POINTERS IN C

POINTERS IN C

•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.

• 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:
type *var-name;

Here,
-- type is the pointer's base type (it must be a valid C
datatype).
-- var-name is the name of the pointer variable.
-- * used to declare a pointer.

• Some of the valid pointer declarations are as follows −
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
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.

USAGE OF POINTERS

(a) We define a pointer variable.
(b) Assign the address of a variable to a pointer.
(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.

• Example:
#include
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, it produces the following result −
-- Address of var variable: bffd8b3c
-- Address stored in ip variable: bffd8b3c
-- Value of *ip variable: 20

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 −

• Example:
#include
int main ()
{
int *ptr = NULL;
printf("The value of ptr is : %x\n", ptr );
return 0;
}

• When the above code is compiled and executed, it produces the following result −
-- The value of ptr is 0
-- 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 −
if(ptr) /* succeeds if p is not null /
if(!ptr) / succeeds if p is null */