On an FC5 box running 2.6.9-34.ELsmp with xorg 7.0.0/1.0.1 rpms, I'm
having an issue getting output from an Xvfb session started with -

Xvfb :2 -screen 0 640x480x16 -ac -shmem &
xclock &

Dumping root window to a file with the utility xwd or IM's import works. I can
see the root window and the xclock window.

I need to include this type of functionality in another program, so to start
with i wrote a simple program (shm2xwd) that would grab the xwd format content
from shmem which is "meant" to be written by Xvfb and output it to a file. In
essence it does the following operations:

...
shmptr = shmat( shmid, (void *)0, SHM_RDONLY ); /* attach to shmem */
int ctlrv = shmctl (shmid, IPC_STAT, &shmbuffer); /* get buffer info */
printf ("Buffer Size: %d\n", shmbuffer.shm_segsz);
printf ("Buffer Attach Time: %s", ctime(&shmbuffer.shm_atime));
printf ("Buffer Change Time: %s", ctime(&shmbuffer.shm_ctime));
fd = open(argv[2], O_WRONLY | O_CREAT, mode);
write (fd, shmptr, shmbuffer.shm_segsz); /* write framebuffer to file */
...

When i try and view file written by shm2xwd, all i see is an empty root window -
I'm wondering if Xvfb is updating the shmem at all because looking at the output
of this program:

./shm2xwd 3342352 x.xwd
Buffer Size: 615328
Buffer Attach Time: Wed Sep 27 01:10:00 2006
Buffer Change Time: Tue Sep 26 17:27:25 2006

I see that the buffer change time was when i first started the Xvfb,
but it hasn't been updated since - i.e. when i started/restarted
xclock multiple times. I believe this is a bug.