Understanding C Language Pointer

Pointer

Pointers are special variables in C used to store address of another variable of matched data-type. Pointers (or Pointer variables) are created in the same way as an ordinary variable however the identifier must be prefixed with an asterisk (or start) mark.

Pointers are useful to provide indirect access to the variable to which it points.

Example:

int *p

float *q

char *r

Unlike ordinary variables, system allocates equally 2 bytes of memory to each pointer irrespective to their data-type. That is, an integer pointer occupies 2 bytes as well as a float pointer occupies 2 bytes as other pointer too.

NOTE: Pointers are always unsigned in nature because the addresses are always sign-less data (memory location numbers).

Diagrammatically,

Let us take a variable and a pointer for it as-

int a=50

int *p

p=&a // Address of a is assigned to p

In the above example system allocates 2 bytes memory to the ordinary integer variable a (let it is the location number 2001-2002 as given in the diagram). Similarly, pointer p also occupies 2 bytes (let it is the location number 4001-5002 as given in the above diagram). Now while the pointer is assigned with address of the variable a, it would now contain 2001.

IMPORTANT: If a pointer is assigned with the address of a variable of unmatched data-type or incompatible type, system generates an error.

Example:

int a=50

float b=2.5

and int *p

float *q

Now if we assign

p=&b

or q=&a both are illegal

Similarly, we can t assign data or a variable of ordinary type to a pointer because pointers are only meant for storing address.

Example:

Suppose there are variables and pointer in a program like

int x

char y

float z

int *m

char *n

float *o

m=x // Is not legal because x contains a value and pointers are to store address.

m=&x // Ok

Again,

m=&z // Not legal because `z` is a float variable but `m` is an integer pointer.

o=&z // Ok.

Generally, a variable has two pieces of information its content (value) and its address. But pointers provide 3 pieces of information. Along with its content (address of the variable to which it points) and its address pointers has de-referenced value. The de-reference value of the pointer (or indirection value) is the value in the variable to which it points.

We can get indirection or de-reference value of a pointer through unary indirection (*) operator.

Example:

int a=50

int *p

p=&a

Now,

printf( %d , *p) // Displays value of a

Or,

*p=100 // Changes variable a

NOTE: * is used for two purposes First, to declare the pointer and Second, to get the indirection data. Asterisk operator is also used as binary multiplication operator. System automatically evaluates the meaning of the operator according to its use (i.e. whether it is used as unary or binary).