[erlang-questions] Passing data between Erlang and C port while preserving padding and alignment?

[Jesper Louis Andersen]

There is also an attribute, packed, which can be useful here:

https://gcc.gnu.org/onlinedocs/gcc-4.0.4/gcc/Type-Attributes.html
http://www.catb.org/esr/structure-packing/

If you are not too concerned about the efficiency, I would just pack the
struct and send it over the wire. The problem with this on a modern CPU:

* While it packs tighter in your L1D cache, and this gives a speedup.
* It requires more instructions on-CPU to get alignment back in order.
* If multiple CPU threads access the data, you will end up bouncing between
the L1D caches of the cores, and this is usually bad. With alignment, this
can be avoided.

My attack on the problem would be to pack first, then start measuring and
see how much more I can get out of a solution as suggested by Mikael in
this thread.

On Sat, Dec 8, 2018 at 12:11 AM Sölvi Páll Ásgeirsson <solvip@REDACTED>
wrote:

> Can you use #pragma pack on the C structs?
> You can then easily deconstruct them in a binary match
>
> Regards
> Sölvi
>
> Sent from my iPhone
>
> On 7 Dec 2018, at 21:02, Roger Lipscombe <roger@REDACTED> wrote:
>
> On Fri, 7 Dec 2018 at 18:02, Frank Muller <frank.muller.erl@REDACTED>
> wrote:
>
>> Thanks Mikael.
>> Any others thoughts ?
>>
>
> I'm assuming that you've considered serializing and deserializing the data
> to an unpadded (or defined padding) format, but that you've presumably
> discounted it for performance reasons?
>
> Just putting it out there in case someone's facing a similar issue, but
> has different constraints.
>
> We once used XDR for this (with some kind of IDL-to-C converter, if I
> recall correctly). Some brief googling suggests that XDR is not dead yet.
>
> Cheers,
> Roger.
>
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-- 
J.
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