.. index::
single: global interpreter lock
single: interpreter lock
single: lock, interpreterUnless on a :term:`free-threaded build` of :term:`CPython`, the Python interpreter is generally not thread-safe. In order to support multi-threaded Python programs, there's a global lock, called the :term:`global interpreter lock` or :term:`GIL`, that must be held by a thread before accessing Python objects. Without the lock, even the simplest operations could cause problems in a multi-threaded program: for example, when two threads simultaneously increment the reference count of the same object, the reference count could end up being incremented only once instead of twice.
As such, only a thread that holds the GIL may operate on Python objects or invoke Python's C API.
.. index:: single: setswitchinterval (in module sys)In order to emulate concurrency, the interpreter regularly tries to switch threads between bytecode instructions (see :func:`sys.setswitchinterval`). This is why locks are also necessary for thread-safety in pure-Python code.
Additionally, the global interpreter lock is released around blocking I/O operations, such as reading or writing to a file. From the C API, this is done by :ref:`detaching the thread state <detaching-thread-state>`.
.. index::
single: PyThreadState (C type)The Python interpreter keeps some thread-local information inside a data structure called :c:type:`PyThreadState`, known as a :term:`thread state`. Each thread has a thread-local pointer to a :c:type:`PyThreadState`; a thread state referenced by this pointer is considered to be :term:`attached <attached thread state>`.
A thread can only have one :term:`attached thread state` at a time. An attached thread state is typically analogous with holding the GIL, except on free-threaded builds. On builds with the GIL enabled, attaching a thread state will block until the GIL can be acquired. However, even on builds with the GIL disabled, it is still required to have an attached thread state, as the interpreter needs to keep track of which threads may access Python objects.
Note
Even on the free-threaded build, attaching a thread state may block, as the GIL can be re-enabled or threads might be temporarily suspended (such as during a garbage collection).
Generally, there will always be an attached thread state when using Python's
C API, including during embedding and when implementing methods, so it's uncommon
to need to set up a thread state on your own. Only in some specific cases, such
as in a :c:macro:`Py_BEGIN_ALLOW_THREADS` block or in a fresh thread, will the
thread not have an attached thread state.
If uncertain, check if :c:func:`PyThreadState_GetUnchecked` returns NULL.
If it turns out that you do need to create a thread state, call :c:func:`PyThreadState_New` followed by :c:func:`PyThreadState_Swap`, or use the dangerous :c:func:`PyGILState_Ensure` function.
Most extension code manipulating the :term:`thread state` has the following simple structure:
Save the thread state in a local variable.
... Do some blocking I/O operation ...
Restore the thread state from the local variable.This is so common that a pair of macros exists to simplify it:
Py_BEGIN_ALLOW_THREADS
... Do some blocking I/O operation ...
Py_END_ALLOW_THREADS.. index::
single: Py_BEGIN_ALLOW_THREADS (C macro)
single: Py_END_ALLOW_THREADS (C macro)The :c:macro:`Py_BEGIN_ALLOW_THREADS` macro opens a new block and declares a hidden local variable; the :c:macro:`Py_END_ALLOW_THREADS` macro closes the block.
The block above expands to the following code:
PyThreadState *_save;
_save = PyEval_SaveThread();
... Do some blocking I/O operation ...
PyEval_RestoreThread(_save);.. index::
single: PyEval_RestoreThread (C function)
single: PyEval_SaveThread (C function)Here is how these functions work:
The attached thread state implies that the GIL is held for the interpreter. To detach it, :c:func:`PyEval_SaveThread` is called and the result is stored in a local variable.
By detaching the thread state, the GIL is released, which allows other threads to attach to the interpreter and execute while the current thread performs blocking I/O. When the I/O operation is complete, the old thread state is reattached by calling :c:func:`PyEval_RestoreThread`, which will wait until the GIL can be acquired.
Note
Performing blocking I/O is the most common use case for detaching the thread state, but it is also useful to call it over long-running native code that doesn't need access to Python objects or Python's C API. For example, the standard :mod:`zlib` and :mod:`hashlib` modules detach the :term:`thread state <attached thread state>` when compressing or hashing data.
On a :term:`free-threaded build`, the :term:`GIL` is usually out of the question, but detaching the thread state is still required, because the interpreter periodically needs to block all threads to get a consistent view of Python objects without the risk of race conditions. For example, CPython currently suspends all threads for a short period of time while running the garbage collector.
