Who moved my cheese?

... er ... pointer ?

Here is another real-life example of buggy code that I found.

This piece of code aims to create a copy of a doubly linked list
// - localHead is the head of the list of blocks being copied
// - new_blk is the block next to the tail the list being copied
// - bodyTail is the tail of the current list,
// at the end of which the newly copied list is appended

listBlk *tmpBlk = localHead;
listBlk *nxtBlk = NULL;
listBlk *dupFirstBlk = NULL;

while (tmpBlk && (tmpBlk != new_block))
{
nxtBlk = tmpBlk->copy();
if (dupFirstBlk)
{
nxtBlk->back(tmpBlk->back());
nxtBlk->next(tmpBlk->next());
}
else
{
nxtBlk->setBackNext(bodyTail);
nxtBlk->next(new_block);
dupFirstBlk = nxtBlk;
}
tmpBlk = tmpBlk->next();
}
nxtBlk->next(new_block);


This has beeing lying around dormant for years .... (just because the list never had more than one blocks, owing to the way the code is structured)

And once again, what is the problem here ?