Custom message definition in ROS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a custom message in ROS?

A custom message in ROS is a user-defined data structure that allows you to send specific information between nodes, beyond the standard message types.

beginner

How do you define a custom message in ROS?

You create a .msg file inside the msg folder of your ROS package, listing the fields and their types line by line.

intermediate

Why do you need to rebuild your ROS package after adding a custom message?

Because ROS generates source code from the .msg files during build, so rebuilding creates the necessary code to use the new message type.

intermediate

What types of fields can you include in a ROS custom message?

You can include primitive types (int32, float64, bool), arrays, strings, and even other message types as fields.

intermediate

How do you use a custom message in your ROS node code?

After building your package, you import the generated message class in your code and use it to publish or subscribe to topics with that message type.

Where do you place the .msg file for a custom message in a ROS package?

A In the config folder

B Inside the msg directory of the package

C In the launch folder

D In the src directory

Custom message files must be placed inside the msg directory to be recognized and processed by ROS.

What command do you run after adding a new custom message to your ROS package?

A roslaunch

B rosrun

C catkin_make or colcon build

D rosmsg show

You must rebuild your package with catkin_make or colcon build to generate code for the new message.

Which of these is NOT a valid field type in a ROS custom message?

A int32

B string

C float64[]

D html

ROS messages support primitive types, arrays, strings, and other messages, but not HTML.

How do you refer to a custom message type in your ROS Python code?

A Import the message class from your package's msg module

B Write the message fields manually

C Use a generic message type

D No import needed, just use strings

You import the generated message class from your package's msg module to use the custom message.

What happens if you forget to rebuild your ROS package after adding a custom message?

A Your code will not recognize the new message type

B The message will work anyway

C ROS will auto-generate the message code

D Your package will crash immediately

Without rebuilding, the generated code for the new message does not exist, so your code cannot use it.

Explain the steps to create and use a custom message in a ROS package.

Think about file creation, build process, and code usage.

Describe why custom messages are useful in ROS and give an example scenario.

Consider how standard messages might be limited.

Practice

1. What is the main purpose of defining a custom message in ROS?

easy

A. To send specific data types tailored to your robot's needs

B. To create new robot hardware components

C. To write Python scripts for robot control

D. To install ROS packages automatically

Solution

1. Step 1: Understand the role of custom messages

   Custom messages allow you to define exactly what data your robot nodes exchange.

2. Step 2: Compare with other options

   Options B, C, and D relate to hardware, scripting, or package management, not message definition.

3. Final Answer:

   To send specific data types tailored to your robot's needs -> Option A

4. Quick Check:

   Custom messages = tailored data [OK]

Hint: Custom messages define data, not hardware or scripts [OK]

Common Mistakes:

• Confusing message definition with hardware setup
• Thinking custom messages are scripts
• Assuming messages install packages

2. Which of the following is the correct syntax to define a field named speed of type float32 in a ROS .msg file?

easy

A. speed float32

B. float32 speed

C. float speed32

D. float speed

Solution

1. Step 1: Recall ROS message field syntax

   In ROS .msg files, the type comes first, then the field name, e.g., float32 speed.

2. Step 2: Check each option

   float32 speed matches the correct syntax. Options B, C, and D have incorrect order or wrong types.

3. Final Answer:

   float32 speed -> Option B

4. Quick Check:

   Type before name in .msg files [OK]

Hint: Type first, then field name in .msg files [OK]

Common Mistakes:

• Swapping field name and type order
• Using incorrect type names
• Omitting the type or name

3. Given this custom message definition in RobotStatus.msg:

bool is_active
float32 battery_level
string status_message

What will be the default value of is_active when a new message is created in a ROS node?

medium

A. false

B. 1

C. null

D. true

Solution

1. Step 1: Understand default values in ROS messages

   Boolean fields default to false when a message is created without explicit initialization.

2. Step 2: Evaluate options

   false is false, which matches the default. Options A and B represent true values, and C is invalid for bool.

3. Final Answer:

   false -> Option A

4. Quick Check:

   Boolean default = false [OK]

Hint: Boolean fields default to false unless set [OK]

Common Mistakes:

• Assuming booleans default to true
• Confusing null with default values
• Using numeric 1 instead of boolean true

4. You wrote this in your SensorData.msg file:

int32 temperature
float64 humidity
string location

But when building your ROS package, you get an error. What is the likely cause?

medium

A. Using float64 instead of float32 is invalid in ROS messages

B. Field names cannot be lowercase

C. Missing blank line at the end of the file

D. No error; the message definition is correct

Solution

1. Step 1: Check ROS message type validity

   ROS supports float64 as a valid type, so Using float64 instead of float32 is invalid in ROS messages is incorrect.

2. Step 2: Verify field naming and formatting

   Lowercase field names are allowed, and no blank line is required at the end.

3. Final Answer:

   No error; the message definition is correct -> Option D

4. Quick Check:

   Valid types and names mean no error [OK]

Hint: ROS supports float64 and lowercase names [OK]

Common Mistakes:

• Thinking float64 is invalid
• Assuming field names must be uppercase
• Expecting blank line at file end

5. You want to define a custom message RobotCommand.msg that includes a nested message Header from the standard ROS messages and a custom field command of type string. Which is the correct way to define this?

hard

A. Header header
string command

B. header Header
command string

C. std_msgs/Header header
string command

D. Header: std_msgs/Header
string command

Solution

1. Step 1: Understand nested message syntax in ROS

   To use a message from another package, you must prefix the type with the package name, e.g., std_msgs/Header.

2. Step 2: Check each option

   std_msgs/Header header string command correctly uses std_msgs/Header header. Header header string command misses the package prefix, C reverses type and name, D uses invalid syntax.

3. Final Answer:

   std_msgs/Header header string command -> Option C

4. Quick Check:

   Use package/type for nested messages [OK]

Hint: Prefix nested types with package name [OK]

Common Mistakes:

• Omitting package name for nested messages
• Swapping field name and type
• Using colon syntax incorrectly