POINTERS
Pointers in C are
easy and fun to learn. Some C programming tasks are
performed more easily with pointers, and other tasks, such as dynamic memory
allocation, cannot be performed without using pointers. So it becomes necessary
to learn pointers to become a perfect C programmer. Let's start learning them
in
simple and easy steps.
As you know, every variable is a memory location and every memory location
has its address defined which can be accessed using ampersand (&) operator,
which denotes an address in memory. Consider the following example, which
prints the address of the variables defined:
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#include <stdio.h>
int main ()
{
int var1;
char var2[10];
printf("Address of var1 variable: %x\n", &var1 );
printf("Address of var2 variable: %x\n", &var2 );
return 0;
}
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When the above code
is compiled and executed, it produces the following result:
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Address of var1 variable: bff5a400
Address of var2 variable: bff5a3f6
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What are Pointers?
A pointer is a variable whose value is the address of another variable,
i.e.,
direct address of the memory location. Like any variable or constant, you must
declare a pointer before using it to store any variable address. The general
form
of a pointer variable declaration is:5. POINTE
Here, type is the pointer's base type; it must be a valid C data type
and varname is the name of the pointer variable. The asterisk * used to
declare a
pointer is the same asterisk used for multiplication. However, in this
statement,
the asterisk is being used to designate a variable as a pointer. Take a look at
some of the valid pointer declarations:
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int *ip;
/*pointer to an integer
*/
double *dp; /* pointer to a double */
float *fp; /*
pointer to a float */
char *ch /* pointer to a character */
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The actual data type
of the value of all pointers, whether integer, float,
character, or otherwise, is the same, a long hexadecimal number that represents
a memory address. The only difference between pointers of different data types
is the data type of the variable or constant that the pointer points to.
How to
Use Pointers?
There are a few important operations, which we will do with the help of
pointers
very frequently. (a) We define a pointer variable, (b) assign the
address of a
variable to a pointer, and (c) finally access the value at the address
available in
the pointer variable. This is done by using unary operator * that
returns the
value of the variable located at the address specified by its operand. The
following example makes use of these operations:
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#include <stdio.h>
int main ()
{
int var = 20; /* actual variable declaration */
int *ip; /* pointer variable declaration */
ip = &var; /* store address of var in pointer variable*/
printf("Address of var variable: %x\n", &var );
/* address stored in pointer variable */
printf("Address stored in ip variable: %x\n", ip );
/* access the value using the pointer */
printf("Value of *ip variable: %d\n", *ip );
return 0;
}
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When the above code is compiled and executed, it produces the following result:
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Address of var variable: bffd8b3c
Address stored in ip variable: bffd8b3c
Value of *ip variable: 20
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NULL
Pointers
It is always a good practice to assign a NULL value to a pointer variable in
case
you do not have an exact address to be assigned. This is done at the time of
variable declaration. A pointer that is assigned NULL is called a null pointer.
The NULL pointer is a constant with a value of zero defined in several standard
libraries. Consider the following program:
|
#include <stdio.h>
int main ()
{
int *ptr = NULL;
printf("The value of ptr is : %x\n", ptr );
return 0;
}
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When the above code
is compiled and executed, it produces the following result:
In most of the
operating systems, programs are not permitted to access
memory at address 0 because that memory is reserved by the operating system.
However, the memory address 0 has special significance; it signals that the
pointer is not intended to point to an accessible memory location. But by
convention, if a
pointer contains the null (zero) value, it is assumed to point to
nothing.
To check for a null pointer, you can use an ‘if’ statement as follows:
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if(ptr) /* succeeds if p is not null */
if(!ptr) /* succeeds if p is null */
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Pointers in Detail
Pointers have many but easy concepts and they are very important to C
programming. The following important pointer concepts should be clear to any C
programmer:
Concept Description
Pointer arithmetic There are four arithmetic operators that
can be used in pointers: ++, --, +, -
Array of pointers You can define arrays to hold a number of
pointers.
Pointer to pointer C allows you to have pointer on a pointer
and so on.
Passing pointers to functions in C Passing an argument by reference or by
address enable the passed argument to be
changed in the calling function by the
called function.
Return pointer from functions in C C allows a function to return a pointer to
the local variable, static variable, and
dynamically allocated memory as well.
Pointer
Arithmetic
A pointer in C is an address, which
is a numeric value. Therefore, you can
perform arithmetic operations on a pointer just as you can on a numeric value.
There are four arithmetic operators that can be used on pointers: ++, --, +,
and
-
To understand pointer arithmetic, let us consider that ptr is an integer
pointer
which points to the address 1000. Assuming 32-bit integers, let us perform the
following arithmetic operation on the pointer:
After the above operation, the ptr will point to the location 1004
because each
time ptr is incremented, it will point to the next integer location which is 4
bytes
next to the current location. This operation will move the pointer to the next
memory location without impacting the actual value at the memory location.
