C memory management with more pointers to the same location

Question

I was playing with this code:

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

void function(int *p){
    free(p);
}

int main(){
    int *i = calloc(3, sizeof(int));
    function(i);
    i[0] = 'a';
    printf("%c\n", i[0]);
    return 0;
}

I expected to get an error, but instead it printed 'a', if 'a' is deallocated why does it print it? What happens in function(int *p) has any effect if the main()?(if you can, explain what happen to that 'p' pointer)

Suppose that I have those two in the same function:

int *a = malloc(...);
int *b = a;

Both of them point to the same piece of memory, but when I have to free it, should I call free on both of them, or just one of them?(again explain why if possible)

Answer 1

That is undefined behavior. Freeing the pointer just tells the OS that the memory allocated can be overwritten. The memory is still there.. for lack of a better word. But anything can happen.. it may print a for you now, at other times it could be another character or number because another program has written to that memory location... anything can happen.

For your last question.. a and b are just symbolic names to an address. Calling free on the address is important, so whether you do free(a) or free(b), you are grand.