[Info-vax] Multiple File Readers/Writers

[chrisj.doran%proemail.co.uk at gtempaccount.com]

On Nov 25, 1:48 am, chrisj.doran%proemail.co... at gtempaccount.com
wrote:
> On Nov 25, 1:29 am, Matt <matt-nos... at 9track.net> wrote:
>
>
>
>
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> > I'm trying to port SQLite to OpenVMS/Integrity. It's mostly working so
> > far with only a few minor changes required thanks to the 'unix
> > compatibility' layer in the C runtime library. However what doesn't work
> > is the ability for multiple processes to access the same database file
> > at the same time.
>
> > I've written two small test programs using the same functions as the
> > SQLite module to demonstrate the problem. One is called 'writer' which
> > opens a file test.dat and writes a counter every second into the first
> > few bytes. The second program 'reader' opens the same file and displays
> > the counter once per second. I've tried them on Windows and Linux and
> > they work as expected, however on VMS the counter never updates and the
> > reader seems to be dealing with a cached version of the file on disk.
> > The only way to get the counter to update is to close and open the file
> > on every read operation.
>
> > I've read the relevant sections of 'Guide to OpenVMS File Applications',
> > 'OpenVMS Record Management Services Reference Manual' and the 'C
> > Run-Time Library Reference Manual' and tried many combinations of
> > options but always get the same result.
>
> > What combination of parameters do I need to pass to the 'open' call to
> > get the file sharing to work properly so that the reader does not get
> > stale data?
>
> > The test programs are located here:
>
> >http://www.9track.net/vms/reader.chttp://www.9track.net/vms/writer.c
>
> > Thanks,
>
> > Matt
>
> It's been a while, but I have a vague recollection of having the same
> problem and needing to replace fsync with fflush, or use both, despite
> the docs implying they do the same thing. Sorry, don't have time to
> investigate at this precise moment.
>
> Chris- Hide quoted text -
>
> - Show quoted text -

Sorry, forget what I said; I must have been thinking of some other
problem/system. As you say, the problem is in the reader, as DUMP
shows the file changing.

I note that my (rather old)  manual says that "file sharing is not
supported for stream files" but setting ctx=rec doesn't help. Nor does
turning off buffering with setvbuf. I've tried various other things,
but still can't get it to work. I'll keep thinking.

Maybe there's a better way to sync. ISTR that when I needed to do
something similar, I checked the last update time of the file.

I found a couple of buglets, though: You've omitted the mode argument
to open which results in funny file protection, and tested fd instead
of rc after lseek() and read(). But these don't affect the problem.

Chris