Warning
Detaching the thread state can lead to unexpected behavior during interpreter finalization. See :ref:`cautions-regarding-runtime-finalization` for more details.
The following macros are normally used without a trailing semicolon; look for example usage in the Python source distribution.
Note
These macros are still necessary on the :term:`free-threaded build` to prevent deadlocks.
.. c:macro:: Py_BEGIN_ALLOW_THREADS
This macro expands to ``{ PyThreadState *_save; _save = PyEval_SaveThread();``.
Note that it contains an opening brace; it must be matched with a following
:c:macro:`Py_END_ALLOW_THREADS` macro. See above for further discussion of this
macro.
.. c:macro:: Py_END_ALLOW_THREADS
This macro expands to ``PyEval_RestoreThread(_save); }``. Note that it contains
a closing brace; it must be matched with an earlier
:c:macro:`Py_BEGIN_ALLOW_THREADS` macro. See above for further discussion of
this macro.
.. c:macro:: Py_BLOCK_THREADS
This macro expands to ``PyEval_RestoreThread(_save);``: it is equivalent to
:c:macro:`Py_END_ALLOW_THREADS` without the closing brace.
.. c:macro:: Py_UNBLOCK_THREADS
This macro expands to ``_save = PyEval_SaveThread();``: it is equivalent to
:c:macro:`Py_BEGIN_ALLOW_THREADS` without the opening brace and variable
declaration.
When threads are created using the dedicated Python APIs (such as the :mod:`threading` module), a thread state is automatically associated with them, However, when a thread is created from native code (for example, by a third-party library with its own thread management), it doesn't hold an attached thread state.
If you need to call Python code from these threads (often this will be part of a callback API provided by the aforementioned third-party library), you must first register these threads with the interpreter by creating a new thread state and attaching it.
The most robust way to do this is through :c:func:`PyThreadState_New` followed by :c:func:`PyThreadState_Swap`.
Note
PyThreadState_New requires an argument pointing to the desired
interpreter; such a pointer can be acquired via a call to
:c:func:`PyInterpreterState_Get` from the code where the thread was
created.
For example:
/* The return value of PyInterpreterState_Get() from the
function that created this thread. */
PyInterpreterState *interp = thread_data->interp;
/* Create a new thread state for the interpreter. It does not start out
attached. */
PyThreadState *tstate = PyThreadState_New(interp);
/* Attach the thread state, which will acquire the GIL. */
PyThreadState_Swap(tstate);
/* Perform Python actions here. */
result = CallSomeFunction();
/* evaluate result or handle exception */
/* Destroy the thread state. No Python API allowed beyond this point. */
PyThreadState_Clear(tstate);
PyThreadState_DeleteCurrent();Warning
If the interpreter finalized before PyThreadState_Swap was called, then
interp will be a dangling pointer!
Creating and destroying a :c:type:`PyThreadState` is not free, and is more expensive on a :term:`free-threaded build`. If a non-Python thread calls into the interpreter many times, creating a fresh thread state on every entry and destroying it on every exit is a performance anti-pattern. Instead, create the thread state once (when the native thread starts, or lazily on its first call into Python), attach and detach it around each call, and destroy it when the native thread exits:
/* Thread startup: create the state once. */
PyThreadState *tstate = PyThreadState_New(interp);
/* Per-call: attach, run Python, detach. */
PyEval_RestoreThread(tstate);
result = CallSomeFunction();
PyEval_SaveThread();
/* ... many more calls ... */
/* Thread shutdown: destroy the state once. */
PyEval_RestoreThread(tstate);
PyThreadState_Clear(tstate);
PyThreadState_DeleteCurrent();The equivalent with the :ref:`PyGILState API <gilstate>` keeps an outer :c:func:`PyGILState_Ensure` outstanding for the thread's lifetime, so nested Ensure/Release pairs never drop the internal nesting counter to zero.
