Hi Volker,
On 02 January 2005 16:35, Volker Wysk wrote:
> I made an error when simplifying the program for the bug report.
> Here's the message again.
>
> I'm building a program which converts file names from ISO8859-1 to
> UTF-8. It calls the recode program to do the actual conversion. This
> part does the work:
>
> pfade <- fmap lines (contents "-")
>
> pipe_to (unlines pfade)
> (execp "recode" ["-f", "latin1..utf8"]
> -|= (do pfade_utf8 <- fmap lines (contents "-") --XX
> error here mapM_ (\(pfad, pfad_utf8) -> do
> ...
> )
> (zip pfade pfade_utf8)
> )
> )
>
> ...
>
> lazy_contents :: String -> IO (String, Handle)
> lazy_contents pfad = do
> h <- if pfad == "-" then return stdin else openFile pfad
> ReadMode txt <- hGetContents h
> return (txt, h)
>
> contents :: String -> IO String
> contents pfad = do
> (txt, h) <- lazy_contents pfad
> seq (length txt) (return ())
> hClose h
> return txt
>
>
> pipe_to and (-|=) fork two processes, connected through a pipe. I get
> this error at the marked point:
>
> In child process, part of a pipe:
> IO-Error
> Error type: illegal operation
> Location: hGetContents
> Description: handle is closed
> File name: "<stdin>"
>
> The problem is, the call of contents "-" at the beginning closes the
> main process' standard input (mere getContents would semi-close it,
> same problem). It is replaced in the child with the pipe from recode,
> but the runtime system doesn't notice that stdin is open again. The
> openness state seems to be duplicated in the file handle.
So, in the child process you actually want to reset the stdin Handle,
right? There is hDuplicateTo, which might be enough for your needs.
Use it like this:
h <- fdToHandle pipe_read_fd
hDuplicateTo h stdin
hDuplicateTo is only available from GHC.Handle at the moment.
> It works when replacing the marked line with the following.
>
> h <- fdToHandle 0
> pfade_utf8 <- fmap lines (hGetContents h)
>
> I'm using only functions from the System.Posix library (e.g. dupTo),
> so I think the runtime system should notice.
I'm not sure if what you're asking could/should be done automatically by
the RTS. If I understand correctly, you'd like System.Posix.dupTo to
automatically update Handles that refer to the destination file
descriptor, right? We don't keep around a table mapping file
descriptors to Handles, so that would be quite hard.
> Apart from that, is there any way to notify the runtime system that
> the file descriptor 0 has changed?
>
> But in the first place, it should be avoided to duplicate the file
> descriptor's state in the handle, if possible.
Which state are you referring to here? The "closed" state? If so,
that's only Haskell-side state, we don't actually close the real file
descriptor.
Cheers,
Simon
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