POINTER
Pointer is a variable just like
other variables of c but only difference is unlike the other variable it stores
the memory address of any other variables of c. This variable may be type
of int, char, array, structure, function or any other pointers.
For
examples:
(1) Pointer p which is storing
memory address of a int type variable:
int i=50;
int *p=&i;
(2) Pointer p which is storing
memory address of an array:
int arr[20];
(3) Pointer p which is storing
memory address of a function:
char display(void);
char(*p)(void)=&display;
(4) Pointer p which is storing
memory address of struct type variable:
struct abc{
int a;
float b;
}var;
struct abc
*p=&var;
Pointer is a user defined data type
which creates special types of variables which can hold the address of
primitive data type like char, int, float, double or
user defined data type like function, pointer etc. or derived data type like
array, structure, union,enum.
Examples:
int *ptr;
int (*ptr)();
int (*ptr)[2];
In c programming every variable
keeps two type of value.
- Contain of variable or value of variable.
- Address of variable where it has stored in the memory.
Meaning of following simple
pointer declaration and definition:
int a=5;
int * ptr;
ptr=&a;
Explanation:
About variable a:
- Name of variable : a
- Value of variable which it keeps: 5
- Address where it has stored in memory : 1025 (assume)
About variable ptr:
- Name of variable : ptr
- Value of variable which it keeps: 1025
- Address where it has stored in memory : 5000 (assume)
Note: A variable where it will be stored in memory is
decided by operating system. We cannot guess at which location a particular
variable will be stored in memory.
Meaning of following pointer
declaration and definition
int a=50;
int *ptr1;
int **ptr2;
ptr1=&a;
ptr2=&pt1;
Explanation:
About variable a:
- Name of variable : a
- Value of variable which it keeps: 50
- Address where it has stored in memory : 5000 (assume)
About variable ptr1:
- Name of variable : ptr1
- Value of variable which it keeps: 5000
- Address where it has stored in memory : 9000 (assume)
About variable ptr2:
- Name of variable : ptr2
- Value of variable which it keeps: 9000
- Address where it has stored in memory : 9555 (assume)
Note: * is know as indirection operator
which gives content of any variable.
& is know as reference
operator which gives address where variable has stored in memory.
Cancellation rule of above two
operators:
* and & operators always cancel
to each other. i.e.
*&p=p
But it is not right to write:
&*p=p
Example:
What will be output of following c
program?
void main()
{
int x=25;
int *ptr=&x; //statement one
int **temp=&ptr; //statement two
printf(“%d %d %d”.x.*ptr,**temp);
}
Output: 25 25 25
Explanation:
As we know value of variable x is
25.
*ptr= *(&x) //from
statement one
=*&x
=x //using cancellation rule
=25
**temp=
**(&ptr)=*(*&ptr)=*ptr=*(&x)=*&x=x=25
Read complex pointers in C
Programming
Rule 1.
Assign
the priority to the pointer declaration considering precedence and associative
according to following table.
(): This operator behaves as bracket operator or function
operator.
[]: This operator behaves as array subscription operator.
*:
This operator behaves as pointer operator not as multiplication operator.
Identifier: It is not an operator but it is name of pointer variable.
You will always find the first priority will be assigned to the name of
pointer.
Data type: It is also not an operator. Data types also includes
modifier (like signed int, long double etc.)
Read following pointer
char (* ptr)[3]
Step 1: () and [] enjoys equal precedence. So rule of
associative will decide the priority. Its associative is left to right So first
priority goes to ().
Step 2: Inside the bracket * and ptr enjoy equal precedence.
From rule of associative (right to left) first priority goes to ptr and second
priority goes to *.
Step3: Assign third priority to [].
Step4: Since data type enjoys least priority so assign fourth
priority to char.
ptr is pointer to such one
dimensional array of size three which content char type
data.
Read following pointer
float (* ptr)(int)
Assign the priority considering
precedence and associative.
ptr is pointer to such function whose
parameter is int type data and return type is float type data.
Rule 2:
Assign the priority of each function parameter separately
and read it also separately.
Understand it through following
example.
Read following pointer
void (*ptr)(int (*)[2],int (*)
void))
Assign the priority considering rule
of precedence and associative.
Now read it following manner:
ptr is pointer to such function which
first parameter is pointer to one dimensional array of
size two which content int type data and second parameter is pointer to
such function which parameter is void and return type is int data
type and return type is void.
Read following pointer
int ( * ( * ptr ) [ 5 ] ) ( )
Assign the priority considering rule
of precedence and associative.
ptr is pointer to such array of
size five which content arepointer to such function which
parameter is void and return type is int type data.
Read following pointer
double*(*(*ptr)(int))(double **,char
c)
ptr is pointer to function which parameter is int type data and return type is pointer to function which first parameter is pointer to pointer of double data type and second parameter is char type data type and return type is pointerto double data type.
Read following pointer
unsigned **(*(*ptr)[8](char const *,
...)
Assign the priority considering rule
of precedence and associative.
ptr is pointer to array of
size eight and content of array is pointer to function which
first parameter is pointer to character constant and second parameter is
variable number of arguments and return type is pointer to pointer ofunsigned int
data type.