# 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.
Note
Architectural Overview > Runtime Environment provides more information about tasks and work queue tasks.
Note
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 (opens new window).
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:
Specify the dependency on the work queue library in the cmake definition file (CMakeLists.txt (opens new window)):
... DEPENDS px4_work_queue
In addition to
ModuleBase
, the task should also derive fromScheduledWorkItem
(included from ScheduledWorkItem.hpp (opens new window))Specify the queue to add the task to in the constructor initialisation. The work_item (opens new window) example adds itself to the
wq_configurations::test1
work queue as shown below:WorkItemExample::WorkItemExample() : ModuleParams(nullptr), ScheduledWorkItem(MODULE_NAME, px4::wq_configurations::test1) { }
Note
The available work queues (
wq_configurations
) are listed in WorkQueueManager.hpp (opens new window).Implement the
ScheduledWorkItem::Run()
method to perform "work".Implement the
task_spawn
method, specifying that the task is a work queue (using thetask_id_is_work_queue
id.Schedule the work queue task using one of the scheduling methods (in the example we use
ScheduleOnInterval
from within theinit
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 (opens new window).
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
. Themodule 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 (opens new window)):- 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.
- They are used to print the command-line usage when entering