Thread states and the global interpreter lock¶

Unless on a free-threaded build of CPython, the Python interpreter is generally not thread-safe. In order to support multi-threaded Python programs, there's a global lock, called the global interpreter lock or 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.

In order to emulate concurrency, the interpreter regularly tries to switch threads between bytecode instructions (see 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 detaching the thread state.

The Python interpreter keeps some thread-local information inside a data structure called PyThreadState, known as a thread state. Each thread has a thread-local pointer to a PyThreadState; a thread state referenced by this pointer is considered to be attached.

A thread can only have one 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.

注釈

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 Py_BEGIN_ALLOW_THREADS block or in a fresh thread, will the thread not have an attached thread state. If uncertain, check if PyThreadState_GetUnchecked() returns NULL.

If it turns out that you do need to create a thread state, call PyThreadState_New() followed by PyThreadState_Swap(), or use the dangerous PyGILState_Ensure() function.

Detaching the thread state from extension code¶

Most extension code manipulating the 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.

この構造は非常に一般的なので、作業を単純にするために2つのマクロが用意されています:

Py_BEGIN_ALLOW_THREADS
... Do some blocking I/O operation ...
Py_END_ALLOW_THREADS

Py_BEGIN_ALLOW_THREADS マクロは新たなブロックを開始し、隠しローカル変数を宣言します; Py_END_ALLOW_THREADS はブロックを閉じます。

上のブロックは次のコードに展開されます:

PyThreadState *_save;

_save = PyEval_SaveThread();
... Do some blocking I/O operation ...
PyEval_RestoreThread(_save);

Here is how these functions work:

The attached thread state implies that the GIL is held for the interpreter. To detach it, 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 PyEval_RestoreThread(), which will wait until the GIL can be acquired.

注釈

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 zlib and hashlib modules detach the thread state when compressing or hashing data.

On a free-threaded build, the 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.

警告

Detaching the thread state can lead to unexpected behavior during interpreter finalization. See Cautions regarding runtime finalization for more details.

APIs¶

以下のマクロは、通常末尾にセミコロンを付けずに使います; Python ソース配布物内の使用例を見てください。

注釈

These macros are still necessary on the free-threaded build to prevent deadlocks.

Py_BEGIN_ALLOW_THREADS¶: 次に属します: Stable ABI.
このマクロを展開すると { PyThreadState *_save; _save = PyEval_SaveThread(); になります。マクロに開き波括弧が入っていることに注意してください; この波括弧は後で Py_END_ALLOW_THREADS マクロと対応させなければなりません。マクロについての詳しい議論は上記を参照してください。

Py_END_ALLOW_THREADS¶: 次に属します: Stable ABI.
このマクロを展開すると PyEval_RestoreThread(_save); } になります。マクロに開き波括弧が入っていることに注意してください; この波括弧は事前の Py_BEGIN_ALLOW_THREADS マクロと対応していなければなりません。マクロについての詳しい議論は上記を参照してください。

Py_BLOCK_THREADS¶: 次に属します: Stable ABI.
このマクロを展開すると PyEval_RestoreThread(_save); になります: 閉じ波括弧のない Py_END_ALLOW_THREADS と同じです。

Py_UNBLOCK_THREADS¶: 次に属します: Stable ABI.
このマクロを展開すると _save = PyEval_SaveThread(); になります: 開き波括弧のない Py_BEGIN_ALLOW_THREADS と同じです。

Python 以外で作られたスレッド¶

When threads are created using the dedicated Python APIs (such as the 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 PyThreadState_New() followed by PyThreadState_Swap().

注釈

PyThreadState_New requires an argument pointing to the desired interpreter; such a pointer can be acquired via a call to PyInterpreterState_Get() from the code where the thread was created.

例えば:

/* 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();

警告

If the interpreter finalized before PyThreadState_Swap was called, then interp will be a dangling pointer!

Legacy API¶

Another common pattern to call Python code from a non-Python thread is to use PyGILState_Ensure() followed by a call to 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);

Cautions about fork()¶

Another important thing to note about threads is their behaviour in the face of the C fork() call. On most systems with 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 os.fork() by acquiring the locks it uses internally before the fork, and releasing them afterwards. In addition, it resets any 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 pthread_atfork() would need to be used to accomplish the same thing. Additionally, when extending or embedding Python, calling fork() directly rather than through 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. 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 os.fork() does. This means finalizing all other PyThreadState objects belonging to the current interpreter and all other PyInterpreterState objects. Due to this and the special nature of the "main" interpreter, fork() should only be called in that interpreter's "main" thread, where the CPython global runtime was originally initialized. The only exception is if exec() will be called immediately after.

高水準 API¶

These are the most commonly used types and functions when writing multi-threaded C extensions.

type PyThreadState¶

次に属します: Limited API (不透明な構造体として).

This data structure represents the state of a single thread. The only public data member is:

PyInterpreterState *interp¶: This thread's interpreter state.

void PyEval_InitThreads()¶

次に属します: Stable ABI.

Deprecated function which does nothing.

In Python 3.6 and older, this function created the GIL if it didn't exist.

