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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, roscp, and rosmake commandline tools.

Tutorial Level: BEGINNER

Next Tutorial: Writing a simple publisher and subscriber (python) (c++)

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.

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

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. 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

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.

Using roscp

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:

Open CMakeLists.txt in your favorite text editor. 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

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

$ rosmake beginner_tutorials

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

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++).