Initializing a pointer variable in C Language

Initializing a pointer variable

Initialization of a pointer variable is the process of assigning the address of a variable or memory to a pointer variable. The initialization of a pointer variable can be done using following three steps :
Step-1 : Declare a data variable.
Step-2 : Declare a pointer variable.
Step-3: Assign address of a data variable to pointer variable using & operator and assignment operator.
Note that the Step1 and 2 can be interchanged i.e. we can first declare a pointer variable, then declare a data variable and then initialize the pointer variable.

Example : Consider the following three statements:
int x; /* Step-1 : x is declared as a data variable */
int *px ; /* Step-2: px is declared as a pointer variable */
px= &x; /* Step-3 : copy address of data variable to pointer variable */
....
.....
Here , the variable x is declared as integer data variable. Since px is pointer variable of type integer, it should contain address of integer variable. So, using the statement : px = & x;
the valid address is stored in the pointer variable and hence the pointer variable px is initialized.

Example: Consider the following three statements:
int x ;  int *px; px=&x;
The above three statements can also be written as shown below:
int x;    int *px=&x;
Note : It is also possible to combine the declaration of data variable, pointer variable and initialization of pointer variable in one step as: int x, *px=&x;
Here, x is declared as a data variable, px is declared as pointer variable and px is initialized to address of data variable x.

Example : Consider the following statements:
int p, *ip;
float d,f;
ip=&p;  /* Valid Initialization */
..............
ip=&d;   /* Invalid initialization */
Note: First initialization is valid. In the second initialization , ip is a pointer to integer. So , it should contain address of integer variable. But , address of floating point variable d is stored in ip and hence it is invalid.

Example : Pointers are flexible i.e. a pointer can point to different data variables by storing the address of approximate variables. Consider the following declaration:
int x=10, y=20,z=30;
int *p;

p= &x;
.........
p=&y;
.........
p=&z;
..........

Note : It is observed from above example that the pointer variable p points to different memory location by storing the addresses of different variables. Thus, same pointer can be pointed to different data variables.