[lldb-dev] qfThreadInfo/qsThreadInfo stub impl and max packet size

Todd Fiala todd.fiala at gmail.com
Fri May 30 08:51:02 PDT 2014

Hey Matthew,

Thanks for your thoughts on this.  Just had a minute to dig around the code
a bit.  I was thinking the potential limit on the stub => client
communication was more oriented towards max packet sizes expected over the
(historically) not-assumed-to-be-reliable communication pipe between the
gdb client and stub.

I found the code that triggered my question on this, in RNBRemote.h:

/* Generally speaking, you can't assume gdb can receive more than 399 bytes
   at a time with a random gdb.  This bufsize constant is only specifying
   how many bytes gdb can *receive* from debugserver -- it tells us nothing
   about how many bytes gdb might try to send in a single packet.  */

So my question was really where did this number come from, and in which
cases does it really matter, and is there some way to negotiate a much
larger number or discard it entirely if - say - the communication medium is
known to be reliable.

On Thu, May 22, 2014 at 12:45 AM, Matthew Gardiner <mg11 at csr.com> wrote:

> Todd Fiala wrote:
>> It also seems like the qSupported request to the stub can take
>> client-side values pushed in as suffixes to the qSupported request.  Is
>> that the way to inform the stub of the client's max accepted incoming
>> packet length?
> I can't see "PacketSize" listed as a gdbfeature, in the "GDB Remote Serial
> Protocol" appendix. (I assuming that the document lists gdbfeature and
> stubfeature as exclusive sets).
> It's possible that the client is intended to tolerate large packets, since
> it is envisaged to run on a machine with sufficient memory to keep
> reallocing it's recv buffer. However, (I'm guessing here) the stub is
> intended to run on a more constrained environment, and where it may not be
> possible to allocate as much memory, and therefore is reliant on a
> statically allocated fixed-size buffer. As a consequence, the designers of
> the protocol may have decided that there must be a way to constrain the
> client, but that the stub need not be similarly constrained? Well, that's
> my take on it.
> Matt
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