This is a set of backends for pw_thread based on FreeRTOS.


This module is still under construction, the API is not yet stable.

Thread Creation Backend

A backend for pw::thread::Thread is offered using xTaskCreateStatic(). Optional dynamic allocation for threads is supported using xTaskCreate(). Optional joining support is enabled via an StaticEventGroup_t in each thread’s context.

This backend always permits users to start threads where static contexts are passed in as an option. As a quick example, a detached thread can be created as follows:

#include "FreeRTOS.h"
#include "pw_thread/detached_thread.h"
#include "pw_thread_freertos/config.h"
#include "pw_thread_freertos/context.h"
#include "pw_thread_freertos/options.h"

constexpr UBaseType_t kFooPriority =
constexpr size_t kFooStackSizeWords =

void StartExampleThread() {

Alternatively when PW_THREAD_FREERTOS_CONFIG_DYNAMIC_ALLOCATION_ENABLED is enabled, dynamic thread allocation can be used. The above example could instead be done as follows:

#include "FreeRTOS.h"
#include "pw_thread/detached_thread.h"
#include "pw_thread_freertos/config.h"
#include "pw_thread_freertos/context.h"
#include "pw_thread_freertos/options.h"

constexpr UBaseType_t kFooPriority =
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 a StaticEventGroup_t to every thread’s pw::thread::freertos::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.


Whether dynamic allocation for threads (stacks and contexts) is enabled. By default this matches the FreeRTOS configuration on whether dynamic allocations are enabled. Note that static contexts must be provided if dynamic allocations are disabled.


The default stack size in words. By default this uses the minimal FreeRTOS stack size based on configMINIMAL_STACK_SIZE.


The default thread priority. By default this uses the minimal FreeRTOS priority level above the idle priority (tskIDLE_PRIORITY + 1).


The maximum thread priority. By default this uses the value below the number of priorities defined by the FreeRTOS configuration (configMAX_PRIORITIES - 1).


The log level to use for this module. Logs below this level are omitted.

FreeRTOS Thread Options

class pw::thread::freertos::Options
set_name(const char *name)

Sets the name for the FreeRTOS task, note that this will be truncated based on configMAX_TASK_NAME_LEN. This is deep copied by FreeRTOS into the task’s task control block (TCB).

set_priority(UBaseType_t priority)

Sets the priority for the FreeRTOS task. This must be a value between 0 to PW_THREAD_FREERTOS_CONFIG_MAXIMUM_PRIORITY. Higher priority values have a higher priority.

Note that the idle task priority, tskIDLE_PRIORITY, is fixed to 0. See the FreeRTOS documentation on the idle task for more details.


set_stack_size(size_t size_words)

Set the stack size in words for a dynamically thread.


Precondition: size_words must be >= configMINIMAL_STACK_SIZE

set_static_context(pw::thread::freertos::Context &context)

Set the pre-allocated context (all memory needed to run a thread). The StaticContext can either be constructed with an externally provided std::span<StackType_t> stack or the templated form of StaticContextWithStack<kStackSizeWords> can be used.

Thread Identification Backend

A backend for pw::thread::Id and pw::thread::get_id() is offerred using xTaskGetCurrentTaskHandle(). It uses DASSERT to ensure that it is not invoked from interrupt context and if possible that the scheduler has started via xTaskGetSchedulerState().

Thread Sleep Backend

A backend for pw::thread::sleep_for() and pw::thread::sleep_until() is offerred using vTaskDelay() if the duration is at least one tick, else taskYIELD() 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 taskYIELD(). 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 and interrupts are disabled.

Calling this before the scheduler has started, via vTaskStartScheduler(), 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.


This uses an unsupported method to iterate the threads in a more efficient manner while also supporting interrupt contexts. This requires linking against internal statics from the FreeRTOS kernel, pw_third_party_freertos_DISABLE_TASKS_STATICS must be used.

Snapshot integration

This pw_thread backend provides helper functions that capture FreeRTOS thread state to a pw::thread::Thread proto.

FreeRTOS tskTCB facade

Unfortunately FreeRTOS entirely hides the contents of the TCB inside of Source/tasks.c, but it’s necessary for snapshot processing in order to access the stack limits from interrupt contexts. For this reason, FreeRTOS snapshot integration relies on the pw_thread_freertos:freertos_tsktcb facade to provide the tskTCB definition.

The selected backend is expected to provide the struct tskTCB definition through pw_thread_freertos_backend/freertos_tsktcb.h. The facade asserts that this definition matches the size of FreeRTOS’s StaticTask_T which is the public opaque TCB type.


SnapshotThread() captures the thread name, state, and stack information for the provided 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::Thread::StreamEncoder& encoder,
       pw::ConstByteSpan stack) -> pw::Status {
  return encoder.WriteRawStack(stack);
pw::thread::threadx::SnapshotThread(my_thread, thread_state, stack_ptr,
                                    snapshot_encoder, cb);

SnapshotThreads() wraps the singular thread capture to instead captures all created threads to a pw::thread::SnapshotThreadInfo message which also captures the thread state for you. 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.


SnapshotThreads() is only safe to use this while the scheduler and interrupts are disabled as it relies on ForEachThread().