Haskell threads & pipes & UNIX processes

Michael Marte marte@pms.informatik.uni-muenchen.de
Thu, 15 Feb 2001 17:34:05 +0100 (CET)

Hello *,

I need to compress the output of my Hakell program. To avoid the creation
of huge files, I want to compress before writing by means of gzip or
bzip2. However, this seems to be quite involved.

I decided to run the compressor with 

runProcess :: FilePath                    -- Command
           -> [String]                    -- Arguments
            -> Maybe [(String, String)]    -- Environment(Nothing -> Inherited)
            -> Maybe FilePath              -- Working directory (Nothing -> inherited)
            -> Maybe Handle                -- stdin (Nothing -> inherited)
            -> Maybe Handle                -- stdout (Nothing -> inherited)
            -> Maybe Handle                -- stderr (Nothing -> inherited)
            -> IO ()

First I wanted to use Posix.createPipe to connect to the compressor but
Posix.createPipe returns a Fd. So this does not fit.

I ended up with using Posix.createNamedPipe. This creates a named pipe
that can be openend with IO.openFile (which again yields the Handle needed
for runProcess).

At this point, it turned out that the pipe has to be opened for reading
before it can be opened for writing (ghc-4.08.1). This seems to be a bug.
(In a shell, the order does not matter; the processes are suspended 
as expected.)

Then, as expected, the problem occured that writing to the pipe and
compressing cannot be sequenced. If the producer is started first, the
producer blocks because there is no consumer. If the consumer is started
first, the consumer blocks because there is no input.

So I played around with threads. If both consumer and producer are
executed as threads, the program terminates immediately without any
output. There seems to be no need to start executing any of the threads.

If either the producer or the compressor are executed in a thread, the
compressor bails out complaining that some resource is not available.

So what's going on? How can the goal be achieved?

Thank you,