Question

This is a school assignment and I have most of it under control, but there is a small part that is creating a memory leak and I have no more ideas on how to fix it.

We have created a memory allocator, and the trouble part are this two functions.

This first one cannot be modified

void destroy (){
  free():
  tot_alloc = 0; }

This second one is the one I’m working on

void free(){
  munmap(pool, PAGE_SIZE);
  Page* f = this;
  f = f->prev;
  if (f != NULL)
    f->destroy();
}

I have written all of the free() function and was asked in the assignment to call destroy().
I realize that this function is not destroying the first “this” because immediately is going to f->prev but I have no clue how to make it first destroy this and the go to prev.

I hope this is not a too silly question.

Many thanks!

Nico

Answer 1

To remove an element from a singly-linked list, you must walk to that element to find the element before it in the list. Then you have to ‘stitch’ around it. Like this:

void free()
{ // Note: This has no error checking!
    Page *f = tail, *next_f = NULL;
    while(f != this)
    { // find this node to find the node after this node
        next_f = f;
        f = f->prev;
    }

    // found it. Now, next_f is the node after this node.
    // close the chain so this link can go away
    if (next_f == NULL) // there is no node after us, we must be the tail
       tail = prev; // the tail is now whatever node is after us
    else // in the node after us, the node before it is now the node before us
       next_f->prev = prev; 

    destroy(); // we are unlinked from the chain so can now be freed
}