Actually, I take that back. If it is a worry that the internal representation will change, it seems also unsafe to be casting the sixteen bytes as a uuid_t when copying into them with uuid_copy(). If the internal representation changed to add a new field, then there would be a buffer overrun when using uuid_copy(). If the internal representation changed the order of the bytes, uuid_copy() would propagate that to the on-disk structure and yield compatibility issues.
Perhaps it would be safer to store the UUIDs on disk as a string. Or add to uuid.[ch] a function to get the raw bytes in a fixed order suitable for storing on disk?
Perhaps it would be safer to store the UUIDs on disk as a string. Or add to uuid.[ch] a function to get the raw bytes in a fixed order suitable for storing on disk?
On Thu, Apr 2, 2020 at 11:52 AM John Dorminy <jdorminy@xxxxxxxxxx> wrote:
That does make sense. May I request, then, using UUID_SIZE instead of 16? Perhaps with a compile-time assertion that UUID_SIZE has not change from 16?On Thu, Apr 2, 2020 at 11:10 AM Hannes Reinecke <hare@xxxxxxx> wrote:On 4/2/20 4:53 PM, John Dorminy wrote:
> I'm worried about hardcoding uuid members as u8[16].
>
> May I ask why you're not using uuid_t to define it in the on-disk
> structure? It would save the casting of the uuid members to (uuid_t *)
> every time you use a uuid.h function.
>
> Possibly it is customary to use only raw datatypes on disk rather than
> opaque types like uuid_t, I'm not sure. But in that case, perhaps using
> the named constant UUID_SIZE instead of 16 would make the meaning clearer?
>
I prefer to use absolute types (like __u8) when describing the on-disk
format.
When using opaque types like uuid_t there always is the risk that the
internal representation will change, leading to an involuntary change of
the on-disk format structure and subsequent compability issues.
Cheers,
Hannes
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