42 lines
3.9 KiB
ReStructuredText
42 lines
3.9 KiB
ReStructuredText
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threading
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=========
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Lua has no thread safety. sol does not force thread safety bottlenecks anywhere. Treat access and object handling like you were dealing with a raw ``int`` reference (``int&``) (no safety or order guarantees whatsoever).
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Assume any access or any call on Lua affects the whole ``sol::state``/``lua_State*`` (because it does, in a fair bit of cases). Therefore, every call to a state should be blocked off in C++ with some kind of access control (when you're working with multiple C++ threads). When you start hitting the same state from multiple threads, race conditions (data or instruction) can happen.
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To handle multithreaded environments, it is encouraged to either spawn a Lua state (``sol::state``) for each thread you are working with and keep inter-state communication to synchronized serialization points. This means that 3 C++ threads should have 3 Lua states, and access between them should be controlled using some kind of synchronized C++ mechanism (actual transfer between states must be done by serializing the value into C++ and then re-pushing it into the other state).
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`Here is an example of a processor that explicitly serializes`_ Lua returns into C++ values, and Lua functions into state-transferrable byte code to do work.
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Using coroutines and Lua's threads might also buy you some concurrency and parallelism (**unconfirmed and likely untrue, do not gamble on this**), but remember that Lua's 'threading' technique is ultimately cooperative and requires explicit yielding and resuming (simplified as function calls for :doc:`sol::coroutine<api/coroutine>`).
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getting the main thread
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-----------------------
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Lua 5.1 does not keep a reference to the main thread, therefore the user has to store it themselves. If you create a ``sol::state`` or follow the :ref:`steps for opening up compatibility and default handlers here<state-automatic-handlers>`, you can work with ``sol::main_thread`` to retrieve you the main thread, given a ``lua_State*`` that is either a full state or a thread: ``lua_state* Lmain = sol::main_thread( Lcoroutine )``; This function will always work in Lua 5.2 and above: in Lua 5.1, if you do not follow the ``sol::state`` instructions and do not pass a fallback ``lua_State*`` to the function, this function may not work properly and return ``nullptr``.
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working with multiple Lua threads
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---------------------------------
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You can mitigate some of the pressure of using coroutines and threading by using the ``lua_xmove`` constructors that sol implements. Simply keep a reference to your ``sol::state_view`` or ``sol::state`` or the target ``lua_State*`` pointer, and pass it into the constructor along with the object you want to copy. Note that there is also some implicit ``lua_xmove`` checks that are done for copy and move assignment operators as well, as noted :ref:`at the reference constructor explanations<lua_xmove-note>`.
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.. note::
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Advanced: For every single ``sol::reference`` derived type, there exists a version prefixed with the word ``main_``, such as ``sol::main_table``, ``sol::main_function``, ``sol::main_object`` and similar. These classes, on construction, assignment and other operations, forcibly obtain the ``lua_State*`` associated with the main thread, if possible. Using these classes will allow your code to be immune when a wrapped coroutine or a lua thread is set to ``nil`` and then garbage-collected.
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.. note::
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This does **not** provide immunity from typical multithreading issues in C++, such as synchronized access and the like. Lua's coroutines are cooperative in nature and concurrent execution with things like ``std::thread`` and similar still need to follow good C++ practices for multi threading.
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Here's an example of explicit state transferring below:
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.. literalinclude:: ../../examples/source/docs/state_transfer.cpp
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:name: state-transfer
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:linenos:
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.. _Here is an example of a processor that explicitly serializes: https://github.com/ThePhD/sol2/blob/develop/examples/source/docs/std_thread.cpp
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