Re: Error checking in XPL

[Carl Distefano CLDistefano@xxxxxxxx]

Reply to note from "R Tennenbaum"  Wed,
03 Jun 98 18:55:29 -0500

-> The other question I've got is surely a long shot: Since I tend
-> to have several Xy sessions open, does anyone happen to know
-> whether there's any way to ensure that if a file is opened in
-> one session, it can only be called up in read-only mode if it's
-> called up in another?

Rafe:

The short answer to your question is: No. The longer answer is,
well, Yes. There's no "native" way to do it, but I can think of at
least one kludge-y, inelegant, non-foolproof procedure that *will*
work. It would involve creating and manipulating a disk file -- a
common resource that's available to concurrent sessions -- that
lists the names of all files open in any XyWrite session.

Here's what I have in mind: Create two XPL routines, Open and
Close. Before CAlling a file, Open would check the file list; if
the filename is there, it would REad rather than CAll the file;
otherwise, it would CAll the file and add the filename to the list.
The companion program Close would first check whether the file is
displayed in Read-Write mode; if it is, it would remove the filename
from the list before SToring or ABorting the file.

It's pretty simple; in fact, I worked out a prototype (appended to
this message in UUencoded form). The procedure is, I think, good;
the main caveat is that you must *unfailingly* use Open and Close
instead of their native equivalents. Once you starting CAlling or
ABorting files in concurrent sessions, the system breaks down.

Attached and UUencoded are a set of routines, which must be run from
a U2 Help file with a Helpkey installed (visit XyWWWeb for
instructions). The usages are:

 OPEN [filename]
and
 CLOSE S[tore]|A[bort]

If you omit the argument from CLOSE, it will prompt you for the
desired option. A third routine, GetFileList, is called by both
Open and Close to read the file list.


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