writing to stdout in an windows app without a console

Krasimir Angelov kr.angelov at gmail.com
Wed Feb 16 02:18:34 EST 2005

Hi Duncan

With ghc-6.4 the output from Debug.Trace.trace will be redirected to
the debug console when the application is compiled with --subsystem
windows. You can see the trace messages with any Windows debuger. The
error messages from any uncaught exception will be displayed in
message box.


On Tue, 15 Feb 2005 21:47:43 +0000, Duncan Coutts
<duncan.coutts at worc.ox.ac.uk> wrote:
> This might be a bug report but it's definitely a question on what the
> behaviour should be:
> On windows if you link using "--subsystem windows" then your program
> starts without popping up a console. This is good for GUI apps (and we
> would like to make it the default when using gtk2hs on win32 by adding
> --subsystem windows to the linker flags in the gtk package file).
> Apparently, Windows default behaviour if a program without a console (ie
> a gui program linked using --subsystem windows) writes to stdout is to
> silently discard the output. Though exactly which bit of the windows
> tech stack implements this behaviour I am not sure.
> With ghc on windows, upon writing to stdout you get an IO exception:
> Fail: <stdout>: hPutChar: failed (Bad file descriptor)
> One can see how this is reasonable behaviour since there really isn't
> any output handle (unlike on unix where there would be a valid stdout fd
> but it'd be connected to /dev/null). On the other hand it is not so
> helpful since it means anyone who's not aware of this behaviour will
> bring their program to a immediate halt without any obvious explanation
> as to what went wrong.
> In fact the only way to see what really happened is from a windows
> console:
> hello.exe 2> err.txt
> So the stdout is still disconnected from the console (despite launching
> the program from the console!) but stderr, where the IO exception
> message gets written, can be captured.
> So I'm wondering if it would be better to arrange that output to
> stdout/stderr gets silently discarded in the case that stdout/stderr are
> not connected rather than raising an exception. I have no idea how
> difficult this hack would be to implement.
> Otherwise for our windows users who ask:
>        "how do I get rid of that ugly black window behind my gui app?"
> we'll have to say
>        "use --subsystem windows BUT!... never use print/putStr and
>        friends or your program will be killed instantly! This includes
>        making sure any other Haskell libraries you use do not use the
>        standard output or print any error messages (eg using
>        Debug.Trace)"
> Duncan
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