Memory Management Functions

Transcription

1 Meory Mngeent Functions Chpter 9 Meory Mngeent Process of binding vlues to eory loctions Vlues y be sttic or dynic Vlues re ssigned t different plces Sttic eory Run-tie stck Hep 1

2 Meory Mngeent Sttic Meory Allocted to vlues whose storge requireents re known t copile tie Rein constnt throughout the life of the progr My be globl or locl to functions Meory Mngeent Run-tie Stck Pivotl structure in ctivtion of ethods Activtion A stck fre is pushed on top of stck Dectivtion A stck fre is popped fro the stck Storge spce for Locl vribles Actul preters Return vlues 2

3 Meory Mngeent Hep Storge for ll dyniclly llocted eory More unstructured Alloction nd delloction y hppen in rbitrry order Meory y becoe frgented Need for grbge collection We will describe grbge collection lgoriths Typicl Run-Tie Meory Structure 3

4 Run-Tie Meory Structure Sttic eory reins fixed throughout Stck grows nd shrinks in structured nner Stck pointer keeps trck of the beginning of current stck Stck size usully defined t copile tie Hep h usully defined t the beginning of run tie Soe progrs use little or no hep spce Soe chines llow h to vry Accessed vi pointers or references Run-Tie Meory Structure Address spce Rnge of ddresses vilble to the progr to use Likely logicl s opposed to physicl Rnge of ddresses 0 n Sttic re strts with 0 Top of the stck: Beginning of the hep : h 4

5 Run-Tie Meory Structure h nd n re defined t the beginning of runtie Size of hep spce y vry Bsed on the need of the progr Size of stck depends on the runtie behvior Level of nesting/recursion 0 h < n Otherwise the dreded stck overflow error Run-Tie Meory Structure Must nge eory explicitly when using stcks nd heps Defined using n Environent for n ctive ethod Pirs of vrible nes nd eory ddresses ccessible within the ethod Meory p Pirs of eory ddresses nd stored vlues Addresses y be unused (loctions to which no vribles hve been llocted) or undefined (loctions llocted vrible but no vlue yet ssigned) 5

6 Run-Tie Meory Structure int i=13 j=-1 k; inside ethod γ = { < i154 < j155 < k156 µ = {... < < < 156 undef... < -1undef < unused... < nunused σ = γ µ Address of vrible is given byγ (v): Vlue of vrible is given by µ(γ ( v)) = σ (v) γ (i) = 154 σ (i) = µ(γ (i)) = 13 γ (j) = 155 σ (j) = µ(γ (j)) = 1 Run-Tie Meory Structure Where cn the eory loction for vrible be? Sttic re Stck Hep (ccessed vi pointer) Addressing function for sttic nd stck vribles ust return vlue between 0 nd -1 0 ( v) 1 γ 6

7 7 Run-Tie Meory Allocte (using Stck) Done using declrtions Assue ech vrible cn fit in single eory loction k undef k undef undef k v v v d d d llocte k k + = + < + < < = + < + < < = = ' { ' { ' ' ' )... ( µ µ γ γ µ γ σ Environent updte uses regulr set union Allows vribles with se nes but different eory loctions Meory p updte uses overriding set union Well-defined s long s +k<h Run-Tie Meory Dellocte (using Stck) k unused k unused unused k v v v d d d dellocte k k = + < + < < = + < + < < = = ' { ' { ' ' ' )... ( µ µ γ γ µ γ σ Environent updte uses regulr set difference Meory p updte uses overriding set union Rel systes often skip this step for purposes of efficiency will just be overwritten with the next llocte

8 Methods Procedures Functions Lots of benefits for using ethods Abstrction Define procedure CoputeIt tht hides internl detils Ipleenttion Hiding Modify innrds of ethod without hving to chnge the clling code Modulrity Sller pieces better understood Esier to develop in isoltion Librries Extending the lnguge e.g. theticl functions or grphics Methods Activtion Invoking ethod Method P clls Method Q P is put on hold Control is pssed to Q Q generlly hs ccess to its own liited scope of vribles Q quits Control is sent bck to P Recursive : ultiple ctivtions of se ethod 8

9 Meory Mngeent All eory needed for ctivtion ust be llocted dyniclly. Stck is typiclly used Liittion Ipertive lnguges require procedures to be declred up front with preters. So we cn t crete functions on the fly e.g. hve the progr crete its own functions nd invoke the Typicl Procedure Activtion Cller evlutes the ctuls nd plces the vlue in the ctivtion record for cllee The stte infortion is sved (to return to cller) Cllee lloctes spce Locl vribles Teporry storge (expressions : interedites) Execute the body of cllee My cll nother procedure (next fre) 9

10 Procedure Activtion Control returns to the cller Return vlues is plced so cller cn find it On stck or in registers Restore the old stte Return control to cller Pop the stck (get redy for the next cll) Activtion Record or Stck Fre Locls Arguents Sttic link Link to the sttic re Dynic link Stck fre of cller Other dt to store Return Address Function result Tep Storge 10

11 Lyout of Activtion Records (C) Incoing Preter n. Incoing Preter 2 Incoing Preter1 This fre Cller Sves Fre Pointer Locl Vribles Teporry Storge Cllee Sves Outgoing Preters Incoing for next fre Next fre Locl vribles re referenced s n offset fro the Fre Pointer Coputed t copile tie Method Activtion Exple Sttic Scoping: Scope deterined by progr s sttic structure t copile tie 11

