Thread: Problem with pointers

Hello,
I was doing some experiments with pointer and wrote the following code :-

Code:

#include <stdio.h>
#include <stdlib.h>

int main()
{
	int *p;
	int *q;

	p = (int*) malloc (sizeof(int) * 3);

	printf ("Address: &p = %u, p = %u\n", &p, p);

	p[0] = 10;
	p[1] = 20;
	p[2] = 30;

	q = p;
	
	printf ("&q = %u, q = %u\n", &q, q);

	free(p);
	
	printf ("After Freeing p:\n&q = %u, q = %u\n", &q, q);

	printf ("%d %d %d\n", q[0], q[1], q[2]);

	return 0;
}

I have allocated the memory to pointer p for 3 integers and then i assigned them values 10, 20, 30. Now for pointer q, I set it to point to the address pointed by p. Now I freed pointer p's allocated memory. After freeing, q will still point to the same address. Now when I print the values again through q, the first value q[0] comes out to be 0 and remaining the same. Why does it happen? Any Idea.

Thanks and Best Regards,
Aakash Johari

In which way, it becomes invalid. Its still pointing to the same address. And after running the whole code, I found the output:

Code:

0 20 30

Means, it just cleared out the first integer space and did not do anything with the other allocated memory part.

And please explain the term "Code Broken".

Thanks and Best Regards,
Aakash Johari

q points to the same memory address, yes. But the memory at that memory address is no longer yours, since you gave it back; it now belongs to some other process on the machine.

And any code that attempts to access memory which it is not entitled to access is broken (it may compile, but that doesn't mean it's right).

This could be a quirk of the os or compiler because of the way they manage dynamic memory.
After free(p) there's no guarantee if the memory is in the process' address space or has been released.
So in this aspect what you are doing is undefined and at best it is implementation-dependent.

Thank you very much.

I got it.

Originally Posted by aakashjohari

Thank you very much.

I got it.

Think of it this way... You burned down the house with both cars still in the driveway. The cars may or may not be damaged by the fire.

Originally Posted by CommonTater

Think of it this way... You burned down the house with both cars still in the driveway. The cars may or may not be damaged by the fire.

Sorry, I couldnt get you.

The house is analogous to the memory location.... the cars are the pointers...