前面我们安装了ROS环境，接着就可以开始进行ROS程序的开发了。在开发之前，我们有几个概念需要厘清一下。第一个是workspace，它相当于一个项目。毕竟，一个电脑上面可以有很多的workspace，也就相当于有很多的项目。第二个是package，它相当于功能块，功能块里面可以有很多的程序，package本身就是很多程序的一个集合。第三个就是node，直接把它看成独立的程序就可以了。

1、创建workspace

创建工作区是所有工作的第一步。可以通过创建workspace下面src目录的方法，直接进行创建，

mkdir -p ./catkin_ws/src

2、编译workspace

创建完成之后，就可以直接编译了。这个时候虽然什么也没有，但是catkin_make命令会帮我们自动创建build和dev这两个目录，

cd ./catkin_ws
catkin_make

3、创建package

有了工作区，就可以开始创建工作包了。工作包的创建方法也不复杂，主要使用catkin_create_pkg命令即可，前提是先进入src目录，

cd ./catkin_ws/src
catkin_create_pkg beginner_tutorials std_msgs rospy roscpp

4、开始准备代码talker.cpp & listener.cpp

package里面也是有src目录的，虽然有点绕，这个我们可以稍微花点时间理解一下。现在假设有两个程序，一个是talker.cpp，一个是listener.cpp，他们都是放在package的src目录里面。所以第一步，先进入package里面的src目录，即，

./catkin_ws/src/beginner_tutorials/src

准备talker.cpp文件，

#include "ros/ros.h"
#include "std_msgs/String.h"

#include <sstream>

/**
 * This tutorial demonstrates simple sending of messages over the ROS system.
 */
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, "talker");

  /**
   * 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 advertise() function is how you tell ROS that you want to
   * publish on a given topic name. This invokes a call to the ROS
   * master node, which keeps a registry of who is publishing and who
   * is subscribing. After this advertise() call is made, the master
   * node will notify anyone who is trying to subscribe to this topic name,
   * and they will in turn negotiate a peer-to-peer connection with this
   * node.  advertise() returns a Publisher object which allows you to
   * publish messages on that topic through a call to publish().  Once
   * all copies of the returned Publisher object are destroyed, the topic
   * will be automatically unadvertised.
   *
   * The second parameter to advertise() is the size of the message queue
   * used for publishing messages.  If messages are published more quickly
   * than we can send them, the number here specifies how many messages to
   * buffer up before throwing some away.
   */
  ros::Publisher chatter_pub = n.advertise<std_msgs::String>("chatter", 1000);

  ros::Rate loop_rate(10);

  /**
   * A count of how many messages we have sent. This is used to create
   * a unique string for each message.
   */
  int count = 0;
  while (ros::ok())
  {
    /**
     * This is a message object. You stuff it with data, and then publish it.
     */
    std_msgs::String msg;

    std::stringstream ss;
    ss << "hello world " << count;
    msg.data = ss.str();

    ROS_INFO("%s", msg.data.c_str());

    /**
     * The publish() function is how you send messages. The parameter
     * is the message object. The type of this object must agree with the type
     * given as a template parameter to the advertise<>() call, as was done
     * in the constructor above.
     */
    chatter_pub.publish(msg);

    ros::spinOnce();

    loop_rate.sleep();
    ++count;
  }


  return 0;
}

接着准备listener.cpp文件，

#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;
}

5、修改CMakeLists.txt

这里需要注意的是，相关的CMakeLists.txt文件不是src目录下面文件，而是package下面的文件。这一点很容易改错的。


cmake_minimum_required(VERSION 2.8.3)
project(beginner_tutorials)

