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 alsobe 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.

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.

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:


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 &operator only applies to objects in memory: variables and array elements. It cannot be applied to expressions,
constants, or register variables.

The unary operator * is the indirection or dereferencing operator; when applied to a pointer, it
accesses the object the pointer points to. Suppose that x and y are integers and ipis a pointer to

This artificial sequence shows how to declare a pointer and how to use &and *:

int x = 1, y = 2, z[10];
 int *ip;
 ip = &x;
 y = *ip;
 *ip = 0;
 ip = &z[0];

The declaration of x, y, and z are what we’ve seen all along. The declaration of the pointer ip.
int *ip; is intended as a mnemonic; it says that the expression *ipis an int. The syntax of the declaration for a variable mimics the syntax of expressions in which the variable might appear. This
reasoning applies to function declarations as well.
For example,
double *dp, atof(char *);

says that in an expression *dpand atof(s) have values of double, and that the argument of
atofis a pointer to char.

You should also note the implication that a pointer is constrained to point to a particular kind of
object: every pointer points to a specific data type.

If ippoints to the integer x, then *ipcan occur in any context where x could, so

*ip = *ip + 10;

increments *ip by 10.

The unary operators * and &bind more tightly than arithmetic operators, so the assignment
y = *ip + 1

takes whatever ippoints at, adds 1, and assigns the result to y, while
*ip += 1

increments what ippoints to, as do
++*ip and (*ip)++

The parentheses are necessary in this last example; without them, the expression would increment
ip instead of what it points to, because unary operators like * and ++ associate right to left.
Finally, since pointers are variables, they can be used without dereferencing. For example, if iq is
another pointer to int,
iq = ip copies the contents of ipinto iq, thus making iqpoint to whatever ippointed to.

Pointers and Function Arguments
Pointers and Function Arguments
Since C passes arguments to functions by value, there is no direct way for the called function to
alter a variable in the calling function. For instance, a sorting routine might exchange two outoforder
arguments with a function called swap.
It is not enough to write swap(a, b);
where the swap function is defined as

void swap(int x, int y)
 int temp;
 temp = x;
 x = y;
 y = temp;

Because of call by value, swap can’t affect the arguments a and b in the routine that called it.
The function above swaps copies of a and b. The way to obtain the desired effect is for the
calling program to pass pointers to the values to be changed:
swap(&a, &b);
Since the operator & produces the address of a variable, &a is a pointer to a. In swap itself, the
parameters are declared as pointers, and the operands are accessed indirectly through them.

void swap(int *px, int *py) /* interchange *px and *py */
 int temp;
 temp = *px;
 *px = *py;
 *py = temp;

Pointer arguments enable a function to access and change objects in the function that called it. As
an example, consider a function getint that performs free-format input conversion by breaking a
stream of characters into integer values, one integer per call. getint has to return the value it
found and also signal end of file when there is no more input. These values have to be passed

back by separate paths, for no matter what value is used for EOF, that could also be the value of an
input integer.
One solution is to have getint return the end of file status as its function value, while using a
pointer argument to store the converted integer back in the calling function. This is the scheme
used by scanfas well
The following loop fills an array with integers by calls to getint:

int n, array[SIZE], getint(int *);
for (n = 0; n < SIZE &&getint(&array[n]) != EOF; n++)



Each call sets array[n] to the next integer found in the input and increments n. Notice that it is
essential to pass the address of array[n] to getint. Otherwise there is no way for getint to
communicate the converted integer back to the caller.
Our version of getintreturns EOF for end of file, zero if the next input is not a number, and a
positive value if the input contains a valid number.

int getch(void);
 void ungetch(int);
 int getint(int *pn)
 int c, sign;
 while (isspace(c = getch()));
if (!isdigit(c) && c != EOF && c != '+' && c != '-')
 ungetch(c); return 0;
sign = (c == '-') ? -1 : 1;
 if (c == '+' || c == '-')
 c = getch();
 for (*pn = 0; isdigit(c), c = getch())
 *pn = 10 * *pn + (c - '0');
 *pn *= sign;
 if (c != EOF)
return c;

Throughout getint, *pnis used as an ordinary intvariable. We have also used getchand
ungetchso the one extra character that must be read can be pushed back onto the input

Try Now – Programming In C MCQs