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Date: Sat, 27 Feb 2010 17:45:23 -0500
Message-ID: <22a0ef081002271445l7a79aaebna536c16fed4495ae@mail.gmail.com>
Subject: Re: LinkList implementation
From: Michael Hwang <michael.a.hwang@gmail.com>
To: Zsh hackers list <zsh-workers@zsh.org>
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Sorry, I made a typo. The second code snippet is from newlinklist(),
not newlinknode(), as I wrote.

Michael Hwang

On Sat, Feb 27, 2010 at 5:33 PM, Michael Hwang
<michael.a.hwang@gmail.com> wrote:
> In the process of writing a new builtin, I discovered an oddity of
> zsh's implementation of LinkList, which is a doubly linked list.
>
> LinkList alist =3D newlinklist();
> LinkNode node;
> for (node =3D lastnode(alist); node; decnode(alist)) {
> =A0 =A0 ((SomeStructPointer) getdata(node))->someField;
> }
>
> Since no nodes have been added to the LinkList, one would expect the
> body of the loop to not run at all. However, it does, and crashes
> because attempting to access someField dereferences a null pointer. I
> think this is due to this line in [z]newlinknode():
>
> list->list.last =3D &list->node;
>
> I'm not exactly sure why LinkLists are set up this way; I assume it's
> a tricky way to allow [z]insertlinknode() to be used in the pushnode()
> macro. However, this makes the lastnode() macro return non-null on an
> empty LinkList. Walking backwards along a LinkList is only done once
> in the entirety of zsh (the decnode() macro is only used once), which
> is why I think no one has run into this bug before. I'm hoping someone
> who has a better understanding of LinkLists can fix this.
>
> Michael Hwang
>