Thread states and the global interpreter lock

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Detaching the thread state from extension code#
APIs#
Non-Python created threads#
Legacy API#
Cautions about fork()#
High-level APIs#
GIL-state APIs#
Low-level APIs#

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.

Nota:

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.

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 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.

Nota:

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.

Advertencia:

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

APIs#

The following macros are normally used without a trailing semicolon; look for example usage in the Python source distribution.

Nota:

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 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()".

Nota:

"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.

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();

Advertencia:

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.

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.

Distinto en la versión 3.9: The function now does nothing.

Distinto en la versión 3.7: This function is now called by "Py_Initialize()", so you don't have to call it yourself anymore.

Distinto en la versión 3.2: This function cannot be called before "Py_Initialize()" anymore.

Obsoleto desde la versión 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.

Nota:

 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.

Distinto en la versión 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").

See also "PyThreadState_GetUnchecked()".

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.

Ver también: "PyEval_ReleaseThread()"

Nota:

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

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.

Advertencia:

 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.

Distinto en la versión 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.

Nota:

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

Ver también: "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.

Nota:

 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

Distinto en la versión 3.9: This function now calls the "PyThreadState.on_delete" callback. Previously, that happened in "PyThreadState_Delete()".

Distinto en la versión 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.

Advertencia:

 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.

Distinto en la versión 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.

Distinto en la versión 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.

Distinto en la versión 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.

Distinto en la versión 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.

Ver también: "threading.get_ident()"

PyObject PyThread_GetInfo(void) * Part of the Stable ABI since version 3.3.

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) * Part of the 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.

Ver también: "threading.get_native_id()"

void PyThread_exit_thread(void) * Part of the 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.

Advertencia:

 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.

Obsoleto desde la versión 3.14.

void PyThread_init_thread(void) * Part of the 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) * Part of the 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) * Part of the 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.