12 Method Activtion Exple Dynic scoping esier to see here e.g. for vrible i in Fre B Considered dngerous tody due to side effects Sttic Vribles Retin their vlues between ctivtions Storge for the is llocted stticlly t copile tie Non-sttic locl vribles hve dynic eory lloction t ctivtion tie 12

13 Exple: Exine Stck for the C Progr int br(int x) { int z=5; return z; int foo(int x) { int y=3; x = x + y; y = br(x); return x; int in(int rgc chr* rgv[]) { int =1 b=2 c=3; b = foo(); printf("%d %d %d\n"bc); return 0; Forl Description of Stck Alloction For C-like lnguge: Vlue ssocited with n ddress cn be Unused Undefined Int Boolen Address Address = Vlue tht references nother eory loction denoted where r is in {0 n Initil stte for previous C-Like progr σ 1 = Ø µ(0) = {<0unused<1unused After declring sttic vribles σ g = γ g µ = {<h0<i1 {<0undef<1undef<2unused 13

14 Stck Alloction After in begins execution: σ in = γ in µ Where γ in = {<h0<i1<slink2<dlink3<4<b5 µ = { <0undef <1undef <2@0 <3@0 <4undef<5undef<6unused <nunused = {<hundef<iundef<slink@0<dlink@0<undef<bundef Stte fter cll to ny ethod with np prs nd nd locls: σ = llocte(slinkdlink.prs.locls σ) = γ µ < slink < dlink + 1 γ = γ < p < pnp + np + 1 < di + np < dnd + np + nd + 1 Active eory: σ(slink in ) σ(dlink in ) Stck Alloction 0 < + 1@ 1 < + 2 v1... < + np + 1 vnp µ = µ ( ) < + np + 2 undef... < + np + nd + 1 undef < + np + nd + 2 unused... < n unused 14

15 Preter Pssing Mpping between forls nd ctuls Cll by vlue : pss the vlue Vlue preters e.g. pssing priitives in Jv Cll by reference : pss the ddress Reference preters e.g. pssing objects in Jv ctully lso cll by vlue since the vlue of n object is relly its ddress Cll by ne : pss the ne s is Ne preters Cll by Vlue Forl preter corresponds to the vlue of the ctul Evlute the rguent t the tie of cll Plce the vlue in the stck corresponding to the rguent No side effect w.r.t. rguents Esy to understnd Priry pssing echnis in C Pscl nd ost other lnguges 15

16 Cll by Vlue Exple swp(int xy); { int z; z=x; x=y; y=z; Does this work? Coon istke de in CS201 No vlues re chnged in clling procedure Cll by Reference Forl preter becoes synonyous with loction of the ctul preter This ens the ctul cn t be n expression Must be vrible or n ssignble coponent of dt structure Loction is coputed nd is pssed to the clled procedure 16

17 Cll by Ref Exple Sy this is pssed by reference Cll by Reference in C/C++ void swp( int *x int *y) { int z; z=*x;*x=*y; *y=; void swp( int &x int &y) { int z; z=x;x=y; y=; Invoction: int =1; b=2; swp(&&b); C Invoction: int =1; b=2; swp(b); C++ 17

18 swp(&&b) Cll by Reference C Pss the ddress of nd b Only preter pssing echnis in C is cll by vlue Cn get cll by reference by pssing pointers explicitly The * opertor is used to dereference pointer Cll by Vlue-Result Copy in /copy out Copy in phse Vlues nd loctions of ctuls coputed Copy vlue to beginning of the ctivtion record At this point just like cll by vlue Copy out phse Upon terintion of the ethod the finl vlues fro the storge loction on the ctivtion record re copied bck to the originl loctions End result is like cll by reference 18

19 Sentics of Cll nd Return We cn extend the ening function M for Clite to include Cll nd Return for ethods M(Cll s Stte σ) = dectivte(s.ne M(s.body ctivte(s.ne s.rgs σ))) In other words we perfor the following steps Activte the cll of s with rguents s.rgs by creting new stck fre nd estblishing the vlue/reference ssocitions between rguents nd preters Deterine the ening of the body s creted by the ctivtion Dectivte the cll by reoving the stck fre creted in the first step See text for detils we lredy sw llocting stck fre to dellocte we reove the newly dded eleents to γ nd µ vi set subtrction set the old vlues in µ to unused Pointers Coonly used in CC++ A pointer is eory ddress or reference to soe other vrible Defined using * by the vrible - used to dereference (*) nd ccess eber of the structure Why? Indirect ccess to dt Intended to point to objects of specific type Fixed size independent of type; single chine loction Efficiency : Avoid ove/copy lrge dt structures Dynic Dt : Allow dt structures to grow nd shrink during execution 19

20 C pointer exple Definition of Node struct Node { like : clss Node { int key; Node *next; int key; Node next; Node *hed *tep; hed=(node *)lloc(sizeof(node)); tep=(node *)lloc(sizeof(node)); (*hed).key = 1; hed-next = tep; hed-next-key = 3; tep=(node *)lloc(sizeof(node)); hed-next-next = tep; hed-next-next-key = 5; hed-next-next-next = null; key next key next 1 hed 3 key next 5 null Pointers // Serch for key x Node *p = hed; while ((p!= NULL) && (p-key!= x)) p=p-next; Pointers often viewed s the bne of relible softwre developent *p could point nywhere in eory e.g.: p=0; *p = 100; // Chnge contents of e[0] to 100 But llows for gret efficiencies. How cn we get by without pointers in Jv? 20