Middleware in Embedded Linux – ZeroMQ

cppzmq is an open-source C++ library based on ZeroMQ, used for building distributed and concurrent applications. It provides a simple interface for communicating with ZeroMQ message queues. This article will introduce the basic concepts, common patterns, and example code of cppzmq.

Basic Concepts

1. 1. ZeroMQ: ZeroMQ is a lightweight messaging library that allows applications to communicate through various messaging patterns. It can communicate between different processes or computers and supports multiple programming languages.

2. 2. cppzmq: cppzmq is the C++ binding for ZeroMQ, providing a simple and easy-to-use interface for communicating with ZeroMQ in C++ programs.

Common Patterns

1. 1. Request-Reply Pattern (REQ-REP): This is a typical client-server pattern. The client sends a request message to the server and waits for a response. The server receives the request message and sends a response message back to the client.

2. 2. Publish-Subscribe Pattern (PUB-SUB): In this pattern, message publishers broadcast messages to all subscribers. Subscribers can choose to subscribe to messages of interest and receive those messages.

3. 3. Push-Pull Pattern (PUSH-PULL): This is a many-to-many communication pattern. Message pushers send messages to available receivers. All receivers can receive messages and process them.

Example Code

Below is a simple cppzmq example code demonstrating the use of the REQ-REP pattern:

// Server.cpp
#include <zmq.hpp>
#include <iostream>

int main() {
    zmq::context_t context(1);
    zmq::socket_t socket(context, zmq::socket_type::rep);
    socket.bind("tcp://*:5555");

    while (true) {
        zmq::message_t request;
        socket.recv(request, zmq::recv_flags::none);

        std::cout << "Received request: " << request.to_string() << std::endl;

        zmq::message_t reply(5);
        memcpy(reply.data(), "Reply", 5);
        socket.send(reply, zmq::send_flags::none);
    }

    return 0;
}

// Client.cpp
#include <zmq.hpp>
#include <iostream>

int main() {
    zmq::context_t context(1);
    zmq::socket_t socket(context, zmq::socket_type::req);
    socket.connect("tcp://localhost:5555");

    std::string request_str = "Hello";
    zmq::message_t request(request_str.size());
    memcpy(request.data(), request_str.c_str(), request_str.size());
    socket.send(request, zmq::send_flags::none);

    zmq::message_t reply;
    socket.recv(reply, zmq::recv_flags::none);
    
    std::cout << "Received reply: " << reply.to_string() << std::endl;

    return 0;
}

The above code is divided into two files, one is Server.cpp and the other is Client.cpp. Server.cpp creates a REP type socket and binds it to the address “tcp://*:5555”. In the server’s infinite loop, it receives request messages from the client and then sends a reply message.

Client.cpp creates a REQ type socket and connects to “tcp://localhost:5555”. The client sends a request message to the server and waits for the server’s response.

This is just an introductory tutorial to cppzmq. More detailed content and more complex patterns can be found in the official documentation. I hope this tutorial is helpful to you!