2.3 Pointers and arrays

From the beginning, we only show how to access or change directly the values of variables through their names. However, the C language provides the developers an effective method to access variables - pointer.

A pointer is a variable that contains the address of a variable. Pointers are much used in C, partly because they are sometimes the only way to express a computation, and partly because they usually lead to more compact and efficient code than can be obtained in other ways. Pointers and arrays are closely related; this Unit also explores this relationship and shows how to exploit it.

Pointers and addresses

Let us begin with a simplified picture of how memory is organized. A typical machine has an array of consecutively numbered or addressed memory cells that may be manipulated individually or in contiguous groups. One common situation is that any byte can be a char, a pair of one-byte cells can be treated as a short integer, and four adjacent bytes form a long.

Any variable in a program is stored at a specific area in memory. If you declare a variable, the compiler will allocate this variable to some consecutive memory cells to hold the value of the variable. The address of the variable is the address of the first memory cell.

One variable always has two properties:

• The address of the variable
• The value of the variable.

Consider the following Example

Type of these two variables is integer so they are stored in 2-byte memory area. Suppose that the compiler allocates i at the FFEC address in memory and j in FFEE, we have:

Two different variables have different addresses. The i = j assignment affects only on the value of variables, that means the content of the memory area for j will be copied to the content of the memory area for i.

Pointers

A pointer is a group of cells (often two or four) that can hold an address. So if c is a char and p is a pointer that points to it, we could represent the situation this way:

Pointer declaration

If you declare a variable, its name is a direct reference to its value. If you have a pointer to a variable or any other object in memory, you have an indirect reference to its value. A pointer variable stores the address of another object or a function. To start out, the declaration of a pointer to an object that is not an array has the following syntax:

In declarations, the asterisk (*) means "pointer to". The identifier name is declared as an object with the type *, or pointer to type. * is the indirection or dereferencing operator; when applied to a pointer, it accesses the object the pointer points to.

Here is a simple Example

The type int is the type of object that the pointer iPtr can point to.

To make a pointer refer to a certain object, assign it the address of the object.

For example, if iVar is an int variable, then the following assignment makes iPtr point to the variable iVar: