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Threads
=======

Wait a minute? Why are we on threads? Aren't event loops supposed to be **the
way** to do *web-scale programming*? Well... no. Threads are still the medium in
which processors do their jobs. Threads are therefore mighty useful sometimes, even
though you might have to wade through various synchronization primitives.

Threads are used internally to fake the asynchronous nature of all of the system
calls. libuv also uses threads to allow you, the application, to perform a task
asynchronously that is actually blocking, by spawning a thread and collecting
the result when it is done.

Today there are two predominant thread libraries: the Windows threads
implementation and POSIX's `pthreads`_. libuv's thread API is analogous to
the pthreads API and often has similar semantics.

A notable aspect of libuv's thread facilities is that it is a self contained
section within libuv. Whereas other features intimately depend on the event
loop and callback principles, threads are complete agnostic, they block as
required, signal errors directly via return values, and, as shown in the
:ref:`first example <thread-create-example>`, don't even require a running
event loop.

libuv's thread API is also very limited since the semantics and syntax of
threads are different on all platforms, with different levels of completeness.

This chapter makes the following assumption: **There is only one event loop,
running in one thread (the main thread)**. No other thread interacts
with the event loop (except using ``uv_async_send``).

Core thread operations
----------------------

There isn't much here, you just start a thread using ``uv_thread_create()`` and
wait for it to close using ``uv_thread_join()``.

.. _thread-create-example:

.. rubric:: thread-create/main.c
.. literalinclude:: ../../code/thread-create/main.c
    :linenos:
    :lines: 26-36
    :emphasize-lines: 3-7

.. tip::

    ``uv_thread_t`` is just an alias for ``pthread_t`` on Unix, but this is an
    implementation detail, avoid depending on it to always be true.

The second parameter is the function which will serve as the entry point for
the thread, the last parameter is a ``void *`` argument which can be used to pass
custom parameters to the thread. The function ``hare`` will now run in a separate
thread, scheduled pre-emptively by the operating system:

.. rubric:: thread-create/main.c
.. literalinclude:: ../../code/thread-create/main.c
    :linenos:
    :lines: 6-14
    :emphasize-lines: 2

Unlike ``pthread_join()`` which allows the target thread to pass back a value to
the calling thread using a second parameter, ``uv_thread_join()`` does not. To
send values use :ref:`inter-thread-communication`.

Synchronization Primitives
--------------------------

This section is purposely spartan. This book is not about threads, so I only
catalogue any surprises in the libuv APIs here. For the rest you can look at
the pthreads `man pages <pthreads>`_.

Mutexes
~~~~~~~

The mutex functions are a **direct** map to the pthread equivalents.

.. rubric:: libuv mutex functions
.. literalinclude:: ../../../include/uv.h
    :lines: 1355-1360

The ``uv_mutex_init()``, ``uv_mutex_init_recursive()`` and ``uv_mutex_trylock()``
functions will return 0 on success, and an error code otherwise.

If `libuv` has been compiled with debugging enabled, ``uv_mutex_destroy()``,
``uv_mutex_lock()`` and ``uv_mutex_unlock()`` will ``abort()`` on error.
Similarly ``uv_mutex_trylock()`` will abort if the error is anything *other
than* ``EAGAIN`` or ``EBUSY``.

Recursive mutexes are supported, but you should not rely on them. Also, they
should not be used with ``uv_cond_t`` variables.

The default BSD mutex implementation will raise an error if a thread which has
locked a mutex attempts to lock it again. For example, a construct like::

    uv_mutex_init(a_mutex);
    uv_mutex_lock(a_mutex);
    uv_thread_create(thread_id, entry, (void *)a_mutex);
    uv_mutex_lock(a_mutex);
    // more things here

can be used to wait until another thread initializes some stuff and then
unlocks ``a_mutex`` but will lead to your program crashing if in debug mode, or
return an error in the second call to ``uv_mutex_lock()``.

.. note::

    Mutexes on Windows are always recursive.