[Haskell-cafe] using createProcess and waitForProcess, but mpg123 plays for only 6 seconds

[Niklas Hambüchen]

Yes, the automatic closing of handles has confused me as well when I
wrote Proctest:

http://hackage.haskell.org/packages/archive/proctest/latest/doc/html/Test-Proctest.html

It might contain useful examples on how to deal with processes.

Niklas

On 15/11/12 14:42, Herbert Valerio Riedel wrote:
> rnons <remotenonsense at gmail.com> writes:
> 
> [...]
> 
>> mpgLoop = do
>>     let sh = "mpg123 -R"
>>     (Just hin, Just hout, _, hdl) <- createProcess (shell sh){ std_in = CreatePipe, std_out=CreatePipe }
>>     --hPutStrLn hin "SILENCE"
>>     hPutStrLn hin "LOAD /home/rnons/Music/test.mp3"
>>     hFlush hin
>>     waitForProcess hdl
>>     return ()
> 
> [...]
> 
>> I expected with "waitForProcess", this program will be able to run
>> till the song ends. However, mpg123 plays for only 6 seconds.
>>
>> Maybe I missed something?
> 
> Just a guess: the problem may be that "a Handle will be automatically
> closed when the garbage collector detects that it has become
> unreferenced by the program"[1], and in the program above, the liveness
> of the hin/hout references ends before mpg123 is finished playing. And
> 'mpg123 -R' shuts down as soon as it detects its stdin (or stdout)
> getting EOF... so that's what you're experiencing most likely here.
> 
> Try to keep hin/hout "alive" a bit longer, and mpg123 should survive
> longer. Also you should most probably consume the output comming from
> 'mpg123 -R' via 'hout', as otherwise it the buffer might build up and
> mpg123 might block.
> 
> hth,
> hvr
> 
>  [1]: http://hackage.haskell.org/packages/archive/base/4.6.0.0/doc/html/System-IO.html#t:Handle
> 
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>