Message-ID: <49A2A68A.5070200@onevision.de>
Date: Mon, 23 Feb 2009 14:37:14 +0100
From: Roland Schwingel <roland DOT schwingel AT onevision DOT de>
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To: cygwin AT cygwin DOT com
Subject: Re: Strange freezes when using gdb (rxvt/mintty but not dos box)
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Hi Corinna, Dave and Christopher...

First thanks all for your replies....

cygwin-owner AT cygwin DOT com wrote on 20.02.2009 19:05:15:
 > On Feb 20 12:45, Christopher Faylor wrote:
 > > On Fri, Feb 20, 2009 at 06:26:17PM +0100, Corinna Vinschen wrote:
 > > >On Feb 20 12:13, Christopher Faylor wrote:
 > > >> On Fri, Feb 20, 2009 at 04:45:00PM +0000, Dave Korn wrote:
 > > >> >Roland Schwingel wrote:
 > > >> >>gdb freezes upon execution of the inferior process when used from
 > > >> >>within mintty/rxvt, but does not freeze when gdb is invoked from a
 > > >> >>(conventional) bash inside of a windows DOS box.
 > > >> >[...]
 > > >> >>But why does it work with DOS box and not using mintty/rxvt???
 > > >> >
 > > >> >Clue here.  You are launching a win32 app, from a cygwin app, in a
 > > >> >(non-console-based) GUI window.  Won't work.  Use a console.  See
 > > >> >discussions around CYGWIN=tty for full details (which, if you 
had it
 > > >> >set, ought to make the crash occur in a console as well as in
 > > >> >rxvt/mintty, but isn't anything you can turn off for rxvt and 
get it
 > > >> >working).
 > > >>
 > > >> Right on, Dave.  There are all sorts of potential problems with 
using
 > > >> a cygwin pty/tty and a mingw process.
 > > >>
 > > >> So: what he said.
 > > >
 > > >What about the new-console option in Cygwin's GDB to create the 
inferior
 > > >process in a new Console window?  Shouldn't that help here, as long as
 > > >the pty is actually running in the local GUI session.
 > >
 > > I don't think that resets stdin/stdout for the inferior process so I
 > > doubt that it would help.
 >
 > Indeed, that could be a problem.  I was just wondering if this win32
 > app doesn't actually opens the console by itself and uses functions
 > like WriteConsole.  That could explain why it hangs.

Your assumption that the differences between cywin and native app are 
the reasons for the hang are true.
I turned my little test app into a native cygwin app by using pthreads 
and I got no hang when debugging it.

While this is a solution for this little test app it is not a solution 
for me in real life. I can't turn my native apps
into cygwin apps. For many years I use cygwin gdb to debug native apps. 
So I started to dig a little deeper...

gdb starting from 6.8 introduced this new feature of printing thread 
events. When I switch them off using
"set print thread-events off" inside of gdb before starting my native 
app it works without block.. Hmmm...
gdb uses its internal function printf_unfiltered() to print the tread 
events (eg. "[New thread 2060.0xca4]").
This function is used all over gdb for printing things. So the problem 
of gdb hangings can reappear for
me at any time if gdb wants to print something while a thread in the 
inferior process also prints.
(BTW: When I kill the native process when cygwin gdb started to hang gdb 
awakes again and is fully
functional). When analyzing gdb I found that the print is sent (after 
some hops) to cygwin's stdout
(what a surprise... ;-)).
Here I am running out of knowledge now how cygwin handles stdout once 
for itself and once for native
applications. Well... I made some more tests. If I force GDB/native 
application to always run on just one
CPU core I never got any hang. I tried it a lot of times without a 
problem. There is IMHO a solution possible
when it would be possible to serialize native and cygwin output so that 
they do not interfere. I do not
know whether this is possible to do or not.

Thanks again for your help,

Roland


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