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 −
#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, 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 −
#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, 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 */
