godot-docs/classes/class_thread.rst

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.. _class_Thread:

Thread
======

**Inherits:** :ref:`RefCounted<class_RefCounted>` **<** :ref:`Object<class_Object>`

A unit of execution in a process.

.. rst-class:: classref-introduction-group

Description
-----------

A unit of execution in a process. Can run methods on :ref:`Object<class_Object>`\ s simultaneously. The use of synchronization via :ref:`Mutex<class_Mutex>` or :ref:`Semaphore<class_Semaphore>` is advised if working with shared objects.

\ **Warning:**\ 

To ensure proper cleanup without crashes or deadlocks, when a **Thread**'s reference count reaches zero and it is therefore destroyed, the following conditions must be met:

- It must not have any :ref:`Mutex<class_Mutex>` objects locked.

- It must not be waiting on any :ref:`Semaphore<class_Semaphore>` objects.

- :ref:`wait_to_finish()<class_Thread_method_wait_to_finish>` should have been called on it.

.. rst-class:: classref-introduction-group

Tutorials
---------

- :doc:`Using multiple threads <../tutorials/performance/using_multiple_threads>`

- :doc:`Thread-safe APIs <../tutorials/performance/thread_safe_apis>`

- `3D Voxel Demo <https://godotengine.org/asset-library/asset/2755>`__

.. rst-class:: classref-reftable-group

Methods
-------

.. table::
   :widths: auto

   +---------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
   | :ref:`String<class_String>`           | :ref:`get_id<class_Thread_method_get_id>`\ (\ ) |const|                                                                                         |
   +---------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
   | :ref:`bool<class_bool>`               | :ref:`is_alive<class_Thread_method_is_alive>`\ (\ ) |const|                                                                                     |
   +---------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
   | :ref:`bool<class_bool>`               | :ref:`is_main_thread<class_Thread_method_is_main_thread>`\ (\ ) |static|                                                                        |
   +---------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
   | :ref:`bool<class_bool>`               | :ref:`is_started<class_Thread_method_is_started>`\ (\ ) |const|                                                                                 |
   +---------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
   | |void|                                | :ref:`set_thread_safety_checks_enabled<class_Thread_method_set_thread_safety_checks_enabled>`\ (\ enabled\: :ref:`bool<class_bool>`\ ) |static| |
   +---------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
   | :ref:`Error<enum_@GlobalScope_Error>` | :ref:`start<class_Thread_method_start>`\ (\ callable\: :ref:`Callable<class_Callable>`, priority\: :ref:`Priority<enum_Thread_Priority>` = 1\ ) |
   +---------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
   | :ref:`Variant<class_Variant>`         | :ref:`wait_to_finish<class_Thread_method_wait_to_finish>`\ (\ )                                                                                 |
   +---------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+

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----

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Enumerations
------------

.. _enum_Thread_Priority:

.. rst-class:: classref-enumeration

enum **Priority**: :ref:`🔗<enum_Thread_Priority>`

.. _class_Thread_constant_PRIORITY_LOW:

.. rst-class:: classref-enumeration-constant

:ref:`Priority<enum_Thread_Priority>` **PRIORITY_LOW** = ``0``

A thread running with lower priority than normally.

.. _class_Thread_constant_PRIORITY_NORMAL:

.. rst-class:: classref-enumeration-constant

:ref:`Priority<enum_Thread_Priority>` **PRIORITY_NORMAL** = ``1``

A thread with a standard priority.

.. _class_Thread_constant_PRIORITY_HIGH:

.. rst-class:: classref-enumeration-constant

:ref:`Priority<enum_Thread_Priority>` **PRIORITY_HIGH** = ``2``

A thread running with higher priority than normally.

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----

.. rst-class:: classref-descriptions-group

Method Descriptions
-------------------

.. _class_Thread_method_get_id:

.. rst-class:: classref-method

:ref:`String<class_String>` **get_id**\ (\ ) |const| :ref:`🔗<class_Thread_method_get_id>`

Returns the current **Thread**'s ID, uniquely identifying it among all threads. If the **Thread** has not started running or if :ref:`wait_to_finish()<class_Thread_method_wait_to_finish>` has been called, this returns an empty string.

.. rst-class:: classref-item-separator

----

.. _class_Thread_method_is_alive:

.. rst-class:: classref-method

:ref:`bool<class_bool>` **is_alive**\ (\ ) |const| :ref:`🔗<class_Thread_method_is_alive>`

Returns ``true`` if this **Thread** is currently running the provided function. This is useful for determining if :ref:`wait_to_finish()<class_Thread_method_wait_to_finish>` can be called without blocking the calling thread.

To check if a **Thread** is joinable, use :ref:`is_started()<class_Thread_method_is_started>`.

