301 lines
10 KiB
ReStructuredText
301 lines
10 KiB
ReStructuredText
.. _lifecycle_v2:
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Lifecycle
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#########
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A :dfn:`thread` is a kernel object that is used for application processing
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that is too lengthy or too complex to be performed by an ISR.
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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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Any number of threads can be defined by an application. Each thread is
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referenced by a :dfn:`thread id` that is assigned when the thread is spawned.
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A thread has the following key properties:
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* A **stack area**, which is a region of memory used for the thread's stack.
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The **size** of the stack area can be tailored to conform to the actual needs
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of the thread's processing. Special macros exist to create and work with
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stack memory regions.
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* A **thread control block** for private kernel bookkeeping of the thread's
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metadata. This is an instance of type :c:type:`struct k_thread`.
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* An **entry point function**, which is invoked when the thread is started.
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Up to 3 **argument values** can be passed to this function.
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* A **scheduling priority**, which instructs the kernel's scheduler how to
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allocate CPU time to the thread. (See :ref:`scheduling_v2`.)
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* A set of **thread options**, which allow the thread to receive special
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treatment by the kernel under specific circumstances.
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(See :ref:`thread_options_v2`.)
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* A **start delay**, which specifies how long the kernel should wait before
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starting the thread.
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* An **execution mode**, which can either be supervisor or user mode.
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By default, threads run in supervisor mode and allow access to
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privileged CPU instructions, the entire memory address space, and
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peripherals. User mode threads have a reduced set of privileges.
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This depends on the :option:`CONFIG_USERSPACE` option. See :ref:`usermode`.
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.. _spawning_thread:
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Thread Creation
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===============
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A thread must be created before it can be used. The kernel initializes
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the thread control block as well as one end of the stack portion. The remainder
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of the thread's stack is typically left uninitialized.
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Specifying a start delay of :c:macro:`K_NO_WAIT` instructs the kernel
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to start thread execution immediately. Alternatively, the kernel can be
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instructed to delay execution of the thread by specifying a timeout
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value -- for example, to allow device hardware used by the thread
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to become available.
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The kernel allows a delayed start to be canceled before the thread begins
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executing. A cancellation request has no effect if the thread has already
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started. A thread whose delayed start was successfully canceled must be
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re-spawned before it can be used.
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Thread Termination
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==================
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Once a thread is started it typically executes forever. However, a thread may
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synchronously end its execution by returning from its entry point function.
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This is known as **termination**.
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A thread that terminates is responsible for releasing any shared resources
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it may own (such as mutexes and dynamically allocated memory)
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prior to returning, since the kernel does *not* reclaim them automatically.
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.. note::
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The kernel does not currently make any claims regarding an application's
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ability to respawn a thread that terminates.
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Thread Aborting
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===============
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A thread may asynchronously end its execution by **aborting**. The kernel
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automatically aborts a thread if the thread triggers a fatal error condition,
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such as dereferencing a null pointer.
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A thread can also be aborted by another thread (or by itself)
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by calling :cpp:func:`k_thread_abort()`. However, it is typically preferable
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to signal a thread to terminate itself gracefully, rather than aborting it.
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As with thread termination, the kernel does not reclaim shared resources
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owned by an aborted thread.
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.. note::
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The kernel does not currently make any claims regarding an application's
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ability to respawn a thread that aborts.
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Thread Suspension
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=================
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A thread can be prevented from executing for an indefinite period of time
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if it becomes **suspended**. The function :cpp:func:`k_thread_suspend()`
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can be used to suspend any thread, including the calling thread.
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Suspending a thread that is already suspended has no additional effect.
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Once suspended, a thread cannot be scheduled until another thread calls
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:cpp:func:`k_thread_resume()` to remove the suspension.
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.. note::
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A thread can prevent itself from executing for a specified period of time
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using :cpp:func:`k_sleep()`. However, this is different from suspending
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a thread since a sleeping thread becomes executable automatically when the
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time limit is reached.
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.. _thread_options_v2:
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Thread Options
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==============
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The kernel supports a small set of :dfn:`thread options` that allow a thread
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to receive special treatment under specific circumstances. The set of options
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associated with a thread are specified when the thread is spawned.
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A thread that does not require any thread option has an option value of zero.
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A thread that requires a thread option specifies it by name, using the
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:literal:`|` character as a separator if multiple options are needed
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(i.e. combine options using the bitwise OR operator).
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The following thread options are supported.
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:c:macro:`K_ESSENTIAL`
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This option tags the thread as an :dfn:`essential thread`. This instructs
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the kernel to treat the termination or aborting of the thread as a fatal
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system error.
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By default, the thread is not considered to be an essential thread.
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:c:macro:`K_FP_REGS` and :c:macro:`K_SSE_REGS`
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These x86-specific options indicate that the thread uses the CPU's
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floating point registers and SSE registers, respectively. This instructs
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the kernel to take additional steps to save and restore the contents
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of these registers when scheduling the thread.
