Re: Bug in IO libraries when sending data through a pipe?

4 Apr 2002

      Jens Petersen  wrote,
...
Jens Petersen  writes:
Also I just rediscovered Manuel Chakravarty's HPL (Haskell
Ports Library), which provides a rather elegant,
sophisticated approach.
http://www.cse.unsw.edu.au/~chak/haskell/ports/
It compiles fine under ghc-5.02.2, and using the BufferMode
patch included at the end output seems to be ok,
Why LineBuffering?  Doesn't really change anything for me
(same GHC version).
...
but input
of more than 2048 bytes doesn't seem to be being handled
reliably.
[..]
mostly gives no output, but occasionally I see
Warning: Ports.listenToPort: Attempted to listen to a closed port!
Needs some debugging I guess. :)
I guess, it needs more documentation.  Your test program is
wrong in two places.

The function `listenToPort' starts to listens at the given
port exactly when `listenToPort' is executed; ie, any data
that goes over the port earlier will not be received in the
stream that this call to `listenToPort' returns.  

You wrote
...
outpt <- newPort ' '
       errpt <- newPort ' '
       let p = proc cmd args
       p inpt outpt errpt
       putStrLn "output:"
       out <- listenToPort outpt
As the processes `cmd' is fork()ed in a separate thread, it
may already have finshed its business until you get to
`listenToPort outpt', which means you have missed all the
data.  For similar reasons
...
errclosed <- isClosedPort errpt
       unless errclosed $
   do
   err <- listenToPort errpt
isn't recommended.  

I changed your program to what I have appended.  It then
works for me for the

        % cat 4096 | test-processes cat

test...except that I sometimes get a "Broken Pipe".  So, I
guess, I have to do some debugging after all (plus improve
the documentation).

Cheers,
Manuel

PS: I'll add your program to the ports/tests/ directory if
    you don't mind.

-=-

module Main
where
import Processes
import Ports
import IO (openFile, hGetContents, IOMode(..), hSetBuffering, BufferMode(..))
import Monad (unless)

main	:: IO()
main = do
     inpt <- getContents
     withPorts [] $ \ (cmd:args) ->
       do
       outpt <- newPort ' '
       errpt <- newPort ' '
       out <- listenToPort outpt
       err <- listenToPort errpt
       let p = proc cmd args
       p inpt outpt errpt
       putStrLn "output:"
       mapM_ putStrLn $ lines out
       putStrLn "error:"
       putStr err
       putStrLn "test finished"

Re: Bug in IO libraries when sending data through a pipe?

Manuel M. T. Chakravarty