Memory management in C programsWord格式.docx

Memory management in C programsWord格式.docx

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ThereareseveraltechniquesavailableformemorymanagementinC.ManyofthemareusedinNetHack3.4.3;

andevenmoreareusedsomewhereinNetHack4.Inthisblogpost,I'

dliketolookatthemanddiscusstheiradvantagesanddisadvantages.I'

mmostlyconcernedaboutcorrectness,ratherthanefficiency,here;

thatmeansthatunlesstheperformancedifferenceisverylarge,IcaremoreaboutcleancodethanIdoaboutfastcode.

Techniquesforallocatingfixedamountsofmemory

Therearebasicallytwodifferentissueshere:

trackingthelifetimeofmemoryallocations,andensuringthatthey'

retherightsize.Assuch,I'

llstartbylookingatallocationswhosesizeisstaticallyknownatcompiletime,andthenmoveontotechniquesthatcanhandlethesizebeingunknown.

Fixed-sizebuffersonthestack

ProbablythesimplestwaytoallocatememoryinCistouseastackallocation;

ifavariableisdeclaredinsideafunctionwithoutspecifyingstaticorextern,enoughmemoryforthatvariablewillbeallocatedwhenthatvariable'

sscopestarts,andfreedagainwhenthatvariable'

sscopeends（sometimesknownasan"

automaticvariable"

）.BecausescopesinCarewell-nested,atypicalCimplementationwillimplementthisviausingastackofmemory;

newlyallocatedvariableswillbegivenspacefromthe"

top"

ofthestack,andthatspacewillbereleasedfromthetopofthestackwhentheyleavescope.（"

Top"

isinquotesbecauseit'

sfarfromrareforastacktobeupside-downinC,withthetopbeingtheendwiththelowestmemoryaddress.）

Therearesomemajoradvantages（andsomeminoradvantages）ofdoingthingsthisway:

∙Itdoesnotrequireanyextrastate,beyondthatprovidedbytheimplementation,towork.Quiteoften,memoryallocationschemeswillthemselvesneedmemorytowork,leadingtosomethingofaninfiniteregress.Stackallocationsareoftenusefultoprovidethatmemory.

∙Unlikethevastmajorityofmemoryallocationschemes,itisexception-safewithnoextraeffort:

longjmpisawareofstackallocations,andwillautomaticallyfreethemfromanyscopeitjumpsoutof.（Somecareisneededinthefunctioncontainingsetjmpitself;

anymemorythatneedstopersistpastalongjmpmustbemarkedasvolatile,arequirementsetbythestandardinordertoallowcompilerstooptimizearoundlongjmpcallsmoreeasily.Technicallyspeaking,thememorydoesnotneedtobemarkedasvolatileifitsvaluedoesnotchangebewtweensetjmpandthematchinglongjmp;

inpractice,althoughcompilerscanhandlethiscase,theytendtospewwarnings,soitmakessensetomarkthememoryasvolatileifitisreadafterthelongjmpatall.）

ThismostlydoesnotmatterinNetHack3.4.3,whichdoesnotuseanyexception-likebehaviour（ifsomethinggoeswrong,itcallspanicwhichattemptstoproduceasavefile,andthenexitstheentireprocess,freeingallthememorythatway）.However,NetHack4usesexception-likebehaviourinternally（implementedusinglongjmpandsetjmp,becauseCdoesnothavetrueexceptions）;

NitroHack（andthusNetHack4）usesexceptionsinordertoexitagamewithoutexitingtheprocess.FromNetHack4.3（thecurrentdevelopmentversion,whichisunreleasedbutavailableusingthesavebreakbranchoftherepository）onwards,theyareusedevenmoreheavily,handlingsituationslikerewindingthegamemid-turn.

∙Itworkscorrectlyinthepresenceofmultithreading,recursion,andasynchronoussignals.

∙ThistechniquedoesnotrequireanyglobalstateapartfromthatmanagedbytheCcompiler;

thus,itisimpossibleto,forinstance,forgettosaveavariablestoredinastackallocation（unlessthesavecodeisrunninginsideascopecontainingit）,ortohaveitaccidentallypersistfromonegameintothenext.

∙Ifyourcodedoesendupoverflowingafixed-sizedbuffer,yourprogramisgoingtobeincorrectnomatterwhat,butifthatbufferisallocatedonthestack,itmaximizesthechancethatyourcompilerwillbeabletogeneratecodetocatchthebufferoverflow（especiallyiftheoverflowhappensinacalltoastandardstringmanipulationfunctionlikesprintforstrcat）.Itisincrediblyunwisetorelyonthistoprotectyourcodefromexploits,butit'

saratherhelpfulfeaturefordebugging.（Staticallyallocatedmemoryisalmostasgoodinthisrespect,butdynamicallocationsarehopeless.）

Itisthusnosurprisethatstackalloc