Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!unido!ira.uka.de!smurf!urlichs
From: urlichs@smurf.ira.uka.de (Matthias Urlichs)
Newsgroups: comp.sys.mac.programmer
Subject: Re: File System Re-entrancy
Message-ID: <1258@smurf.ira.uka.de>
Date: 29 Nov 89 22:17:54 GMT
References: <924@maytag.waterloo.edu>
Reply-To: urlichs@smurf.ira.uka.de (Matthias Urlichs)
Organization: Uiversity of Karlsruhe, FRG
Lines: 46

In comp.sys.mac.programmer jb@aries5.UUCP, writes:
< This may seem like a strange question but seeing the discussion on
< multi-tasking, I have a related question.
< 
< Is there any reliable way to make the device manager re-entrant.
<  
That should be the file manager..?

< In other words if I do a PBRead() to a device driver can that device
< driver synchronously do another PBRead() to another device driver?  
< 
One simple answer: No. Not even asyncronously.

I assume that you want to do something like what I tried to do a few years
back: the straight-ahead way to partition a disk would of course be to
open a file and then write a "disk driver" which simply does PB{read,write}()
calls on that file.
No such luck. Apart from the obvious (the file manager queue), the file
manager implements its own internal stack for async requests, plus state
variables and (since HFS) two special files which make this even more complex.

My hack of a solution was to allocate the file contiguously (using
AllocContig) and then, after opening the file, calculate its beginning on disk
from the information from GetVolInfo() and from the FCB (_not_ generally
recommended).
You will have to test that the file is still contiguous (if somebody copies
it, it is likely not to be anymore); this is easy because the FCB contains the
first three extents: the last two should be zero.  You will also run into the
fact that doing this right (ie. if your driver is called asynchronously; it
may well be) is not easy.
This "solution" (apart from being a hack anyway) only works on local disks
which indicates that there should be a better solution.
< 
< Jim Bruyn

I suggest that you post a note describing exactly what you want to do; then
we'll all come up with some better ideas. ;-)

NB: Some rather vague Sytem 7.0 information (reliability level and source
attribution: grapevine) says that the paging file for virtual memory will not
have to be contiguous. I wonder if Apple will do this right (ie. make the
emergency file manager call available for those who need it), or not (ie. hack
it in, using heaps of undocumented file manager features). Based on past
experience I tend to expect the latter. :-(
-- 
Matthias Urlichs