POSIX Threads Support

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Overview

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.

RTOS Integration

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, vTaskDelay(), or usleep.

Note

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 one FreeRTOS tick period. If the time is shorter, the thread will busy-wait instead of yielding to another RTOS task.

By default, all POSIX Threads have the same RTOS priority, but it is possible to change this by calling a custom API.

Standard Features

The following standard APIs are implemented in ESP-IDF.

Refer to standard POSIX Threads documentation, 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.

Thread APIs

• pthread_create()

  • 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.

• pthread_join()

• pthread_detach()

• pthread_exit()

• sched_yield()

• pthread_self()

  • An assert will fail if this function is called from a FreeRTOS task which is not a pthread.

• pthread_equal()

Thread Attributes

• pthread_attr_init()

• pthread_attr_destroy()

  • 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().

• pthread_attr_getstacksize() / pthread_attr_setstacksize()

• pthread_attr_getdetachstate() / pthread_attr_setdetachstate()

Once

• pthread_once()

Static initializer constant PTHREAD_ONCE_INIT is supported.

Note

This function can be called from tasks created using either pthread or FreeRTOS APIs.

Mutexes

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 xSemaphoreCreateMutex().

• pthread_mutex_init()

• pthread_mutex_destroy()

• pthread_mutex_lock()

• pthread_mutex_timedlock()

• pthread_mutex_trylock()

• pthread_mutex_unlock()

• pthread_mutexattr_init()

• pthread_mutexattr_destroy()

• pthread_mutexattr_gettype() / pthread_mutexattr_settype()

Static initializer constant PTHREAD_MUTEX_INITIALIZER is supported, but the non-standard static initializer constants for other mutex types are not supported.

Note

These functions can be called from tasks created using either pthread or FreeRTOS APIs.

Condition Variables

• pthread_cond_init()

  • The attr argument is not implemented and is ignored.

• pthread_cond_destroy()

• pthread_cond_signal()

• pthread_cond_broadcast()

• pthread_cond_wait()

• pthread_cond_timedwait()

Static initializer constant PTHREAD_COND_INITIALIZER is supported.

• The resolution of pthread_cond_timedwait() timeouts is the RTOS tick period (see CONFIG_FREERTOS_HZ). Timeouts may be delayed up to one tick period after the requested timeout.

Note

These functions can be called from tasks created using either pthread or FreeRTOS APIs.

Semaphores

In ESP-IDF, POSIX unnamed semaphores are implemented. The accessible API is described below. It implements semaphores as specified in the POSIX standard, unless specified otherwise.

• sem_init()

• sem_destroy()

  • pshared is ignored. Semaphores can always be shared between FreeRTOS tasks.

• sem_post()

  • If the semaphore has a value of SEM_VALUE_MAX already, -1 is returned and errno is set to EAGAIN.

• sem_wait()

• sem_trywait()

• sem_timedwait()

  • 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.

• sem_getvalue()

Read/Write Locks

The following API functions of the POSIX reader-writer locks specification are implemented:

• pthread_rwlock_init()

  • The attr argument is not implemented and is ignored.

• pthread_rwlock_destroy()

• pthread_rwlock_rdlock()

• pthread_rwlock_tryrdlock()

• pthread_rwlock_wrlock()

• pthread_rwlock_trywrlock()

• pthread_rwlock_unlock()

The static initializer constant PTHREAD_RWLOCK_INITIALIZER is supported.

Note

These functions can be called from tasks created using either pthread or FreeRTOS APIs.

Thread-Specific Data

• pthread_key_create()

  • 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 vTaskDelete().

• pthread_key_delete()

• pthread_setspecific() / pthread_getspecific()

Note

These functions can be called from tasks created using either pthread or FreeRTOS APIs. When calling these functions from tasks created using FreeRTOS APIs, CONFIG_FREERTOS_TLSP_DELETION_CALLBACKS config option must be enabled to ensure the thread-specific data is cleaned up before the task is deleted.

Note

There are other options for thread local storage in ESP-IDF, including options with higher performance. See Thread Local Storage.

Not Implemented

The pthread.h header is a standard header and includes additional APIs and features which are not implemented in ESP-IDF. These include:

• pthread_cancel() returns ENOSYS if called.

• pthread_condattr_init() returns ENOSYS if called.

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 with the details.

ESP-IDF Extensions

The API 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:

• Default stack size of new threads, if not specified when calling pthread_create() (overrides CONFIG_PTHREAD_TASK_STACK_SIZE_DEFAULT).

• RTOS priority of new threads (overrides CONFIG_PTHREAD_TASK_PRIO_DEFAULT).

• Core affinity / core pinning of new threads (overrides CONFIG_PTHREAD_TASK_CORE_DEFAULT).

• FreeRTOS task name for new threads (overrides CONFIG_PTHREAD_TASK_NAME_DEFAULT)

This configuration is scoped to the calling thread (or FreeRTOS task), meaning that 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.

Examples

• system/pthread demonstrates using the pthreads API to create threads.

• cxx/pthread demonstrates using C++ Standard Library functions with threads.

API Reference

Header File

• components/pthread/include/esp_pthread.h

• This header file can be included with:

  #include "esp_pthread.h"
  

• This header file is a part of the API provided by the pthread component. To declare that your component depends on pthread, add the following to your CMakeLists.txt:

  REQUIRES pthread
  

  or

  PRIV_REQUIRES pthread
  

Functions

esp_pthread_cfg_t esp_pthread_get_default_config(void)

Creates a default pthread configuration based on the values set via menuconfig.

Returns

A default configuration structure.

esp_err_t esp_pthread_set_cfg(const esp_pthread_cfg_t *cfg)

Configure parameters for creating pthread.

This API allows you to configure how the subsequent pthread_create() call will behave. This call can be used to setup configuration parameters like stack size, priority, configuration inheritance etc.

If the 'inherit' flag in the configuration structure is enabled, then the same configuration is also inherited in the thread subtree.

Note

Passing non-NULL attributes to pthread_create() will override the stack_size parameter set using this API

Parameters

cfg -- The pthread config parameters

Returns

• ESP_OK if configuration was successfully set

• ESP_ERR_NO_MEM if out of memory

• ESP_ERR_INVALID_ARG if stack_size is less than PTHREAD_STACK_MIN

esp_err_t esp_pthread_get_cfg(esp_pthread_cfg_t *p)

Get current pthread creation configuration.

This will retrieve the current configuration that will be used for creating threads.

Parameters

p -- Pointer to the pthread config structure that will be updated with the currently configured parameters

Returns

• ESP_OK if the configuration was available

• ESP_ERR_NOT_FOUND if a configuration wasn't previously set

esp_err_t esp_pthread_init(void)

Initialize pthread library.

Structures

struct esp_pthread_cfg_t

pthread configuration structure that influences pthread creation

Public Members

size_t stack_size

The stack size of the pthread.

size_t prio

The thread's priority.

bool inherit_cfg

Inherit this configuration further.

const char *thread_name

The thread name.

int pin_to_core

The core id to pin the thread to. Has the same value range as xCoreId argument of xTaskCreatePinnedToCore.

Macros

PTHREAD_STACK_MIN

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