Hi Jonathan,
I think the problem is with this function:
std::shared_ptr<capnp::MessageReader> uda_capnp_deserialise(const char*
bytes, size_t size)
{
// ArrayPtr requires non-const ptr, but we are only using this to read
from the bytes array
kj::ArrayPtr<kj::byte>
buffer(reinterpret_cast<kj::byte*>(const_cast<char*>(bytes)), size);
kj::ArrayInputStream in(buffer);
return std::make_shared<capnp::PackedMessageReader>(in);
}
Here, you are returning a PackedMessageReader that is wrapping an
ArrayInputStream, but the ArrayInputStream is allocated on the stack, so is
no longer valid once the function returns. The stream needs to remain valid
until the PackedMessageReader is destroyed. Cap'n Proto can actually lazily
read segments of the message from the stream; it doesn't read the entire
thing in the constructor.
-Kenton
On Thu, Apr 6, 2023 at 3:26 PM Jonathan Hollocombe <jholloco...@gmail.com>
wrote:
> Hi,
>
> I am using Cap'n Proto to serialise a data tree which is then sent as a
> blob to a client to be unpacked. I was using something similar to:
>
> kj::VectorOutputStream out;
> capnp::writePackedMessage(out, builder);
> auto arr = out.getArray();
> char* buffer = (char*)malloc(arr.size());
> memcpy(buffer, arr.begin(), arr.size());
>
> and
>
> kj::ArrayPtr<kj::byte>
> buffer(reinterpret_cast<kj::byte*>(const_cast<char*>(bytes)),
> size);
> kj::ArrayInputStream in(buffer);
> capnp::PackedMessageReader reader{in};
>
> This is working for small tree sizes but segfaults when I get to larger
> number of elements.
>
> I've found a solution that works by switching to messageToFlatArray and
> FlatArrayMessageReader but this dramatically increases the amount of data
> that needs to be sent, especially for small trees.
>
> I've also found that writing the buffer to a file and then using
> PackedFdMessageReader also works but i'd rather avoid creating files
> unnessarily.
>
> Is there a way to use the PackedMessageReader with a byte buffer?
>
> Cheers,
> Jonathan.
>
> p.s. my example code can be found at:
> https://github.com/jholloc/capnp_test
>
>
>
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