March 12, 2017


Pointers are the one of the best feature of C language.They are a powerful tool and handy to use once they are mastered. There are a number of reasons for using pointers.

  1. A pointer enables us to access that is defined outside the function.
  2. Poniters are more efficient in handling the data tables.
  3. Pointers reduse the length and complexity of a program.
  4. They increase the execution speed.
  5. The use of a pointer array to character strings result in saving of data storage space in memory.
What are Pointers?  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 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 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 − /* pointer to an integer */double *dp;    /* pointer to a double */
/* 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. 

 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 − 
  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 program 
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 */ 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 − S.N. Concept & Description 1 Pointer arithmetic There are four arithmetic operators that can be used in pointers: ++, --, +, - 2 Array of pointers You can define arrays to hold a number of pointers. 3 Pointer to pointer C allows you to have pointer on a pointer and so on. 4 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. 5 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.

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  1. Pointers to information altogether enhance execution for monotonous operations, for example, crossing strings, query tables, control tables and tree structures. Specifically, it is regularly considerably less expensive in time and space to duplicate and dereference pointers than it is to duplicate and get to the information to which the pointers point.

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