Looking at
http://xprint.freedesktop.org/cgi-bin/viewcvs.cgi/*checkout*/xprint/xprint/src/xprint_main/xc/programs/Xserver/Xprint/attributes.c?content-type=text%2Fplain&rev=1.5

The code calls |fork()| but doesn't |flush()| any files/sockets etc. before
doing that
-- snip --
/*
 * ExecuteCommand takes two pointers - the command to execute,
 * and the "argv" style NULL-terminated vector of arguments for the command.
 * We wait for the command to terminate before continuing to ensure that
 * we don't delete the job file before the spooler has made a copy.
 */
static void
ExecCommand(pCommand, argVector)
    char *pCommand;
    char **argVector;
{
    pid_t childPid;
    int status;

    if((childPid = fork()) == 0)
    {
        /* return BadAlloc? */
        if (execv(pCommand, argVector) == -1) {
            FatalError("unable to exec '%s'", pCommand);
        }
    }
    else
    {
        (void) waitpid(childPid, &status, 0);
    }
    return;
}
-- snip --

I've seen some complains about problems when printing with some spoolers, the
error message on the client side was |Xlib: unexpected async reply (sequence
0x4000)!|.

Alan:
Is it possible that calling |fork()| may require to flush the streams/files/etc.
before calling it to prevent that the child process may flush cloned data back
to the client ?