In thread startup, pin the state and immediately detach so the thread
does not hold the GIL while off doing non-Python work. Stash outer
and saved somewhere that survives for the thread's lifetime (for
example, in thread-local storage):
PyGILState_STATE outer = PyGILState_Ensure();
PyThreadState *saved = PyEval_SaveThread();Each subsequent call into Python from this thread reuses the pinned state; the inner Release decrements the nesting counter but does not destroy the thread state because the outer Ensure is still outstanding:
PyGILState_STATE inner = PyGILState_Ensure();
result = CallSomeFunction();
PyGILState_Release(inner);At thread shutdown, re-attach and drop the outer reference to destroy the thread state:
PyEval_RestoreThread(saved);
PyGILState_Release(outer);The embedding code must arrange for the shutdown sequence to run before the native thread exits, and before :c:func:`Py_FinalizeEx` is called. If interpreter finalization begins first, the shutdown :c:func:`PyEval_RestoreThread` call will hang the thread (see :c:func:`PyEval_RestoreThread` for details) rather than return. If the native thread exits without running the shutdown sequence, the thread state is leaked for the remainder of the process.
Another common pattern to call Python code from a non-Python thread is to use :c:func:`PyGILState_Ensure` followed by a call to :c:func:`PyGILState_Release`.
These functions do not work well when multiple interpreters exist in the Python
process. If no Python interpreter has ever been used in the current thread (which
is common for threads created outside Python), PyGILState_Ensure will create
and attach a thread state for the "main" interpreter (the first interpreter in
the Python process).
Additionally, these functions have thread-safety issues during interpreter
finalization. Using PyGILState_Ensure during finalization will likely
crash the process.
Usage of these functions look like such:
PyGILState_STATE gstate;
gstate = PyGILState_Ensure();
/* Perform Python actions here. */
result = CallSomeFunction();
/* evaluate result or handle exception */
/* Release the thread. No Python API allowed beyond this point. */
PyGILState_Release(gstate);Another important thing to note about threads is their behaviour in the face of the C :c:func:`fork` call. On most systems with :c:func:`fork`, after a process forks only the thread that issued the fork will exist. This has a concrete impact both on how locks must be handled and on all stored state in CPython's runtime.
The fact that only the "current" thread remains means any locks held by other threads will never be released. Python solves this for :func:`os.fork` by acquiring the locks it uses internally before the fork, and releasing them afterwards. In addition, it resets any :ref:`lock-objects` in the child. When extending or embedding Python, there is no way to inform Python of additional (non-Python) locks that need to be acquired before or reset after a fork. OS facilities such as :c:func:`!pthread_atfork` would need to be used to accomplish the same thing. Additionally, when extending or embedding Python, calling :c:func:`fork` directly rather than through :func:`os.fork` (and returning to or calling into Python) may result in a deadlock by one of Python's internal locks being held by a thread that is defunct after the fork. :c:func:`PyOS_AfterFork_Child` tries to reset the necessary locks, but is not always able to.
The fact that all other threads go away also means that CPython's runtime state there must be cleaned up properly, which :func:`os.fork` does. This means finalizing all other :c:type:`PyThreadState` objects belonging to the current interpreter and all other :c:type:`PyInterpreterState` objects. Due to this and the special nature of the :ref:`"main" interpreter <sub-interpreter-support>`, :c:func:`fork` should only be called in that interpreter's "main" thread, where the CPython global runtime was originally initialized. The only exception is if :c:func:`exec` will be called immediately after.
These are the most commonly used types and functions when writing multi-threaded C extensions.
.. c:type:: PyThreadState
This data structure represents the state of a single thread. The only public
data member is:
.. c:member:: PyInterpreterState *interp
This thread's interpreter state.
.. c:function:: void PyEval_InitThreads()
.. index::
single: PyEval_AcquireThread()
single: PyEval_ReleaseThread()
single: PyEval_SaveThread()
single: PyEval_RestoreThread()
Deprecated function which does nothing.
In Python 3.6 and older, this function created the GIL if it didn't exist.
.. versionchanged:: 3.9
The function now does nothing.
.. versionchanged:: 3.7
This function is now called by :c:func:`Py_Initialize()`, so you don't
have to call it yourself anymore.
.. versionchanged:: 3.2
This function cannot be called before :c:func:`Py_Initialize()` anymore.
.. deprecated:: 3.9
.. index:: pair: module; _thread
.. c:function:: PyThreadState* PyEval_SaveThread()
Detach the :term:`attached thread state` and return it.
The thread will have no :term:`thread state` upon returning.
.. c:function:: void PyEval_RestoreThread(PyThreadState *tstate)
Set the :term:`attached thread state` to *tstate*.
The passed :term:`thread state` **should not** be :term:`attached <attached thread state>`,
otherwise deadlock ensues. *tstate* will be attached upon returning.
.. note::
Calling this function from a thread when the runtime is finalizing will
hang the thread until the program exits, even if the thread was not
created by Python. Refer to
:ref:`cautions-regarding-runtime-finalization` for more details.
