c - Does free() remove the data stored in the dynamically allocated memory?

Question

I wrote a simple program to test the contents of a dynamically allocated memory after free() as below. (I know we should not access the memory after free. I wrote this to check what will be there in the memory after free)

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

main()
{
    int *p = (int *)malloc(sizeof(int));

    *p = 3;
    printf("%d
", *p);
    free(p);

    printf("%d
", *p);
}

output: 3 0

I thought it will print either junk values or crash by 2nd print statement. But it is always printing 0.

1) Does this behaviour depend on the compiler?

2) if I try to deallocate the memory twice using free(), core dump is getting generated. In the man pages, it is mentioned that program behaviour is abnormal. But I am always getting core dump. Does this behaviour also depend on the compiler?

Answer 1

As described in gnu website

Freeing a block alters the contents of the block. Do not expect to find any data (such as a pointer to the next block in a chain of blocks) in the block after freeing it.

So, accessing a memory location after freeing it results in undefined behaviour, although free doesnt change the data in the memory location. U may be getting 0 in this example, u might as well get garbage in some other example.

And, if you try to deallocate the memory twice, on the second attempt you would be trying to free a memory which is not allocated, thats why you are gettin the core dump.