Re: [Qemu-devel] [PATCH v2] qemu_file: use fwrite() correctly

[Eric Blake <eblake@redhat.com>]

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On 02/11/2014 05:00 PM, Juan Quintela wrote:
> fwrite() returns the number of items written.  But when there is one
> error, it can return a short write.
> 
> In the particular bug that I was tracking, I did a migration to a
> read-only filesystem.  And it was able to finish the migration
> correctly.  fwrite() never returned a negative error code, the 1st
> time it returns 0, after that it returns 4096. (migration writes
> chunks of about 14000 bytes).  And it was able to "complete" the
> migration with success (yes, reading the file was a bit more
> difficult).  On the 1st fwrite() for the read-only filesystem,
> it returns an errno of -EPIPE, that is exactly what has failed.
> 
> To add insult to injury, if your amount of memory was big enough (12GB
> on my case), it overwrote some important structure, and from them,
> malloc failed.  This check makes the problem go away.
> 
> Signed-off-by: Juan Quintela <quintela@redhat.com>
> ---
> 
> v2: a.k.a Paolo was right
> 
>     On the first call to fwrite() it returns 0, and errno is setup to
>     EPIPE, exactly what we wanted.
> 
>     Once here, improve the commit message.

Reviewed-by: Eric Blake <eblake@redhat.com>

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


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