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Re: [Qemu-devel] [PATCH COLO-BLOCK v7 02/17] quorum: implement block dri

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH COLO-BLOCK v7 02/17] quorum: implement block driver interfaces add/delete a BDS's child
Date: Mon, 27 Jul 2015 08:45:02 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.1.0

On 07/02/2015 09:21 AM, Alberto Garcia wrote:

>>> 3) I don't think it's necessary to set to NULL the pointers in
>>> s->bs[i] when i >= num_children. There's no way to access those
>>> pointers anyway. Same for the ' s->bs[s->num_children] = NULL; ' bit
>>> in quorum_del_child(). I also think that using memset() for setting
>>> NULL pointers is not portable, although QEMU is already doing this in
>>> a few places.
>> OK, will remove it in the next version. Just a question: why is using
>> memset() for setting NULL pointers is not prtable?
> The standard allows for null pointers to be internally represented by
> nonzero bit patterns. However I'm not aware of any system that we
> support that does that.
>    http://c-faq.com/null/confusion4.html
>    http://c-faq.com/null/machexamp.html

What's more, POSIX has very recently taken the stance that memset() to 0
will work on all POSIX systems, even if someone is insane enough to use
a non-zero bit pattern for NULL on modern hardware:


Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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