Introduction to Introduction to ProgrammingProgramming

Lecture 24Lecture 24

Today’s Today’s AgendaAgenda Memory AllocationMemory Allocation

– Dynamic memory allocationDynamic memory allocation– Advantages/disadvantages of Advantages/disadvantages of

Dynamic and static memory Dynamic and static memory allocationallocation

– Common programming errors Common programming errors while using Dynamic memory while using Dynamic memory allocationallocation

Static Memory Static Memory AllocationAllocation

int i , j , k ;int i , j , k ;

char s [ 20 ] ;char s [ 20 ] ;

Compile Time Compile Time AllocationAllocation

Dynamic Dynamic Memory Memory

AllocationAllocation

HeapHeap

PointersPointers

void void PointerPointer

int *i ; int *i ;

char *s ;char *s ;

i is a pointer to an integer

void void *ptr ;*ptr ;

CastCast

void *ptr ;void *ptr ;

( int * ) ( int * ) ptr ;ptr ;

NULLNULL

calloc ( n , m ) calloc ( n , m ) ;;

Space in terms of numbers of elements

Space in terms of size each of elements

calloc ( 1000 , sizeof calloc ( 1000 , sizeof ( int ) ) ;( int ) ) ;

( int * ) calloc ( 1000 , sizeof ( int ) ) ;( int * ) calloc ( 1000 , sizeof ( int ) ) ;

void * calloc ( size_t n , size_t el-size ) ;void * calloc ( size_t n , size_t el-size ) ;

Example 1Example 1int *iPtr ;int *iPtr ;

iPtr = ( int * ) calloc ( 1000 , sizeof ( int ) ) ;iPtr = ( int * ) calloc ( 1000 , sizeof ( int ) ) ;

if ( iPtr == NULL )if ( iPtr == NULL )

exit ( ) ;exit ( ) ;

void * void * malloc ( n ) ;malloc ( n ) ;

Number of bytes required

malloc (1000 *sizeof ( int ) ) ) ;malloc (1000 *sizeof ( int ) ) ) ;

malloc ( n ( sizeof ( float ) ) ) ;malloc ( n ( sizeof ( float ) ) ) ;

Static Memory Static Memory AllocationAllocation

#define MAXSTUDENT 100#define MAXSTUDENT 100

int Student [ MAXSTUDENT ] ;int Student [ MAXSTUDENT ] ;

Problem Problem statementstatement

Find the average age of Find the average age of

the students in your classthe students in your class

Example 2Example 2int numstd ;int numstd ;int *iPtr , *sPtr ;int *iPtr , *sPtr ;cout << "Enter the number of students " << endl ;cout << "Enter the number of students " << endl ;cin >> numstd ;cin >> numstd ;iPtr = malloc ( numstd * ( sizeof ( int ) ) ) ;iPtr = malloc ( numstd * ( sizeof ( int ) ) ) ;if ( iPtr == NULL )if ( iPtr == NULL ){{        cout << "Error on malloc " ;        cout << "Error on malloc " ;        return 1 ;        return 1 ;                /* Use a nonzero return to indicate an error has occurred *//* Use a nonzero return to indicate an error has occurred */}}

// a while loop to read the ages of the student and place them in // a while loop to read the ages of the student and place them in the memorythe memory sPtr = iPtr ;sPtr = iPtr ; sPtr++ ;sPtr++ ;

free ( iPtr ) free ( iPtr ) ;;

realloc(void *iPtr, size_t size);realloc(void *iPtr, size_t size);

UnreferenceUnreferenced Memoryd Memory

Memory Memory LeaksLeaks

Example Example main ( )main ( ){{

funct ( ) ;funct ( ) ;}}funct ( )funct ( ){{ int *iPtr ;int *iPtr ;

iPtr = malloc ( 1000 * ( sizeof ( int ) ) ) ; iPtr = malloc ( 1000 * ( sizeof ( int ) ) ) ; // used the memory// used the memory

}}

Dangling Dangling PointersPointers

int *ptr1 , *ptr2 ;int *ptr1 , *ptr2 ;

ptr1 = malloc (1000 * ( sizeof ( int ) ) ) ; ptr1 = malloc (1000 * ( sizeof ( int ) ) ) ;

ptr2 = ptr1 ;ptr2 = ptr1 ;

- - -- - -

free ( ptr1 ) ;free ( ptr1 ) ;

ExampleExample

Multi-Multi-taskingtasking

ReviewReview Dynamic Memory AllocationDynamic Memory Allocation

– Efficient usage of computers Efficient usage of computers

resourcesresources– Have to do memoryHave to do memory

managementmanagement