Re: [libvirt] locking down struct size/layout in remote-protocol.x

On 05/06/2010 12:35 PM, Jim Meyering wrote: > This week we noticed that a change to struct remote_error > was causing trouble (new member addition changed the size of > the object being decoded -- bad for the protocol). > > In order to ensure that no such changes sneak in again, > I'm planning to do the following. > > pdwtags src/libvirt_driver_remote_la-remote_protocol.o > > prints, among other things, detailed type info for every struct: > > /* 89 */ > struct remote_nonnull_domain { > remote_nonnull_string name; /* 0 8 */ > remote_uuid uuid; /* 8 16 */ > int id; /* 24 4 */ > > /* size: 32, cachelines: 1, members: 3 */ > /* last cacheline: 32 bytes */ > > /* BRAIN FART ALERT! 32 != 28 + 0(holes), diff = 4 */ > > }; /* size: 32 */ Ouch. Architecture sizing plays a role. On a 32-bit machine, the first struct is: /* 86 */ struct remote_nonnull_domain { remote_nonnull_string name; /* 0 4 */ remote_uuid uuid; /* 4 16 */ int id; /* 20 4 */ /* size: 24, cachelines: 1, members: 3 */ /* last cacheline: 24 bytes */ }; /* size: 24 */ Are we sure migration between 32-bit and 64-bit hypervisors works? And if it does, then these structs don't quite match what is actually sent over the wire. At any rate,