Hi Jitesh,
Yes, you can avoid the allocation as follows:
1. Construct a kj::ArrayOutputStream that uses your destination buffer as
the output.
2. Use capnp::writeMessage() to write the message to that stream.
-Kenton
On Fri, Feb 25, 2022 at 12:31 PM Jitesh Khandelwal
wrote:
> I am trying to implement a middle layer which serialises some data from a
> data structure into a buffer, which is owned by and whose contents are
> written on the wire by an outer layer.
>
> Currently, my code looks as follows
>
>
> constexpr size_t N = 2048;
> char arena_buffer[N]{0};
> auto arena = arrayPtr(reinterpret_cast(arena_buffer), N /
> sizeof(word));
>
> Encode (some_data_structure, arena, buffer)
> {
> auto builder = MallocMessageBuilder(arena);
>
> // Step 1. set fields from some_data_structure using builder into arena
>
>
> // Step 2. convert to flat array (this allocates another array)
> auto message = messageToFlatArray(builder);
> auto bytes = message.asBytes();
>
> // Step 3. copy to destination
> memcpy(buffer, bytes.begin(), bytes.size());
> }
>
>
>
> My question is, Can I do better?
>
> My understanding is that the extra allocation in Step 2, is because of the
> segment framing protocol ?
>
> Can I bypass it somehow, as I am sure that the arena is big enough for my
> message and there'll be only 1 segment ?
>
> And if yes, then may be I can use the buffer as the arena, as in build
> directly into the output buffer?
>
>
>
> Thanks,
> Jitesh
>
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