changing I/O channels

From: Eric Smith (eric-no-spam-for-me_at_brouhaha.com)
Date: 02/28/04 

Date: 27 Feb 2004 15:04:08 -0800

I'm embedding Tcl as a debugger language into a simulator, similar to what
I've done in the Altogether simulator for the Xerox Alto:
    http://altogether.brouhaha.com/

In this simulator, Tcl runs in its own thread. I need the Tcl stdin,
stdout, and stderr channels to be directed to a PTY, but the PTY is on
file descriptors other than 0, 1, and 2. I can't play games with
rearranging the Unix file descriptors, because it would interfere with
standard I/O in my other two threads.

I tried the code below, but it segfaults in the call to Tcl_RegisterChannel().
I also wasn't sure whether Tcl_UnregisterChannel() was the right thing to
call. Initially I tried Tcl_Close(), but that complained "called Tcl_Close
on channel with refCount > 0".

I'm also somewhat confused by the documentation for Tcl_MakeFileChannel().
The man page says that you have to register the channel with
Tcl_RegisterChannel. It also says that if one of the stnadard channels
was previously closed, the act of creating the channel also assigns it as
a replacement. But surely it must be the act of registering the channel,
not the act of creating it, that assigns it as the replacement?

Thanks!
Eric Smith
[To reply by private email, please remove the obvious spam-proofing from
my email address.]

Tcl_Channel tcl_reopen (Tcl_Interp *interp, char *name, int fd, int rw)
{
  Tcl_Channel chan;

  chan = Tcl_GetChannel (interp, name, NULL);
  if (chan)
    {
      if (Tcl_UnregisterChannel (interp, chan) != TCL_OK)
fprintf (stderr, "error unregistering Tcl %s channel\n", name);
    }
  else
    fprintf (stderr, "error getting Tcl %s channel\n", name);
  chan = Tcl_MakeFileChannel ((void *) fd, rw);
  Tcl_RegisterChannel (interp, chan);
  /* need to register? */
  return (chan);
}

void init_tcl (dbg_t *dbg)
{
  cmd_entry *ce;

  dbg->tcl_interp = Tcl_CreateInterp ();

  dbg->tcl_stdin = tcl_reopen (dbg->tcl_interp,
                                "stdin",
                                dbg->slave_pty,
                                TCL_READABLE);
  dbg->tcl_stdout = tcl_reopen (dbg->tcl_interp,
                                "stdout",
                                dbg->slave_pty,
                                TCL_WRITABLE);
  dbg->tcl_stderr = tcl_reopen (dbg->tcl_interp,
                                "stderr",
                                dbg->slave_pty,
                                TCL_WRITABLE);

  for (ce = & cmd_table [0]; ce->name != NULL; ce++)
    Tcl_CreateCommand (dbg->tcl_interp,
       ce->name,
       ce->handler,
       dbg,
       NULL);
}

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