Article 155784 of comp.os.vms:
In article <527572$bum@test-sun.erols.com>, dhowell@erols.com writes...
> Can anybody tell me the system service used by the show display command?

     An IO$_SENSEMODE QIO call is used to query a WSAx pseudo-
device.  But the SHOW command uses the translation of logical name
DECW$DISPLAY to find your default WSAx device.  Without having
access to that translation for some other process, knowing how
to use $QIO won't be very helpful.

> I have a need to find a WSA device that translates into an X-server
> screen over DECnet transport on a remote node in a network (as opposed
> to a cluster), and I must get it from inside a detached process run
> from loginout.

     I'm curious as to why identifying the WSAx pseudo-device
would be useful.  It is not involved in any active X11 connections,
only in providing information to XOpenDisplay() when a connection
is first established.  The WSAx device can later to modified at
will, so querying it doesn't necessarily yield any information
about active connections.

     To answer the question you've asked--which seems to me like
it might not be what you really want to know--you can use
SYS$DEVICE_SCAN() to run through "_WSA*", and then use SYS$QIOW
to obtain the transport value for each one.  Code to do the
latter has been posted here quite a few times over the years,
so is available in the Info-VAX archives if you look hard enough.
This C header file is the most interesting bit of some code I
first posted here six years ago:

-----
/* ws_io.h */
/* defs for obtaining data from _WSAn: via $QIO */
/*        [ derived from V5.2 listings ]        */

#define DECW$C_WS_DSP_NODE      1
#define DECW$C_WS_DSP_TRANSPORT 2
#define DECW$C_WS_DSP_SERVER    3
#define DECW$C_WS_DSP_SCREEN    4

#define IO$V_WS_DISPLAY 6
#define IO$V_WS_PERM    7
#define IO$M_WS_DISPLAY (1<<IO$V_WS_DISPLAY)    /* 0x0040 */
#define IO$M_WS_PERM    (1<<IO$V_WS_PERM)       /* 0x0080 */

/* $QIO[W] ( efn, chan,
	     func [== IO$_SENSEMODE | IO$M_WS_DISPLAY],  iosb, astrtn, astprm,
	     p1 [== &buffer], p2 [== sizeof buffer],
	     p3 [== DECW$C_WS_DSP_xxx],  p4 [0], p5 [0], p6 [0] )
*/

#define DT$_DECW_PSEUDO 7       /* from WSDRVRDEF.SDL */
-----