[Haskell-cafe] runInteractiveCommand: program ends before writing or reading all the output

[Philip Weaver]

2008/5/15 Olivier Boudry <olivier.boudry at gmail.com>:
> Hi all,
>
> It's the first time I use the runInteractiveCommand and I was probably
> bitten by laziness.
>
> When I run the following program and send its output to a file using '>'
> redirection I get the full output of the called process. But if I run it in
> the console I get only half of the output. As console is slower than disk I
> assume the called process terminates before all data has been read from it
> or the main process terminates before data has been written to stdout. I
> thought using waitForProcess, closing called process output and flushing
> stdout would solve the problem but it doesn't.
>
>> -- Compile with -threaded option
>> module Main where
>>
>> import Control.Concurrent (forkIO)
>> import System.Environment (getArgs)
>> import System.FilePath (dropExtension, takeFileName)
>> import System.IO (Handle, hClose, hFlush, hGetContents, stdout)
>> import System.Process (runInteractiveCommand, waitForProcess)
>>
>> main :: IO ()
>> main = do
>>   (file:_) <- getArgs
>>   (_, out, _, pid) <- runInteractiveCommand $ "dumpbin /EXPORTS " ++ file
>>   forkIO (createDefFile file out)
>>   waitForProcess pid
>>   hClose out
>>   hFlush stdout
>>
>> createDefFile :: String -> Handle -> IO ()
>> createDefFile file inp = do
>>   putStrLn $ "LIBRARY " ++ (dropExtension . takeFileName) file ++ ".dll"
>>   putStrLn "EXPORTS"
>>   text <- hGetContents inp
>>   mapM_ writeExport $ keepExports $ map words $ lines text
>>   where
>>     keepExports :: [[String]] -> [String]
>>     keepExports = map head
>>                   . filter (not . null)
>>                   . takeWhile (["Summary"]/=)
>>                   . drop 1
>>                   . dropWhile (["ordinal","name"]/=)
>>     writeExport ('_':xs) = putStrLn xs
>>     writeExport xs = putStrLn xs
>
> Any idea regarding the cause of this problem?
>

I think I've encountered the same problem several times, and it was
because I was reading from the handle lazily, like this:

   (_, out, _, pid) <- runInteractiveProcess ...
   str <- hGetContents out
   waitForProcess pid

But I didn't use 'str' until after the process finishes.  My solution
was to use strict IO, usually by replacing String with a strict
ByteString.  I hear there is now a library that lets you do strict IO
with Strings....

Hope this helps.

> Thanks,
>
> Olivier.
>
>
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