.. versionchanged:: 3.14
Hangs the current thread, rather than terminating it, if called while the
interpreter is finalizing... c:function:: PyThreadState* PyThreadState_Get()
Return the :term:`attached thread state`. If the thread has no attached
thread state, (such as when inside of :c:macro:`Py_BEGIN_ALLOW_THREADS`
block), then this issues a fatal error (so that the caller needn't check
for ``NULL``).
See also :c:func:`PyThreadState_GetUnchecked`... c:function:: PyThreadState* PyThreadState_GetUnchecked()
Similar to :c:func:`PyThreadState_Get`, but don't kill the process with a
fatal error if it is NULL. The caller is responsible to check if the result
is NULL.
.. versionadded:: 3.13
In Python 3.5 to 3.12, the function was private and known as
``_PyThreadState_UncheckedGet()``.
.. c:function:: PyThreadState* PyThreadState_Swap(PyThreadState *tstate)
Set the :term:`attached thread state` to *tstate*, and return the
:term:`thread state` that was attached prior to calling.
This function is safe to call without an :term:`attached thread state`; it
will simply return ``NULL`` indicating that there was no prior thread state.
.. seealso::
:c:func:`PyEval_ReleaseThread`
.. note::
Similar to :c:func:`PyGILState_Ensure`, this function will hang the
thread if the runtime is finalizing.
The following functions use thread-local storage, and are not compatible with sub-interpreters:
.. c:type:: PyGILState_STATE
The type of the value returned by :c:func:`PyGILState_Ensure` and passed to
:c:func:`PyGILState_Release`.
.. c:enumerator:: PyGILState_LOCKED
The GIL was already held when :c:func:`PyGILState_Ensure` was called.
.. c:enumerator:: PyGILState_UNLOCKED
The GIL was not held when :c:func:`PyGILState_Ensure` was called... c:function:: PyGILState_STATE PyGILState_Ensure()
Ensure that the current thread is ready to call the Python C API regardless
of the current state of Python, or of the :term:`attached thread state`. This may
be called as many times as desired by a thread as long as each call is
matched with a call to :c:func:`PyGILState_Release`. In general, other
thread-related APIs may be used between :c:func:`PyGILState_Ensure` and
:c:func:`PyGILState_Release` calls as long as the thread state is restored to
its previous state before the Release(). For example, normal usage of the
:c:macro:`Py_BEGIN_ALLOW_THREADS` and :c:macro:`Py_END_ALLOW_THREADS` macros is
acceptable.
The return value is an opaque "handle" to the :term:`attached thread state` when
:c:func:`PyGILState_Ensure` was called, and must be passed to
:c:func:`PyGILState_Release` to ensure Python is left in the same state. Even
though recursive calls are allowed, these handles *cannot* be shared - each
unique call to :c:func:`PyGILState_Ensure` must save the handle for its call
to :c:func:`PyGILState_Release`.
When the function returns, there will be an :term:`attached thread state`
and the thread will be able to call arbitrary Python code. Failure is a fatal error.
.. warning::
Calling this function when the runtime is finalizing is unsafe. Doing
so will either hang the thread until the program ends, or fully crash
the interpreter in rare cases. Refer to
:ref:`cautions-regarding-runtime-finalization` for more details.
.. versionchanged:: 3.14
Hangs the current thread, rather than terminating it, if called while the
interpreter is finalizing... c:function:: void PyGILState_Release(PyGILState_STATE)
Release any resources previously acquired. After this call, Python's state will
be the same as it was prior to the corresponding :c:func:`PyGILState_Ensure` call
(but generally this state will be unknown to the caller, hence the use of the
GILState API).
Every call to :c:func:`PyGILState_Ensure` must be matched by a call to
:c:func:`PyGILState_Release` on the same thread... c:function:: PyThreadState* PyGILState_GetThisThreadState()
Get the :term:`attached thread state` for this thread. May return ``NULL`` if no
GILState API has been used on the current thread. Note that the main thread
always has such a thread-state, even if no auto-thread-state call has been
made on the main thread. This is mainly a helper/diagnostic function.
.. note::
This function may return non-``NULL`` even when the :term:`thread state`
is detached.
Prefer :c:func:`PyThreadState_Get` or :c:func:`PyThreadState_GetUnchecked`
for most cases.
