Pointer Basics

COMP281 Lectures 2022-02-24

Pointers are a variable holding the address of another variable of the same data type.

You can use them to create:

• Shared variables between different functions without copying.
• Linked data structures such as linked lists.

Addresses vs Pointers

• You can get the address of a variable using &.
• You can store that address in a pointer using *.

int x = 9;
int *y = &x;

Variable	Address	Value
x	0xA0C1549	9
y	0xA0C1549	0xA0C1549

Pointers have their own addresses as they are a variable that holds an address.

The * Operator

The star operator has several functions:

• Multiplication:

    a = b * c;
  

• Declaring a pointer variable:

    int *a;
  

• Dereferencing a pointer:

    printf("%d", *a);
  

Dereferencing

This operation follows the pointer’s reference to get the value of its pointee.

printf("%d", *a);

If we didn’t dereference we will just get the value of the pointee’s address.

If you are pointing to a pointer that points to the value (and so on), you need to dereference an appropriate amount of times to get the value you want:

int x = 9;
int *y = &x;
int **z = &y;
printf("%d", **z);

Initialisation

If you are not sure about which variable’s address to assign to a pointer, use:

int *ptr = NULL;

This ensures that the pointer is not pointing to some random place in memory.

Call by Reference

To use a reference to a variable in a function, instead of calling by value, you can use the following:

void incr(int *z) {
	(*z)++;
}

int main(void) {
	int x = 10;
	incr(&x);
	printf("&d", &x);
	return 0;
}

The brackets need to be used around z: (*z)++ to avoid incrementing the pointer instead of the value.