.. rst-class:: classref-item-separator

----

.. _class_Thread_method_is_main_thread:

.. rst-class:: classref-method

:ref:`bool<class_bool>` **is_main_thread**\ (\ ) |static| :ref:`🔗<class_Thread_method_is_main_thread>`

Returns ``true`` if the thread this method was called from is the main thread.

\ **Note:** This is a static method and isn't associated with a specific **Thread** object.

.. rst-class:: classref-item-separator

----

.. _class_Thread_method_is_started:

.. rst-class:: classref-method

:ref:`bool<class_bool>` **is_started**\ (\ ) |const| :ref:`🔗<class_Thread_method_is_started>`

Returns ``true`` if this **Thread** has been started. Once started, this will return ``true`` until it is joined using :ref:`wait_to_finish()<class_Thread_method_wait_to_finish>`. For checking if a **Thread** is still executing its task, use :ref:`is_alive()<class_Thread_method_is_alive>`.

.. rst-class:: classref-item-separator

----

.. _class_Thread_method_set_thread_safety_checks_enabled:

.. rst-class:: classref-method

|void| **set_thread_safety_checks_enabled**\ (\ enabled\: :ref:`bool<class_bool>`\ ) |static| :ref:`🔗<class_Thread_method_set_thread_safety_checks_enabled>`

Sets whether the thread safety checks the engine normally performs in methods of certain classes (e.g., :ref:`Node<class_Node>`) should happen **on the current thread**.

The default, for every thread, is that they are enabled (as if called with ``enabled`` being ``true``).

Those checks are conservative. That means that they will only succeed in considering a call thread-safe (and therefore allow it to happen) if the engine can guarantee such safety.

Because of that, there may be cases where the user may want to disable them (``enabled`` being ``false``) to make certain operations allowed again. By doing so, it becomes the user's responsibility to ensure thread safety (e.g., by using :ref:`Mutex<class_Mutex>`) for those objects that are otherwise protected by the engine.

\ **Note:** This is an advanced usage of the engine. You are advised to use it only if you know what you are doing and there is no safer way.

\ **Note:** This is useful for scripts running on either arbitrary **Thread** objects or tasks submitted to the :ref:`WorkerThreadPool<class_WorkerThreadPool>`. It doesn't apply to code running during :ref:`Node<class_Node>` group processing, where the checks will be always performed.

\ **Note:** Even in the case of having disabled the checks in a :ref:`WorkerThreadPool<class_WorkerThreadPool>` task, there's no need to re-enable them at the end. The engine will do so.

.. rst-class:: classref-item-separator

----

.. _class_Thread_method_start:

.. rst-class:: classref-method

:ref:`Error<enum_@GlobalScope_Error>` **start**\ (\ callable\: :ref:`Callable<class_Callable>`, priority\: :ref:`Priority<enum_Thread_Priority>` = 1\ ) :ref:`🔗<class_Thread_method_start>`

Starts a new **Thread** that calls ``callable``.

If the method takes some arguments, you can pass them using :ref:`Callable.bind()<class_Callable_method_bind>`.

The ``priority`` of the **Thread** can be changed by passing a value from the :ref:`Priority<enum_Thread_Priority>` enum.

Returns :ref:`@GlobalScope.OK<class_@GlobalScope_constant_OK>` on success, or :ref:`@GlobalScope.ERR_CANT_CREATE<class_@GlobalScope_constant_ERR_CANT_CREATE>` on failure.

.. rst-class:: classref-item-separator

----

.. _class_Thread_method_wait_to_finish:

.. rst-class:: classref-method

:ref:`Variant<class_Variant>` **wait_to_finish**\ (\ ) :ref:`🔗<class_Thread_method_wait_to_finish>`

Joins the **Thread** and waits for it to finish. Returns the output of the :ref:`Callable<class_Callable>` passed to :ref:`start()<class_Thread_method_start>`.

Should either be used when you want to retrieve the value returned from the method called by the **Thread** or before freeing the instance that contains the **Thread**.

To determine if this can be called without blocking the calling thread, check if :ref:`is_alive()<class_Thread_method_is_alive>` is ``false``.

.. |virtual| replace:: :abbr:`virtual (This method should typically be overridden by the user to have any effect.)`
.. |required| replace:: :abbr:`required (This method is required to be overridden when extending its base class.)`
.. |const| replace:: :abbr:`const (This method has no side effects. It doesn't modify any of the instance's member variables.)`
.. |vararg| replace:: :abbr:`vararg (This method accepts any number of arguments after the ones described here.)`
.. |constructor| replace:: :abbr:`constructor (This method is used to construct a type.)`
.. |static| replace:: :abbr:`static (This method doesn't need an instance to be called, so it can be called directly using the class name.)`
.. |operator| replace:: :abbr:`operator (This method describes a valid operator to use with this type as left-hand operand.)`
.. |bitfield| replace:: :abbr:`BitField (This value is an integer composed as a bitmask of the following flags.)`
.. |void| replace:: :abbr:`void (No return value.)`