ESP-IDF is based on FreeRTOS but offers a range of POSIX-compatible APIs that allow easy porting of third-party code. This includes support for common parts of the POSIX Threads ``pthread`` API.
POSIX Threads are implemented in ESP-IDF as wrappers around equivalent FreeRTOS features. The runtime memory or performance overhead of using the pthreads API is quite low, but not every feature available in either pthreads or FreeRTOS is available via the ESP-IDF pthreads support.
Pthreads can be used in ESP-IDF by including standard ``pthread.h`` header, which is included in the toolchain libc. An additional ESP-IDF specific header, ``esp_pthread.h``, provides additional non-POSIX APIs for using some ESP-IDF features with pthreads.
C++ Standard Library implementations for ``std::thread``, ``std::mutex``, ``std::condition_variable``, etc., are realized using pthreads (via GCC libstdc++). Therefore, restrictions mentioned here also apply to the equivalent C++ standard library functionality.
Unlike many operating systems using POSIX Threads, ESP-IDF is a real-time operating system with a real-time scheduler. This means that a thread will only stop running if a higher priority task is ready to run, the thread blocks on an OS synchronization structure like a mutex, or the thread calls any of the functions ``sleep``, :cpp:func:`vTaskDelay`, or ``usleep``.
When calling a standard libc or C++ sleep function, such as ``usleep`` defined in ``unistd.h``, the task will only block and yield the core if the sleep time is longer than :ref:`one FreeRTOS tick period <CONFIG_FREERTOS_HZ>`. If the time is shorter, the thread will busy-wait instead of yielding to another RTOS task.
Refer to `standard POSIX Threads documentation <https://man7.org/linux/man-pages/man7/pthreads.7.html>`__, or ``pthread.h``, for details about the standard arguments and behaviour of each function. Differences or limitations compared to the standard APIs are noted below.
- The ``attr`` argument is supported for setting stack size and detach state only. Other attribute fields are ignored.
- Unlike FreeRTOS task functions, the ``start_routine`` function is allowed to return. A detached type thread is automatically deleted if the function returns. The default joinable type thread will be suspended until ``pthread_join()`` is called on it.
- This function does not need to free any resources and instead resets the ``attr`` structure to defaults. The implementation is the same as ``pthread_attr_init()``.
POSIX Mutexes are implemented as FreeRTOS Mutex Semaphores (normal type for "fast" or "error check" mutexes, and Recursive type for "recursive" mutexes). This means that they have the same priority inheritance behavior as mutexes created with :cpp:func:`xSemaphoreCreateMutex`.
Static initializer constant ``PTHREAD_MUTEX_INITIALIZER`` is supported, but the non-standard static initializer constants for other mutex types are not supported.
* The resolution of ``pthread_cond_timedwait()`` timeouts is the RTOS tick period (see :ref:`CONFIG_FREERTOS_HZ`). Timeouts may be delayed up to one tick period after the requested timeout.
In ESP-IDF, POSIX **unnamed** semaphores are implemented. The accessible API is described below. It implements `semaphores as specified in the POSIX standard <https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/semaphore.h.html>`_, unless specified otherwise.
- The time value passed by abstime will be rounded up to the next FreeRTOS tick.
- The actual timeout happens after the tick that the time was rounded to and before the following tick.
- It is possible, though unlikely, that the task is preempted directly after the timeout calculation, delaying the timeout of the following blocking operating system call by the duration of the preemption.
- The ``destr_function`` argument is supported and will be called if a thread function exits normally, calls ``pthread_exit()``, or if the underlying task is deleted directly using the FreeRTOS function :cpp:func:`vTaskDelete`.
These functions can be called from tasks created using either pthread or FreeRTOS APIs. When calling these functions from tasks created using FreeRTOS APIs, :ref:`CONFIG_FREERTOS_TLSP_DELETION_CALLBACKS` config option must be enabled to ensure the thread-specific data is cleaned up before the task is deleted.
Other POSIX Threads functions (not listed here) are not implemented and will produce either a compiler or a linker error if referenced from an ESP-IDF application. If you identify a useful API that you would like to see implemented in ESP-IDF, please open a `feature request on GitHub <https://github.com/espressif/esp-idf/issues>`_ with the details.
The API :cpp:func:`esp_pthread_set_cfg` defined in the ``esp_pthreads.h`` header offers custom extensions to control how subsequent calls to ``pthread_create()`` behaves. Currently, the following configuration can be set:
- Stack memory capabilities determine which kind of memory is used for allocating pthread stacks. The field takes ESP-IDF heap capability flags, as defined in :component_file:`heap/include/esp_heap_caps.h`. The memory must be 8-bit accessible (MALLOC_CAP_8BIT), besides other custom flags the user can choose from. The user is responsible for ensuring the correctness of the stack memory capabilities. For more information about memory locations, refer to the documentation of :ref:`memory_capabilities`.
This configuration is scoped to the calling thread (or FreeRTOS task), meaning that :cpp:func:`esp_pthread_set_cfg` can be called independently in different threads or tasks. If the ``inherit_cfg`` flag is set in the current configuration then any new thread created will inherit the creator's configuration (if that thread calls ``pthread_create()`` recursively), otherwise the new thread will have the default configuration.
