.. _threading:

Threading and synchronization utilities
=======================================

libuv provides cross-platform implementations for multiple threading and
synchronization primitives. The API largely follows the pthreads API.


Data types
----------

.. c:type:: uv_thread_t

    Thread data type.

.. c:type:: void (*uv_thread_cb)(void* arg)

    Callback that is invoked to initialize thread execution. `arg` is the same
    value that was passed to :c:func:`uv_thread_create`.

.. c:type:: uv_key_t

    Thread-local key data type.

.. c:type:: uv_once_t

    Once-only initializer data type.

.. c:type:: uv_mutex_t

    Mutex data type.

.. c:type:: uv_rwlock_t

    Read-write lock data type.

.. c:type:: uv_sem_t

    Semaphore data type.

.. c:type:: uv_cond_t

    Condition data type.

.. c:type:: uv_barrier_t

    Barrier data type.


API
---

Threads
^^^^^^^

.. c:function:: int uv_thread_create(uv_thread_t* tid, uv_thread_cb entry, void* arg)
.. c:function:: unsigned long uv_thread_self(void)
.. c:function:: int uv_thread_join(uv_thread_t *tid)
.. c:function:: int uv_thread_equal(const uv_thread_t* t1, const uv_thread_t* t2)

Thread-local storage
^^^^^^^^^^^^^^^^^^^^

.. note::
    The total thread-local storage size may be limited. That is, it may not be possible to
    create many TLS keys.

.. c:function:: int uv_key_create(uv_key_t* key)
.. c:function:: void uv_key_delete(uv_key_t* key)
.. c:function:: void* uv_key_get(uv_key_t* key)
.. c:function:: void uv_key_set(uv_key_t* key, void* value)

Once-only initialization
^^^^^^^^^^^^^^^^^^^^^^^^

Runs a function once and only once. Concurrent calls to :c:func:`uv_once` with the
same guard will block all callers except one (it's unspecified which one).
The guard should be initialized statically with the UV_ONCE_INIT macro.

.. c:function:: void uv_once(uv_once_t* guard, void (*callback)(void))

Mutex locks
^^^^^^^^^^^

Functions return 0 on success or an error code < 0 (unless the
return type is void, of course).

.. c:function:: int uv_mutex_init(uv_mutex_t* handle)
.. c:function:: void uv_mutex_destroy(uv_mutex_t* handle)
.. c:function:: void uv_mutex_lock(uv_mutex_t* handle)
.. c:function:: int uv_mutex_trylock(uv_mutex_t* handle)
.. c:function:: void uv_mutex_unlock(uv_mutex_t* handle)

Read-write locks
^^^^^^^^^^^^^^^^

Functions return 0 on success or an error code < 0 (unless the
return type is void, of course).

.. c:function:: int uv_rwlock_init(uv_rwlock_t* rwlock)
.. c:function:: void uv_rwlock_destroy(uv_rwlock_t* rwlock)
.. c:function:: void uv_rwlock_rdlock(uv_rwlock_t* rwlock)
.. c:function:: int uv_rwlock_tryrdlock(uv_rwlock_t* rwlock)
.. c:function:: void uv_rwlock_rdunlock(uv_rwlock_t* rwlock)
.. c:function:: void uv_rwlock_wrlock(uv_rwlock_t* rwlock)
.. c:function:: int uv_rwlock_trywrlock(uv_rwlock_t* rwlock)
.. c:function:: void uv_rwlock_wrunlock(uv_rwlock_t* rwlock)

Semaphores
^^^^^^^^^^

Functions return 0 on success or an error code < 0 (unless the
return type is void, of course).

.. c:function:: int uv_sem_init(uv_sem_t* sem, unsigned int value)
.. c:function:: void uv_sem_destroy(uv_sem_t* sem)
.. c:function:: void uv_sem_post(uv_sem_t* sem)
.. c:function:: void uv_sem_wait(uv_sem_t* sem)
.. c:function:: int uv_sem_trywait(uv_sem_t* sem)

Conditions
^^^^^^^^^^

Functions return 0 on success or an error code < 0 (unless the
return type is void, of course).

.. note::
    Callers should be prepared to deal with spurious wakeups on :c:func:`uv_cond_wait` and
    :c:func:`uv_cond_timedwait`.

.. c:function:: int uv_cond_init(uv_cond_t* cond)
.. c:function:: void uv_cond_destroy(uv_cond_t* cond)
.. c:function:: void uv_cond_signal(uv_cond_t* cond)
.. c:function:: void uv_cond_broadcast(uv_cond_t* cond)
.. c:function:: void uv_cond_wait(uv_cond_t* cond, uv_mutex_t* mutex)
.. c:function:: int uv_cond_timedwait(uv_cond_t* cond, uv_mutex_t* mutex, uint64_t timeout)

Barriers
^^^^^^^^

Functions return 0 on success or an error code < 0 (unless the
return type is void, of course).

.. note::
    :c:func:`uv_barrier_wait` returns a value > 0 to an arbitrarily chosen "serializer" thread
    to facilitate cleanup, i.e.

    ::

        if (uv_barrier_wait(&barrier) > 0)
            uv_barrier_destroy(&barrier);

.. c:function:: int uv_barrier_init(uv_barrier_t* barrier, unsigned int count)
.. c:function:: void uv_barrier_destroy(uv_barrier_t* barrier)
.. c:function:: int uv_barrier_wait(uv_barrier_t* barrier)