CODEWITHSUNDEEP
c++zeromq
Michael Schubert
Michael Schubert

Reputation: 2796

How to copy messages in ZeroMQ without copying the memory?

I'm trying to initialize a message in ZeroMQ that I can send multiple times without copying memory every time.

According to the documentation, this can be done by using the zmq_msg_copy function (or message_t::copy() in C++):

#include <cstdio>
#include <zmq.hpp>
using namespace zmq;

int main() {
    int payload = 5;
    message_t* msg = new message_t((void*)&payload, sizeof(payload));
    message_t copy = message_t(sizeof(payload));
//    msg->copy(&copy); // this fails but does not produce an error
    int status = zmq_msg_copy((zmq_msg_t*)copy.data(), (zmq_msg_t*)msg->data());
    printf("%i\n", status);
}

I compile with

gcc -lzmq -lstdc++ file.cpp

and run

./a.out

which produces

-1

What am I doing wrong here?

As for source and destination variables:

zmq_msg_copy has destination first, source second:

int zmq_msg_copy (zmq_msg_t *dest, zmq_msg_t *src);

message_t::copy is implemented the following way:

inline void copy (message_t const *msg_)
{
    int rc = zmq_msg_copy (&msg, const_cast<zmq_msg_t*>(&(msg_->msg)));
    if (rc != 0)
        throw error_t ();
}

so @vasek is exactly right, the supplied argument needs to be the source.

Upvotes: 0

Views: 2026

Answers (1)

vasek
vasek

Reputation: 2849

According to documentation for message_t argument to copy should be source message, not the destination.

This code should work (tested under Slackware 14.2/GCC6.3 and MSVC 2017):

message_t copy;
copy.copy(msg);

Also note you don't have to preallocate message buffer for copied message.

Upvotes: 2

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