Embedded Motion Control/Tutorials/Creating a ROS msg and srv

Description: This tutorial covers how to create and build msg and srv files as well as the rosmsg, rossrv and roscp commandline tools.

Introduction to msg and srv

• msg: msg files are simple text files that describe the fields of a ROS message. They are used to generate source code for messages in different languages.
• srv: an srv file describes a service. It is composed of two parts: a request and a response.

msg files are stored in the msg directory of a package, and srv files are stored in the srv directory.

msgs are just simple text files with a field type and field name per line. The field types you can use are:

• int8, int16, int32, int64 (plus uint*)
• float32, float64
• string
• time, duration
• other msg files
• variable-length array[] and fixed-length array[C]

There is also a special type in ROS: Header, the header contains a timestamp and coordinate frame information that are commonly used in ROS. You will commonly see the first line in a msg file have Header header.

Here is an example of a msg that uses a Header, a string primitive, and two other msgs :

Header header
string child_frame_id
geometry_msgs/PoseWithCovariance pose
geometry_msgs/TwistWithCovariance twist

srv files are just like msg files, except they contain two parts: a request and a response. The two parts are separated by a '---' line. Here is an example of a srv file:

int64 A
int64 B
---
int64 Sum

In the above example, A and B are the request, and Sum is the response.

Using msg

Creating a msg

Let's define a new msg in the package that was created in the previous tutorial.

roscd beginner_tutorials_<YOUR_NAME>
mkdir msg
echo "int64 num" > msg/Num.msg

The example above is the simplest where .msg file contains only 1 line. You can, of course, create more complex file by adding multiple elements per line like this:

string first_name
string last_name
uint8 age
uint32 score

There's one more step, though. We need to make sure that the msg files are turned into source code for C++, Python, and other languages. Open CMakeLists.txt in your favorite text editor (rosed from the previous tutorial is a good option) and remove # to uncomment the following line:

# rosbuild_genmsg()

Using rosmsg

That's all you need to do to create a msg. Let's make sure that ROS can see it using the rosmsg show command.

Usage:

rosmsg show [message type]

Example:

rosmsg show beginner_tutorials/Num

You will see:

int64 num<//pre>

In the previous example, the message type consists of two parts:

beginner_tutorials -- the package where the message is defined Num -- The name of the msg Num. If you can't remember which Package a msg is in, you can leave out the package name. Try:

$ rosmsg show Num You will see:

[beginner_tutorials/Num]: int64 num

Using srv

Creating a srv

Let's use the package we just created to create a srv:

$ roscd beginner_tutorials $ mkdir srv Instead of creating a new srv definition by hand, we will copy an existing one from another package.

For that, roscp is a useful commandline tool for copying files from one package to another.

Usage:

$ roscp [package_name] [file_to_copy_path] [copy_path] Now we can copy a service from the rospy_tutorials package:

$ roscp rospy_tutorials AddTwoInts.srv srv/AddTwoInts.srv There's one more step, though. We need to make sure that the srv files are turned into source code for C++, Python, and other languages.

Once again, open CMakeLists.txt and remove # to uncomment the following line:

1. rosbuild_gensrv()

Using rossrv

That's all you need to do to create a srv. Let's make sure that ROS can see it using the rossrv show command.

Usage:

$ rossrv show <service type> Example:

$ rossrv show beginner_tutorials/AddTwoInts You will see:

int64 a int64 b --- int64 sum Similar to rosmsg, you can find service files like this without specifying package name:

$ rossrv show AddTwoInts [beginner_tutorials/AddTwoInts]: int64 a int64 b --- int64 sum

[rospy_tutorials/AddTwoInts]: int64 a int64 b --- int64 sum

Common step for msg and srv

Now that we have made some new messages we need to make our package again:

$ rosmake beginner_tutorials Any .msg file in the msg directory will generate code for use in all supported languages. The C++ message header file will be generated in ~/catkin_ws/devel/include/beginner_tutorials/. The Python script will be created in ~/catkin_ws/devel/lib/python2.7/dist-packages/beginner_tutorials/msg. The lisp file appears in ~/catkin_ws/devel/share/common-lisp/ros/beginner_tutorials/msg/.

The full specification for the message format is available at the Message Description Language page.

Getting Help

We've seen quite a few ROS tools already. It can be difficult to keep track of what arguments each command requires. Luckily, most ROS tools provide their own help.

Try:

$ rosmsg -h You should see a list of different rosmsg subcommands. Commands:

 rosmsg show Show message description
 rosmsg users  Find files that use message
 rosmsg md5  Display message md5sum
 rosmsg package  List messages in a package
 rosmsg packages List packages that contain messages

You can also get help for subcommands

$ rosmsg show -h This shows the arguments that are needed for rosmsg show: Usage: rosmsg show [options] <message type>

Options:

 -h, --help  show this help message and exit
 -r, --raw   show raw message text, including comments

Review

Lets just list some of the commands we've used so far:

rospack = ros+pack(age) : provides information related to ROS packages rosstack = ros+stack : provides information related to ROS stacks roscd = ros+cd : changes directory to a ROS package or stack rosls = ros+ls : lists files in a ROS package roscp = ros+cp : copies files from/to a ROS package rosmsg = ros+msg : provides information related to ROS message definitions rossrv = ros+srv : provides information related to ROS service definitions rosmake = ros+make : makes (compiles) a ROS package You should use catkin_make if you are using a catkin workspace.

Next Tutorial

Now that you've made a new ROS msg and srv, let's look at writing a simple publisher and subscriber (python) (c++).