Beyond volatile

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1、BeyondvolatileorHowtosavedaysofdebuggingtimeWhendebuggingembeddedapplications,intermittentproblemsareamongthemoredifficultones.Thistextdiscussesonesourceofsuchproblems:variablesthatareaccessedasynchronously,e.g.,byinterruptroutinesorbycoderunninginseparatethreads.Suchvariablesmu

2、stbeproperlydefinedandhaveadequateprotection.Thedefinitionmustincludethevolatilekeyword—thisinformsthecompilerthatthevariablecanbechangedfromotherplacesthanthecurrentlyexecutingthread.Thecompilerwillthenavoidcertainoptimizationsonthisvariable,obeyingtheaccessestothevariableasgiv

3、eninthesourcecodeandmakingsurethevariableisstableateach"sequencepoint".Thusthecompilerwillnotrelyonknowledgeofvaluesfrompreviousaccessestoreusethesameregister,moveoperationsacrosssequencepointsandsoon.Toprotectasharedvariable,eachaccesstothevariablemustalsobemadeatomicfromothera

4、ccesses.ReadingfromorwritingtoavariableinCisnotguaranteedtobeanatomicoperationassuch.Accessinga32-bitvariableonan8-bitarchitecturewouldtypicallybenon-atomic,aswouldanyobjectreadorwritethatrequiresmorethanoneinstruction.EvenifyouinspectthecompilergeneratedcodeandfindittobeOK,itis

5、notcertainthatthenextcompilationwillgivethesameresultifyouhavemadechangestothecodeortothecompilersettings.CExampleAtypicalexampleofasharedvariableiswhenaninterruptroutineupdatesatime-keepingvariablethatisreadfrommaincode.IftheInterruptServiceRoutine(ISR)runswithinterruptsdisable

6、d(whichoftenisthecase),theinterruptfunctionisatomic.Inotherwords,itwillnotbeinterruptedandthusthisaccessshouldnotneedfurtherprotection.NotethatforthebelowISRyouwillalsoneedtoenableandinitializetheinterrupt,butthatisnotofinterestforthisexample.volatilelongtick_count=0;__interrupt

7、voidtick_timer(){tick_count++;}Themaincodeontheotherhandmustpreventtheinterruptfromhappeningwhilethevariableisbeingaccessed.Asimplifiedunsafeexamplewouldbe:...if(tick_count>=next_stop)/*Unsafe!*/{next_stop+=100;do_stuff();}Thiscodeisunsafeasreadingtick_countisnotguaranteedtobean

8、atomicoperation.Atick_timerinterruptmayoccurwhe