| .. _module-pw_thread: |
| |
| ========= |
| pw_thread |
| ========= |
| The ``pw_thread`` module contains utilities for thread creation and thread |
| execution. |
| |
| .. contents:: |
| :local: |
| :depth: 2 |
| |
| .. Warning:: |
| This module is still under construction, the API is not yet stable. |
| |
| --------------- |
| Thread Creation |
| --------------- |
| The class ``pw::thread::Thread`` can represent a single thread of execution. |
| Threads allow multiple functions to execute concurrently. |
| |
| The Thread's API is C++11 STL |
| `std::thread <https://en.cppreference.com/w/cpp/thread/thread>`_ like, meaning |
| the object is effectively a thread handle and not an object which contains the |
| thread's context. Unlike ``std::thread``, the API requires |
| ``pw::thread::Options`` as an argument and is limited to only work with |
| ``pw::thread::ThreadCore`` objects and functions which match the |
| ``pw::thread::Thread::ThreadRoutine`` signature. |
| |
| We recognize that the C++11's STL ``std::thread``` API has some drawbacks where |
| it is easy to forget to join or detach the thread handle. Because of this, we |
| offer helper wrappers like the ``pw::thread::DetachedThread``. Soon we will |
| extend this by also adding a ``pw::thread::JoiningThread`` helper wrapper which |
| will also have a lighter weight C++20 ``std::jthread`` like cooperative |
| cancellation contract to make joining safer and easier. |
| |
| Threads may begin execution immediately upon construction of the associated |
| thread object (pending any OS scheduling delays), starting at the top-level |
| function provided as a constructor argument. The return value of the |
| top-level function is ignored. The top-level function may communicate its |
| return value by modifying shared variables (which may require |
| synchronization, see :ref:`module-pw_sync`) |
| |
| Thread objects may also be in the state that does not represent any thread |
| (after default construction, move from, detach, or join), and a thread of |
| execution may be not associated with any thread objects (after detach). |
| |
| No two Thread objects may represent the same thread of execution; Thread is |
| not CopyConstructible or CopyAssignable, although it is MoveConstructible and |
| MoveAssignable. |
| |
| .. list-table:: |
| |
| * - *Supported on* |
| - *Backend module* |
| * - FreeRTOS |
| - :ref:`module-pw_thread_freertos` |
| * - ThreadX |
| - :ref:`module-pw_thread_threadx` |
| * - embOS |
| - :ref:`module-pw_thread_embos` |
| * - STL |
| - :ref:`module-pw_thread_stl` |
| * - Zephyr |
| - Planned |
| * - CMSIS-RTOS API v2 & RTX5 |
| - Planned |
| |
| |
| Options |
| ======= |
| The ``pw::thread::Options`` contains the parameters or attributes needed for a |
| thread to start. |
| |
| Pigweed does not generalize options, instead we strive to give you full control |
| where we provide helpers to do this. |
| |
| Options are backend specific and ergo the generic base class cannot be |
| directly instantiated. |
| |
| The attributes which can be set through the options are backend specific |
| but may contain things like the thread name, priority, scheduling policy, |
| core/processor affinity, and/or an optional reference to a pre-allocated |
| Context (the collection of memory allocations needed for a thread to run). |
| |
| Options shall NOT permit starting as detached, this must be done explicitly |
| through the Thread API. |
| |
| Options must not contain any memory needed for a thread to run (TCB, |
| stack, etc.). The Options may be deleted or re-used immediately after |
| starting a thread. |
| |
| Please see the thread creation backend documentation for how their Options work. |
| |
| .. Note:: |
| Options have a default constructor, however default options are not portable! |
| Default options can only work if threads are dynamically allocated by default, |
| meaning default options cannot work on backends which require static thread |
| allocations. In addition on some schedulers, default options will not work |
| for other reasons. |
| |
| Detaching & Joining |
| =================== |
| The ``Thread::detach()`` API is always available, to let you separate the |
| thread of execution from the thread object, allowing execution to continue |
| independently. |
| |
| The joining API, more specifically ``Thread::join()``, is conditionally |
| available depending on the selected backend for thread creation and how it is |
