CSE 30: Computer Organization and Systems Programming

Lecture 10: Dynamic memory allocation:

A cautionary taleC struct

Diba MirzaUniversity of California, San Diego

1

Dynamic memory allocation

void* realloc (void * p, size_t s);

• Resize an existing block of memory pointed to by ‘p’ to a total

size of ‘s’ bytes.

• On failure: returns NULL on failure

• Are the contents copied over to the new block?

2

Dynamic memory allocation

void free(void * ptr);

• Frees the heap block pointed to by p

• What happens in the following code?

int *p=malloc(8);

free(p);

free(p);

3

Dangling pointers and memory leaks

– Dangling pointer: Pointer points to a memory

location that no longer exists

– Memory leaks (tardy free): Memory allocated by

program is not freed.

• The problem is particularly acute if memory allocated

in heap can no longer be accessed

Q: Which of the following functions returns a dangling pointer?

int * f1(int num){ int *mem1 =(int *)malloc(num*sizeof(int));return(mem1);

}

A. f1

B. f2

C. Both

int * f2(int num){int mem2[num];return(mem2);}

Which of the following is an example of a dangling pointer?

A. void foo(int bytes) {char *ch =(char *) malloc(bytes);ch[0]=‘a’;free (ch);. . . .

}

B. int * foo(int bytes) {int i=14;. . . .return (&i);

}main () {

int *p = foo(10);}C. char* foo(int bytes) {

char *ch =(char *) malloc(bytes);return (ch);

}

D. A combination of the above

Memory Leak?int * createArray (int n) {

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

return ptr;

}

void main() {

int *arr=createArray(5);

}

We have a memory leak! Why?

Memory Leak?int * createArray (int n)

{

int *ptr = malloc(size*sizeof(int)); //line 1

return ptr;

}

int * resizeArray (int *ptr, int n)

{

ptr = realloc(ptr, n*sizeof(int)); //line 2

return ptr;

}

void main() {

int *arr=createArray(5); //line 3

arr= resizeArray(arr, 10); //line 4

free(arr);

}

Which of the following is true about the given code?A. There is a definite memory leak B. There is a possible memory leakC. There is no memory leak

C structures : Overview

• A struct is a data structure composed of simpler data types.

–Like a class in Java/C++ but without methods or inheritance.

struct point {int x;int y;

}

void PrintPoint(struct point p){

printf(“(%d,%d)”, p.x, p.y);}

Pointers to structures

• The C arrow operator (->) dereferences and extracts a structure

field with a single operator.

• The following are equivalent:

struct point *p;

printf(“x is %d\n”, (*p).x);printf(“x is %d\n”, p->x);

Representation in memorystruct p {

int y;

char x;};

struct p sp;

x (1byte)y (4 bytes)

sp

0x100 0x104 0x105