Thread states and the global interpreter lock¶

Unless on a free-threaded build of CPython, the Python interpreter is not fully 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 the current thread before it can safely access 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.

Therefore, the rule exists that only the thread that has acquired the GIL may operate on Python objects or call Python/C API functions. In order to emulate concurrency of execution, the interpreter regularly tries to switch threads (see sys.setswitchinterval()). The lock is also released around potentially blocking I/O operations like reading or writing a file, so that other Python threads can run in the meantime.

The Python interpreter keeps some thread-specific bookkeeping information inside a data structure called PyThreadState, known as a thread state. Each OS 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 to call most of the C API.

In general, there will always be an attached thread state when using Python’s C API. Only in some specific cases (such as in a Py_BEGIN_ALLOW_THREADS block) will the thread not have an attached thread state. If uncertain, check if PyThreadState_GetUnchecked() returns NULL.

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.

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

The Py_BEGIN_ALLOW_THREADS macro opens a new block and declares a hidden local variable; the 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);

Here is how these functions work:

The attached thread state holds the GIL for the entire interpreter. When detaching the attached thread state, the GIL is released, allowing other threads to attach a thread state to their own thread, thus getting the GIL and can start executing. The pointer to the prior attached thread state is stored as a local variable. Upon reaching Py_END_ALLOW_THREADS, the thread state that was previously attached is passed to PyEval_RestoreThread(). This function will block until another releases its thread state, thus allowing the old thread state to get re-attached and the C API can be called again.

For free-threaded builds, the GIL is normally out of the question, but detaching the thread state is still required for blocking I/O and long operations. The difference is that threads don’t have to wait for the GIL to be released to attach their thread state, allowing true multi-core parallelism.

Note

Calling system I/O functions is the most common use case for detaching the thread state, but it can also be useful before calling long-running computations which don’t need access to Python objects, such as compression or cryptographic functions operating over memory buffers. For example, the standard zlib and hashlib modules detach the thread state when compressing or hashing data.

APIs¶

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 free-threaded build to prevent deadlocks.

Py_BEGIN_ALLOW_THREADS¶: Part of the Stable ABI.
This macro expands to { PyThreadState *_save; _save = PyEval_SaveThread();. Note that it contains an opening brace; it must be matched with a following Py_END_ALLOW_THREADS macro. See above for further discussion of this macro.

Py_END_ALLOW_THREADS¶: Part of the Stable ABI.
This macro expands to PyEval_RestoreThread(_save); }. Note that it contains a closing brace; it must be matched with an earlier Py_BEGIN_ALLOW_THREADS macro. See above for further discussion of this macro.

Py_BLOCK_THREADS¶: Part of the Stable ABI.
This macro expands to PyEval_RestoreThread(_save);: it is equivalent to Py_END_ALLOW_THREADS without the closing brace.

Py_UNBLOCK_THREADS¶: Part of the Stable ABI.
This macro expands to _save = PyEval_SaveThread();: it is equivalent to Py_BEGIN_ALLOW_THREADS without the opening brace and variable declaration.

Non-Python created threads¶

When threads are created using the dedicated Python APIs (such as the threading module), a thread state is automatically associated to them and the code shown above is therefore correct. However, when threads are created from C (for example by a third-party library with its own thread management), they don’t hold the GIL, because they don’t have 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 an attached thread state before you can start using the Python/C API. When you are done, you should detach the thread state, and finally free it.

The PyGILState_Ensure() and PyGILState_Release() functions do all of the above automatically. The typical idiom for calling into Python from a C thread is:

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);

Note that the PyGILState_* functions assume there is only one global interpreter (created automatically by Py_Initialize()). Python supports the creation of additional interpreters (using Py_NewInterpreter()), but mixing multiple interpreters and the PyGILState_* API is unsupported. This is because PyGILState_Ensure() and similar functions default to attaching a thread state for the main interpreter, meaning that the thread can’t safely interact with the calling subinterpreter.

