# Header Message Errors
When wanting to introduce [headers](http://docs.ros.org/en/melodic/api/std_msgs/html/msg/Header.html) do the following steps.  
  
You should have a CMake package created already (as only Cmake packages can implement custom msgs as of now).

## Custom Message File
* Create a *.mgs file inside the `msg/` directory and start the file name with a capital letter
* Add the line `std_msgs/Header header` in order to use the header message, this msg type is from standard messages ([std_msgs](http://docs.ros.org/en/melodic/api/std_msgs/html/index-msg.html)) 
* Only introduce absolutely necessary comments in *.msg file (eg. #Header) in order to minimize possible errors.

## Configure package.xml
* Add _buildtool_depend_:  
```xml
<buildtool_depend>ament_cmake</buildtool_depend>
<buildtool_depend>rosidl_default_generators</buildtool_depend>
```
* Add _build_depend_:  
```xml
<build_depend>builtin_interfaces</build_depend>
<build_depend>std_msgs</build_depend>
```
* Add _exec_depend_:  
```xml
<exec_depend>rosidl_default_runtime</exec_depend>
<exec_depend>builtin_interfaces</exec_depend>
<exec_depend>std_msgs</exec_depend>
```

## Configure CMakeLists.txt
* Add required packages:  
```cmake
find_package(ament_cmake REQUIRED)
find_package(rosidl_default_generators REQUIRED)
find_package(builtin_interfaces REQUIRED)
find_package(std_msgs REQUIRED)
```
* Create msg files set (exchange with your own `msg/*.msg` files):  
```cmake
set(msg_files
  "msg/IMUInfo.msg"
  "msg/Extrinsics.msg"
)
```
* Load set into _rosidl_generate_interfaces_ and add the dependency for std_msgs:  
```cmake
rosidl_generate_interfaces(${PROJECT_NAME}
  ${msg_files}
  DEPENDENCIES builtin_interfaces std_msgs
)
```

## Build Colcon Workspace
* Move to root of workspace: `cd ~/<path_to_ws>`
* Run build: `colcon build`

## Test and Verify
* Run publisher: `ros2 topic pub /chatter <pkg_name>/msg/<custom_msg_file>`
* Run subscriber: `ros2 topic echo /chatter`  
  
You should see your custom msg being published and received, including the header.

## Source
[Link to the problem's solution at ros.org forum](https://answers.ros.org/question/326008/ros2-run-symbol-not-found-on-custom-msg/)  
Example Files:
* ROS2 Realsense: 
  * [Custom msg file](https://github.com/IntelRealSense/realsense-ros/blob/eloquent/realsense2_camera_msgs/msg/IMUInfo.msg)
  * [CMakeLists.txt](https://github.com/IntelRealSense/realsense-ros/blob/eloquent/realsense2_camera_msgs/CMakeLists.txt)
  * [package.xml](https://github.com/IntelRealSense/realsense-ros/blob/eloquent/realsense2_camera_msgs/package.xml)
  * [Write timestamp](https://answers.ros.org/question/354203/timestamp-message-ros2-python/)