89 lines
3.0 KiB
ReStructuredText
89 lines
3.0 KiB
ReStructuredText
.. _system_threads_v2:
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System Threads
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##############
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.. contents::
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:local:
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:depth: 2
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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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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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:c:func:`main` function (if one is defined).
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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 or executing the application's :c:func:`main` function;
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this means a fatal system error is raised if the thread aborts. If
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:c:func:`main` is not defined, or if it executes and then does a normal
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return, the main thread terminates normally and no error is raised.
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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. (See :ref:`workqueues_v2`.)
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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 ``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, unless ``CONFIG_BOOTARGS`` is selected. In such case the
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kernel passes arguments to it and ``main(int, char **)`` can be used.
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The following code outlines a trivial ``main(void)`` 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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int 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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