Supporting subinterpreters in non-Python threads¶

If you would like to support subinterpreters with non-Python created threads, you must use the PyThreadState_* API instead of the traditional PyGILState_* API.

In particular, you must store the interpreter state from the calling function and pass it to PyThreadState_New(), which will ensure that the thread state is targeting the correct interpreter:

/* The return value of PyInterpreterState_Get() from the
   function that created this thread. */
PyInterpreterState *interp = ThreadData->interp;
PyThreadState *tstate = PyThreadState_New(interp);
PyThreadState_Swap(tstate);

/* GIL of the subinterpreter is now held.
   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();

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.

High-level APIs¶

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

type PyThreadState¶

Part of the Limited API (as an opaque struct).

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()¶

Part of the Stable ABI.

Deprecated function which does nothing.

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

Changed in version 3.9: The function now does nothing.

Changed in version 3.7: This function is now called by Py_Initialize(), so you don’t have to call it yourself anymore.

Changed in version 3.2: This function cannot be called before Py_Initialize() anymore.

Deprecated since version 3.9.

PyThreadState *PyEval_SaveThread()¶: Part of the Stable ABI.
Detach the attached thread state and return it. The thread will have no thread state upon returning.

void PyEval_RestoreThread(PyThreadState *tstate)¶: Part of the 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.

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 Cautions regarding runtime finalization for more details.

Changed in version 3.14: Hangs the current thread, rather than terminating it, if called while the interpreter is finalizing.

PyThreadState *PyThreadState_Get()¶

Part of the 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)¶

Part of the 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.

GIL-state APIs¶

The following functions use thread-local storage, and are not compatible with sub-interpreters:

type PyGILState_STATE¶

Part of the 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()¶

Part of the 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.

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 Cautions regarding runtime finalization for more details.

Changed in version 3.14: Hangs the current thread, rather than terminating it, if called while the interpreter is finalizing.

void PyGILState_Release(PyGILState_STATE)¶

Part of the Stable ABI.

Release any resources previously acquired. After this call, Python’s state will be the same as it was prior to the corresponding PyGILState_Ensure() call (but generally this state will be unknown to the caller, hence the use of the GILState API).

Every call to PyGILState_Ensure() must be matched by a call to PyGILState_Release() on the same thread.

PyThreadState *PyGILState_GetThisThreadState()¶: Part of the 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.

Note

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

See also

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.

Note

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.

Low-level APIs¶

PyThreadState *PyThreadState_New(PyInterpreterState *interp)¶: Part of the 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)¶: Part of the Stable ABI.
Reset all information in a thread state object. tstate must be attached

Changed in version 3.9: This function now calls the PyThreadState.on_delete callback. Previously, that happened in PyThreadState_Delete().

Changed in version 3.13: The PyThreadState.on_delete callback was removed.

void PyThreadState_Delete(PyThreadState *tstate)¶: Part of the 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)¶

Part of the Stable ABI since version 3.10.

Get the current frame of the Python thread state tstate.

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

Asynchronous notifications¶

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.

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

Part of the Stable ABI.

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 bytecode boundary;
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.

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 PyGILState API.

Added in version 3.1.

Changed in version 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.

Changed in version 3.12: This function now always schedules func to be run in the main interpreter.

int Py_MakePendingCalls(void)¶

Part of the 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.

Changed in version 3.12: This function only runs pending calls in the main interpreter.

int PyThreadState_SetAsyncExc(unsigned long id, PyObject *exc)¶: Part of the 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.

Changed in version 3.7: The type of the id parameter changed from long to unsigned long.

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)¶

Part of the 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)¶

Part of the 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.

Thread states and the global interpreter lock¶

Detaching the thread state from extension code¶

APIs¶

Non-Python created threads¶

Supporting subinterpreters in non-Python threads¶

Cautions about fork()¶

High-level APIs¶

GIL-state APIs¶

Low-level APIs¶

Asynchronous notifications¶

Operating system thread APIs¶

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