On 08/29/2014 03:45 PM, Max Reitz wrote: > A refblock entry may have a different size than 16 bits, it may even be > smaller than a byte. Correct the refcount_block_entries calculation > accordingly. > > Signed-off-by: Max Reitz <mre...@redhat.com> > --- > docs/specs/qcow2.txt | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt > index cfbc8b0..531c478 100644 > --- a/docs/specs/qcow2.txt > +++ b/docs/specs/qcow2.txt > @@ -183,7 +183,7 @@ blocks and are exactly one cluster in size. > Given a offset into the image file, the refcount of its cluster can be > obtained > as follows: > > - refcount_block_entries = (cluster_size / sizeof(uint16_t)) > + refcount_block_entries = (cluster_size / (refcount_bits / 8)) >
Consider refcount_order == 0 (that is, no shared blocks, ALL blocks have at most refcount 1). Then, refcount_bits is (1 << 0) == 1. But 1/8 in integer math truncates to 0 (oops, division by zero is undefined); when in reality, the expression you want here is (cluster_size * 8 / refcount_bits). -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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