[M3devel] random-acces I/O

[Dragiša Durić]

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>