Ros2 node communication realizes zero copy

The following example demonstrates how to realize zero copy communication between two nodes .

Be careful , The following test example ROS2 Version is Galactic.

#include <chrono>
#include <cinttypes>
#include <cstdio>
#include <memory>
#include <string>
#include <utility>

#include "rclcpp/rclcpp.hpp"
#include "std_msgs/msg/int32.hpp"

using namespace std::chrono_literals;

// Node that produces messages.
struct Producer : public rclcpp::Node
{
  Producer(const std::string & name, const std::string & output)
  : Node(name, rclcpp::NodeOptions().use_intra_process_comms(true))
  {
    // Create a publisher on the output topic.
    pub_ = this->create_publisher<std_msgs::msg::Int32>(output, 10);
    std::weak_ptr<std::remove_pointer<decltype(pub_.get())>::type> captured_pub = pub_;
    // Create a timer which publishes on the output topic at ~1Hz.
    auto callback = [captured_pub]() -> void {
        auto pub_ptr = captured_pub.lock();
        if (!pub_ptr) {
          return;
        }
        static int32_t count = 0;
        std_msgs::msg::Int32::UniquePtr msg(new std_msgs::msg::Int32());
        msg->data = count++;
        printf(
          "Published message with value: %d, and address: 0x%" PRIXPTR "\n", msg->data,
          reinterpret_cast<std::uintptr_t>(msg.get()));
        pub_ptr->publish(std::move(msg));
      };
    timer_ = this->create_wall_timer(1s, callback);
  }

  rclcpp::Publisher<std_msgs::msg::Int32>::SharedPtr pub_;
  rclcpp::TimerBase::SharedPtr timer_;
};

// Node that consumes messages.
struct Consumer : public rclcpp::Node
{
  Consumer(const std::string & name, const std::string & input)
  : Node(name, rclcpp::NodeOptions().use_intra_process_comms(true))
  {
    // Create a subscription on the input topic which prints on receipt of new messages.
    sub_ = this->create_subscription<std_msgs::msg::Int32>(
      input,
      10,
      [](std_msgs::msg::Int32::UniquePtr msg) {
        printf(
          " Received message with value: %d, and address: 0x%" PRIXPTR "\n", msg->data,
          reinterpret_cast<std::uintptr_t>(msg.get()));
      });
  }

  rclcpp::Subscription<std_msgs::msg::Int32>::SharedPtr sub_;
};

int main(int argc, char * argv[])
{
  setvbuf(stdout, NULL, _IONBF, BUFSIZ);
  rclcpp::init(argc, argv);
  rclcpp::executors::SingleThreadedExecutor executor;

  auto producer = std::make_shared<Producer>("producer", "number");
  auto consumer = std::make_shared<Consumer>("consumer", "number");

  executor.add_node(producer);
  executor.add_node(consumer);
  executor.spin();

  rclcpp::shutdown();

  return 0;
}

There are several points to pay attention to in the establishment of nodes in the example ：

1. Can make intra_process

Node(name, rclcpp::NodeOptions().use_intra_process_comms(true))

Enable when configuring nodes intra_process.

2. Use when publishing messages UniquePtr and std::move(msg)

In addition to the writing in the above example , You can also write in the following way ：

auto dis = std::make_unique<std_msgs::msg::Float32>();
dis->data = 10.0;
pub_->publish(std::move(dis));

3. Add the nodes that need to communicate to the same process

  rclcpp::executors::SingleThreadedExecutor executor;

  auto producer = std::make_shared<Producer>("producer", "number");
  auto consumer = std::make_shared<Consumer>("consumer", "number");

  executor.add_node(producer);
  executor.add_node(consumer);

The complete function code can be obtained through the following link ：

https://github.com/shoufei403/ros2_galactic_tutorials

The corresponding code is in demos/intra_process_demo Catalog .

After compiling the code, use the following command to start the sample program

source install/setup.bash
ros2 run intra_process_demo two_node_pipeline

Output results ：

Published message with value: 0, and address: 0x5625E3159130
 Received message with value: 0, and address: 0x5625E3159130
Published message with value: 1, and address: 0x5625E3159130
 Received message with value: 1, and address: 0x5625E3159130
Published message with value: 2, and address: 0x5625E3159130
 Received message with value: 2, and address: 0x5625E3159130
Published message with value: 3, and address: 0x5625E3159130
 Received message with value: 3, and address: 0x5625E3159130
Published message with value: 4, and address: 0x5625E3159130
 Received message with value: 4, and address: 0x5625E3159130
Published message with value: 5, and address: 0x5625E3159130
 Received message with value: 5, and address: 0x5625E3159130
Published message with value: 6, and address: 0x5625E3159130
 Received message with value: 6, and address: 0x5625E3159130
Published message with value: 7, and address: 0x5625E3159130
 Received message with value: 7, and address: 0x5625E3159130
Published message with value: 8, and address: 0x5625E3159130
 Received message with value: 8, and address: 0x5625E3159130
Published message with value: 9, and address: 0x5625E3159130
 Received message with value: 9, and address: 0x5625E3159130

You can find , The address of the sending end data is consistent with that of the receiving end data . Therefore, the sender just sends the address where the data is stored to the receiver, and there is no data copy .

The zero copy feature is particularly useful for transmitting image data . For an example of image transmission, please refer to demos/intra_process_demo Other examples in the catalog .

Reference resources ：

https://docs.ros.org/en/galactic/Tutorials/Demos/Intra-Process-Communication.html

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