Stack vs Heap Memory Allocation - GeeksforGeeks

Memory in a C/C++/Java program can either be allocated on a stack or a heap. Prerequisite: Memory layout of C program. Stack Allocation: The ... 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13Jun,2022MemoryinaC/C++/Javaprogramcaneitherbeallocatedonastackoraheap.Prerequisite:MemorylayoutofCprogram.StackAllocation:Theallocationhappensoncontiguousblocksofmemory.Wecallitastackmemoryallocationbecausetheallocationhappensinthefunctioncallstack.Thesizeofmemorytobeallocatedisknowntothecompilerandwheneverafunctioniscalled,itsvariablesgetmemoryallocatedonthestack.Andwheneverthefunctioncallisover,thememoryforthevariablesisde-allocated.Thisallhappensusingsomepredefinedroutinesinthecompiler.Aprogrammerdoesnothavetoworryaboutmemoryallocationandde-allocationofstackvariables.ThiskindofmemoryallocationalsoknownasTemporarymemoryallocationbecauseassoonasthemethodfinishesitsexecutionallthedatabelongstothatmethodflushesoutfromthestackautomatically.Means,anyvaluestoredinthestackmemoryschemeisaccessibleaslongasthemethodhasn’tcompleteditsexecutionandcurrentlyinrunningstate.KeyPoints:It’satemporarymemoryallocationschemewherethedatamembersareaccessibleonlyifthemethod()thatcontainedthemiscurrentlyisrunning.Itallocatesorde-allocatesthememoryautomaticallyassoonasthecorrespondingmethodcompletesitsexecution.WereceivethecorrespondingerrorJava.lang.StackOverFlowErrorbyJVM,Ifthestackmemoryisfilledcompletely.Stackmemoryallocationisconsideredsaferascomparedtoheapmemoryallocationbecausethedatastoredcanonlybeaccessbyownerthread.Memoryallocationandde-allocationisfasterascomparedtoHeap-memoryallocation.Stack-memoryhaslessstoragespaceascomparedtoHeap-memory.CPPintmain(){   //Allthesevariablesgetmemory   //allocatedonstack   inta;   intb[10];   intn=20;   intc[n];}HeapAllocation:Thememoryisallocatedduringtheexecutionofinstructionswrittenbyprogrammers.Notethatthenameheaphasnothingtodowiththeheapdatastructure.Itiscalledheapbecauseitisapileofmemoryspaceavailabletoprogrammerstoallocatedandde-allocate.EverytimewhenwemadeanobjectitalwayscreatesinHeap-spaceandthereferencinginformationtotheseobjectsarealwaysstoredinStack-memory.Heapmemoryallocationisn’tassafeasStackmemoryallocationwasbecausethedatastoredinthisspaceisaccessibleorvisibletoallthreads.Ifaprogrammerdoesnothandlethismemorywell,amemoryleakcanhappenintheprogram.TheHeap-memoryallocationisfurtherdividedintothreecategories:-Thesethreecategorieshelpustoprioritizethedata(Objects)tobestoredintheHeap-memoryorintheGarbagecollection.YoungGeneration–It’stheportionofthememorywhereallthenewdata(objects)aremadetoallocatethespaceandwheneverthismemoryiscompletelyfilledthentherestofthedataisstoredinGarbagecollection.OldorTenuredGeneration–ThisisthepartofHeap-memorythatcontainstheolderdataobjectsthatarenotinfrequentuseornotinuseatallareplaced.PermanentGeneration–ThisistheportionofHeap-memorythatcontainstheJVM’smetadatafortheruntimeclassesandapplicationmethods.KeyPoints:WereceivethecorrespondingerrormessageifHeap-spaceisentirelyfull, java.lang.OutOfMemoryErrorbyJVM.ThismemoryallocationschemeisdifferentfromtheStack-spaceallocation,herenoautomaticde-allocationfeatureisprovided.WeneedtouseaGarbagecollectortoremovetheoldunusedobjectsinordertousethememoryefficiently.Theprocessingtime(Accessingtime)ofthismemoryisquiteslowascomparedtoStack-memory.Heap-memoryisalsonotthreaded-safeasStack-memorybecausedatastoredinHeap-memoryarevisibletoallthreads.SizeofHeap-memoryisquitelargerascomparedtotheStack-memory.Heap-memoryisaccessibleorexistsaslongasthewholeapplication(orjavaprogram)runs.CPPintmain(){   //Thismemoryfor10integers   //isallocatedonheap.   