50 Chapter 4 Memory Managemen t In Ja v a the task of - PDF document

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Chapter 4 Memory Managemen t In Ja v a the task of memory managemen t is largely conducted in the bac kground, just b e- y ond the issues of concern to the t ypical programmer. This b e an adv an tage, as it c an remo v es a complex task from the programmers range of vision, th us allo wing him or her to concen trate on more application sp eci�c details. On the other hand, it also means that the Ja v a programmer has little con trol o v er ho w memory managemen t is p erformed. W arning Using C , in con trast, memory managemen t is explicitly under the direction of the ++ programmer. If prop erly used this can w ork to the programmers adv an tage, p ermitting The C pr o- ++ gr ammer m uc h more e�cien t use of memory than is p ossible with Ja v a. But if improp erly used (or, must always more often, ignored) memory managemen t issues can bloat a program at run-time, or mak e b e awar e a program v ery ine�cien t. F or this reason, to write e�ectiv e C programs it is imp ortan t ++ of how mem- to understand ho w the memory managemen t system op erates. ory manage- Because in C the programmer is resp onsible for memory managemen t, there are a ++ ment is b eing v ariet y of errors that are p ossible in C programs but are rare or un usual in Ja v a. These ++ p erforme d include the follo wing: � Using a v alue b efore it has b een initialized. (The Ja v a language requires a data�o w analysis that will detect man y of these errors. The C language mak es no suc h W arning ++ requiremen t). C do es not ++ p erform � Allo cating memory for a v alue, then not deleting it once it is no longer b eing used. data�ow (This will cause a long running program to consume more and more memory , un til it analysis fails in a catastrophic fashion.) to dete ct p o- tential ly un- � Using a v alue after it has b een freed. (Once returned to the memory managemen t de�ne d lo c al system, the con ten ts of a freed lo cation are t ypically o v erwritten in an unpredictable variables fashion. Th us, the con ten ts of a freed v alue will b e unpredictable.) Eac h of these p oten tial errors will b e illustrated b y examples in the remainder of this c hapter. 51

52 CHAPTER 4. MEMOR Y MANA GEMENT 4.1 The Memory Mo del T o understand ho w memory managemen t is p erformed in C it is �rst necessary to appre- ++ ciate the C memory mo del, whic h di�ers in a fundamen tal fashion from the Ja v a memory ++ mo del. In man y w a ys, the C mo del is m uc h closer to the actual mac hine represen tation ++ than is the Ja v a memory mo del. This closeness p ermits an e�cien t implemen tation, at the exp ense of needing increased diligence on the part of the programmer. Define In C there is a clear distinction b et w een memory v alues that are esident , and stack-r ++ Stack- those that are esident . Stac k residen t v alues are those created automatically when a he ap-r r esident val- pro cedure is en tered or exited, while heap residen t v alues m ust b e explicitly created using ues ar e the new op erator. In the follo wing sections w e will describ e some of the c haracteristics of cr e ate d when b oth of these, and the problems the programmer should lo ok out for. a pr o c e dur e is enter e d, and deletete d 4.2 Stac k Residen t Memory V alues when the pr o- c e dur e exits Both the stac k and the heap are in ternal data structures managed b y a run-time system. Note A n Of the t w o, the stac k is m uc h more orderly . V alues on a stac k are strongly tied to pro cedure acti- en try and exit. When a pro cedure b egins execution, a new section of the stac k is created. vation r e c or d This stac k segmen t holds parameters, the return address for the caller, sa v ed in ternal reg- is sometimes isters and other mac hine sp eci�c information, and space for lo cal v ariables. The section of c al le d a stac k sp eci�cally dev oted to one pro cedure is often termed an d . activation r e c or an activation In Ja v a, only primitiv e v alues are truly stac k residen t. Ob jects and arra ys are alw a ys fr ame, or a allo cated on the heap. Compare the follo wing t w o example pro cedures. Eac h creates a lo cal stack fr ame in teger, a lo cal arra y , and an ob ject instance. Ja v a void test () // f int i; int a[ ] = new int[10]; anObject ao = new anObject(); ... g void test () // C++ f int i; int a[10]; anObject ao; ... g

4.2. ST A CK RESIDENT MEMOR Y V ALUES 53 In the Ja v a pro cedure only the primitiv e in teger v alue is actually allo cated space on the stac k b y the declaration. Both the arra y and the ob ject v alue m ust b e explicitly created (as heap-residen t v alues) using the new op erator. In C , on the other hand, all three v alues ++ will reside on the stac k, and none require an y actions b ey ond the declaration to bring them in to existence. Pro cedure in v o cations execute in a v ery orderly fashion. If pro cedure A in v ok es pro ce- dure B, and B in turn in v ok es C, then pro cedure C m ust terminate b efore B will con tin ue with execution, and B in turn m ust terminate b efore A will resume. This prop ert y allo ws activ ation records to b e managed in a v ery e�cien t and orderly manner. When a pro cedure is called, an activ ation record is created and pushed on the stac k, and when the pro cedure returns, the activ ation record can b e p opp ed from the stac k. A p oin ter can b e used to p oin t to the curren t top of stac k. Allo cation of a section of the stac k then simply means mo ving this p oin ter v alue forw ard b y the required amoun t. The e�ciencies of stac k memory are not without cost. There are t w o ma jor dra wbac ks to the use of the stac k for v ariable storage: Define � The lifetime of stac k residen t memory v alues is tied to pro cedure en try and exit. This means that stac k residen t v alues cease to exist when a pro cedure returns. An attempt The lifetime is the to use a stac k residen t v alue after it has b een deleted will t ypically result in error. p erio d of time a value c an b e � The size of stac k residen t memory v alues m ust b e kno wn at compile time, whic h is use d when the structure of the activ ation record is laid out. The follo wing sections will describ e some of the implications of these t w o issues. 4.2.1 Lifetime errors An imp ortan t fact to remem b er is that once a pro cedure returns from execution, an y stac k residen t v alues are deleted and no longer accessible. A r efer enc e to suc h a v alue (see Chap- ter 3) will no longer b e v alid, although it ma y for a time app ear to w ork. Here are some examples. // W ARNING { Program con tains an error char � readALine () f declare a bu�er for the line char buffer[1000]; // gets(buffer); // read the line return buffer; // return text of line g