Remote protocol question: the documentation says '?' is not required, but maybe it is?

[Maciej W. Rozycki]

On Tue, 21 Jul 2020, Reuben Thomas via Gdb wrote:

> Also, whether or not I send T does not affect GDB's behaviour. In fact, I
> shortened my code by changing it to send an S packet instead, which also
> works fine, but still GDB needs me to implement '?'. The "invalid remote
> reply" is in response to the stub sending an empty reply to "?".

 FYI, I do believe `?' is indeed mandatory, as GDB needs to figure out the 
initial state of the remote target as it has connected to it, and there is 
no other way.  Documentation may be incomplete/incorrect here, and fallout 
from the lack of response (a protocol violation) might be better.

  Maciej