Date:	Sat, 29 Jan 94 02:07:35 JST
From:	Stephen Turnbull <turnbull AT shako DOT sk DOT tsukuba DOT ac DOT jp>
To:	brssp2!twuyts AT brgtw DOT att DOT com
Cc:	bergman AT panix DOT com, djgpp AT sun DOT soe DOT clarkson DOT edu
Subject:	fseek / share

   I'm not an expert on DOS and SHARE, but could somebody explain how it
   exactly works ? What's the effect of the unlink when SHARE is running ?

I'm no expert either, but here goes...

Basically, share sets up some kind of semaphore that prevents
processes that don't own a file from doing dangerous things to it.  I
don't know quite what groff is doing, but I assume it's creating a
temporary file of some kind.  This kind of behavior is fine under
Unix, because under Unix temporary files are a system resource.  (I'm
not sure if this is technically true, but there are utility functions
for handling them, at least for getting tmpfile names.  In fact, groff
may use the unlink to completely hide the tmpfile from any other
process.  This would depend on Unix not touching any of the file's
resources, like the inode, until it's closed.  This seems reasonable
to me.)
    But under DOS, temporary files are not different in any way from
ordinary files; and worse, DOS *keeps the file's allocation
information in the directory entry*, not in a separte resource like an
inode under Unix.  This means that there are two bad things that can
happen if groff is allowed to unlink the file.  One is that other
programs could do the same thing.  Well actually, the problem is that
the *same* program in a different process could do it.  This is
typical of DOS programs which typically call their tmpfiles "tmpfile":
and the second copy of such a program would then go overwrite it!
Worse yet, if groff were allowed to unlink the file, then the
directory entry would be indicated as deleted, and very probably the
next file opened would reuse that space.  This is a good way to fill
up your disk with "lost chains."  This is one of many reasons why DOS
Windows can't do real multitasking very well.

   it is made by Martin Filteau (fm59 AT music DOT mus DOT polymtl DOT ca) and fools any
   program that needs share into beleiving it is running (such as Word 6).
   I don't know what the possible dangers are, but it works for me.

See above.  As long as you only run one copy of each program you
should be in good shape.  Just pray that none of your other programs
running in the background ever open a file using the space in the
directory that groff's tmpfile uses.

    --Steve

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