[lldb-dev] thread state question on Ctrl-C

Todd Fiala tfiala at google.com
Thu Sep 11 10:00:45 PDT 2014

I'll need to go through it in detail.  I suspect I'm actually sending a
SIGSTOP to each thread  on Ctrl-C which I think is the root of the issue -
I only need to do that to one.

You guys gave me enough info to work through it from here.  Thanks!

On Thu, Sep 11, 2014 at 9:50 AM, Greg Clayton <gclayton at apple.com> wrote:

> > On Sep 11, 2014, at 8:41 AM, Todd Fiala <tfiala at google.com> wrote:
> >
> > Hey guys,
> >
> > When we hit Ctrl-C in lldb while debugging a multithreaded inferior with
> 5 threads, at a low level we'll end up stopping all 5 threads.  It looks
> like our expectation is that one of the 5 threads gets marked with a stop
> reason, while the other 4 threads in this case would simply report they are
> stopped for no reason (gdb-remote T00 stop reason).  Is that correct?
> You should show what actually happens in the system. If all 5 threads
> actually stop due to a signal, show that. So try to show the truth as much
> as you can. It would be great if we actually were able to stop the program
> some other way that doesn't involve a signal being thrown, but none of us
> have that as far as I know. It would be great to get all threads stopped
> with no stop reason, but we currently should show the truth about how the
> program stopped (SIGSTOP) in case when you resume the SIGSTOP got delivered
> to your program you would want to be able to understand why it did such a
> thing.
> >
> > Right now in llgs I believe I'm doing this in a non-compliant way.  I am
> marking all threads as stopped with the stop reason for an interrupt.
> You should tell the truth and just tell us which thread got the signal if
> you can determine this.
> > This seems to translate to lldb thinking it needs multiple restarts to
> get it going again.
> Signals shouldn't require multiple starts. They don't on MacOSX. We
> determine, using the current UnixSignals from the process, if we suppress
> them or not and just resume the program.
> >
> > Related - if I do a qThreadStopInfo gdb-remote command and the thread is
> *not* stopped, that is an error that gets an E response.  But if it is
> stopped but not for any user-visible reason, that is a T00 response.  Is
> that correct?
> qThreadStopInfo should always return a valid response for any real thread.
> It can't be sent while the process is running because when the process is
> running you are in the middle of a "c" or "vCont..." packet and waiting for
> the stop reply. T00 should be returned for any thread was simply suspended
> because we wanted to stop the process, yes that is correct.
> >
> > The difference between MacOSX/debugserver and Linux/llgs is visible in
> the test/tools/lldb-gdbserver/TestGdbRemote_qThreadStopInfo.py test if you
> log the stop signals returned by the qThreadStopInfo that loops over all
> threads.  MacOSX is only marking one with a stop reason of non-zero
> We aren't marking anything, we are just telling the truth by converting
> the actual system exception information into a stop reply packet.
> > whereas Linux/llgs is marking all threads with the SIGSTOP stop reason.
> So the question is: when you interrupt your target, how are you doing it?
> Sending a SIGSTOP? Does any thread actually have a stop reason? If you have
> no threads with no real stop reasons, you will need to mark at least one
> thread with a stop reason of "reason:interrupted;" (and add corresponding
> code to ProcessGDBRemote::SetThreadStopInfo(...) to handle the new
> "interrupted" reason.
> >  That particular test isn't checking that aspect, but obviously I want
> to add a test that verifies we're handling the stop reason marking
> correctly on the llgs side.
> >
> So tell the truth as much as possible, and if you are interrupting and
> have to make up a new stop reason, use the "interrupted" on a single thread
> (the first one probably) if no threads actually have a stop reason to
> report after halting your process.
> Greg

Todd Fiala | Software Engineer | tfiala at google.com | 650-943-3180
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