This is a set of backends for pw_thread based on embOS v4.


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 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:

#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 module documentation 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#

class pw::thread::embos::Options#
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 based on PW_THREAD_EMBOS_CONFIG_MAX_THREAD_NAME_LEN.

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.

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.

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:

// Capture PSP.
void* stack_ptr = 0;
asm volatile("mrs %0, psp\n" : "=r"(stack_ptr));
pw::thread::ProcessThreadStackCallback cb =
    [](pw::thread::proto::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::proto::SnapshotThreadInfo message. This proto message overlays a snapshot, so it is safe to static cast a pw::snapshot::Snapshot::StreamEncoder to a pw::thread::proto::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.