54c
Subj : Re: Reader Writer Puzzle
To   : comp.programming.threads
From : bonnie.caballero
Date : Wed Jun 15 2005 03:25 pm

I have some small code here for the Reader and Delete tasks.  But I
realized that after the Reader increments p to p->next and unlocks the
read lock, the Delete task might delete the node pointed to by
(Reader's) p and hence, the Reader would fail when it tries to access p
in its next loop iteration.  I'm thinking of using reference counts to
prevent this situation but it's not yet in the code.  What do you
think?

TIA

----
// Note: H is the list head, is always not NULL and its data is not
used.
void Reader(void)
{
  Node * p = NULL;
  Node pa;
  Boolean got1 = FALSE;

  ASSERT(H != NULL);
  while (1)
  {
    read_lock(lock);
    if (p == NULL)
    {
      p = H->next;
    }
    if (p != NULL)  // Wrong: p might have been previously deleted.
    {
      memcpy(&pa, p, sizeof(Node));
      got1 = TRUE;
      p = p->next;
    }
    else
      got1 = FALSE;
    read_unlock(lock);
    if (got1)
      Access(&pa);
  }
}

void Delete(Node * T)
{
  Node * p = H;

  ASSERT(p != NULL);
  while (p->next != NULL)
  {
    if (p->next == T)
    {
      write_lock(lock);
      p->next = T->next;
      free(T);
      write_unlock(lock);
      break;
    }
  }
}

.

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