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From: giguere@csg.uwaterloo.ca (Eric Giguere)
Newsgroups: comp.sys.amiga.programmer
Subject: Re: Arexx article in AC Tech Amiga - good but ???
Keywords: ARexx, AC Tech Amiga
Message-ID: <1991Mar27.200758.1432@maytag.waterloo.edu>
Date: 27 Mar 91 20:07:58 GMT
Sender: daemon@maytag.waterloo.edu (Admin)
Organization: University of Waterloo
Lines: 41

Though I haven't seen the article in question, the original poster 
is quite correct in that hard-coded port names can lead to problems.
It's not a simple problem, though, and it shows up in any system
with some kind of public interprocess communication:  how do I identify
an IPC port?

The ARexx resident process (the REXX server) is a good example.  Running
ARexx programs and host applications NEED to know how to locate it so
that they can sent it messages.  So the resident process opens two ports
("REXX" and "AREXX") as its "mailboxes".  The names are well-known and
won't change.  This solution works quite nicely because only one copy of
the resident process will ever be running at any given time.  (It's easy
to check, too -- just see if the port you want to allocate has already
been allocated...)

But then there's the case of programs that can be run multiple times.
What should you do then?  Let's take a text editor as a classic example.
A text editor could be run twice, and each copy can open one or more files
for editing.  The solution advocated in the Amiga User Interface Style
Guide is to open an ARexx port for each open document, with each ARexx
port being of the form

				 <APPNAME>.<NUMBER>

where APPNAME is the base application name ("MYEDIT") and NUMBER is a
"slot number", a simple integer.  The integer is chosen so that the
combination of say "MYEDIT.3" is unique from any other port name.  Thus
a text editor could open port "MYEDIT.1", "MYEDIT.2", etc., as required,
re-using slot numbers whenever possible.

Of course, now you have a different problem... if you want to talk to
a text editor, which port name do you use...?  Well ARexx scripts usually
don't have to worry because the default host address of an ARexx script
will be set to the correct port by the application that starts it.  It's
your own independent scripts that have to worry... In those situations
it may be wise for the application to always open a well-known port for
independent communication.

--
Eric Giguere                                       giguere@csg.UWaterloo.CA
           Unlike the cleaning lady, I have to do Windows.