[Haskell-cafe] Reading and writing to handles made with System.Process far too slow

Mateusz Kowalczyk fuuzetsu at fuuzetsu.co.uk
Mon May 12 14:56:46 UTC 2014


I'm have some business in piping some data and reading some data back
out of a socket so I thought that I'd just use the ‘socat’ tool. I went
off to System.Process just to find out that reading and writing are
taking far too long.

I put together a small example which only requires that you have ‘cat’
on your system:

{-# LANGUAGE UnicodeSyntax #-}
module Uzbl.WithSource where

import GHC.IO.Handle ( hPutStr, hGetContents, hSetBuffering
                     , BufferMode(..))
import System.Process ( createProcess, proc
                      , StdStream(CreatePipe), std_out, std_in)

gs ∷ IO String
gs = do
  let sp = (proc "cat" [])
              { std_out = CreatePipe, std_in = CreatePipe }
  (Just hin, Just hout, _, _) ← createProcess sp
  -- hSetBuffering hin NoBuffering
  -- hSetBuffering hout NoBuffering
  hPutStr hin "Test data"
  hGetContents hout

All this should effectively do is to give you back "Test data". While it
*does* do that, it takes far too long. When I run ‘gs’, it will start to
(lazily) print the result, printing nothing but opening ‘"’ and then
after about 2-3 seconds printing the rest and finishing.

If we set buffering on the in-handle (hin) to NoBuffering, we get a
slightly different behaviour: pretty much straight away we'll have
‘"Test data’ but then it will wait for the same amount of time to
conclude that it's the end of the response. Changing buffering mode on
‘hout’ seems to make no difference. Setting precise number in a
BlockBuffering seems to be no improvement and in the actual application
I will not know how long the data I'm piping in and out will be.

GHC 7.8.2, process-; I'm running ‘gs’ in GHCi. It seems that if I
change the module name to Main, make ‘main = gs >>= putStrLn’, compile
the file and run it, it just hangs there! If I add a newline at the end,
it will print but the program will not finish. This makes me think that
perhaps I should be closing handles somewhere (but if I try inside the
function,  I get no output, thanks lazy I/O).

What I would expect this program to do is to produce same result as
‘print "Test data" | cat’.

Mateusz K.

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