.. seealso:: :c:func:`PyThreadState_Get`.. c:function:: int PyGILState_Check()
Return ``1`` if the current thread is holding the :term:`GIL` and ``0`` otherwise.
This function can be called from any thread at any time.
Only if it has had its :term:`thread state <attached thread state>` initialized
via :c:func:`PyGILState_Ensure` will it return ``1``.
This is mainly a helper/diagnostic function. It can be useful
for example in callback contexts or memory allocation functions when
knowing that the :term:`GIL` is locked can allow the caller to perform sensitive
actions or otherwise behave differently.
.. note::
If the current Python process has ever created a subinterpreter, this
function will *always* return ``1``. Prefer :c:func:`PyThreadState_GetUnchecked`
for most cases.
.. versionadded:: 3.4
.. c:function:: PyThreadState* PyThreadState_New(PyInterpreterState *interp)
Create a new thread state object belonging to the given interpreter object.
An :term:`attached thread state` is not needed... c:function:: void PyThreadState_Clear(PyThreadState *tstate)
Reset all information in a :term:`thread state` object. *tstate*
must be :term:`attached <attached thread state>`
.. versionchanged:: 3.9
This function now calls the :c:member:`!PyThreadState.on_delete` callback.
Previously, that happened in :c:func:`PyThreadState_Delete`.
.. versionchanged:: 3.13
The :c:member:`!PyThreadState.on_delete` callback was removed.
.. c:function:: void PyThreadState_Delete(PyThreadState *tstate)
Destroy a :term:`thread state` object. *tstate* should not
be :term:`attached <attached thread state>` to any thread.
*tstate* must have been reset with a previous call to
:c:func:`PyThreadState_Clear`.
.. c:function:: void PyThreadState_DeleteCurrent(void)
Detach the :term:`attached thread state` (which must have been reset
with a previous call to :c:func:`PyThreadState_Clear`) and then destroy it.
No :term:`thread state` will be :term:`attached <attached thread state>` upon
returning... c:function:: PyFrameObject* PyThreadState_GetFrame(PyThreadState *tstate)
Get the current frame of the Python thread state *tstate*.
Return a :term:`strong reference`. Return ``NULL`` if no frame is currently
executing.
See also :c:func:`PyEval_GetFrame`.
*tstate* must not be ``NULL``, and must be :term:`attached <attached thread state>`.
.. versionadded:: 3.9
.. c:function:: uint64_t PyThreadState_GetID(PyThreadState *tstate)
Get the unique :term:`thread state` identifier of the Python thread state *tstate*.
*tstate* must not be ``NULL``, and must be :term:`attached <attached thread state>`.
.. versionadded:: 3.9
.. c:function:: PyInterpreterState* PyThreadState_GetInterpreter(PyThreadState *tstate)
Get the interpreter of the Python thread state *tstate*.
*tstate* must not be ``NULL``, and must be :term:`attached <attached thread state>`.
.. versionadded:: 3.9
.. c:function:: void PyThreadState_EnterTracing(PyThreadState *tstate)
Suspend tracing and profiling in the Python thread state *tstate*.
Resume them using the :c:func:`PyThreadState_LeaveTracing` function.
.. versionadded:: 3.11
.. c:function:: void PyThreadState_LeaveTracing(PyThreadState *tstate)
Resume tracing and profiling in the Python thread state *tstate* suspended
by the :c:func:`PyThreadState_EnterTracing` function.
See also :c:func:`PyEval_SetTrace` and :c:func:`PyEval_SetProfile`
functions.
.. versionadded:: 3.11
.. c:function:: int PyUnstable_ThreadState_SetStackProtection(PyThreadState *tstate, void *stack_start_addr, size_t stack_size)
Set the stack protection start address and stack protection size
of a Python thread state.
On success, return ``0``.
On failure, set an exception and return ``-1``.
CPython implements :ref:`recursion control <recursion>` for C code by raising
:py:exc:`RecursionError` when it notices that the machine execution stack is close
to overflow. See for example the :c:func:`Py_EnterRecursiveCall` function.
For this, it needs to know the location of the current thread's stack, which it
normally gets from the operating system.
When the stack is changed, for example using context switching techniques like the
Boost library's ``boost::context``, you must call
:c:func:`~PyUnstable_ThreadState_SetStackProtection` to inform CPython of the change.
Call :c:func:`~PyUnstable_ThreadState_SetStackProtection` either before
or after changing the stack.
