Malloc | Free | Dangling Pointer | Prog. & DS

Question

MSQ

 

Consider the following C snippet:

#include <stdio.h>

int main() 
{
    int *ptr = (int*) malloc(100*sizeof(int));
    *ptr=33;
    printf("%d %d\n",ptr,*ptr); // Line X
    
    free(ptr);
    *ptr=37;
    printf("%d %d\n",ptr,*ptr);  // Line Y
    
return 0;

}

Which of the following is TRUE regarding “Line X” and “Line Y”

1. Printed value of ptr (heap address) is same for both “Line X” and “Line Y”
2. Printed value of ptr (heap address) is not same for both “Line X” and “Line Y”, as in Line Y, ptr is a dangling pointer and will print garbage value
3. The value of *ptr (value at *ptr) is 37 in Line ‘Y’
4. The value of ptr (value at *ptr) is 0 as in Line Y, the allocated heap is freed, and we cannot update it anymore, as ptr is now a dangling pointer

Answer 1

Since in the code after freeing up the pointer variable ptr we are not assigning NULL to it, it is behaving in an undefined manner. So the given answer might not always be true because it will depend on the platform. 

Source of the Image: Dynamic Memory Allocation - How to play with pointers in C (codingame.com)… 

If we assign NULL to the ptr variable after freeing it up it works in the expected manner. 

Point to be Noted: If we assign NULL to the ptr variable before freeing it up, it can cause Memory Leak.

 

Comments

so, according to this, Option 2 is the best choice here?

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Yes 2 seems to be the best choice but that is also not entirely correct.

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@Sunnidhya Roy

according to the details given in question.

Answer 2

The answer is ‘1’ and ‘3’ any idea why?

Comments

For Line X it will print the address of ptr and value stored at that address(*ptr)

But then the address which is allocated to ptr is freed, and thus we don’t have any  access to *ptr(Dangling pointer).

$Option$ $4$ cannot be true .

Thus $Option$ $2$ is the answer according to this.

how 1 and 3?

anything wrong in this?