Module Template for Full Applications

An application can be written to run as either a task (a module with its own stack and process priority) or as a work queue task (a module that runs on a work queue thread, sharing the stack and thread priorit with other tasks on the work queue). In most cases a work queue task can be used, as this minimizes resource usage.

Architectural Overview > Runtime Environment provides more information about tasks and work queue tasks.

All the things learned in the First Application Tutorial are relevant for writing a full application.

Work Queue Task

PX4-Autopilot contains a template for writing a new application (module) that runs as a work queue task: src/examples/work_item.

A work queue task application is just the same as an ordinary (task) application, except that it needs to specify that it is a work queue task, and schedule itself to run during initialisation.

The example shows how. In summary:

  1. Specify the dependency on the work queue library in the cmake definition file (CMakeLists.txt):
    ...
    DEPENDS
       px4_work_queue
    
  2. In addition to ModuleBase, the task should also derive from ScheduledWorkItem (included from ScheduledWorkItem.hpp)
  3. Specify the queue to add the task to in the constructor initialisation. The work_item example adds itself to the wq_configurations::test1 work queue as shown below:

    WorkItemExample::WorkItemExample() :
        ModuleParams(nullptr),
        ScheduledWorkItem(MODULE_NAME, px4::wq_configurations::test1)
    {
    }
    

    The available work queues (wq_configurations) are listed in WorkQueueManager.hpp.

  4. Implement the ScheduledWorkItem::Run() method to perform "work".

  5. Implement the task_spawn method, specifying that the task is a work queue (using the task_id_is_work_queue id.
  6. Schedule the work queue task using one of the scheduling methods (in the example we use ScheduleOnInterval from within the init method).

Tasks

PX4/PX4-Autopilot contains a template for writing a new application (module) that runs as a task on its own stack: src/templates/template_module.

The template demonstrates the following additional features/aspects that are required or are useful for a full application:

  • Accessing parameters and reacting to parameter updates.
  • uORB subscriptions and waiting for topic updates.
  • Controlling the task that runs in the background via start/stop/status. The module start [<arguments>] command can then be directly added to the startup script.
  • Command-line argument parsing.
  • Documentation: the PRINT_MODULE_* methods serve two purposes (the API is documented in the source code):
    • They are used to print the command-line usage when entering module help on the console.
    • They are automatically extracted via script to generate the Modules & Commands Reference page.

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