Do not call any other Python C API between the call and the stack
change.
See :c:func:`PyUnstable_ThreadState_ResetStackProtection` for undoing this operation.
.. versionadded:: 3.15
.. c:function:: void PyUnstable_ThreadState_ResetStackProtection(PyThreadState *tstate)
Reset the stack protection start address and stack protection size
of a Python thread state to the operating system defaults.
See :c:func:`PyUnstable_ThreadState_SetStackProtection` for an explanation.
.. versionadded:: 3.15
.. c:function:: PyObject* PyThreadState_GetDict()
Return a dictionary in which extensions can store thread-specific state
information. Each extension should use a unique key to use to store state in
the dictionary. It is okay to call this function when no :term:`thread state`
is :term:`attached <attached thread state>`. If this function returns
``NULL``, no exception has been raised and the caller should assume no
thread state is attached.
.. c:function:: void PyEval_AcquireThread(PyThreadState *tstate)
:term:`Attach <attached thread state>` *tstate* to the current thread,
which must not be ``NULL`` or already :term:`attached <attached thread state>`.
The calling thread must not already have an :term:`attached thread state`.
.. note::
Calling this function from a thread when the runtime is finalizing will
hang the thread until the program exits, even if the thread was not
created by Python. Refer to
:ref:`cautions-regarding-runtime-finalization` for more details.
.. versionchanged:: 3.8
Updated to be consistent with :c:func:`PyEval_RestoreThread`,
:c:func:`Py_END_ALLOW_THREADS`, and :c:func:`PyGILState_Ensure`,
and terminate the current thread if called while the interpreter is finalizing.
.. versionchanged:: 3.14
Hangs the current thread, rather than terminating it, if called while the
interpreter is finalizing.
:c:func:`PyEval_RestoreThread` is a higher-level function which is always
available (even when threads have not been initialized).
.. c:function:: void PyEval_ReleaseThread(PyThreadState *tstate)
Detach the :term:`attached thread state`.
The *tstate* argument, which must not be ``NULL``, is only used to check
that it represents the :term:`attached thread state` --- if it isn't, a fatal error is
reported.
:c:func:`PyEval_SaveThread` is a higher-level function which is always
available (even when threads have not been initialized).
A mechanism is provided to make asynchronous notifications to the main interpreter thread. These notifications take the form of a function pointer and a void pointer argument.
.. c:function:: int Py_AddPendingCall(int (*func)(void *), void *arg)
Schedule a function to be called from the main interpreter thread. On
success, ``0`` is returned and *func* is queued for being called in the
main thread. On failure, ``-1`` is returned without setting any exception.
When successfully queued, *func* will be *eventually* called from the
main interpreter thread with the argument *arg*. It will be called
asynchronously with respect to normally running Python code, but with
both these conditions met:
* on a :term:`bytecode` boundary;
* with the main thread holding an :term:`attached thread state`
(*func* can therefore use the full C API).
*func* must return ``0`` on success, or ``-1`` on failure with an exception
set. *func* won't be interrupted to perform another asynchronous
notification recursively, but it can still be interrupted to switch
threads if the :term:`thread state <attached thread state>` is detached.
This function doesn't need an :term:`attached thread state`. However, to call this
function in a subinterpreter, the caller must have an :term:`attached thread state`.
Otherwise, the function *func* can be scheduled to be called from the wrong interpreter.
.. warning::
This is a low-level function, only useful for very special cases.
There is no guarantee that *func* will be called as quick as
possible. If the main thread is busy executing a system call,
*func* won't be called before the system call returns. This
function is generally **not** suitable for calling Python code from
arbitrary C threads. Instead, use the :ref:`PyGILState API<gilstate>`.
.. versionadded:: 3.1
.. versionchanged:: 3.9
If this function is called in a subinterpreter, the function *func* is
now scheduled to be called from the subinterpreter, rather than being
called from the main interpreter. Each subinterpreter now has its own
list of scheduled calls.
.. versionchanged:: 3.12
This function now always schedules *func* to be run in the main
interpreter.
.. c:function:: int Py_MakePendingCalls(void)
Execute all pending calls. This is usually executed automatically by the
interpreter.
This function returns ``0`` on success, and returns ``-1`` with an exception
set on failure.
If this is not called in the main thread of the main
interpreter, this function does nothing and returns ``0``.
The caller must hold an :term:`attached thread state`.
