[lldb-dev] debugserver inferior launch, stopped, $qC and $? thread not matching

Todd Fiala todd.fiala at gmail.com
Tue May 6 08:27:24 PDT 2014

I see the issue.

In debugserver and lldb-platform's impl (which llgs uses), qC is returning
the process id.  I *think* this is wrong according to the gdb remote
protocol, per the protocol documentation

‘qC’Return the current thread ID.

‘QC thread-id’Where thread-id is a thread ID as documented in thread-id
‘(anything else)’Any other reply implies the old thread ID.
This raises a general question.  If we have the protocol documented and
we're not following it exactly (and provided the documentation isn't just
plain wrong even for gdb), do we want to fix up lldb to match the protocol?
 Or do we keep things the same, and document that we're deviating form the
protocol as written?  I'd prefer to match the protocol for spec-following
and iteroperability reasons.

In the case of Linux, this issue wouldn't be discovered in the case of
launching a process since the process id happens to also be the thread id
of the first thread - hence this case the actual use of the pid instead of
the tid in the $QC response packet wasn't detected.

Let me know what you think and I'll fix it up (including the RNBRemote
side).  I may swing over to gdb/gdbremote from MacOSX homebrew/MacPorts to
see if gdb really uses the thread id or the process id - that will at least
rule out if the docs are correct.


On Mon, May 5, 2014 at 10:45 PM, Todd Fiala <todd.fiala at gmail.com> wrote:

> Hi Greg,
> I'm about to add a protocol-level test for llgs and debugserver that
> verifies that a launched inferior process's initial reported thread (i.e.
> response to $qC) is the same thread that reports when asking for stop state
> ($?), or at least that the thread-id is present in the threads listed when
> QListThreadsInStopReply is available.  What I'm finding on debugserver on
> MacOSX is that right after the successful launch with $A, the $qC query
> responds with a $QC{thread-id}.  The very next $?, though, without any
> intervening resume operation, lists the threads but doesn't contain the
> {thread-id} from the $QC.
> Here's a real transcript (with non-interesting bits removed).  It's from a
> debugserver started with no inferior, then attached to by lldb, then
> launching the first inferior process.
> ...
> <lldb.driver.main-thread> <  27> send packet: $QListThreadsInStopReply#21
> <lldb.driver.main-thread> <   6> read packet: $OK#00
> <lldb.driver.main-thread> <  13> send packet: $qHostInfo#9b
> <lldb.driver.main-thread> < 122> read packet:
> $cputype:16777223;cpusubtype:3;ostype:macosx;watchpoint_exceptions_received:after;vendor:apple;endian:little;ptrsize:8;#00
> ...
> <lldb.driver.main-thread> <  66> send packet:
> $A56,0,2f55736572732f746669616c612f706c61792f6370702f68656c6c6f#a0
> <lldb.driver.main-thread> <   6> read packet: $OK#00
> <lldb.driver.main-thread> <  18> send packet: $qLaunchSuccess#a5
> <lldb.driver.main-thread> <   6> read packet: $OK#00
> <lldb.driver.main-thread> <   6> send packet: $qC#b4
> <lldb.driver.main-thread> <   9> *read packet: $QC980#00*   <<< Doesn't
> this say the app is launched, stopped, and thread-id 980 is selected?
> <lldb.driver.main-thread> <   5> send packet: $?#3f
> <lldb.driver.main-thread> < 503> read packet: $T11*thread:63f5*;qaddr:a0;
> *threads:63f5*
> ;00:0000000000000000;01:0000000000000000;02:0000000000000000;03:0000000000000000;04:0000000000000000;05:0000000000000000;06:0000000000000000;07:68f8bf5fff7f0000;08:0000000000000000;09:0000000000000000;0a:0000000000000000;0b:0000000000000000;0c:0000000000000000;0d:0000000000000000;0e:0000000000000000;0f:0000000000000000;10:2810c05fff7f0000;11:0002000000000000;12:2b00000000000000;13:0000000000000000;14:0000000000000000;metype:5;mecount:2;medata:10003;medata:11;#00
> ...
> The $? response seems to say it only has one thread, 63f5.  I would expect
> at least the 980 thread-id reported in the initial qC packet to exist
> somewhere.
> What am I missing?
> Thanks!
> --
> -Todd

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