075ee6b9fd
Note that a new section will be created to discuss workqueues, including the system workqueue. Change-Id: I921511f117acb07768619418539bef5b6a2a0a72 Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
106 lines
3.1 KiB
ReStructuredText
106 lines
3.1 KiB
ReStructuredText
.. _system_threads_v2:
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System Threads
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##############
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A :dfn:`system thread` is a thread that the kernel spawns automatically
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during system initialization.
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.. contents::
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:local:
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:depth: 2
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Concepts
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********
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The kernel spawns the following system threads.
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**Main thread**
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This thread performs kernel initialization, then calls the application's
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:cpp:func:`main()` function. If the application does not supply a
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:cpp:func:`main()` function, the main thread terminates once initialization
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is complete.
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By default, the main thread uses the highest configured preemptible thread
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priority (i.e. 0). If the kernel is not configured to support preemptible
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threads, the main thread uses the lowest configured cooperative thread
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priority (i.e. -1).
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The main thread is an essential thread while it is performing kernel
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initialization, which means a fatal system error is raised if the thread
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aborts. Once initialization is complete the thread is considered
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non-essential, so the termination or aborting of a :cpp:func:`main()`
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function supplied by the application does not cause a fatal system error.
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**Idle thread**
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This thread executes when there is no other work for the system to do.
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If possible, the idle thread activates the board's power management support
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to save power; otherwise, the idle thread simply performs a "do nothing"
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loop. The idle thread remains in existence as long as the system is running
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and never terminates.
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The idle thread always uses the lowest configured thread priority.
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If this makes it a cooperative thread, the idle thread repeatedly
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yields the CPU to allow the application's other threads to run when
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they need to.
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The idle thread is an essential thread, which means a fatal system error
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is raised if the thread aborts.
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Additional system threads may also be spawned, depending on the kernel
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and board configuration options specified by the application. For example,
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enabling the system workqueue spawns a system thread
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that services the work items submitted to it.
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Implementation
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**************
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Writing a main() function
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=========================
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An application-supplied :cpp:func:`main()` function begins executing once
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kernel initialization is complete. The kernel does not pass any arguments
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to the function.
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The following code outlines a trivial :cpp:func:`main()` function.
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The function used by a real application can be as complex as needed.
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.. code-block:: c
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void main(void)
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{
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/* initialize a semaphore */
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...
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/* register an ISR that gives the semaphore */
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...
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/* monitor the semaphore forever */
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while (1) {
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/* wait for the semaphore to be given by the ISR */
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...
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/* do whatever processing is now needed */
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...
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}
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}
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Suggested Uses
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**************
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Use the main thread to perform thread-based processing in an application
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that only requires a single thread, rather than defining an additional
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application-specific thread.
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Configuration Options
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*********************
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Related configuration options:
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* :option:`CONFIG_MAIN_THREAD_PRIORITY`
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* :option:`CONFIG_MAIN_STACK_SIZE`
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APIs
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****
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None.
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