In c,a pointer is a variable that points to or references a memory location in which data is stored. In the computer,each memory cell has an address that can be used to access that location so a pointer variable points to a memory location we can access and change the contents of this memory location via the pointer.

A pointer is a variable that contains the memory location of another variable in shich data is stored. Using pointer,you start by specifying the type of data stored in the location. The asterisk helps to tell the compiler that you are creating a pointer variable. Finally you have to give the name of the variable. The syntax is as shown below.

Once we declare a pointer variable then we must point it to something we can do this by assigning to the pointer the address of the variable you want to point as in the following example:

The above code tells that the address where num is stores into the variable ptr. The variable ptr has the value 21260,if num is stored in memory 21260 address then

C allows us to subtract integers to or add integers from pointers as well as to subtract one pointer from the other. We can also use short hand operators with pointers p1+=; sum+=*p2; etc., By using relational operators,we can also compare pointers like the expressions such as p1 >p2 , p1==p2 and p1!=p2 are allowed.

In a function declaration,the pointer are very much used . Sometimes,only with a pointer a complex function can be easily represented and success. In a function definition,the usage of the pointers may be classified into two groups.

1. Call by reference
2. Call by value.

We have seen that there will be a link established between the formal and actual parameters when a function is invoked. As soon as temporary storage is created where the value of actual parameters is stored. The formal parameters picks up its value from storage area the mechanism of data transfer between formal and actual parameters allows the actual parameters mechanism of data transfer is referred as call by value. The corresponding formal parameter always represents a local variable in the called function. The current value of the corresponding actual parameter becomes the initial value of formal parameter. In the body of the actual parameter,the value of formal parameter may be changed. In the body of the subprogram,the value of formal parameter may be changed by assignment or input statements. This will not change the value of the actual parameters.

The address should be pointers,when we pass address to a function the parameters receiving . By using pointers,the process of calling a function to pass the address of the variable is known as call by reference. The function which is called by reference can change the value of the variable used in the call.

an array is actually very much similar like pointer. We can declare as int *a is an address,because a[0] the arrays first element as a[0] and *a is also an address the form of declaration is also equivalent. The difference is pointer can appear on the left of the assignment operator and it is a is a variable that is lvalue. The array name cannot appear as the left side of assignment operator and is constant.

We know the name of an array stands for address of its zeroth element the same concept applies for names of arrays of structures. Suppose item is an array variable of the struct type. Consider the following declaration:

this statement ptr as a pointer data objects of type struct products and declares item as array of two elements, each type struct products.

A pointer contains garbage value until it is initialized. Since compilers cannot detect the errors may not be known until we execute the program remember that even if we are able to locate a wrong result,uninitialized or wrongly initialized pointers it may not provide any evidence for us to suspect problems in pointers. For example the expressions such as