C Pointers.md

C Pointers

What are Pointers?  A 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 *var-name;

Here, 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 −

💡 int *ip; /* pointer to an integer */ double *dp; /* pointer to a double */ float *fp; /* 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?

Example:

#include <stdio.h>

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 following program −

#include <stdio.h>

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 */

Examples:

int i=42 , j=107;
int *p=&i;
int *q =&j
printf("%d\n" , *p)//

Address	Memory
9016	9004	q
9008	9000	p
9004	107	j
9000	42	i

what do these lines do?

1)p=q

answer:

Memory	Memory
9016	9004	q
9008	9004	p
9004	107	j
9000	42	i

2)*p=q

answer:

Address	Memory
9016	9004	q
9008	900	p
9004	107	j
9000	9004	i

3)p=*q

answer:

Address	Memory
9016	9004	q
9008	107	p
9004	107	j
9000	42	i

4)*p=*q

answer:

Address	Memory
9016	9004	q
9008	9000	p
9004	107	j
9000	107	i