[M3devel] random-acces I/O

[Mika Nystrom]

Yes, I am aware of that.  My point was simply that if you want
a reader and writer accessing the same data it's probably easiest
to connect together the reader and writer directly, somehow, and write
some sort of "watcher" that flushes modifications to disk "on the side"
of the interaction between the reader and writer.  

Btw, I do have some code that does this sort of thing (not entirely
random access), and to run both on Unix and NT386GNU, yes, I do
close the file every time I "turn it around" from reading to writing 
or vice versa.

    Mika


=?UTF-8?Q?Dragi=C5=A1a_Duri=C4=87?= writes:
>rdwr is package by Blair MacIntyre and it's not related to anything
>File.* except indirectly, ie if you make File Wr part of TeeWr. 
>
>On Sat, 2008-03-01 at 14:49 -0800, Mika Nystrom wrote:
>> Hi Hendrik,
>> 
>> The answer is, "it depends."  As a matter of fact this was my biggest
>> area of trouble when porting a program from PM3 on Unix to
>> PM3/Klagenfurt for NT386GNU.  On NT386GNU, the default is for
>> Modula-3 to avail itself of Windows's mandatory file locking, whereas
>> POSIX has no such thing.  This means that a Modula-3 program on
>> NT386GNU can't even open a file for reading if you are "lessing"
>> it for instance.  I am sure it can be worked around, but I never
>> bothered to figure out how, sorry (because the behavior I needed
>> was in fact strict locking of this kind, which I had to go to special
>> lengths to get on Unix!)
>> 
>> On Unix, things are a bit more as you describe them, but not exactly.
>> You can definitely have an Rd.T and a Wr.T on a single file, but I 
>> do not believe that the Rd.T and Wr.T will be aware of each other's
>> existence (I could be wrong).  However, there is a package called
>> rdwr(?) that I think may have the sort of functionality you're
>> looking for.  Maybe.
>> 
>> File locking is strictly an OS issue as far as I know and that is not
>> something that Rd and Wr concern themselves with.  
>> 
>> If you need this kind of low-level access I have a feeling Rd and Wr
>> are too high-level for you.  I would suspect you want to build something
> with rdwr (or some other means of getting an Rd out of a Wr or vice 
>> versa) and then extend it with routines for maintaining a faithful
>> disk-based copy of what you want... using lower-level access methods.
>> 
>> Oh yes Rd and Wr are documented in detail in chapter 6 of the Green Book.
>> 
>>     Mika
>> 
>> hendrik at topoi.pooq.com writes:
>> >I need to do direct-access I/O from a Modula 3 program with a file 
>> >format that is externally specified.  It is necessary to both read and 
>> >write in random fashion during a run of the program, prefereably without 
>> >having to close or open the file each time.
>> >
>> >I expect the answers to the following questions are "Yes", but I'd 
>> >appreciate confirmation:
>> >
>> >  ? If I have a File.T (obtained from FS.OpenFile) can I have both a 
>> >    Rd.T and a Wr.T active on it simultaeously?
>> >
>> >  ? Will things written using the Wr.T be immediately available 
>> >    for reading with the Rd.T, with no buffereing problems?
>> >
>> >  ? Are there methods for locking the file -- or portions of it --  
>> >    against simutaneous access by other programs, including ones written 
>> >    in other languages?
>> >
>> >-- hendrik
>-- 
>Dragiša Durić <dragisha at m3w.org>