int*ptr =newint[10];}IntermixedexampleofbothkindofmemoryallocationHeapandStackinjava:JavaclassEmp{    intid;    Stringemp_name;     publicEmp(intid,Stringemp_name){        this.id=id;        this.emp_name=emp_name;    }} publicclassEmp_detail{    privatestaticEmpEmp_detail(intid,Stringemp_name){        returnnewEmp(id,emp_name);    }     publicstaticvoidmain(String[]args){        intid=21;        Stringname="Maddy";        Empperson_=null;        person_=Emp_detail(id,emp_name);    }}Followingaretheconclusionsonwhichwe’llmakeafteranalyzingtheaboveexample:Aswestartexecutionofthehaveprogram,alltherun-timeclassesarestoredintheHeap-memoryspace.Thenwefindthemain()methodinthenextlinewhichisstoredintothestackalongwithallit’sprimitive(orlocal)andthereferencevariableEmpoftypeEmp_detailwillalsobestoredintheStackandwillpointouttothecorrespondingobjectstoredinHeapmemory.ThenthenextlinewillcalltotheparameterizedconstructorEmp(int,String)frommain()andit’llalsoallocatetothetopofthesamestackmemoryblock.Thiswillstore:Theobjectreferenceoftheinvokedobjectofthestackmemory.Theprimitivevalue(primitivedatatype)intidinthestackmemory.ThereferencevariableofStringemp_nameargumentwhichwillpointtotheactualstringfromstringpoolintotheheapmemory.ThenthemainmethodwillagaincalltotheEmp_detail()staticmethod,forwhichallocationwillbemadeinstackmemoryblockontopofthepreviousmemoryblock.So,forthenewlycreatedobjectEmpoftypeEmp_detailandallinstancevariableswillbestoredinheapmemory.PictorialrepresentationasshownintheFigure.1below:Fig.1KeyDifferencesBetweenStackandHeapAllocations  Inastack,theallocationandde-allocationareautomaticallydonebythecompilerwhereasinheap,itneedstobedonebytheprogrammermanually.HandlingofHeapframeiscostlierthanthehandlingofthestackframe.Memoryshortageproblemismorelikelytohappeninstackwhereasthemainissueinheapmemoryisfragmentation.Stackframeaccessiseasierthantheheapframeasthestackhaveasmallregionofmemoryandiscache-friendly,butincaseofheapframeswhicharedispersedthroughoutthememorysoitcausesmorecachemisses.Astackisnotflexible,thememorysizeallottedcannotbechangedwhereasaheapisflexible,andtheallottedmemorycanbealtered.Accessingtimeofheaptakesismorethanastack.ComparisonChartParameterSTACKHEAPBasicMemoryisallocatedinacontiguousblock.Memoryisallocatedinanyrandomorder.AllocationandDe-allocationAutomaticbycompilerinstructions.Manualbytheprogrammer.CostLessMoreImplementationEasyHardAccesstimeFasterSlowerMainIssueShortageofmemoryMemoryfragmentationLocalityofreferenceExcellentAdequateSafetyThreadsafe,datastoredcanonlybeaccessedbyownerNotThreadsafe,datastoredvisibletoallthreadsFlexibilityFixed-sizeResizingispossibleDatatypestructureLinearHierarchicalPreferredStaticmemoryallocationispreferredinarray.Heapmemoryallocationispreferredinthelinkedlist.SizeSmallthanheapmemory.Largerthanstackmemory.MyPersonalNotes 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