## Find catkin and any catkin packages
find_package(catkin REQUIRED COMPONENTS roscpp rospy std_msgs genmsg)

## Declare ROS messages and services
#add_message_files(FILES Num.msg)
#add_service_files(FILES AddTwoInts.srv)

## Generate added messages and services
generate_messages(DEPENDENCIES std_msgs)

## Declare a catkin package
catkin_package()

## Build talker and listener
include_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)

6、开始编译

编译就非常简单了。需要回到workspace的目录，然后输入catkin_make就可以了，

cd ./catkin_ws
catkin_make

不出意外的话，我们在./catkin_ws/devel/lib/beginner_tutorials看到对应的可执行文件。

7、启动程序

启动程序需要三个步骤。第一个步骤，设置一下rosrun的环境，即，

source devel/setup.sh

这样后期rosrun的时候，就可以直接启动node程序了。第二个步骤，输入roscore，这个相当于整个底层信息交换的大脑。第三个步骤，就是启动talker和listener了，

rosrun beginner_tutorials talker
rosrun beginner_tutorials listener

所有操作都完成之后，我们就可以看到talker程序这边在不断发送消息，而listener这边在不断地接收消息。这是一个完整的publish和subscribe程序对。

整个过程看上去自己要做的内容很少。这是因为ros已经帮助我们完成了大部分的工作，用ldd命令看下生成的程序就知道了，

feixiaoxing@feixiaoxing-VirtualBox:~/Desktop/catkin_ws/devel/lib/beginner_tutorials$ ldd listener 
	linux-vdso.so.1 (0x00007ffeb4bfe000)
	libroscpp.so => /opt/ros/noetic/lib/libroscpp.so (0x00007f2381bc9000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f2381b69000)
	librosconsole.so => /opt/ros/noetic/lib/librosconsole.so (0x00007f2381b06000)
	libroscpp_serialization.so => /opt/ros/noetic/lib/libroscpp_serialization.so (0x00007f2381b01000)
	libstdc++.so.6 => /lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007f238191f000)
	libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f2381904000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f2381710000)
	libxmlrpcpp.so => /opt/ros/noetic/lib/libxmlrpcpp.so (0x00007f23816ec000)
	librostime.so => /opt/ros/noetic/lib/librostime.so (0x00007f23816bf000)
	libcpp_common.so => /opt/ros/noetic/lib/libcpp_common.so (0x00007f23816b1000)
	libboost_thread.so.1.71.0 => /lib/x86_64-linux-gnu/libboost_thread.so.1.71.0 (0x00007f2381685000)
	libboost_chrono.so.1.71.0 => /lib/x86_64-linux-gnu/libboost_chrono.so.1.71.0 (0x00007f2381677000)
	libboost_filesystem.so.1.71.0 => /lib/x86_64-linux-gnu/libboost_filesystem.so.1.71.0 (0x00007f2381657000)
	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f2381508000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f2381db2000)
	librosconsole_log4cxx.so => /opt/ros/noetic/lib/librosconsole_log4cxx.so (0x00007f23814e8000)
	librosconsole_backend_interface.so => /opt/ros/noetic/lib/librosconsole_backend_interface.so (0x00007f23814e3000)
	liblog4cxx.so.10 => /lib/x86_64-linux-gnu/liblog4cxx.so.10 (0x00007f2381305000)
	libboost_regex.so.1.71.0 => /lib/x86_64-linux-gnu/libboost_regex.so.1.71.0 (0x00007f2381205000)
	libconsole_bridge.so.0.4 => /lib/x86_64-linux-gnu/libconsole_bridge.so.0.4 (0x00007f23811fd000)
	libapr-1.so.0 => /lib/x86_64-linux-gnu/libapr-1.so.0 (0x00007f23811c4000)
	libaprutil-1.so.0 => /lib/x86_64-linux-gnu/libaprutil-1.so.0 (0x00007f2381196000)
	libicui18n.so.66 => /lib/x86_64-linux-gnu/libicui18n.so.66 (0x00007f2380e97000)
	libicuuc.so.66 => /lib/x86_64-linux-gnu/libicuuc.so.66 (0x00007f2380cb1000)
	libuuid.so.1 => /lib/x86_64-linux-gnu/libuuid.so.1 (0x00007f2380ca6000)
	libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f2380ca0000)
	libcrypt.so.1 => /lib/x86_64-linux-gnu/libcrypt.so.1 (0x00007f2380c65000)
	libexpat.so.1 => /lib/x86_64-linux-gnu/libexpat.so.1 (0x00007f2380c37000)
	libicudata.so.66 => /lib/x86_64-linux-gnu/libicudata.so.66 (0x00007f237f176000)