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 "pthreads" 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 implemented 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
, :cppvTaskDelay
, or usleep
.
Note
If calling a standard libc or C++ sleep function, such as usleep
defined in unistd.h
, then the task will only block and yield the CPU if the sleep time is longer than one FreeRTOS tick period <CONFIG_FREERTOS_HZ>
. 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 <esp-pthread>
.
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.
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.
- The
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()
pthread_attr_init()
pthread_attr_destroy()
- This function doesn't need to free any resources and instead resets the
attr
structure to defaults (implementation is same aspthread_attr_init()
).
- This function doesn't need to free any resources and instead resets the
pthread_attr_getstacksize()
/pthread_attr_setstacksize()
pthread_attr_getdetachstate()
/pthread_attr_setdetachstate()
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
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 behaviour as mutexes created with :cppxSemaphoreCreateMutex
.
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
pthread_cond_init()
- The
attr
argument is not implemented and is ignored.
- The
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 (seeCONFIG_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
pthread_rwlock_init()
- The
attr
argument is not implemented and is ignored.
- The
pthread_rwlock_destroy()
pthread_rwlock_rdlock()
pthread_rwlock_wrlock()
pthread_rwlock_unlock()
Static initializer constant PTHREAD_RWLOCK_INITIALIZER
is supported.
Note
These functions can be called from tasks created using either pthread or FreeRTOS APIs. Note also that the current pthread reader-writer-locks implementation is based on the condition variable API. The performance is not optimal when locking only for a minimal amount of time like e.g. accessing a simple variable. In these cases with minimal locking time, a simple mutex might be faster.
pthread_key_create()
- The
destr_function
argument is supported and will be called if a thread function exits normally, callspthread_exit()
, or if the underlying task is deleted directly using the FreeRTOS function :cppvTaskDelete
.
- The
pthread_key_delete()
pthread_setspecific()
/pthread_getspecific()
Note
These functions can be called from tasks created using either pthread or FreeRTOS APIs
Note
There are other options for thread local storage in ESP-IDF, including options with higher performance. See /api-guides/thread-local-storage
.
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()
returnsENOSYS
if called.pthread_condattr_init()
returnsENOSYS
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 <https://github.com/espressif/esp-idf/issues> with the details.
The API :cppesp_pthread_set_cfg
defined in the esp_pthreads.h
header offers custom extensions to control how subsequent calls to pthread_create()
will behave. 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
).
- 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 :cppesp_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.
system/pthread
demonstrates using the pthreads API to create threadscxx/pthread
demonstrates using C++ Standard Library functions with threads
inc/esp_pthread.inc