[capnproto] Having trouble with Github CI

[Jens Alfke]

I’m trying to get my (C++) project to build in Github Actions, but getting 
compile errors because apparently there’s a version mismatch between the Cap’n 
Proto headers and tool.

I don’t build Cap’n Proto as part of the project; instead I use a package 
manager to install it. My CMakeLists.txt has a step to compile my interface:
find_package(CapnProto CONFIG REQUIRED)
capnp_generate_cpp(myCapnPSources myCapnPHeaders src/interface.capnp)

This works fine in local builds, both macOS and Ubuntu.

In my Github Actions yaml file, I added a build step that uses the get-package 
action:
  - name: Install Capn Proto
uses: mstksg/get-package@v1
with:
  brew: capnp
  apt-get: capnproto libcapnp-dev

Unfortunately the C++ compile then fails with the error: "Version mismatch 
between generated code and library headers.  You must use the same version of 
the Cap'n Proto compiler and library.”

I assume this means that on the Github runner the `capnp` tool is from a 
different version than the headers. I don’t know why that would be, when I had 
apt install both packages. (And why would there be a second installation of 
Cap’n Proto at all?)

Has anyone successfully gotten Github Actions to build C++ based Cap’n Proto 
projects?

—Jens

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Re: [capnproto] Can I do better?

[Jitesh Khandelwal]

Thanks Kenton, this is working well. Now the allocation in 
messageToFlatArray is not happening.

Now, is it possible to build the message in the buffer directly ? 
Currently, I'm building in arena and then calling writeMessage to write 
into the buffer.
--
auto builder = MallocMessageBuilder(*arena*);
...
...
auto message = arrayPtr(reinterpret_cast(*buffer*), N);  

   
auto aos = kj::ArrayOutputStream(message);  

 
capnp::writeMessage(aos, builder);
--
On Saturday, 26 February 2022 at 07:07:44 UTC+5:30 ken...@cloudflare.com 
wrote:

> 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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>>  
>> 
>> .
>>
>

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