バージョン 3.9 で変更: The function now does nothing.

バージョン 3.7 で変更: この関数は Py_Initialize() から呼び出されるようになり、わざわざ呼び出す必要はもう無くなりました。

バージョン 3.2 で変更: この関数は Py_Initialize() より前に呼び出すことができなくなりました。

バージョン 3.9 で非推奨.

PyThreadState *PyEval_SaveThread()¶: 次に属します: Stable ABI.
Detach the attached thread state and return it. The thread will have no thread state upon returning.

void PyEval_RestoreThread(PyThreadState *tstate)¶: 次に属します: Stable ABI.
Set the attached thread state to tstate. The passed thread state should not be attached, otherwise deadlock ensues. tstate will be attached upon returning.

注釈

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 Cautions regarding runtime finalization for more details.

バージョン 3.14 で変更: Hangs the current thread, rather than terminating it, if called while the interpreter is finalizing.

PyThreadState *PyThreadState_Get()¶

次に属します: Stable ABI.

Return the attached thread state. If the thread has no attached thread state, (such as when inside of Py_BEGIN_ALLOW_THREADS block), then this issues a fatal error (so that the caller needn't check for NULL).

PyThreadState *PyThreadState_GetUnchecked()¶: Similar to 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.

Added in version 3.13: In Python 3.5 to 3.12, the function was private and known as _PyThreadState_UncheckedGet().

PyThreadState *PyThreadState_Swap(PyThreadState *tstate)¶

次に属します: Stable ABI.

Set the attached thread state to tstate, and return the thread state that was attached prior to calling.

This function is safe to call without an attached thread state; it will simply return NULL indicating that there was no prior thread state.

参考

PyEval_ReleaseThread()

注釈

Similar to PyGILState_Ensure(), this function will hang the thread if the runtime is finalizing.

GIL-state APIs¶

以下の関数はスレッドローカルストレージを利用していて、サブインタプリタとの互換性がありません:

type PyGILState_STATE¶

次に属します: Stable ABI.

The type of the value returned by PyGILState_Ensure() and passed to PyGILState_Release().

enumerator PyGILState_LOCKED¶: The GIL was already held when PyGILState_Ensure() was called.

enumerator PyGILState_UNLOCKED¶: The GIL was not held when PyGILState_Ensure() was called.

PyGILState_STATE PyGILState_Ensure()¶

次に属します: Stable ABI.

Ensure that the current thread is ready to call the Python C API regardless of the current state of Python, or of the 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 PyGILState_Release(). In general, other thread-related APIs may be used between PyGILState_Ensure() and PyGILState_Release() calls as long as the thread state is restored to its previous state before the Release(). For example, normal usage of the Py_BEGIN_ALLOW_THREADS and Py_END_ALLOW_THREADS macros is acceptable.

The return value is an opaque "handle" to the attached thread state when PyGILState_Ensure() was called, and must be passed to 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 PyGILState_Ensure() must save the handle for its call to PyGILState_Release().

When the function returns, there will be an attached thread state and the thread will be able to call arbitrary Python code. Failure is a fatal error.

警告

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 Cautions regarding runtime finalization for more details.

バージョン 3.14 で変更: Hangs the current thread, rather than terminating it, if called while the interpreter is finalizing.

void PyGILState_Release(PyGILState_STATE)¶

次に属します: Stable ABI.

獲得したすべてのリソースを解放します。この関数を呼び出すと、Pythonの状態は対応する PyGILState_Ensure() を呼び出す前と同じとなります (通常、この状態は呼び出し元でははわかりませんので、GILState APIを利用するようにしてください)。

PyGILState_Ensure() を呼び出す場合は、必ず同一スレッド内で対応する PyGILState_Release() を呼び出してください。

PyThreadState *PyGILState_GetThisThreadState()¶: 次に属します: Stable ABI.
Get the 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.

注釈

This function may return non-NULL even when the thread state is detached. Prefer PyThreadState_Get() or PyThreadState_GetUnchecked() for most cases.

参考

PyThreadState_Get()

int PyGILState_Check()¶: Return 1 if the current thread is holding the GIL and 0 otherwise. This function can be called from any thread at any time. Only if it has had its thread state initialized via 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 GIL is locked can allow the caller to perform sensitive actions or otherwise behave differently.

注釈

If the current Python process has ever created a subinterpreter, this function will always return 1. Prefer PyThreadState_GetUnchecked() for most cases.

Added in version 3.4.

低水準 API¶

PyThreadState *PyThreadState_New(PyInterpreterState *interp)¶: 次に属します: Stable ABI.
Create a new thread state object belonging to the given interpreter object. An attached thread state is not needed.

void PyThreadState_Clear(PyThreadState *tstate)¶: 次に属します: Stable ABI.
Reset all information in a thread state object. tstate must be attached

バージョン 3.9 で変更: This function now calls the PyThreadState.on_delete callback. Previously, that happened in PyThreadState_Delete().

