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What is a Pointer?

A pointer is a variable which contains the address in memory of another variable. We can have a pointer to any variable type.

The unary or monadic operator & gives the ``address of a variable''.

The indirection or dereference operator * gives the ``contents of an object pointed to by a pointer''.

To declare a pointer to a variable do:

int *pointer;

NOTE: We must associate a pointer to a particular type: You can't assign the address of a short int to a long int, for instance.

Consider the effect of the following code:

It is worth considering what is going on at the machine level in memory to fully understand how pointer work. Consider Fig. . Assume for the sake of this discussion that variable x resides at memory location 100, y at 200 and ip at 1000. Note A pointer is a variable and thus its values need to be stored somewhere. It is the nature of the pointers value that is new.

Fig. Pointer, Variables and Memory

Now the assignments x = 1 and y = 2 obviously load these values into the variables. ip is declared to be a pointer to an integer and is assigned to the address of x (&x). So ip gets loaded with the value 200.

Next y gets assigned to the contents of ip. In this example ip currently points to memory location 200 - the location of x. So y gets assigned to the values of x - which is 1.

We have already seen that C is not too fussy about assigning values of different type. Thus it is perfectly legal (although not all that common) to assign the current value of ip to x. The value of ip at this instant is 200.

Finally we can assign a value to the contents of a pointer (ip).

IMPORTANT: When a pointer is declared it does not point anywhere. You must set it to point somewhere before you use it.

So ...

will generate an error (program crash!!).

The correct use is:

We can do integer arithmetic on a pointer:

NOTE: A pointer to any variable type is an address in memory - which is an integer address. A pointer is definitely NOT an integer.

The reason we associate a pointer to a data type is so that it knows how many bytes the data is stored in. When we increment a pointer we increase the pointer by one ``block'' memory.

So for a character pointer ++ch_ptradds 1 byte to the address.

For an integer or float ++ip or ++flp adds 4 bytes to the address.

Consider a float variable (fl) and a pointer to a float (flp) as shown in Fig. .

Fig. Pointer Arithmetic

Assume that flp points to fl then if we increment the pointer (++flp) it moves to the position shown 4 bytes on. If on the other hand we added 2 to the pointer then it moves 2 float positions i.e 8 bytes as shown in the Figure.

Next: Pointer and Functions Up: Pointers Previous: Pointers
Wed Sep 14 10:06:31 BST 1994