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.. _module-pw_thread_threadx:
This is a set of backends for pw_thread based on ThreadX.
.. Warning::
This module is still under construction, the API is not yet stable.
Thread Creation Backend
A backend for ``pw::thread::Thread`` is offered using ``tx_thread_create``.
Optional joining support is enabled via an ``TX_EVENT_FLAGS_GROUP`` in each
thread's context.
This backend permits users to start threads where contexts must be explicitly
allocated and passed in as an option. As a quick example, a detached thread can
be created as follows:
.. code-block:: cpp
#include "pw_thread/detached_thread.h"
#include "pw_thread_threadx/config.h"
#include "pw_thread_threadx/context.h"
#include "pw_thread_threadx/options.h"
#include "tx_api.h"
constexpr UINT kFooPriority =
constexpr ULONG kFooTimeSliceInterval =
constexpr size_t kFooStackSizeWords =
void StartExampleThread() {
.. list-table::
* - :ref:`module-pw_thread` Facade
- Backend Target
- Description
* - ``pw_thread:id``
- ``pw_thread_threadx:id``
- Thread identification.
* - ``pw_thread:yield``
- ``pw_thread_threadx:yield``
- Thread scheduler yielding.
* - ``pw_thread:sleep``
- ``pw_thread_threadx:sleep``
- Thread scheduler sleeping.
* - ``pw_thread:thread``
- ``pw_thread_threadx:thread``
- Thread creation.
Module Configuration Options
The following configurations can be adjusted via compile-time configuration of
this module, see the
:ref:`module documentation <module-structure-compile-time-configuration>` for
more details.
Whether thread joining is enabled. By default this is disabled.
We suggest only enabling this when thread joining is required to minimize
the RAM and ROM cost of threads.
Enabling this grows the RAM footprint of every pw::thread::Thread as it adds
a TX_EVENT_FLAGS_GROUP to every thread's pw::thread::threadx::Context. In
addition, there is a minute ROM cost to construct and destroy this added
PW_THREAD_JOINING_ENABLED gets set to this value.
The default stack size in words. By default this uses the minimal ThreadX
stack size.
The maximum length of a thread's name, not including null termination. By
default this is arbitrarily set to 15. This results in an array of characters
which is this length + 1 bytes in every pw::thread::Thread's context.
The round robin time slice tick interval for threads at the same priority.
By default this is disabled as not all ports support this, using a value of 0
The minimum priority level, this is normally based on the number of priority
The default priority level. By default this uses the minimal ThreadX
priority level, given that 0 is the highest priority.
The log level to use for this module. Logs below this level are omitted.
ThreadX Thread Options
.. cpp:class:: pw::thread::threadx::Options
.. cpp:function:: set_name(const char* name)
Sets the name for the ThreadX thread, note that this will be deep copied
into the context and may be truncated based on
.. cpp:function:: set_priority(UINT priority)
Sets the priority for the ThreadX thread from 0 through 31, where a value
of 0 represents the highest priority, see ThreadX tx_thread_create for
more detail.
**Precondition**: priority <= ``PW_THREAD_THREADX_CONFIG_MIN_PRIORITY``.
.. cpp:function:: set_preemption_threshold(UINT preemption_threshold)
Optionally sets the preemption threshold for the ThreadX thread from 0
through 31.
Only priorities higher than this level (i.e. lower number) are allowed to
preempt this thread. In other words this allows the thread to specify the
priority ceiling for disabling preemption. Threads that have a higher
priority than the ceiling are still allowed to preempt while those with
less than the ceiling are not allowed to preempt.
Not setting the preemption threshold or explicitly specifying a value
equal to the priority disables preemption threshold.
Time slicing is disabled while the preemption threshold is enabled, i.e.
not equal to the priority, even if a time slice interval was specified.
The preemption threshold can be adjusted at run time, this only sets the
initial threshold.
**Precondition**: preemption_threshold <= priority
.. cpp:function:: set_time_slice_interval(UINT time_slice_interval)
Sets the number of ticks this thread is allowed to run before other ready
threads of the same priority are given a chance to run.
Time slicing is disabled while the preemption threshold is enabled, i.e.
not equal to the priority, even if a time slice interval was specified.
A value of ``TX_NO_TIME_SLICE`` (a value of 0) disables time-slicing of
this thread.
Using time slicing results in a slight amount of system overhead, threads
with a unique priority should consider ``TX_NO_TIME_SLICE``.
.. cpp:function:: set_context(pw::thread::embos::Context& context)
Set the pre-allocated context (all memory needed to run a thread). Note
that this is required for this thread creation backend! The Context can
either be constructed with an externally provided ``std::span<ULONG>``
stack or the templated form of ``ContextWihtStack<kStackSizeWords`` can be
Thread Identification Backend
A backend for ``pw::thread::Id`` and ``pw::thread::get_id()`` is offerred using
``tx_thread_identify()``. It uses ``DASSERT`` to ensure that a thread is
executing via ``TX_THREAD_GET_SYSTEM_STATE()``.
Thread Sleep Backend
A backend for ``pw::thread::sleep_for()`` and ``pw::thread::sleep_until()`` is
offerred using ``tx_thread_sleep()`` if the duration is at least one tick, else
``tx_thread_relinquish()`` is used. It uses
``pw::this_thread::get_id() != thread::Id()`` to ensure it invoked only from a
Thread Yield Backend
A backend for ``pw::thread::yield()`` is offered using via
``tx_thread_relinquish()``. It uses
``pw::this_thread::get_id() != thread::Id()`` to ensure it invoked only from a
In cases where an operation must be performed for every thread,
``ForEachThread()`` can be used to iterate over all the created thread TCBs.
Note that it's only safe to use this while the scheduler is disabled.
An ``Aborted`` error status is returned if the provided callback returns
``false`` to request an early termination of thread iteration.
*Return values*
* ``Aborted``: The callback requested an early-termination of thread iteration.
* ``OkStatus``: The callback has been successfully run with every thread.
Snapshot integration
This ``pw_thread`` backend provides helper functions that capture ThreadX thread
state to a ``pw::thread::Thread`` proto.
``SnapshotThread()`` captures the thread name, state, and stack information for
the provided ThreadX TCB to a ``pw::thread::Thread`` protobuf encoder. To ensure
the most up-to-date information is captured, the stack pointer for the currently
running thread must be provided for cases where the running thread is being
captured. For ARM Cortex-M CPUs, you can do something like this:
.. Code:: cpp
// Capture PSP.
void* stack_ptr = 0;
asm volatile("mrs %0, psp\n" : "=r"(stack_ptr));
pw::thread::ProcessThreadStackCallback cb =
[](pw::thread::Thread::StreamEncoder& encoder,
pw::ConstByteSpan stack) -> pw::Status {
return encoder.WriteRawStack(stack);
pw::thread::threadx::SnapshotThread(my_thread, stack_ptr,
snapshot_encoder, cb);
``SnapshotThreads()`` wraps the singular thread capture to instead captures
all created threads to a ``pw::thread::SnapshotThreadInfo`` message. This proto
message overlays a snapshot, so it is safe to static cast a
``pw::snapshot::Snapshot::StreamEncoder`` to a
``pw::thread::SnapshotThreadInfo::StreamEncoder`` when calling this function.