ZeroMQ pub-sub socket receiving error of a serialized message object using cap'n proto (C++)

Question

I am using ZeroMQ and Cap'n Proto to send serialized messages between two processes, but I am getting the following error when I try to receive the message on the receiving end:

terminate called after throwing an instance of 'kj::ExceptionImpl'
  what():  capnp/message.c++:99: failed: expected segment != nullptr && segment->checkObject(segment->getStartPtr(), ONE * WORDS); Message did not contain a root pointer.
stack: 7efc84cd8dd4 558b95e72030 558b95e71488 7efc848ebd09 558b95e71259
Aborted

I have checked that the message and message builder are properly initialized and that the serialization and ZeroMQ message creation steps are being executed successfully. I am not sure what could be causing the "no root pointer" error on the receiving end. Can anyone suggest what might be causing this error and how I can fix it?

Note (in my code): The message.capnp.h file is the generated code from my Cap'n Proto schema file, which defines the Message struct with the preparation and heading fields.

My code for Receiver.cpp

#include "../schema/message.capnp.h"

#include <capnp/message.h>
#include <capnp/serialize.h>
#include <kj/std/iostream.h>
#include <zmq.hpp>

int main() {
  
zmq::context_t context(1);
zmq::socket_t socket(context, ZMQ_SUB);

socket.connect("tcp://localhost:5555"); //or *
socket.set(zmq::sockopt::subscribe, "");

zmq::message_t zmq_message;
socket.recv(zmq_message);

// create a memory buffer from the received message
kj::ArrayPtr<capnp::word> buffer(reinterpret_cast <capnp::word*>(zmq_message.data()), zmq_message.size() / sizeof(capnp::word));

// create an input stream from the memory buffer
capnp::FlatArrayMessageReader message_reader(buffer); 
Message::Reader message = message_reader.getRoot<Message>();

// print the preparation and heading fields
std::cout << "Preparation: " << message.getPreparation().cStr() << std::endl;
std::cout << "Heading: " << message.getHeading().cStr() << std::endl;

return 0;
}

My code for Sender.cpp


  #include "../schema/message.capnp.h"

  #include <capnp/message.h>
  #include <capnp/serialize.h>
  #include <kj/std/iostream.h>
  #include <zmq.hpp>

int main() {
  
  // create a message builder
  capnp::MallocMessageBuilder message;
  Message::Builder messageBuilder = message.initRoot<Message>();

  // set the preparation and heading fields
  messageBuilder.setPreparation("prep");
  messageBuilder.setHeading("heading");

  zmq::context_t context(1);
  zmq::socket_t socket(context, ZMQ_PUB);

  socket.bind("tcp://*:5555");

  // serialize the message to a memory buffer
  kj::Array<capnp::word> serialized_message = capnp::messageToFlatArray(message); 

  // create a ZeroMQ message from the serialized buffer
  zmq::message_t zmq_message(serialized_message.size() * sizeof(capnp::word));
  memcpy(zmq_message.data(), serialized_message.begin(), serialized_message.size() * sizeof(capnp::word));

  while(true)
  {
    socket.send(zmq_message);
  }

  return 0;
}

My cap'n proto schema language Message.capnp

@0xbf5147cbbecf40c1;

struct Message {
  preparation @0 :Text;
  heading @1 :Text;
}

score 0 · Answer 1 · answered Jan 01 '23 at 16:56

0

In the two places in your sender code where you have /** sizeof(capnp::word)*/, you should un-comment this, to * sizeof(capnp::word).

This multiplication is needed because the array's size is counted in words, but ZeroMQ wants a size in bytes. Without this, the message is being truncated to 1/8 its actual size. Probably, the size is ending up less than 8 bytes total. On the receiving end, since you (correctly) divide the size by the size of a word, you'd then end up with a zero-size array. Cap'n Proto is then complaining because the array contains no data.

answered Jan 01 '23 at 16:56

Kenton Varda

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Thank you for your answer! I tried doing this the first time I wrote the code, and I got the same error. That is why I commented it out as an experiment, hoping maybe that would fix it. Could it possibly be that I have the wrong schema language structure in capnp file? @0xbf5147cbbecf40c1; struct Message { preparation @0 :Text; heading @1 :Text; } – CuriousProgrammer Jan 01 '23 at 17:35
The error is what you'd see if `buffer.size()` were zero. So it sounds like you are not receiving the data you thought you sent. I'd suggest writing some code that logs the exact bytes on the sending and receiving end, then examine it to verify they match. I don't personally know anything about ZeroMQ so I'm not sure why the bytes would be disappearing, but it doesn't look like Cap'n Proto is the problem. – Kenton Varda Jan 02 '23 at 21:11

ZeroMQ pub-sub socket receiving error of a serialized message object using cap'n proto (C++)

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