Re: Changed behaviour when reading from FIFOs!
On Mon, Oct 29, 2001 at 02:52:25PM -, Simon Marlow wrote: > > The blocking is essential since I need to be able to use MVars > > between the threadWaitRead & the hGetLine (remember the note I > > sent about fork()ing). > > Sorry, I can't remember that - could you remind me? The hGetLine > already blocks if there's no data in the FIFO, the extra threadWaitRead > will only work if the Handle is in NoBuffering mode, because otherwise > there might be data in the handle buffer waiting to be read which > threadWaitRead would be unable to detect. I'm using 'fork' (the real thing, not forkIO), in a Concurrent Haskell programm and I need a way to lock out multiple readers from the same file handle because of the sharing when forking. -- Volker Stolz * [EMAIL PROTECTED] * PGP + S/MIME ___ Glasgow-haskell-bugs mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
RE: Changed behaviour when reading from FIFOs!
> On Mon, Oct 29, 2001 at 02:08:23PM -, Simon Marlow wrote:
> > This appears to be because the dummy Handle opened in
> WriteMode is being
> > garbage collected and closed, which leaves the FIFO with no
> writers so
> > you get EOF. 5.02 is behaving correctly here. You can add an extra
> > 'hClose dummy' at the end of 'main' to prevent the EOF exception.
>
> Argh, that was exactly what I feared when I read the file-handles
> discussion on the list. Only I didn't realize that the feature
> got indeed introduced into ghc-5.02.
Just to clarify: this isn't really new behaviour, but there might be a
difference in the timing of GC and the scheduling of finalizers which
means the observable behaviour between 5.02 and 5.00.2 is different. Of
course, there might also have been a bug in 5.00.2's IO implementation.
> > You have some extra blocking in there: just comment out the
> first two
> > lines of fifoReadLoop, and everything should be fine.
> This also fixes
> > the other problem you mentioned above ("varying numbers of last").
>
> The blocking is essential since I need to be able to use MVars
> between the threadWaitRead & the hGetLine (remember the note I
> sent about fork()ing).
Sorry, I can't remember that - could you remind me? The hGetLine
already blocks if there's no data in the FIFO, the extra threadWaitRead
will only work if the Handle is in NoBuffering mode, because otherwise
there might be data in the handle buffer waiting to be read which
threadWaitRead would be unable to detect.
Cheers,
Simon
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Re: Changed behaviour when reading from FIFOs!
On Mon, Oct 29, 2001 at 02:08:23PM -, Simon Marlow wrote:
> This appears to be because the dummy Handle opened in WriteMode is being
> garbage collected and closed, which leaves the FIFO with no writers so
> you get EOF. 5.02 is behaving correctly here. You can add an extra
> 'hClose dummy' at the end of 'main' to prevent the EOF exception.
Argh, that was exactly what I feared when I read the file-handles
discussion on the list. Only I didn't realize that the feature
got indeed introduced into ghc-5.02.
> You have some extra blocking in there: just comment out the first two
> lines of fifoReadLoop, and everything should be fine. This also fixes
> the other problem you mentioned above ("varying numbers of last").
The blocking is essential since I need to be able to use MVars
between the threadWaitRead & the hGetLine (remember the note I
sent about fork()ing).
--
Volker Stolz * [EMAIL PROTECTED] * PGP + S/MIME
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RE: Changed behaviour when reading from FIFOs!
Volker Stolz writes:
> Something broke when reading from FIFOs in the transition from
> 5.00 to 5.02. The following program behaves as it should in
> 5.00, but with 5.02 it fails after printing the last line
> (strangely for varying numbers of "last"!) with
>
> Fail: end of file
> Action: hGetChar
> Handle: {loc=foo,type=readable,binary=False,buffering=none}
> File: foo
This appears to be because the dummy Handle opened in WriteMode is being
garbage collected and closed, which leaves the FIFO with no writers so
you get EOF. 5.02 is behaving correctly here. You can add an extra
'hClose dummy' at the end of 'main' to prevent the EOF exception.
> Another issue is that it won't work at all with LineBuffering:
> It will just print one line and then sit around doing nothing.
You have some extra blocking in there: just comment out the first two
lines of fifoReadLoop, and everything should be fine. This also fixes
the other problem you mentioned above ("varying numbers of last").
Cheers,
Simon
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