If ptr points to a character whose address is 1000, then the above
operation will
point to the location 1001 because the next character will be available at
1001.
Incrementing
a Pointer
We prefer using a pointer in our
program instead of an array because the
variable pointer can be incremented, unlike the array name which cannot be
incremented because it is a constant pointer. The following program increments
the variable pointer to access each succeeding element of the array:
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#include <stdio.h>
const int MAX = 3;
int main ()
{
int var[] = {10, 100, 200};
int i, *ptr;
/* let us have array address in pointer */
ptr = var;
for ( i = 0; i < MAX; i++)
{
printf("Address of var[%d] = %x\n", i, ptr );
printf("Value of var[%d] = %d\n", i, *ptr );
/* move to the next location */
ptr++;
}
return 0;
}
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When the above code
is compiled and executed, it produces the following result:
|
Address of var[0] = bf882b30
Value of var[0] = 10
Address of var[1] = bf882b34
Value of var[1] = 100
Address of var[2] = bf882b38
Value of var[2] = 200
|
Decrementing
a Pointer
The same considerations apply to decrementing a pointer, which decreases its
value by the number of bytes of its data type as shown below:
|
#include <stdio.h>
const int MAX = 3;
int main ()
{
int var[] = {10, 100, 200};
int i, *ptr;
/* let us have array address in pointer */
ptr = &var[MAX-1];
for ( i = MAX; i > 0; i--)
{
printf("Address of var[%d] = %x\n", i, ptr );
printf("Value of var[%d] = %d\n", i, *ptr );
/* move to the previous location */
ptr--;
}
return 0;
}
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When the above code
is compiled and executed, it produces the following result:
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Address of var[3] = bfedbcd8
Value of var[3] = 200
Address of var[2] = bfedbcd4
Value of var[2] = 100
Address of var[1] = bfedbcd0
Value of var[1] = 10
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Pointer
Comparisons
Pointers may be compared by using relational operators, such as ==, <, and
>.
If p1 and p2 point to variables that are related to each other, such as
elements
of the same array, then p1 and p2 can be meaningfully compared.
The following program modifies the previous example - one by incrementing the
variable pointer so long as the address to which it points is either less than
or
equal to the address of the last element of the array, which is &var[MAX -
1]:
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#include <stdio.h>
const int MAX = 3;
int main ()
{
int var[] = {10, 100, 200};
int i, *ptr;
/* let us have address of the first element in pointer */
ptr = var;
i = 0;
while ( ptr <= &var[MAX - 1] )
{
printf("Address of var[%d] = %x\n", i, ptr );
printf("Value of var[%d] = %d\n", i, *ptr );
/* point to the previous location */
ptr++;
i++;
}
return 0;
}
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When the above code
is compiled and executed, it produces the following result:
|
Address of var[0] = bfdbcb20
Value of var[0] = 10
Address of var[1] = bfdbcb24
Value of var[1] = 100
Address of var[2] = bfdbcb28
Value of var[2] = 200
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Array of Pointers
Before we understand the concept of arrays of pointers, let us consider the
following example, which uses an array of 3 integers:
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#include <stdio.h>
const int MAX = 3;
int main ()
{
int var[] = {10, 100, 200};
int i;
for (i = 0; i < MAX; i++)
{
printf("Value of var[%d] = %d\n", i, var[i] );
}
return 0;
}
|
When the above code
is compiled and executed, it produces the following result:
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Value of var[0] = 10
Value of var[1] = 100
Value of var[2] = 200
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There may be a situation when we want to maintain an array, which can store
pointers to an int or char or any other data type available. Following is the
declaration of an array of pointers to an integer:
It declares ptr as an array of MAX integer pointers. Thus, each element
in ptr
holds a pointer to an int value. The following example uses three integers,
which
are stored in an array of pointers, as follows:
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#include <stdio.h>
const int MAX = 3;
int main ()
{
int var[] = {10, 100, 200};
int i, *ptr[MAX];
for ( i = 0; i < MAX; i++)
{
ptr[i] = &var[i]; /* assign the address of integer. */
}
for ( i = 0; i < MAX; i++)
{
printf("Value of var[%d] = %d\n", i, *ptr[i] );
}
return 0;
}
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When the above code
is compiled and executed, it produces the following result:
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Value of var[0] = 10
Value of var[1] = 100
Value of var[2] = 200
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You can also use an array of pointers to character to store a list of strings
as
follows:
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#include <stdio.h>
const int MAX = 4;
int main ()
{
char *names[] = {
"Zara Ali",
"Hina Ali",
"Nuha Ali",
"Sara Ali",
};
int i = 0;
for ( i = 0; i < MAX; i++)
{
printf("Value of names[%d] = %s\n", i, names[i] );
}
return 0;
}
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When the above code is compiled and executed, it produces the following result:
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Value of names[0] = Zara Ali
Value of names[1] = Hina Ali
Value of names[2] = Nuha Ali
Value of names[3] = Sara Ali
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Pointer to Pointer
A pointer to a pointer is a form of multiple indirection, or a chain of
pointers.
