From:	SMTP%"RELAY-INFO-VAX@CRVAX.SRI.COM"  5-AUG-1994 13:27:51.06
To:	EVERHART
CC:	
Subj:	add lines at fiole start

Date: Fri, 5 Aug 1994 11:32:15 -0400 (EDT)
From: EVERHART@arisia.gce.com
To:   INFOVAX@arisia.gce.com
Message-Id: <940805113215.62@arisia.gce.com>
Subject: add lines at fiole start

Adding lines of text or the like at the start of a file normally
does mean copying it all. 

However, depending on what's going on maybe a little kludge is at
least possible in principle.

If the material to be added occupies an even number of blocks, either
by design or padding, then in principle one can append an old file
to it by creating a file whose retrieval pointers start with the
new file and then have all the pointers for the old one. During
this operation one would have two aliases for some disk blocks in one's
index file...dangerous, but, hey, if it's YOUR system and you're aware
of what's going on maybe it can be dealt with.

This requires skill in messing with internals, but could be an efficient
way to tack material at the beginning of a file.

I suppose that even the double allocation problem could be "solved"
(sort of) by creating a header for the first bit, and then turning the
old file into an extension header.

*I* wouldn't much like to do it this way, though...it is exceedingly
ugly and a design that uses such a (mis)feature deserves the opprobrium
it will surely get. Nonetheless, there DO arise weird requirements and
this might be a way to deal with this one. If you get it wrong, be
prepared for lots of file structure rebuilding though, and don't expect
all the normal bits to work right with ACLs and the like unless you
can really, REALLY, convince VMS that the extension header is one and
that the new primary header is really full.

In other words, "good luck...you'll need it."

Glenn
Everhart@Arisia.gce.com