Re: Substitute EOF marker

[Dorothy Day]

Robert,
    Thanks for the clearest explanation I've ever seen of why the
Ctrl-Z EOF marker is useful. Some of it I've seen before, but never pulled together so
well. I hope the rest of "the public" appreciates your effort too.

    --Dorothy Day

On Wed, 15 May 1996, Robert Holmgren wrote:

> There are some very unsettling problems when you read past the 'EOF'
> Ascii-26 character in certain XyWrite files, and I don't mean that you see
> the Document Info either! (I.e., if you have DocInfo turned on, with
> default IO). DocInfo is harmless, who cares. The real danger is with
> Help files.
> > Normally, when you LOAD a Help file, data about each routine within it is
> indexed by XyWrite, and this index is stored on disk at the end of the
> file, right after the Ascii-26. You can easily verify this just by LISTing
> any LOADed Help file. These indexes are what make the Help files so
> incredibly fast: the index gets cached, and you get direct disk access.
> > (In case you're interested, and just so this gets publicly
> documented someplace once, the indexes list information in this order [all
> values in hex]: Length of FrameName(s), FrameName(s) as a string, Frame
> Type in hex, Offset of opening Ascii-1 or Ascii-2, an Ascii-0 separator,
> number of bytes to read until first "{" character of next-following frame,
> length of Index entry itself, Ascii-0 separator. Whereas this isn't all
> that interesting, it tells you fairly clearly what data gets checked -- and
> what does not get checked -- when XyWrite LOADs a Help file, and therefore
> represents a short list of _all_ the possible causes of bad LOADs: bad
> frame types, bad sets of {{ and }} curly braces, bad opening Ascii-2s,
> illegal FrameNames like e.g. "{{53rdroutine}}" -- its gotta be
> "{{5,3rdroutine}}" or "{{5Thirdroutine}}". In other words, it ain't bad
> syntax that's crashing your LOAD, it isn't mismatched IF...ENDIFs, it isn't
> a whole raft of things, but just an item on the short list above.)
> > Obviously, XyWrite can't function without these indexes! Now, if you
> enable reading past EOF, the index will be visible to EDITOR, and even
> though some characters (e.g. Ascii-0) will not display, or will be
> literally edited out of the copy in the work window, Help will still
> function properly -- _until_ you make changes to Help and attempt to SAve
> those changes and re-LOAD! At that point, Help will no longer work
> properly, at least from the location of the edit to the end of the file.
> User will commence to go nuts at this point, because user will not know
> what is wrong.
> > This anyway is the way it works in Xy4. Active writers/manipulators of
> Help files must not set default 1A=1! Somebody else should check NB for
> this behavior.
> > My point is that 'EOF' is no frivolous little thing, no anachronism, but
> a key mechanism for maintaining internal control over some of Xy
> function. Hate to repeat myself, but we should have the benefit of a Tech
> Support person here, if this list is going to be the main avenue of
> communication between TTG and users. TTG shouldn't skimp on this. Ken is
> no substitute for the real thing!
> > > -----------
> Robert Holmgren
> -----------
> > >

---
Dorothy Day
School of Library and Information Science
Indiana University