Normally, a pointer contains the address of a variable. When we define a
pointer
to a pointer, the first pointer contains the address of the second pointer,
which
points to the location that contains the actual value as shown below.
A variable that is a pointer to a pointer must be declared as such. This is
done
by placing an additional asterisk in front of its name. For example, the
following
declaration declares a pointer to a pointer of type int:
When a target value is indirectly pointed to by a pointer to a pointer,
accessing
that value requires that the asterisk operator be applied twice, as is shown
below in the example:
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#include <stdio.h>
int main ()
{
int var;
int *ptr;
int **pptr;
var = 3000;
/* take the address of var */
ptr = &var;
/* take the address of ptr using address of operator & */
pptr = &ptr;
/* take the value using pptr */
printf("Value of var = %d\n", var );
printf("Value available at *ptr = %d\n", *ptr );
printf("Value available at **pptr = %d\n", **pptr);
return 0;
}
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When the above code
is compiled and executed, it produces the following result:
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Value of var = 3000
Value available at *ptr = 3000
Value available at **pptr = 3000
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Passing Pointers to Functions
C programming allows
passing a pointer to a function. To do so, simply declare
the function parameter as a pointer type.
Following is a simple example where we pass an unsigned long pointer to a
function and change the value inside the function which reflects back in the
calling function:
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#include <stdio.h>
#include <time.h>
void getSeconds(unsigned long *par);
int main ()
{
unsigned long sec;
getSeconds( &sec );
/* print the actual value */
printf("Number of seconds: %ld\n", sec );
return 0;
}
void getSeconds(unsigned long *par)
{
/* get the current number of seconds */
*par = time( NULL );
return;
}
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When the above code
is compiled and executed, it produces the following result:
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Number of seconds :1294450468
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The function, which can accept a pointer, can also accept an array as shown in
the following example:
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#include <stdio.h>
/* function declaration */
double getAverage(int *arr, int size);
int main ()
{
/* an int array with 5 elements */
int balance[5] = {1000, 2, 3, 17, 50};
double avg;
/* pass pointer to the array as an argument */
avg = getAverage( balance, 5 ) ;
/* output the returned value */
printf("Average value is: %f\n", avg );
return 0;
}
double getAverage(int *arr, int size)
{
int i, sum = 0;
double avg;
for (i = 0; i < size; ++i)
{
sum += arr[i];
}
avg = (double)sum / size;
return avg;
}
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When the above code
is compiled together and executed, it produces the
following result:
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Average value is: 214.40000
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Return Pointer from Functions
We have seen in the last chapter how C programming allows to return an array
from a function. Similarly, C also allows to return a pointer from a function.
To
do so, you would have to declare a function returning a pointer as in the
following example:
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int * myFunction()
{ . . . }
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Second point to remember is that, it is not a good idea to return the address
of
a local variable outside the function, so you would have to define the local
variable as static variable.
Now, consider the following function which will generate 10 random numbers
and return them using an array name which represents a pointer, i.e., address
of first array element.
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#include <stdio.h>
#include <time.h>
/* function to generate and retrun random numbers. */
int * getRandom( )
{
static int r[10];
int i;
/* set the seed */
srand( (unsigned)time( NULL ) );
for ( i = 0; i < 10; ++i)
{
r[i] = rand();
printf("%d\n", r[i] );
}
return r;
}
/* main function to call above defined function */
int main ()
{
/* a pointer to an int */
int *p;
int i;
p = getRandom();
for ( i = 0; i < 10; i++ )
{
printf("*(p + [%d]) : %d\n", i, *(p + i) );
}
return 0;
}
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When the above code is compiled together and executed, it produces the
following result:
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1523198053
1187214107
1108300978
430494959
1421301276
930971084
123250484
106932140
1604461820
149169022
*(p + [0]) : 1523198053
*(p + [1]) : 1187214107
*(p + [2]) : 1108300978
*(p + [3]) : 430494959
*(p + [4]) : 1421301276
*(p + [5]) : 930971084
*(p + [6]) : 123250484
*(p + [7]) : 106932140
*(p + [8]) : 1604461820
*(p + [9]) : 149169022
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