Hi Stephan,
For some reason Google Groups thought your message was spam, but I fished
it out of the spam bucket. (It looked like you sent three different
versions. I accepted this one and deleted the other two.)
I think your problem may be here:
auto byteArray = capnp::messageToFlatArray(msgBuilder).asBytes();
asBytes() returns an ArrayPtr which points back at the array on which it
was called. But in this line, you're calling it on a temporary value (the
return value of messageToFlatArray), so you end up with `byteArray` being a
dangling pointer (and so its contents will be garbage). You could fix that
like this:
auto wordArray = capnp::messageToFlatArray(msgBuilder);
auto byteArray = wordArray.asBytes();
Or like this:
auto byteArray = capnp::messageToFlatArray(msgBuilder).releaseAsBytes();
-Kenton
On Wed, Aug 30, 2017 at 2:36 AM, <op...@vs.uni-kassel.de> wrote:
> Hi all,
>
> like some other people before me, I would like to send Cap'n Proto
> Messages via ZeroMQ. Nevertheless I did not manage to make it work, yet.
> The exception I receive on the receiver side is:
>
> Exception catched: Receiver - src/capnp/serialize.c++:43: failed:
> expected array.size() >= offset; Message ends prematurely in segment table.
>
> *Here is the sending code:*
>
> // cap'n proto part
> ::capnp::MallocMessageBuilder msgBuilder;
> discovery_msgs::Beacon::Builder beaconMsgBuilder =
> msgBuilder.initRoot<discovery_msgs::Beacon>();
> beaconMsgBuilder.setIp(this->wirelessIpAddress);
> beaconMsgBuilder.setPort(6666);
> beaconMsgBuilder.setUuid(kj::arrayPtr(this->uuid, sizeof(this->uuid)));
> auto byteArray = capnp::messageToFlatArray(msgBuilder).asBytes();
>
> // zmq part
>
> this->ctx = zmq_ctx_new();
> this->socket = zmq_socket(ctx, ZMQ_RADIO);
> zmq_connect(this->socket, "udp://224.0.0.1:5555");
> zmq_msg_t msg;
> zmq_msg_init_data(&msg, byteArray.begin(), byteArray.size(), NULL, NULL);
> zmq_msg_set_group(&msg, "TestMCGroup");
> zmq_msg_send(&msg, this->socket, 0);
> zmq_msg_close(&msg);
>
>
> *On the receiving side it is:*
>
> this->ctx = zmq_ctx_new();
> this->socket = zmq_socket(ctx, ZMQ_DISH);
> zmq_bind(socket, "udp://224.0.0.1:5555");
> zmq_join(this->socket, "TestMCGroup")
>
>
> zmq_msg_t msg;
> zmq_msg_init(&msg);
> int nbytes = zmq_msg_recv(&msg, this->socket, 0);
>
> // Received message must contain an integral number of words.
> assert(zmq_msg_size(&msg) % sizeof(capnp::word) == 0);
> auto num_words = zmq_msg_size(&msg) / sizeof(capnp::word);
>
> if (reinterpret_cast<uintptr_t>(zmq_msg_data(&msg)) % sizeof(capnp::word)
> == 0) {
> // message is aligned
> auto wordArray = kj::ArrayPtr<capnp::word
> const>(reinterpret_cast<capnp::word const *>(zmq_msg_data(&msg)),
> num_words);
> ::capnp::FlatArrayMessageReader msgReader = ::capnp::
> FlatArrayMessageReader(wordArray); // <-- Throws Exception: Receiver -
> src/capnp/serialize.c++:43: failed: expected array.size() >= offset;
> Message ends prematurely in segment table.
> auto beacon = msgReader.getRoot<discovery_msgs::Beacon>();
> check(zmq_msg_close(&msg), "zmq_msg_close");
> } else {
> // message is not aligned
> std::cerr << "Agent: receive(): Not aligned " << std::endl;
> }
>
> In my own words, I simply try to send a message via UDP-Multicast over a
> zmq RADIO socket to a zmq DISH socket. My experience is that the exact same
> bytes are received, that are also sended before. The message is also always
> memory aligned. The only problem is the recreation of the Cap'n Proto
> message. The constructor of the FlatArrayMessageReader throws an exception
> as indicated in the code above:
> Receiver - src/capnp/serialize.c++:43: failed: expected array.size() >=
> offset; Message ends prematurely in segment table.
>
> My interpretation would be, that the message is not correctly encoded, or
> not correctly aligned in memory (maybe my alignment check is wrong).
>
> Does somebody have a clue, what I am doing wrong?
>
> Kind Regards
> Stephan
>
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