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
$ 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 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:
- 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++).