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Subject: Re: [Qemu-devel] [PATCH 4/5] block: cow - used QEMU_PACKED for
 on-disk structures
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To: Jeff Cody <jcody@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Markus Armbruster <armbru@redhat.com>, stefanha@redhat.com, qemu-devel@nongnu.org

On 09/25/2013 12:01 PM, Jeff Cody wrote:
> It is from UML, right?  Is there an official spec that is still around
> (most of the links I have found suffer from link rot)?  The closest I
> could find to a spec were old UML patches for x86_64 that cleaned up
> some data types, so that the following was defined:
> 
> struct cow_header_v2 {                                                                                                    
>    __u32 magic;
>    __u32 version;
>    char backing_file[PATH_LEN_V2];
>    time_t mtime;
>    __u64 size;
>    int sectorsize;
> };
> 
> That remains ambiguous, although given the era I suppose it could be
> argued that 32-bit architecture and alignment is assumed.  But if this
> is the original spec, then it seems like a non-portable one.

Indeed.  Not just the potential padding there, but of course
the size of time_t varies between hosts.  And since we use
an int32_t not time_t, we're not even necessarily compatible
with UML.


r~