CSSE 332CSSE 332Explicit Memory Explicit Memory

Allocation, Parameter Allocation, Parameter passing, and GDBpassing, and GDB

22

Example 11#include <stdio.h>

int main(int argc, char *argv[]){int *ptr;/* allocate space to hold an int */ptr = (int*)malloc(4 * sizeof(int));

/* do stuff with the space */*ptr=4; //ptr[0] = 4;

/* free up the allocated space */free(ptr);return 0;

}

Explicit memory allocation Explicit allocation and de-allocationExplicit allocation and de-allocation

33

int *ptr;

?

ptr

4000

ptr = (int*)malloc(4 * sizeof(int));

??? ?

6012600860046000

6000

*ptr=4;

4

free(ptr);

Explicit memory allocation

44

Dynamic arrayDynamic array

int *ptr, i,size;int *ptr, i,size;

printf(“Enter the size of the array”);printf(“Enter the size of the array”);

scanf(“%d”, &scanf(“%d”, &sizesize))

ptr = (int *) malloc( ptr = (int *) malloc( size * size * sizeof(int)sizeof(int) ); );

for(i=0; i<size; i++){for(i=0; i<size; i++){

ptr[i] = i;ptr[i] = i;

}}

55

Array of PointersArray of Pointers Variable length stringsVariable length strings

/* card[4] => array of 4 elements/* card[4] => array of 4 elements char* => element is a pointer to a character.char* => element is a pointer to a character. *card[4] => array of 4 pointers */*card[4] => array of 4 pointers */char *card[4]; char *card[4];

card[3]

4012

card[2]

4008

card[1]

4004

card[0]

4000 NULL

NULL

NULL

NULL

66

card[0] = card[0] = (char*)malloc(6*sizeof(char));(char*)malloc(6*sizeof(char));

card[1] = card[1] = (char*)malloc(3*sizeof(char)); (char*)malloc(3*sizeof(char));

. . . . . .

Static allocation of a 2D array:Static allocation of a 2D array:

char card[4][10]; char card[4][10]; //waste of //waste of spacespace

Array of PointersArray of Pointers

77

Common errors - Memory Common errors - Memory leakleak

int *ptr, x; int *ptr, x; //ptr points to newly allocated memory//ptr points to newly allocated memory

ptr = (int*)malloc(10*sizeof(int)); ptr = (int*)malloc(10*sizeof(int)); // ptr now points away from allocated memory // ptr now points away from allocated memory

ptr = &x; ptr = &x;

Memory allocated with malloc is no longer Memory allocated with malloc is no longer available for use by the program.available for use by the program.

Released only when program quits.Released only when program quits. Becomes a problem in large programs where a Becomes a problem in large programs where a

large number of variables are created and large number of variables are created and destroyed during the execution of the program.destroyed during the execution of the program.

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Common errors - Dangling Common errors - Dangling pointerspointers

int *i, *x;int *i, *x;i = (int*)malloc( 5 x sizeof(int));i = (int*)malloc( 5 x sizeof(int));x = i; x = i; //both point to the same address.//both point to the same address.

/* both i and x are dangling pointers; trying to /* both i and x are dangling pointers; trying to accessaccess

either of them can cause logical errors */either of them can cause logical errors */

free(x);free(x);

x = NULL;x = NULL; // One way to prevent incorrect access // One way to prevent incorrect access i = NULL;i = NULL;

99

Functions – pointers as arguments

Example 12#include <stdio.h>int sumAndInc(int *pa, int *pb,int* pc);

int main(int argc, char *argv[]){ int a=4, b=5, c=6; int *ptr = &b;

/* call the function */ int total = sumAndInc(&a,ptr,&c);

printf(“The sum of %d and %d is %d and c is %p\n”,

a, b, total, c);}/* pointers as arguments */int sumAndInc(int *pa, int *pb,int *pc ){ *pc = *pc+1; // return a pointee value; NOT *(pc+1)

return (*pa+*pb); /* return by value */}

1010

a

4

4000

b

5

4004

c

6

4008

ptr

4004

4012

In main()

pa

4000

6000

pb

4004

6004

pc

4008

6008

In function

1111

a

4

4000

b

5

4004

c

7

4008

ptr

4004

4012

In main() after the function call

1212

What’s wrong with this ?Example 13#include <stdio.h>

void DoSomething(int *ptr);

int main(int argc, char *argv[]) {int *p;DoSomething(p);printf(“%d”, *p); /* will this work ? */return 0;

}/* passed and returned using pointers */void DoSomething(int *ptr){ int temp= 5+3; ptr = &temp;}

/* compiles correctly, but gives incorrect output */

1313

p

?

4000

ptr

6000

?temp

6004

86004

In main()

In the function

1414

p

?

4000

In main() after the function call

int main(int argc, char *argv[]) {int main(int argc, char *argv[]) {int *p;int *p;DoSomething( DoSomething( &p&p ); );printf(“%d”, *p); printf(“%d”, *p); /* will this work ? *//* will this work ? */return 0;return 0;

}}/* passed and returned using pointers *//* passed and returned using pointers */void DoSomething(int void DoSomething(int **ptr**ptr){ ){ int temp= 8;int temp= 8; *ptr = (int*)malloc(sizeof(int));*ptr = (int*)malloc(sizeof(int)); **ptr = temp;**ptr = temp;}}

1616

p

?

4000

ptr

6000

temp

6004

84000

In main()

In the function

?

8000

On the heap8000

8

1717

p

?

4000

In main()

In the function

?

8000

On the heap8000

8

1818

Functions - Passing and returning arraysExample 14#include <stdio.h>

void init_array( int array[], int size ) ;

int main(int argc, char *argv[] ){ int list[5]; init_array(list, 5); //No & because list //is already an address for (i = 0; i < 5; i++) printf(“next:%d”, list[i]);}

/* why size ? */void init_array(int array[], int size) {

/* arrays ALWAYS passed as pointers */ int i; for (i = 0; i < size; i++) array[i] = i; }

1919

Passing/Returning a structure

/* pass struct by value */void displayYear_1(struct birthday mybday) {

printf(“I was born in %d\n”, mybday.year);}/* - inefficient: why ? */

/* pass pointer to struct */void displayYear_2(struct birthday *pmybday) {

printf(“I was born in %d\n”, pmybday->year);/* Note: ‘->’, not ‘.’, after a struct pointer */

}

/* return struct by value */struct birthday get_bday(void){

struct birthday newbday;newbday.year=1971; /* ‘.’ after a struct */

return newbday;}/* - also inefficient: why ? */

2020

GNU debugger (gdb)GNU debugger (gdb)

Tutorial and reference sheet on class Tutorial and reference sheet on class websitewebsite– ……/Homework/Sec-01/gdbtutorial.htm/Homework/Sec-01/gdbtutorial.htm– ……/Homework/Sec-01/gdb-reference-/Homework/Sec-01/gdb-reference-

card.pdfcard.pdf

Homework 4 and 5Homework 4 and 5

More practice on C.More practice on C. Available on class’s Angel pageAvailable on class’s Angel page

– follow link from the schedule pagefollow link from the schedule page

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