRECTORIESWETOOKABRIEFLOOKATTHE/PROCFILESYSTEMBACKINCHAPTER3FINALLY,BECAUSELINUX,LIKEUNIX,HASAVIRTUALMEMORYSYSTEMTHATPAGESCODEANDDATAOUTTOANAREAOFTHEHARDDISK,MANYMOREPROCESSESCANBEMANAGEDTHANWOULDFITINTOTHEPHYSICALMEMORYTHEPROCESSTABLETHELINUXPROCESSTABLEISLIKEADATASTRUCTUREDESCRIBINGALLOFTHEPROCESSESTHATARECURRENTLYLOADEDWITH,FOREXAMPLE,THEIRPID,STATUS,ANDCOMMANDSTRING,THESORTOFINFORMATIONOUTPUTBYPSTHEOPERATINGSYSTEMMANAGESPROCESSESUSINGTHEIRPIDS,ANDTHEYAREUSEDASANINDEXINTOTHEPROCESSTABLETHETABLEISOFLIMITEDSIZE,SOTHENUMBEROFPROCESSESASYSTEMWILLSUPPORTISLIMITEDEARLYUNIXSYSTEMSWERELIMITEDTO256PROCESSESMOREMODERNIMPLEMENTATIONSHAVERELAXEDTHISRESTRICTIONCONSIDERABLYANDMAYBELIMITEDONLYBYTHEMEMORYAVAILABLETOCONSTRUCTAPROCESSTABLEENTRYVIEWINGPROCESSESTHEPSCOMMANDSHOWSTHEPROCESSESWERERUNNING,THEPROCESSANOTHERUSERISRUNNING,ORALLTHEPROCESSESONTHESYSTEMHEREISSOMESAMPLEOUTPUTWHILEITISWAITINGFORSOMEINPUTFROMTHEUSER,THESYSTEMINCREASESITSPRIORITY,SOTHATWHENITSREADYTORESUME,ITHASAHIGHPRIORITYWECANSETTHEPROCESSNICEVALUEUSINGNICEANDADJUSTITUSINGRENICETHENICECOMMANDINCREASESTHENICEVALUEOFAPROCESSBY10,GIVINGITALOWERPRIORITYWECANVIEWTHENICEVALUESOFACTIVEPROCESSESUSINGTHE1ORFFORLONGOUTPUTOPTIONTOPSTHEVALUEWEAREINTERESTEDINISSHOWNINTHENINICECOLUMNHEREWECANSEETHATTHEOCLOCKPROGRAMISRUNNINGASPROCESS1362WITHADEFAULTNICEVALUEIFITHADBEENSTARTEDWITHTHECOMMANDNICEOCLOCKTHESYSTEMFUNCTIONRUNSTHECOMMANDPASSEDTOITASASTRINGANDWAITSFORITTOCOMPLETETHECOMMANDISEXECUTEDASIFTHECOMMANDSHCSTRINGHASBEENGIVENTOASHELLSYSTEMRETURNS127IFASHELLCANTBESTARTEDTORUNTHECOMMANDANDIIFANOTHERERROROCCURSOTHERWISE,SYSTEMRETURNSTHEEXITCODEOFTHECOMMANDTRYITOUTSYSTEMWECANUSESYSTEMTOWRITEAPROGRAMTORUNPSFORUSWHILETHISISNOTTREMENDOUSLYUSEFULINANDOFITSELF,WELLSEEHOWTODEVELOPTHISTECHNIQUEINLATEREXAMPLESWEALSODONTCHECKTHATTHESYSTEMCALLACTUALLYWORKEDFORTHESAKEOFSIMPLICITYINTHEEXAMPLEWHENWECOMPILEANDRUNTHISPROGRAM,SYSTEM1C,WEGETTHEFOLLOWINGBECAUSETHESYSTEMFUNCTIONUSESASHELLTOSTARTTHEDESIREDPROGRAM,WECOULDPUTITINTHEBACKGROUNDBYCHANGINGTHEFUNCTIONCALLINSYSTEM1C,CTOTHEFOLLOWINGSYSTEM“PSAXWHENWECOMPILEANDRUNTHISVERSIONOFTHEPROGRAM,WEGETHOWITWORKSINTHEFIRSTEXAMPLE,THEPROGRAMCALLSSYSTEMWITHTHESTRING”PSAX”,WHICHEXECUTESTHEPSPROGRAMOURPROGRAMRETURNSFROMTHECALLTOSYSTEMWHENTHEPSCOMMANDHASFINISHEDTHESYSTEMFUNCTIONCANBEQUITEUSEFULBUTISALSOLIMITEDBECAUSEOURPROGRAMHASTOWAITUNTILTHEPROCESSSTARTEDBYTHECALLTOSYSTEMFINISHES,WECANTGETONWITHOTHERTASKSINTHESECONDEXAMPLE,THECALLTOSYSTEMRETURNSASSOONASTHESHELLCOMMANDFINISHESBECAUSEITSAREQUESTTORUNAPROGRAMINTHEBACKGROUND,THESHELLRETURNSASSOONASTHEPSPROGRAMISSTARTED,JUSTASWOULDHAPPENIFWEHADTYPEDPSAXSYSTEM功能执行传递给它的命令并等待其完成。如果该命令SHCSTRING已经给了SHELL，其将被执行，如果SHELL不能启动SYSTEM，将返回127，而如果发生其他错误将返回1，否则SYSTEM将返回其命令的出口代码。尝试运行SYSTEM我们可以用SYSTEM来写个程序为我们运行PS，而这对它本身作用不大，我们会在后面的例子中弄清怎样发展这一技术，在该例子中为了简单性，我们也不用检测SYSTEM的实际工作。如果我们编译并运行SYSTEM1C，将会得到如下结果因为SYSTEM功能使用SHELL来启动目标程序，我们把SYSTEM1C功能变为如下后将其放入背景SYSTEM（“PSAX当我们编译和运行此类程序时将得到如下结果。它怎样工作在第一个例子中，用执行PS程序的PSAX呼叫SYSTEM，当PS命令停止时我们的程序返回到SYSTEM。SYSTEM功能非常有用也很有限。因为我们的程序必须等到SYSTEM命令停止时才能开始我们可以继续其他任务。第二个例子中，SYSTEM命令一等到SHELL命令结束就会返回，因为这是在背景中运行程序的要求，当PS程序开始时，SHELL便返回，如果我们输入PSAX便会发生SYSTEM1程序然后打印出DONE。这将会发生在PS命令有机会完成所有输出之前。当SYSTEM1存在后，PS继续输出，这种程序行为将迷惑使用者，为了更好的使用程序，我们需要更好的控制其行为，让我们看一下低水平程序创作EXEC。代替程序