| configured. The backend is responsible for providing the |
| ``PW_THREAD_JOINING_ENABLED`` macro through |
| ``pw_thread_backend/thread_native.h``. This ensures that any users which include |
| ``pw_thread/thread.h`` can use this macro if needed. |
| |
| Please see the selected thread creation backend documentation for how to |
| enable joining if it's not already enabled by default. |
| |
| .. Warning:: |
| A constructed ``pw::thread::Thread`` which represents a thread of execution |
| must be EITHER detached or joined, else the destructor will assert! |
| |
| DetachedThread |
| ============== |
| To make it slightly easier and cleaner to spawn detached threads without having |
| to worry about thread handles, a wrapper ``DetachedThread()`` function is |
| provided which creates a ``Thread`` and immediately detaches it. For example |
| instead of: |
| |
| .. code-block:: cpp |
| |
| Thread(options, foo).detach(); |
| |
| You can instead use this helper wrapper to: |
| |
| .. code-block:: cpp |
| |
| DetachedThread(options, foo); |
| |
| The arguments are directly forwarded to the Thread constructor and ergo exactly |
| match the Thread constuctor arguments for creating a thread of execution. |
| |
| |
| ThreadRoutine & ThreadCore |
| ========================== |
| Threads must either be invoked through a |
| ``pw::thread::Thread::ThreadRoutine``` style function or implement the |
| ``pw::thread::ThreadCore`` interface. |
| |
| .. code-block:: cpp |
| |
| namespace pw::thread { |
| |
| // This function may return. |
| using Thread::ThreadRoutine = void (*)(void* arg); |
| |
| class ThreadCore { |
| public: |
| virtual ~ThreadCore() = default; |
| |
| // The public API to start a ThreadCore, note that this may return. |
| void Start() { Run(); } |
| |
| private: |
| // This function may return. |
| virtual void Run() = 0; |
| }; |
| |
| } // namespace pw::thread; |
| |
| |
| To use the ``pw::thread::Thread::ThreadRoutine``, your function must have the |
| following signature: |
| |
| .. code-block:: cpp |
| |
| void example_thread_entry_function(void *arg); |
| |
| |
| To invoke a member method of a class a static lambda closure can be used |
| to ensure the dispatching closure is not destructed before the thread is |
| done executing. For example: |
| |
| .. code-block:: cpp |
| |
| class Foo { |
| public: |
| void DoBar() {} |
| }; |
| Foo foo; |
| |
| static auto invoke_foo_do_bar = [](void *void_foo_ptr) { |
| // If needed, additional arguments could be set here. |
| static_cast<Foo*>(void_foo_ptr)->DoBar(); |
| }; |
| |
| // Now use the lambda closure as the thread entry, passing the foo's |
| // this as the argument. |
| Thread thread(options, invoke_foo_do_bar, &foo); |
| thread.detach(); |
| |
| |
| Alternatively, the aforementioned ``pw::thread::ThreadCore`` interface can be |
| be implemented by an object by overriding the private |
| ``void ThreadCore::Run();`` method. This makes it easier to create a thread, as |
| a static lambda closure or function is not needed to dispatch to a member |
| function without arguments. For example: |
| |
| .. code-block:: cpp |
| |
| class Foo : public ThreadCore { |
| private: |
| void Run() override {} |
| }; |
| Foo foo; |
| |
| // Now create the thread, using foo directly. |
| Thread(options, foo).detach(); |
| |
| .. Warning:: |
| Because the thread may start after the pw::Thread creation, an object which |
| implements the ThreadCore MUST meet or exceed the lifetime of its thread of |
| execution! |
| |
| ----------------------- |
| pw_snapshot integration |
| ----------------------- |
| ``pw_thread`` provides some light, optional integration with pw_snapshot through |
| helper functions for populating a ``pw::thread::Thread`` proto. Some of these |
| are directly integrated into the RTOS thread backends to simplify the thread |
| state capturing for snapshots. |
| |
| SnapshotStack() |
| =============== |
| The ``SnapshotStack()`` helper captures stack metadata (stack pointer and |
| bounds) into a ``pw::thread::Thread`` proto. After the stack bounds are |
| captured, execution is passed off to the thread stack collection callback to |
| capture a backtrace or stack dump. Note that this function does NOT capture the |
| thread name: that metadata is only required in cases where a stack overflow or |
| underflow is detected. |
| |
| .. Warning:: |
| Snapshot integration is a work-in-progress and may see significant API |
| changes. |