CODEWITHSUNDEEP
cpointers
Bharat Kul Ratan
Bharat Kul Ratan

Reputation: 1003

What does int *p = (int*) 60 mean?

#include <stdio.h>

int main()
{
    int *p = (int*) 60;    --- Line 1
    int *q = (int*) 40;    --- Line 2
    printf("%d", p-q);    //Output is 5
    return 0;
}

Could anybody please explain to me the output of this program?

Upvotes: 15

Views: 13643

Answers (6)

A B
A B

Reputation: 4148

The statement declares a pointer to an integer at address 60

int *p = (int*) 60;

You probably already know this; The danger of doing this is: how do you know there is actually an integer stored at address 60?

Upvotes: 1

varagrawal
varagrawal

Reputation: 3032

The int pointer initialization is to ensure that the pointer is pointing to the memory address of an integer, in this case memory location 60 and 40 for pointers p and q respectively.

What the output is giving you is the difference in memory locations. Usually you expect 60-40 to be 20, but in this case, you are getting 5 because in your machine each integer occupies 4 bytes or 32 bits.

So you can think of it like this: The first integer at 40 takes 4 places, so the next integer is at 44, then 48, then 52. Thus when getting the difference of memory locations, the program takes each 4 byte block as 1 block and there is a difference of 5 blocks between 40 and 60.

In Pointer math, this can be obtained like abs(mem_location1 - mem_location2)/sizeof(int) (i.e. no. of bytes occupied by an integer).

HTH. :)

Upvotes: 0

aaazalea
aaazalea

Reputation: 7910

Each pointer, p and q, is a pointer to an int. p points to memory address 60, and q to memory address 40. When you subtract q from p, the result is how many 4-byte ints fit in-between, in this case 5. that is done to make using pointers with arrays easier, if they were in the same array.

See this site for more information about pointer arithmetic.

Upvotes: 1

more tension
more tension

Reputation: 3352

You're creating two pointer values and then doing pointer math. Apparently sizeof(int) on your system is 4 bytes, so the distance between the two pointer values is 5.

Upvotes: 1

mattjgalloway
mattjgalloway

Reputation: 34912

It's making p point to the memory address 60 and q point to the memory address 40. Then presumably your architecture has 4-byte ints and so p - q equals 5 ((60 - 40) / 4).

Upvotes: 8

orlp
orlp

Reputation: 117681

It means the (implementation-defined) action of assigning an integral value to a pointer happens. This often means that p points to the memory address at 60 and q to the address at 40. These memory addresses could be in virtual memory, hardware memory, and many implementations have different conversion routines for these.

Since this is implementation-defined anything could happen, as described by your implementation.

But isn't this entirely worthless?

It's most certainly not, it is used a lot in embedded hardware programming to access certain features or call built-in functions.

Most likely on your system int is 4 bytes wide, so p - q equals (60 - 40) / 4 == 5.

Upvotes: 10

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