打开一个终端，进入到beginner_tutorials包下面：
cd ~/catkin_ws/src/beginner_tutorials
建立文件src/listener.cpp：
vim src/listener.cpp
将下面的代码复制到文件中：
#include "ros/ros.h"#include "std_msgs/String.h"/** * This tutorial demonstrates simple receipt of messages over the ROS system. */void chatterCallback(const std_msgs::String::ConstPtr& msg){ ROS_INFO("I heard: [%s]", msg->data.c_str());}int main(int argc, char **argv){ /** * The ros::init() function needs to see argc and argv so that it can perform * any ROS arguments and name remapping that were provided at the command line. For programmatic * remappings you can use a different version of init() which takes remappings * directly, but for most command-line programs, passing argc and argv is the easiest * way to do it. The third argument to init() is the name of the node. * * You must call one of the versions of ros::init() before using any other * part of the ROS system. */ ros::init(argc, argv, "listener"); /** * NodeHandle is the main access point to communications with the ROS system. * The first NodeHandle constructed will fully initialize this node, and the last * NodeHandle destructed will close down the node. */ ros::NodeHandle n; /** * The subscribe() call is how you tell ROS that you want to receive messages * on a given topic. This invokes a call to the ROS * master node, which keeps a registry of who is publishing and who * is subscribing. Messages are passed to a callback function, here * called chatterCallback. subscribe() returns a Subscriber object that you * must hold on to until you want to unsubscribe. When all copies of the Subscriber * object go out of scope, this callback will automatically be unsubscribed from * this topic. * * The second parameter to the subscribe() function is the size of the message * queue. If messages are arriving faster than they are being processed, this * is the number of messages that will be buffered up before beginning to throw * away the oldest ones. */ ros::Subscriber sub = n.subscribe("chatter", 1000, chatterCallback); /** * ros::spin() will enter a loop, pumping callbacks. With this version, all * callbacks will be called from within this thread (the main one). ros::spin() * will exit when Ctrl-C is pressed, or the node is shutdown by the master. */ ros::spin(); return 0;} 保存退出。
下面看一下代码的解释：
void chatterCallback(const std_msgs::String::ConstPtr& msg){ ROS_INFO("I heard: [%s]", msg->data.c_str());} 当一个消息到达chatter话题时，这个回调函数将会被调用。
ros::Subscriber sub = n.subscribe("chatter", 1000, chatterCallback); 订阅chatter话题，当一个新的消息到达时，ROS将会调用chatterCallback()函数。第二个参数是对列的长度，会将收到的消息缓冲下来，一共可以缓冲1000条消息，满1000之后，后面的到达的消息将会覆盖前面的消息。
NodeHandle::subscribe()将会返回一个ros::Subscriber类型的对象，当订阅对象被销毁以后，它将会自动从chatter话题上撤销。
ros::spin();
ros::spin()进入了一个循环，非常快的调用消息的回调函数。不要担心，如果它没有什么事情可做时，它也不会浪费太多的CPU。
当ros::ok()返回false时，ros::spin()将会退出。
这就意味着，当ros::shutdown()被调用，或按下CTRL+C等情况，都可以退出。
下面总结一下写一个订阅者的步骤：（1）初始化ROS系统（2）订阅chatter话题（3）Spin，等待消息的到来（4）当一个消息到达时，chatterCallback()函数被调用。
下面看一下如何构建节点。我们之前用过catkin_create_pkg创建过package.xml和CMakeLists.txt(目录：catkin_ws/src/beginner_tutorials/CMakeLists.txt)，这时候你的CMakeLists.txt看起来应该是下面这个样子，包括前面所做的修改，注释部分可以除去：
cmake_minimum_required(VERSION 2.8.3)project(beginner_tutorials)## Find catkin and any catkin packagesfind_package(catkin REQUIRED COMPONENTS roscpp rospy std_msgs genmsg)## Declare ROS messages and servicesadd_message_files(DIRECTORY msg FILES Num.msg)add_service_files(DIRECTORY srv FILES AddTwoInts.srv)## Generate added messages and servicesgenerate_messages(DEPENDENCIES std_msgs)## Declare a catkin packagecatkin_package()
将下面几行代码添加到CMakeLists.txt的最后。最终你的CMakeLists.txt文件看起来样该是下面这个样子：
cmake_minimum_required(VERSION 2.8.3)project(beginner_tutorials)## Find catkin and any catkin packagesfind_package(catkin REQUIRED COMPONENTS roscpp rospy std_msgs genmsg)## Declare ROS messages and servicesadd_message_files(FILES Num.msg)add_service_files(FILES AddTwoInts.srv)## Generate added messages and servicesgenerate_messages(DEPENDENCIES std_msgs)## Declare a catkin packagecatkin_package()## Build talker and listenerinclude_directories(include ${catkin_INCLUDE_DIRS})add_executable(talker src/talker.cpp)target_link_libraries(talker ${catkin_LIBRARIES})add_dependencies(talker beginner_tutorials_generate_messages_cpp)add_executable(listener src/listener.cpp)target_link_libraries(listener ${catkin_LIBRARIES})add_dependencies(listener beginner_tutorials_generate_messages_cpp) 构建完之后，这将会创建两个可执行文件，talker和listener。它们将会产生在~/catkin_ws/devel/lib/share/<package name>目录下，这也就是说构建好的东西，会放在devel的目录中，原本的东西会留在src文件中。
下面开始构建，在你的工作空间根目录下输入：
catkin_make
这样我们就完成了，在某个话题上的消息的发布者和订阅者的可执行模块的创建。