Remote Serial Protocol -- reply to '?' when target running

Michael Eager
Tue Aug 9 19:31:00 GMT 2016

On 08/09/2016 12:14 PM, Gareth McMullin wrote:
> On Wed, Aug 10, 2016 at 5:33 AM, Michael Eager <> wrote:
>> When gdb connects to a remote target, it asks about
>> features, and then sends a '?' packet, asking why the
>> target is stopped.  When the target connection is through
>> a JTAG pod, the pod may be able to respond to the feature
>> request, but not be able to determine what the current state of
>> the target is, likely because it is not stopped.  What should a
>> JTAG pod reply in this case?
>> The GDB Remote Protocol doc doesn't seem to be entirely consistent.
>> The description for '?' is
>>    Indicate the reason the target halted. The reply is the
>>    same as for step and continue.
>> The description for Stop Reply Packet says the following:
>>    The ‘C’, ‘c’, ‘S’, ‘s’, ‘vCont’, ‘vAttach’, ‘vRun’, ‘vStopped’,
>>    and ‘?’ packets can receive any of the below as a reply. Except
>>    for ‘?’ and ‘vStopped’, that reply is only returned when the
>>    target halts.
>> This suggests that the target can reply to '?' even if it is not
>> stopped, which seems to contradict the description for '?'.  But
>> none of the listed replies seem to be appropriate.  None of the
>> replies look like "Don't bother me, I'm busy".
>> How should the JTAG pod respond?  What do most do in this situation?
> I can't comment on others, but the Black Magic debug project that I maintain [1]
> implements the extended-remote prototcol, and initially responds to
> '?' with 'W00' indicating
> the target exited (there is no attached target).  The user initiates a
> scan for connected
> targets with 'monitor jtag_scan' from GDB, and then uses the GDB
> 'attach' command
> to connect, sending the 'vAttach' packet.  This interrupts the target
> and the reply comes
> back as 'T05'.
> [1]


Michael Eager
1960 Park Blvd., Palo Alto, CA 94306  650-325-8077

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