[Xcb] [PATCH] Force XCB event structures with 64-bit extended fields to be packed.

Josh Triplett josh at joshtriplett.org
Tue Dec 31 00:45:14 PST 2013

On Tue, Dec 31, 2013 at 09:06:17AM +0100, Mark Kettenis wrote:
> >  With the advent of the Present extension, some events (such as
> >  PresentCompleteNotify) now use native 64-bit types on the wire.
> >
> >  For XGE events, we insert an extra "uint32_t full_sequence" field
> >  immediately after the first 32 bytes of data.  Normally, this causes
> >  the subsequent fields to be shifted over by 4 bytes, and the structure
> >  to grow in size by 4 bytes.  Everything works fine.
> >
> >  However, if event contains 64-bit extended fields, this may result in
> >  the compiler adding an extra 4 bytes of padding so that those fields
> >  remain aligned on 64-bit boundaries.  This causes the structure to grow
> >  by 8 bytes, not 4.  Unfortunately, XCB doesn't realize this, and
> >  always believes that the length only increased by 4.  read_packet()
> >  then fails to malloc enough memory to hold the event, and the event
> >  processing code uses the wrong offsets.
> >
> >  To fix this, mark any event structures containing 64-bit extended
> >  fields with __attribute__((__packed__)).
> The problem with that approach is that this is a GCC-ism that isn't
> portable.  I believe the sun studio compiler supports something similar. 
> But other compilers might not.

Do you know of a specific target system that XCB actually supports which
has no mechanism for specifying packed structures?

> I also don't quite trust the compiler to get to emit the right
> instructions for unaligned access on all platforms.

Compilers sometimes have bugs, but this is an area where I'd expect them
to generally get the details right.

- Josh Triplett

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