Custom message definition in ROS

Introduction

Custom message definitions let you create your own data types to send between parts of a robot system. This helps you share exactly the information you need.

You want to send sensor data that is not covered by standard messages.

You need to communicate complex information like robot status with multiple fields.

You want to define a message with custom units or formats.

You are building a new robot feature that requires special data.

You want to organize related data together for easier handling.

Syntax

ROS

# Example of a custom message file (MyMessage.msg)
int32 id
string name
float64[] values
bool is_active

Each line defines one field with its type and name.

Use standard ROS message types like int32, string, float64, bool, or arrays with [].

Examples

A simple message with a number and a text label.

ROS

int32 count
string label

A weather message with numbers and a true/false value.

ROS

float64 temperature
float64 humidity
bool is_raining

A message with lists of strings and numbers.

ROS

string[] names
int32[] scores

Sample Program

This example shows a custom message called MyRobotStatus with three fields. The Python node publishes this message every second with example data. The log shows the sent values.

ROS

# File: MyRobotStatus.msg
int32 battery_level
bool is_moving
string current_task

# Usage in a ROS Python node
import rclpy
from rclpy.node import Node
from my_robot_interfaces.msg import MyRobotStatus

class StatusPublisher(Node):
    def __init__(self):
        super().__init__('status_publisher')
        self.publisher_ = self.create_publisher(MyRobotStatus, 'robot_status', 10)
        self.timer = self.create_timer(1.0, self.publish_status)

    def publish_status(self):
        msg = MyRobotStatus()
        msg.battery_level = 85
        msg.is_moving = True
        msg.current_task = 'Exploring'
        self.publisher_.publish(msg)
        self.get_logger().info(f'Published: battery={msg.battery_level}, moving={msg.is_moving}, task={msg.current_task}')


def main(args=None):
    rclpy.init(args=args)
    node = StatusPublisher()
    rclpy.spin(node)
    node.destroy_node()
    rclpy.shutdown()

Important Notes

Remember to add your custom message files to your package's package.xml and CMakeLists.txt so ROS can build them.

After defining a custom message, build your workspace and source the setup file before using it.

Use descriptive names and clear field types to make your messages easy to understand.

Summary

Custom messages let you send exactly the data your robot needs.

Define fields with types and names in a .msg file.

Build and import your messages to use them in ROS nodes.

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