バージョン 3.13 で変更: The PyThreadState.on_delete callback was removed.

void PyThreadState_Delete(PyThreadState *tstate)¶: 次に属します: Stable ABI.
Destroy a thread state object. tstate should not be attached to any thread. tstate must have been reset with a previous call to PyThreadState_Clear().

void PyThreadState_DeleteCurrent(void)¶

Detach the attached thread state (which must have been reset with a previous call to PyThreadState_Clear()) and then destroy it.

No thread state will be attached upon returning.

PyFrameObject *PyThreadState_GetFrame(PyThreadState *tstate)¶

次に属します: Stable ABI (バージョン 3.10 より).

Get the current frame of the Python thread state tstate.

Return a strong reference. Return NULL if no frame is currently executing.

Asynchronous notifications¶

インタプリタのメインスレッドに非同期な通知を行うために提供されている仕組みです。これらの通知は関数ポインタと void ポインタ引数という形態を取ります。

int Py_AddPendingCall(int (*func)(void*), void *arg)¶

次に属します: Stable ABI.

インタプリタのメインスレッドから関数が呼び出される予定を組みます。成功すると 0 が返り、func はメインスレッドの呼び出しキューに詰められます。失敗すると、例外をセットせずに -1 が返ります。

無事にキューに詰められると、func は いつかは必ず インタプリタのメインスレッドから、arg を引数として呼び出されます。この関数は、通常の実行中の Python コードに対して非同期に呼び出されますが、次の両方の条件に合致したときに呼び出されます:

bytecode 境界上にいるとき、
with the main thread holding an 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 thread state is detached.

This function doesn't need an attached thread state. However, to call this function in a subinterpreter, the caller must have an attached thread state. Otherwise, the function func can be scheduled to be called from the wrong interpreter.

警告

これは、非常に特別な場合にのみ役立つ、低レベルな関数です。 func が可能な限り早く呼び出される保証はありません。メインスレッドがシステムコールを実行するのに忙しい場合は、 func はシステムコールが返ってくるまで呼び出されないでしょう。この関数は一般的には、任意の C スレッドから Python コードを呼び出すのには 向きません 。これの代わりに、 PyGILState API を使用してください。

Added in version 3.1.

バージョン 3.9 で変更: 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.

バージョン 3.12 で変更: This function now always schedules func to be run in the main interpreter.

int Py_MakePendingCalls(void)¶

次に属します: Stable ABI.

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 attached thread state.

Added in version 3.1.

バージョン 3.12 で変更: This function only runs pending calls in the main interpreter.

int PyThreadState_SetAsyncExc(unsigned long id, PyObject *exc)¶: 次に属します: Stable ABI.
Asynchronously raise an exception in a thread. The id argument is the thread id of the target thread; exc is the exception object to be raised. This function does not steal any references to exc. To prevent naive misuse, you must write your own C extension to call this. Must be called with an attached thread state. Returns the number of thread states modified; this is normally one, but will be zero if the thread id isn't found. If exc is NULL, the pending exception (if any) for the thread is cleared. This raises no exceptions.

バージョン 3.7 で変更: id 引数の型が long から unsigned long へ変更されました。

Operating system thread APIs¶

PYTHREAD_INVALID_THREAD_ID¶

Sentinel value for an invalid thread ID.

This is currently equivalent to (unsigned long)-1.

unsigned long PyThread_start_new_thread(void (*func)(void*), void *arg)¶

次に属します: Stable ABI.

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 PYTHREAD_INVALID_THREAD_ID.

The caller does not need to hold an attached thread state.

unsigned long PyThread_get_thread_ident(void)¶

次に属します: Stable ABI.

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 attached thread state.

参考

threading.get_ident()

PyObject *PyThread_GetInfo(void)¶

次に属します: Stable ABI (バージョン 3.3 より).

Get general information about the current thread in the form of a struct sequence object. This information is accessible as sys.thread_info in Python.

On success, this returns a new strong reference to the thread information; on failure, this returns NULL with an exception set.

The caller must hold an attached thread state.

PY_HAVE_THREAD_NATIVE_ID¶: This macro is defined when the system supports native thread IDs.

unsigned long PyThread_get_thread_native_id(void)¶

次に属します: Stable ABI on platforms with native thread IDs.

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 PY_HAVE_THREAD_NATIVE_ID is defined.

This function cannot fail, and the caller does not need to hold an attached thread state.

参考

threading.get_native_id()

void PyThread_exit_thread(void)¶

次に属します: Stable ABI.

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.

警告

If the current system uses POSIX threads (also known as "pthreads"), this calls 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.

バージョン 3.14 で非推奨.

void PyThread_init_thread(void)¶: 次に属します: Stable ABI.
Initialize PyThread* APIs. Python executes this function automatically, so there's little need to call it from an extension module.

int PyThread_set_stacksize(size_t size)¶

次に属します: Stable ABI.

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 attached thread state.

size_t PyThread_get_stacksize(void)¶

次に属します: Stable ABI.

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 attached thread state.

Thread states and the global interpreter lock¶

Detaching the thread state from extension code¶

APIs¶

Python 以外で作られたスレッド¶

Legacy API¶

Cautions about fork()¶

高水準 API¶

GIL-state APIs¶

低水準 API¶

Asynchronous notifications¶

Operating system thread APIs¶

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