Re: [Qemu-devel] [PATCH 1/1] parallels: add format spec

[Eric Blake <eblake@redhat.com>]

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On 11/17/2015 11:15 PM, Vladimir Sementsov-Ogievskiy wrote:

>> You may want to compare to the qcow2 spec, which also lists expected
>> byte offsets for each field (rather than having to count how many
>> earlier fields of which widths were specified).
> 
> I've compared qed spec with qcow2 spec and like the first one. What the
> need of specifying each offset? Creating a c-structure is simpler when
> you see types. and than offsetof and sizeof may be used if needed.
> Nobody will #define numeric offsets I think.

You'd be surprised (libvirt has some hard-coded numeric offsets:
https://libvirt.org/git/?p=libvirt.git;a=blob;f=src/util/virstoragefile.c;h=2aa1d90;hb=5ed7afa9d#l159)

Also, listing offsets makes it obvious that you are NOT relying on
compiler padding, and makes it obvious whether you have been careful
that all 64-bit quantities are 8-byte-aligned without wasting space.

The original cow format (not qcow or qcow2) has the awful distinction of
NOT having specified offsets, and had different layouts on 32-bit
platforms than it did on 64-bit platforms; hence, we retired it in
commit 550830f9 as unsupportable.

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


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