Pointers
As we know, every
variable has a memory location and every location has its address defined which
can be accessed using ‘&’ (ampersand operator), which denotes an address in
memory. Consider the below given example, which prints the address of the
variables defined:-
#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;
}
When the above code is compiled and executed, it
produces the following result −
Address
of var1 variable: bff5a400
Address
of var2 variable: bff5a3f6
What
are Pointers?
A Pointer is
basically a variable whose value denotes the address of another variable, that
is, direct address of the memory location. Like any other variable or constant,
we need to declare a pointer before using it to store any variable address. The
general form of a pointer variable declaration is -
type *var-name;
Here, type is the pointer's base
type; it must be a valid C data type and var-name is the name
of the pointer variable. The asterisk * used to declare a pointer is the same
asterisk which is used for multiplication. However, here 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 –
int *ip; /* pointer to an integer */
double *dp; /* pointer to a double */
float *fp; /* pointer to a float */
char *ch /* pointer to a character */
The actual data type of the value of all
pointers, whether it is an integer, float, or a character, 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 some of a very important operations,
which we will be doing with the help of pointers very frequently.
1) We define a pointer variable.
2) Assign the address of a variable to a pointer.
3) Finally access the value at the address available
in the pointer variable.
This is done with the help of unary operator * that returns the value of the
variable located at the address specified by its operand. The below given
examples makes use of these operations -
#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;
}
When the above code is compiled and executed, after
that it produces the following result
−
Address
of var variable: bffd8b3c
Address
stored in ip variable: bffd8b3c
Value
of *ip variable: 20
NULL Pointers
We all should have a good practice of assigning a
NULL value to pointer variable in case we do not have an exact address to be
assigned. This is done at the time of declaration of variable. A pointer which
is assigned NULL pointer.
The NULL pointer is a constant with a value of zero
defined in several standard libraries. Let’s consider the following program −
#include <stdio.h>
int main ()
{
int *ptr =
NULL;
printf("The value of ptr is : %x\n", ptr );
return 0;
}
When the above code is compiled and executed, after
that it produces the following result −
The
value of ptr is 0
In most of the OS, programs are not permitted to
access memory at address 0 because that memory is reserved by the OS. However,
the memory address 0 has special significance i.e., 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, we can use an 'if'
statement as follows −
if(ptr) /* succeeds if p is not null */
if(!ptr) /* succeeds if p is null */
Pointers in Detail
Pointers have many but really easy concepts and
they are very important concept for C programming. The following important
pointer concepts must be clear to us as a C programmer −
|
Sr.No.
|
Concept & Description
|
|
1
|
There are four arithmetic operators that can be
used in pointers: ++, --, +, -
|
|
2
|
we can define arrays to hold a number of
pointers.
|
|
3
|
C allows us to have pointer on a pointer and so
on.
|
|
4
|
Passing an argument by reference or by address
enable the passed argument to be changed in the calling function by the
called function.
|
|
5
|
C allows a function to return a pointer to the static
variable, local variable, and dynamically allocated memory as well.
|
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