gmane.comp.lang.haskell.libraries

runCommand/waitForProcess don't respect text printing order when stdout is redirected

Hi all,

I'm running ghc 6.8.2 in OSX and Linux.

The following program behaves as expected when run on a terminal.

==
module Main where

import System.Process

main = sequence $ replicate 3 command
 where command = do putStrLn "foo"
                    waitForProcess =<< runCommand "echo echo"
                    putStrLn "bar"
==

$ ghc --make Main.hs -o main
$./main
foo
echo
bar
foo
echo
bar
foo
echo
bar

However, when stdout is redirected to a file, the order is no longer respected:

(Continue reading)

headers

Alfonso Acosta | 1 Aug 16:20

Re: runCommand/waitForProcess don't respect text printing order when stdout is redirected

In case it helps, I have just confirmed the same behaviour under Windows.

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Bayley, Alistair | 1 Aug 16:25

RE: runCommand/waitForProcess don't respect text printing order whenstdout is redirected

> From: libraries-bounces <at> haskell.org 
> [mailto:libraries-bounces <at> haskell.org] On Behalf Of Alfonso Acosta
> 
> The following program behaves as expected when run on a terminal.
> 
> However, when stdout is redirected to a file, the order is no 
> longer respected:

Possibly buffering? I think the terminal has line buffering by default,
whereas files are usually block-buffered. Try changing the buffering to
line:

  hSetBuffering stdout LineBuffering

Alistair
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headers

Alfonso Acosta | 1 Aug 16:32

Re: runCommand/waitForProcess don't respect text printing order whenstdout is redirected

On Fri, Aug 1, 2008 at 4:25 PM, Bayley, Alistair
<Alistair.Bayley <at> invesco.com> wrote:
> Possibly buffering? I think the terminal has line buffering by default,
> whereas files are usually block-buffered. Try changing the buffering to
> line:
>
>  hSetBuffering stdout LineBuffering

Thanks! That worked.

Sorry for the noise. I assumend that files, just like terminals, where
line-buffered.

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Johannes Waldmann | 1 Aug 17:51

Re: runCommand/waitForProcess don't respect text printing order whenstdout is redirected


>>  hSetBuffering stdout LineBuffering

I run into this from time to time as well.
I think the ghc (runtime) runtime behaviour is confusing here,
as it tries to be too clever. I don't think it should
even try to find out whether it's writing to terminal or file.

Best regards, J.W.

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Henning Thielemann | 5 Aug 13:04

Re: runCommand/waitForProcess don't respect text printing order whenstdout is redirected


On Fri, 1 Aug 2008, Johannes Waldmann wrote:

> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
>
> >>  hSetBuffering stdout LineBuffering
>
> I run into this from time to time as well.
> I think the ghc (runtime) runtime behaviour is confusing here,
> as it tries to be too clever. I don't think it should
> even try to find out whether it's writing to terminal or file.

As far as I know it's the same behaviour as that of C standard libraries.