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.. _module-pw_thread_embos:
This is a set of backends for pw_thread based on embOS v4.
.. Warning::
This module is still under construction, the API is not yet stable.
Thread Creation Backend
A backend or ``pw::thread::Thread`` is offered using ``OS_CreateTaskEx()``.
Optional joining support is enabled via an ``OS_EVENT`` in each thread's
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_embos/config.h"
#include "pw_thread_embos/context.h"
#include "pw_thread_embos/options.h"
#include "RTOS.h" // For the embOS types.
constexpr OS_PRIO kFooPriority =
constexpr OS_UINT kFooTimeSliceInterval =
constexpr size_t kFooStackSizeWords =
void StartExampleThread() {
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
an OS_EVENT to every thread's pw::thread::embos::Context. In addition, there
is a minute ROM cost to construct and destroy this added object.
PW_THREAD_JOINING_ENABLED gets set to this value.
The minimum stack size in words. By default this uses Segger's recommendation
of 68 bytes.
The default stack size in words. By default this uses Segger's recommendation
of 256 bytes to start.
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 minimum priority level, this is normally 1, since 0 is not a valid
priority level.
The default priority level. By default this uses the minimal embOS priority.
The round robin time slice tick interval for threads at the same priority.
By default this is set to 2 ticks based on the embOS default.
The log level to use for this module. Logs below this level are omitted.
embOS Thread Options
.. cpp:class:: pw::thread::embos::Options
.. cpp:function:: set_name(const char* name)
Sets the name for the embOS task, this is optional.
Note that this will be deep copied into the context and may be truncated
.. cpp:function:: set_priority(OS_PRIO priority)
Sets the priority for the embOS task. Higher values are higher priority,
see embOS OS_CreateTaskEx for more detail.
Precondition: This must be >= ``PW_THREAD_EMBOS_CONFIG_MIN_PRIORITY``.
.. cpp:function:: set_time_slice_interval(OS_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.
A value of 0 disables time-slicing of this thread.
Precondition: This must be <= 255 ticks.
.. 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 ``pw::span<OS_UINT>``
stack or the templated form of ``ContextWithStack<kStackSizeWords>`` can
be used.
Thread Identification Backend
A backend for ``pw::thread::Id`` and ``pw::thread::get_id()`` is offerred using
``OS_GetTaskID()``. It uses ``DASSERT`` to ensure that the scheduler has started
via ``OS_IsRunning()``.
Thread Sleep Backend
A backend for ``pw::thread::sleep_for()`` and ``pw::thread::sleep_until()`` is
offerred using ``OS_Delay()`` if the duration is at least one tick, else
``OS_Yield()`` is used. It uses ``pw::this_thread::get_id() != thread::Id()`` to
ensure it invoked only from a thread.
Thread Yield Backend
A backend for ``pw::thread::yield()`` is offered using via ``OS_Yield()``.
It uses ``pw::this_thread::get_id() != thread::Id()`` to ensure it invoked only
from a thread.
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 suspended, and this
should only be used after ``OS_Start()`` has been called. Calling this before
the scheduler has started is non-fatal, but will result in no action and a
``FailedPrecondition`` error code.
An ``Aborted`` error status is returned if the provided callback returns
``false`` to request an early termination of thread iteration.
*Return values*
* ``FailedPrecondition``: Returned when ``ForEachThread()`` is run before the OS
has been initialized.
* ``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 embOS thread
info to a ``pw::thread::Thread`` proto.
``SnapshotThread()`` captures the thread name, state, and stack information for
the provided embOS 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::embos::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.
Thread Name Capture
In order to capture thread names when snapshotting a thread, embOS must have
``OS_TRACKNAME`` enabled. If ``OS_TRACKNAME`` is disabled, no thread name
is captured. Enabling this is strongly recommended for debugability.
Thread State Capture
embOS thread state is not part of embOS's public API. Despite this, the
snapshot integration captures thread state based on information on how the
thread state is represented from
`Segger's public forum <>`_.
This has been tested on embOS 4.22, and was initially
reported for embOS 5.06. The logic Pigweed uses to interpret thread state may
be incorrect for other versions of embOS.
Thread Stack Capture
Full thread stack information capture is dependent on embOS tracking the stack
bounds for each task. When either ``OS_SUPPORT_MPU`` or ``OS_CHECKSTACK`` are
enabled, stack bounds are tracked and the callback for thread stack dumping
will be called. If both of these options are disabled, ``stack_start_pointer``
and ``stack_end_pointer`` will not be captured, and the
``ProcessThreadStackCallback`` will not be called.