.. versionadded:: 3.1
.. versionchanged:: 3.12
This function only runs pending calls in the main interpreter.
.. c:function:: int PyThreadState_SetAsyncExc(unsigned long id, PyObject *exc)
Schedule an exception to be raised asynchronously in a thread.
If the thread has a previously scheduled exception, it is overwritten.
The *id* argument is the thread id of the target thread, as returned by
:c:func:`PyThread_get_thread_ident`.
*exc* is the class of the exception to be raised, or ``NULL`` to clear
the pending exception (if any).
Return the number of affected thread states.
This is normally ``1`` if *id* is found, even when no change was
made (the given *exc* was already pending, or *exc* is ``NULL`` but
no exception is pending).
If the thread id isn't found, return ``0``. This raises no exceptions.
To prevent naive misuse, you must write your own C extension to call this.
This function must be called with an :term:`attached thread state`.
This function does not steal any references to *exc*.
This function does not necessarily interrupt system calls such as
:py:func:`~time.sleep`.
.. versionchanged:: 3.7
The type of the *id* parameter changed from :c:expr:`long` to
:c:expr:`unsigned long`.
.. c:macro:: PYTHREAD_INVALID_THREAD_ID
Sentinel value for an invalid thread ID.
This is currently equivalent to ``(unsigned long)-1``.
.. c:function:: unsigned long PyThread_start_new_thread(void (*func)(void *), void *arg)
Start function *func* in a new thread with argument *arg*.
The resulting thread is not intended to be joined.
*func* must not be ``NULL``, but *arg* may be ``NULL``.
On success, this function returns the identifier of the new thread; on failure,
this returns :c:macro:`PYTHREAD_INVALID_THREAD_ID`.
The caller does not need to hold an :term:`attached thread state`.
.. c:function:: unsigned long PyThread_get_thread_ident(void)
Return the identifier of the current thread, which will never be zero.
This function cannot fail, and the caller does not need to hold an
:term:`attached thread state`.
.. seealso::
:py:func:`threading.get_ident` and :py:attr:`threading.Thread.ident`
expose this identifier to Python.
.. c:function:: PyObject *PyThread_GetInfo(void)
Get general information about the current thread in the form of a
:ref:`struct sequence <struct-sequence-objects>` object. This information is
accessible as :py:attr:`sys.thread_info` in Python.
On success, this returns a new :term:`strong reference` to the thread
information; on failure, this returns ``NULL`` with an exception set.
The caller must hold an :term:`attached thread state`.
.. c:macro:: PY_HAVE_THREAD_NATIVE_ID
This macro is defined when the system supports native thread IDs.
.. c:function:: unsigned long PyThread_get_thread_native_id(void)
Get the native identifier of the current thread as it was assigned by the operating
system's kernel, which will never be less than zero.
This function is only available when :c:macro:`PY_HAVE_THREAD_NATIVE_ID` is
defined.
This function cannot fail, and the caller does not need to hold an
:term:`attached thread state`.
.. seealso::
:py:func:`threading.get_native_id`
.. c:function:: void PyThread_exit_thread(void)
Terminate the current thread. This function is generally considered unsafe
and should be avoided. It is kept solely for backwards compatibility.
This function is only safe to call if all functions in the full call
stack are written to safely allow it.
.. warning::
If the current system uses POSIX threads (also known as "pthreads"),
this calls :manpage:`pthread_exit(3)`, which attempts to unwind the stack
and call C++ destructors on some libc implementations. However, if a
``noexcept`` function is reached, it may terminate the process.
Other systems, such as macOS, do unwinding.
On Windows, this function calls ``_endthreadex()``, which kills the thread
without calling C++ destructors.
In any case, there is a risk of corruption on the thread's stack.
.. deprecated:: 3.14
.. c:function:: void PyThread_init_thread(void)
Initialize ``PyThread*`` APIs. Python executes this function automatically,
so there's little need to call it from an extension module.
.. c:function:: int PyThread_set_stacksize(size_t size)
Set the stack size of the current thread to *size* bytes.
This function returns ``0`` on success, ``-1`` if *size* is invalid, or
``-2`` if the system does not support changing the stack size. This function
does not set exceptions.
The caller does not need to hold an :term:`attached thread state`.
.. c:function:: size_t PyThread_get_stacksize(void)
Return the stack size of the current thread in bytes, or ``0`` if the system's
default stack size is in use.
The caller does not need to hold an :term:`attached thread state`.