Bug in IO libraries when sending data through a pipe?
20 Mar 2002 15:00:32 +0900
Jens Petersen <firstname.lastname@example.org> writes:
> > The problem is that the child process doesn't receive all the data which
> > the parent sends. It's as if "hPutStr vonh txt" sends the data lazily
> > somehow, and "hClose vonh" closes the pipe prematurely.
> > It varies from run to run exactly which data gets through. If I cause the
> > child process to read all its input immediately, the problem doesn't
> > seem to occur. Normally, it does so gradually, which takes a few seconds.
> > I'm using GHC 5.02.2
> Quite possibly could be a bug. Lazy IO is rather subtle I
> think, specially when done across pipes. I faced some
> similar problem with in POpen recently. You can see how I
> solved it (worked round it?) by comparing the latest release
> 1.00 with the previous one 0.00.1:
> In comparison Posix.runProcess allows attaching file handles
> to the in, out and error pipes, which can be written to and
> read from eagerly I suppose.
Also I just rediscovered Manuel Chakravarty's HPL (Haskell
Ports Library), which provides a rather elegant,
It compiles fine under ghc-5.02.2, and using the BufferMode
patch included at the end output seems to be ok, but input
of more than 2048 bytes doesn't seem to be being handled
Eg with the test program below:
% test-processes ping localhost
works (with the aforementioned patch below to Processes.hs), but
% cat 4096 | test-processes cat
[4096 is a file of 4096 chars]
mostly gives no output, but occasionally I see
Warning: Ports.listenToPort: Attempted to listen to a closed port!
Needs some debugging I guess. :)
-- ghc -o test-processes `ports-config --cflags --libs` test-processes.hs
import IO (openFile, hGetContents, IOMode(..), hSetBuffering, BufferMode(..))
import Monad (unless)
main :: IO()
main = do
inpt <- getContents
withPorts  $ \ (cmd:args) ->
outpt <- newPort ' '
errpt <- newPort ' '
let p = proc cmd args
p inpt outpt errpt
out <- listenToPort outpt
mapM_ putStrLn $ lines out
errclosed <- isClosedPort errpt
unless errclosed $
err <- listenToPort errpt
putStrLn "test finished"
RCS file: /home/chakcvs/cvs/ports/lib/Processes.hs,v
retrieving revision 1.7
diff -u -r1.7 Processes.hs
--- Processes.hs 2001/07/04 16:15:52 1.7
+++ Processes.hs 2002/03/20 05:28:33
@@ -145,6 +145,8 @@
stdoutWriteHandle <- fdToHandle stdoutWriteFD
stderrReadHandle <- fdToHandle stderrReadFD
stderrWriteHandle <- fdToHandle stderrWriteFD
+ hSetBuffering stdoutReadHandle LineBuffering
+ hSetBuffering stderrReadHandle LineBuffering
-- the child must close the pipe ends that it doesn't use (especially, the
-- write end)