CODEWITHSUNDEEP
cpointers
csounder5
csounder5

Reputation: 55

Difference between the address of a pointer and the address that a pointer contains

#include<stdio.h> 

main() {
  int a,*p;
  a=2;
  p = &a;

  printf("The address %p contains the address of a=2:%p\n",&p,p);

  printf("The address of %d is %p\n",a,&a);

  return 0;
}

The result is:

The address 0028FF18 contains the address of a=2: 0028FF1C

is this correct? if not, what's the correct statement??

The address of 2 is 0028FF1C

is this correct? if not, what's the correct statement?? */

Upvotes: 2

Views: 135

Answers (7)

Deepak Uniyal
Deepak Uniyal

Reputation: 89

Look at the diagram below:

+---------+
|         |
|    2    | Value of a=2
|         |
+---------+
Address=0028FF1C


+---------+
|         |
|0028FF1C |  P stores address of a
|         |  So p=0028FF1C
+---------+
Address=0028FF18

So now your first question is

1. The address 0028FF18 contains the address of a=2: 0028FF1C. Is this correct?

Answer: Yes, it is correct as 0028FF18 is address of p and it contains &a i.e. address of a which is 0028FF1C.

Now coming to second question

2. The address of 2 is 0028FF1C. Is this correct?

Answer: This is little bit confusing statement (can't say it's perfectly correct) as address of a is 0028FF1C and value of a is 2. So it might be better if you print it like:

printf("The address of a is %p\n",&a);

So it will give output:

The address of a is 0028FF1C.

This statement seems better as here we are printing address of variable not the address of a value which that variable stores.

Upvotes: 1

Sourav Ghosh
Sourav Ghosh

Reputation: 134336

Maybe a simple analogy will help you understand better.

int a = 5;
int *p = &a;
  • 5 is the value of int variable a.
  • &a is the value of int * variable p.
  • *p is the int value at the address held by int * variable p.

Upvotes: 0

cizixs
cizixs

Reputation: 13981

In your situation, both are correct!

How to understand pointer easily?

Just imagine a variable as a box with label(variable name) on it, and the box itself has an address(memory address),such as Room 304 on 3rd floor, for anyone to locate it. In the box lays a note(value) tells you what it is(a int 3, a char 's', a float 3.14 etc).

While a pointer is also a box with the exception that it contains a note with another box's address on it.

How you read a int?

How it works on an variable int a=2?

You just say, hey I want to access a(the box with label a on it), someone(your compiler along with OS) just find the box and give it to you. You open the box, and see a note reading 2(might as well as type int). You can change it with another note you just scribbled, say, 3.

How you read a int *?

Similarly, say we have int *p = a. We get the box with label p on it, open it, and it says Room 305 on 5th floor. So instead of saying 'I want the box a', this time we say 'bring the box in Room 305 on 5th floor'(which is what *p means).

  • *p = 3; means: bring the box which box p points to and change its value to 3.
  • int *q = &a: bring a new box(put it somewhere), and fill it with a note reads the address of a
  • &p: give me the address of box p

Conclusion

  1. Every variable has a memory address, and a value
  2. A normal variable just deals with its value directly
  3. A pointer has a value points to another address(most time, the address of another variable).
  4. The above Box Model also works on int **p(pointer to pointer) and more.

Upvotes: 0

simon_xia
simon_xia

Reputation: 2544

A picture is worth a thousand words, here is what p and a like in memory.

left are address, and right are variable names. 

            +---------------+      
  0028FF18  |   0028FF1C    |  p               
            +---------------+                  
  0028FF1C  |      2        |  a                
            +---------------+

Upvotes: 0

Ethan Lynn
Ethan Lynn

Reputation: 1009

A pointer is really just a special kind of number — an address used to locate another piece of data. In your example, p is the numeric address to a, and &p is a pointer to the address stored in p. &p is a pointer to a pointer.

Upvotes: 0

John Castleman
John Castleman

Reputation: 1561

Regarding a:

printf("The value of a is %d.", a); // The value of a is 2.
printf("The address of a is %p.", &a); // The address of a is 0028FF1C

Regarding p:

printf("The value of p is the address of a is %p.", p); // The value of p is the address of a is 0028FF1C
printf("The address of p is %p.", &p); // The address of p is 0028FF18

I think of memory on a computer as a street full of addresses (a really long street), and at each address lives a single value. The variables a and p are simply syntax needed for us as programmers to conceive of these memory locations. So, a is a representation for the address 0028FF1C and the current resident of that address is 2 ... in the future, 2 could move out and 4 could move in, but the address would be the same.

Likewise with p - his address will always be 0028FF18, and it just so happens that the address of a is the current resident at this location, but later a different value could move in here (e.g., p = &b) and p's address would still be 0028FF18.

Upvotes: 1

mchant
mchant

Reputation: 330

p has its own address which is &p, but the value of p is the address of a.

Upvotes: 1

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