Re: [Qemu-devel] [PATCH] opengl rendering in the sdl window

[Jamie Lokier <jamie@shareable.org>]

Anthony Liguori wrote:
> The difficulty is deleting the shared memory segment reliably.
> 
> Normally, what you want to do is:
> 
> shmget()
> shmat()
> shmctl(IPC_RMID)
> 
> ...
> 
> shmdt()
> 
> And the first sequence has to be as reliable and quick as possible 
> because if you exit before the shmctl(), you'll leak the shared memory.

Yes, it's ugly, but it's the same problem faced by Gtk apps etc. which
are writing images - doesn't even SDL have this problem?

The nearest thing to "right" I found in X11-SHM apps is:

   - Catch many signals and shmctl(SHM_RMID) in the handler before
     re-raising the signal to kill the program.
   - Try to recognise old, stale shm segments when starting anew
     and remove them.  This can be done by putting a signature into
     them somewhere which won't be overridden in use.

(There is also some trickiness in ensuring that the connection really
is local, and you're not doing a remote connection which just happens
to successfully attach to a SHM segment on a different machine with
the same id.  With SSH X11 forwarding and other ways X11 is proxied, I
found the only way to be sure is create a segment, write strong random
values to it, ask the X server to read from it and return the pixel
values over the normal connection, and verify if they match.)

But that's X11-SHM.  For a new protocol, you can do this:

   - fd = shm_open()
   - shm_unlink()
   - pass file descriptor over the connection.

Or even this:

   - fd = open("/tmp/XXX")
   - unlink()
   - pass file descriptor over the connection.

Both ends call mmap().

It's not perfect (there really should be a create-shmem-with-no-name
syscall), but the window for leaks is quite small, and it's fine to
use in a library.

-- Jamie