Re: [Qemu-devel] Win32 stdio not working if SDL is enabled

[Daniel P. Berrange]

On Thu, Aug 13, 2015 at 07:48:47PM +0200, Stefan Weil wrote:
> Am 13.08.2015 um 14:06 schrieb Daniel P. Berrange:
> > When debugging some patches on Windows, I discovered that nothing printed
> > to stderr ever appears on the console. Eventually I discovered that if I
> > build with --disable-sdl, then stderr appears just fine.
> > 
> > Looking at the code in vl.c I see a hack for SDL introduced in
> > 
> >   commit 59a36a2f6728081050afc6ec97d0018467999f79
> >   Author: Stefan Weil <address@hidden>
> >   Date:   Thu Jun 18 20:11:03 2009 +0200
> > 
> >     Win32: Fix compilation with SDL.
> > 
> > 
> > If I mostly kill the hack from vl.c, and just leave a plain '#undef main'
> > then I get working console stderr once again.
> > 
> 
> Hi Daniel,
> 
> that's a feature of SDL 1.2: stdout and stderr are by default
> redirected to files stdout.txt and stderr.txt in the executable's
> directory.
> 
> This redirection can be disabled by an environment variable
> (SDL_STDIO_REDIRECT="no"). On my Linux machines, I always
> set this variable, so when I run QEMU for Windows with
> wine32 or wine64, stdout and stderr work.
> 
> Printing to stdout / stderr on Windows can be an adventure:
> depending on your shell (command.exe, cmd.exe, MinGW shell,
> MinGW rxvt, Cygwin shell, ...) it works different, and I also
> had application crashes when a GUI application which was
> not started from a shell tried to print to stdout.

I see it is something intentional done by SDL, but I don't think it is
desirable in general.  I rather doubt it would crash as that would imply
that code is checking the return value of fprintf() and taking some action
on error. Instead we exclusive ignore fprintf() return values, so if the
OS is reporting an I/O error we'll be ignoring it. In any case, it is
possible to build QEMU on Win32 without SDL, or set that env variable,
at which point QEMU will be printing to stdio anyway. So in the unlikely
case there is a crash scenario, we need to fix that regardless.

IMHO we should be disabling this bogus behaviour of SDL so QEMU does not
have different behaviour wrt stdio depending on what libraries you happen
to build against, or what platform you choose. Expecting people to know
about a magic env variable to make QEMU work as it does everywhere else
is just broken.

Regards,
Daniel
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