Re: [dev] network protocol packing

From: Steve Dee <>
Date: Tue, 1 Jul 2014 14:40:22 -0700

On Tue, Jul 1, 2014 at 8:01 AM, Markus Teich <> wrote:
> Rob wrote:
>> You've got alignment issues here - msg will be aligned to support any
>> type (as malloc's interface specifies) so msg+1 will most likely be on
>> an odd address, one byte off a highly aligned address. This means if
>> your struct contains anything other than chars, you'll have UB. This is
>> fine on x86, which allows unaligned access with a performance penalty
>> but on something like an ARM machine you'll have issues.
> Heyho,
> so if every field in the message is a multiple of 4 bytes long, this should fix
> the problem? In this case I would just make op an uint32_t and htonl it before
> sending, ntohl after receiving.

It may be worth taking a look at capnproto as I mentioned above if you
haven't yet. It's taken a very similar approach to the problem, and
come up with a rather low-suck solution. Messages are all aligned to
64-bit words and sent that way over the wire, with an optional
lightweight packing format to reduce message sizes. There are C
bindings in a mostly-working state. You don't need to touch C++ except
to compile their protocol format, which you shouldn't have to do most
of the time. The only thing from your strawman that capnproto doesn't
do directly in a similarly simple manner is message signing, and it's
not hard to imagine adding it.

That said, I think your strawman is great, and I'm not trying to shoot
it down -- just pointing in the direction of similar work that might
be inspiring or might save you some effort.

> --Markus
Received on Tue Jul 01 2014 - 23:40:22 CEST

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