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(For more information see :ref:`float_v2`.)
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By default, the kernel does not attempt to save and restore the contents
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of these registers when scheduling the thread.
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:c:macro:`K_USER`
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If :option:`CONFIG_USERSPACE` is enabled, this thread will be created in
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user mode and will have reduced privileges. See :ref:`usermode`. Otherwise
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this flag does nothing.
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:c:macro:`K_INHERIT_PERMS`
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If :option:`CONFIG_USERSPACE` is enabled, this thread will inherit all
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kernel object permissions that the parent thread had, except the parent
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thread object. See :ref:`usermode`.
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Implementation
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**************
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Spawning a Thread
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=================
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A thread is spawned by defining its stack area and its thread control block,
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and then calling :cpp:func:`k_thread_create()`. The stack area must be defined
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using :c:macro:`K_THREAD_STACK_DEFINE` to ensure it is properly set up in
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memory.
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The thread spawning function returns its thread id, which can be used
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to reference the thread.
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The following code spawns a thread that starts immediately.
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.. code-block:: c
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#define MY_STACK_SIZE 500
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#define MY_PRIORITY 5
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extern void my_entry_point(void *, void *, void *);
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K_THREAD_STACK_DEFINE(my_stack_area, MY_STACK_SIZE);
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struct k_thread my_thread_data;
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k_tid_t my_tid = k_thread_create(&my_thread_data, my_stack_area,
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K_THREAD_STACK_SIZEOF(my_stack_area),
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my_entry_point,
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NULL, NULL, NULL,
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MY_PRIORITY, 0, K_NO_WAIT);
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Alternatively, a thread can be spawned at compile time by calling
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:c:macro:`K_THREAD_DEFINE`. Observe that the macro defines
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the stack area, control block, and thread id variables automatically.
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The following code has the same effect as the code segment above.
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.. code-block:: c
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#define MY_STACK_SIZE 500
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#define MY_PRIORITY 5
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extern void my_entry_point(void *, void *, void *);
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K_THREAD_DEFINE(my_tid, MY_STACK_SIZE,
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my_entry_point, NULL, NULL, NULL,
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MY_PRIORITY, 0, K_NO_WAIT);
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User Mode Constraints
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---------------------
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This section only applies if :option:`CONFIG_USERSPACE` is enabled, and a user
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thread tries to create a new thread. The :c:func:`k_thread_create()` API is
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still used, but there are additional constraints which must be met or the
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calling thread will be terminated:
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* The calling thread must have permissions granted on both the child thread
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and stack parameters; both are tracked by the kernel as kernel objects.
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* The child thread and stack objects must be in an uninitialized state,
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i.e. it is not currently running and the stack memory is unused.
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* The stack size parameter passed in must be equal to or less than the
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bounds of the stack object when it was declared.
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* The :c:macro:`K_USER` option must be used, as user threads can only create
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other user threads.
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* The :c:macro:`K_ESSENTIAL` option must not be used, user threads may not be
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considered essential threads.
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* The priority of the child thread must be a valid priority value, and equal to
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or lower than the parent thread.
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Dropping Permissions
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====================
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If :option:`CONFIG_USERSPACE` is enabled, a thread running in supervisor mode
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may perform a one-way transition to user mode using the
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:cpp:func:`k_thread_user_mode_enter()` API. This is a one-way operation which
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will reset and zero the thread's stack memory. The thread will be marked
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as non-essential.
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Terminating a Thread
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====================
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A thread terminates itself by returning from its entry point function.
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The following code illustrates the ways a thread can terminate.
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.. code-block:: c
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void my_entry_point(int unused1, int unused2, int unused3)
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{
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while (1) {
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...
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if (<some condition>) {
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return; /* thread terminates from mid-entry point function */
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}
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...
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}
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/* thread terminates at end of entry point function */
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}
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If CONFIG_USERSPACE is enabled, aborting a thread will additionally mark the
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thread and stack objects as uninitialized so that they may be re-used.
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Suggested Uses
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**************
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Use threads to handle processing that cannot be handled in an ISR.
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Use separate threads to handle logically distinct processing operations
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that can execute in parallel.
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Configuration Options
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*********************
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Related configuration options:
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* :option:`CONFIG_USERSPACE`
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APIs
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****
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The following thread APIs are provided by :file:`kernel.h`:
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* :c:macro:`K_THREAD_DEFINE`
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* :cpp:func:`k_thread_create()`
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* :cpp:func:`k_thread_cancel()`
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* :cpp:func:`k_thread_abort()`
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* :cpp:func:`k_thread_suspend()`
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* :cpp:func:`k_thread_resume()`
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* :c:macro:`K_THREAD_STACK_DEFINE`
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* :c:macro:`K_THREAD_STACK_ARRAY_DEFINE`
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* :c:macro:`K_THREAD_STACK_MEMBER`
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* :c:macro:`K_THREAD_STACK_SIZEOF`
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* :c:macro:`K_THREAD_STACK_BUFFER`
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