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Back to Chapter 4 -- Index -- Chapter 6

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 chapter also explores this relationship and shows how to exploit

it.

Pointers have been lumped with the goto statement as a marvelous way to create impossible-to-understand programs. This is certainly true when they are used carelessly, and it is easy to create pointers that point somewhere unexpected. With discipline, however, pointers can also be used to achieve clarity and simplicity. This is the aspect that we will try to illustrate.

The main change in ANSI C is to make explicit the rules about how pointers can be manipulated, in effect mandating what good programmers already practice and good compilers already enforce. In addition, the type void * (pointer to void) replaces char * as the proper type for a generic pointer.

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

. 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:

_[image omitted]_

The unary operator & gives the address of an object, so the statement

p = &c;

assigns the address of

c

to the variable

p

, and

p

is said to point to''

c

. The

&