Different valid uses of Pointers

The following examples help to distinguish between the use of a pointer and of the pointer's value:

void main()
{
int *p, *q;

p = (int *)malloc(sizeof(int));
q = p;
*p = 10;
printf("%d\n", *q);
*q = 20;
printf("%d\n", *q);
}

The final output of this code would be 10 from line 4 and 20 from line 6. Here's a diagram:


The following code is slightly different:

void main()
{
int *p, *q;

p = (int *)malloc(sizeof(int));
q = (int *)malloc(sizeof(int));
*p = 10;
*q = 20;
*p = *q;
printf("%d\n", *p);
}

The final output from this code would be 20 from line 6. Here's a diagram:


Notice that the compiler will allow *p = *q, because *p and *q are both integers. This statement says, "Move the integer value pointed to by q into the integer value pointed to by p." The statement moves the values. The compiler will also allow p = q, because p and q are both pointers, and both point to the same type (if s is a pointer to a character, p = s is not allowed because they point to different types). The statement p = q says, "Point p to the same block q points to." In other words, the address pointed to by q is moved into p, so they both point to the same block. This statement moves the addresses.

From all of these examples, you can see that there are four different ways to initialize a pointer. When a pointer is declared, as in int *p, it starts out in the program in an uninitialized state. It may point anywhere, and therefore to dereference it is an error. Initialization of a pointer variable involves pointing it to a known location in memory.

1. One way, as seen already, is to use the malloc statement. This statement allocates a block of memory from the heap and then points the pointer at the block. This initializes the pointer, because it now points to a known location. The pointer is initialized because it has been filled with a valid address -- the address of the new block.

2. The second way, as seen just a moment ago, is to use a statement such as p = q so that p points to the same place as q. If q is pointing at a valid block, then p is initialized. The pointer p is loaded with the valid address that q contains. However, if q is uninitialized or invalid, p will pick up the same useless address.

3. The third way is to point the pointer to a known address, such as a global variable's address. For example, if i is an integer and p is a pointer to an integer, then the statement p=&i initializes p by pointing it to i.

4. The fourth way to initialize the pointer is to use the value zero. Zero is a special values used with pointers, as shown here:

p = 0;

or:

p = NULL;

What this does physically is to place a zero into p. The pointer p's address is zero. This is normally diagrammed as:


Any pointer can be set to point to zero. When p points to zero, however, it does not point to a block. The pointer simply contains the address zero, and this value is useful as a tag. You can use it in statements such as:

if (p == 0)
{
...
}

or:

while (p != 0)
{
...
}

The system also recognizes the zero value, and will generate error messages if you happen to dereference a zero pointer. For example, in the following code:

p = 0;
*p = 5;

The program will normally crash. The pointer p does not point to a block, it points to zero, so a value cannot be assigned to *p. The zero pointer will be used as a flag when we get to linked lists.

The malloc command is used to allocate a block of memory. It is also possible to deallocate a block of memory when it is no longer needed. When a block is deallocated, it can be reused by a subsequent malloc command, which allows the system to recycle memory. The command used to deallocate memory is called free, and it accepts a pointer as its parameter. The free command does two things:

1. The block of memory pointed to by the pointer is unreserved and given back to the free memory on the heap. It can then be reused by later new statements.
2. The pointer is left in an uninitialized state, and must be reinitialized before it can be used again.

The free statement simply returns a pointer to its original uninitialized state and makes the block available again on the heap.