7. Creating Ros MSG and SRV

I. MSG and SRV INTRODUCTION

The Msg:msg file declares the individual fields of a ROS message using a simple text format. Just create an msg file that you can use to generate the message definition code for different languages.

The Srv:srv file declares a service, including request and response (response).

msg files and SRV files are stored in the packet's MSG directory and the SRV directory, respectively.

1. msg File

Msgs is a simple TXT file, each line defines an information field, with the format of each line:

信息域的名称 信息域的类型

There are several types of information fields:

int8int16int32int64uintfloat32float64string

Here is an example, this example uses a header type, an original string type, and two other predefined other types:

Headerheaderstring child_frame_idgeometry_msgs/PoseWithCovariance posegeometry_msgs/TwistWithCovariance twist

2. srv file

An SRV file is similar to an MSG file, except that it contains two parts--request and response. Two parts --- separated by:

int64 Aint64 B---int64 Sum

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

Second, use MSG1, create a msg

Let's use the msg file in the previously created package to beginner_tutorials define a msg, and if this package has been removed, please refer to the first article in this series to recreate the package.

First enter the package directory:

roscd beginner_tutorials

Create a msg directory:

mkdir msg

Then create the file using the following method Num.msg (you can also create it yourself and save it with the editor):

echo"int64 num" > msg/Num.msg

The file created above contains only one row, and of course you can add more information fields to each row, for example:

string first_namestring last_nameuint8 ageuint32 score

Once created, we need to make sure that the msg file can be converted to code in C++/python or other languages.

Open the package.xml file and make sure there are two lines in it and remove their comments:

<build_depend>message_generation</build_depend><run_depend>message_runtime</run_depend>

It needs to be pointed out that we need it during the build phase "message_generation" , and we need it at runtime "message_runtime" .

Then open the file under the package directory CMakeLists.txt and find_message add dependencies in the call message_generation so that you can generate ROS information.

As shown below, add an entry in parentheses message_generation :

find_package(catkin REQUIRED COMPONENTS   roscpp   rospy   std_msgs   message_generation)

You may notice that sometimes when you build a project, even if you do not CMakeLists.txt call find_package it in, the build is still normal. This is because catkin all of your projects are integrated, and as long as your previous project is called find_package , the current project will be configured for the same call. But this is a hidden problem, and when you build a project on your own, the lack of a find_package call can cause the build to fail.

Then similar to the above, make sure you join the message_runtime dependency:

Then find the following code snippet:

Remove the comment and replace it with:

add_message_files(  FILES  Num.msg)

Then, make sure that the generate_messages() function is called:

Remove the comment and modify it to:

generate_messages(  DEPENDENCIES  std_msgs)

Now that all the files are ready, sum up, there are probably a few steps:

1. Use the MSG file syntax to define a MSG in the MSG directory
2. Modify CmakeLists.txt , in the find_package call, add the message_generation dependent
3. Modify CmakeLists.txt , catkin_message add dependencies under message_runtime
4. Modify CmakeLists.txt , remove add_message_files comments, add our own definition of msg file
5. Modify CmakeLists.txt , remove generate_messages() a comment

Third, the use of rosmsg

We can use the rosmsg command to view the details of our defined MSG to ensure that it can be correctly identified by Ros:

show beginner_tutorials/Num

Will get:

int64 num

If you forget which packet the MSG is in, you can also omit the package name:

show Num

Get:

[beginner_tutorials/Num]:int64 num

Iv. using SRV1, creating an SRV

Also define the SRV file first:

roscd beginner_tutorialsmkdir srv

To avoid the hassle of manual creation, we can use roscp commands to copy a file from another package srv , using:

roscp[package_name][file_to_copy_path][copy_path]

Here, we rospy_tutorials copy a srv file from:

roscp rospy_tutorials AddTwoInts.srv srv/AddTwoInts.srv

Similar to the above, to ensure that the SRV file can be converted to code, we need to package.xml add content to:

  <build_depend>message_generation</build_depend>  <run_depend>message_runtime</run_depend>

The previous step in creating the MSG has already been done, so we can omit this step here.

The same, adding message_generation dependencies, we've done too:

find_package(catkin REQUIRED COMPONENTS  roscpp  rospy  std_msgs message_generation)

The next step is to add our SRV file to the project:

Find this section:

# add_service_files(#   FILES#   Service1.srv#   Service2.srv# )

Remove the comment and modify it to:

add_service_files(  FILES  AddTwoInts.srv)

It can be found that the process of creating SRV is basically consistent with MSG.

2. Using Rossrv

We can use the rossrv command to view information about the services we create to ensure that it is correctly identified by Ros:

show beginner_tutorials/AddTwoInts

Get:

int64 aint64 b---int64 sum

Similarly rosmsg , you can omit the package name:

$ rossrv show AddTwoInts[beginner_tutorials/AddTwoInts]:int64 aint64 b---int64 sum[rospy_tutorials/AddTwoInts]:int64 aint64 b---int64 sum

V. Basic process of using MSG and SRV 1, msg dependency

We have done this in the previous step to modify CMakeLists.txt :

# generate_messages(#   DEPENDENCIES# #  std_msgs  # Or other packages containing msgs# )

Remove the comments and add any packages that we depend on:

generate_messages(  DEPENDENCIES  std_msgs)

Here to explain the dependencies, as described earlier, when we write msg files, we can not only use the original int, string and other types, and sometimes use other packages of predefined information types, such as in the previous example geometry_msgs/PoseWithCovariance pose , when we need to use the other package to define the information , we have to declare our dependence on other packages, such as in the example above, to declare a geometry_msgs dependency.

In the previous example, only this line is in our msg file:

int64 num

Only the native int type is used, so we just need to declare the std_msgs dependency.

2. Start building

All MSG and SRV files have been defined, and the CmakeLists.txt file has been modified so that we can build our package now beginner_tutorial .

Now that we are in the beginner_tutorial package directory, execute the following command into the top-level directory of the workspace:

cd ../..

Perform:

install

Now, all the. msg files in the MSG directory will generate the source code for the Ros-supported language, the C + + header file is in ~/catkin_ws/devel/include/beginner_tutorials/ , Python is basically in ~/catkin_ws/devel/lib/python2.7/dist-packages/beginner_tutorials/msg , and the Lisp file is in ~/catkin_ws/devel/share/common-lisp/ros/beginner_tutorials/msg/ .

Similar to the location of MSG, the SRV header file for C + + is located in the same directory as the MSG header file, while Python and Lisp exist outside the MSG directory where an SRV directory holds the code generated by the SRV file.

Vi. Getting Help

We've learned a lot of ROS commands, and it's hard to remember the parameters and usage of each command, and we can use help to see the specifics.

For example:

-h

Or:

show -h

Seven, review

7. Creating Ros MSG and SRV