Reword and restructure the GIL API doc
diff --git a/Doc/c-api/init.rst b/Doc/c-api/init.rst
index 8d793a4..f920909 100644
--- a/Doc/c-api/init.rst
+++ b/Doc/c-api/init.rst
@@ -366,48 +366,47 @@
single: lock, interpreter
The Python interpreter is not fully thread-safe. In order to support
-multi-threaded Python programs, there's a global lock, called the :dfn:`global
-interpreter lock` or :dfn:`GIL`, that must be held by the current thread before
+multi-threaded Python programs, there's a global lock, called the :term:`global
+interpreter lock` or :term:`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.
-.. index:: single: setcheckinterval() (in module sys)
+.. index:: single: setswitchinterval() (in module sys)
-Therefore, the rule exists that only the thread that has acquired the global
-interpreter lock may operate on Python objects or call Python/C API functions.
-In order to support multi-threaded Python programs, the interpreter regularly
-releases and reacquires the lock --- by default, every 100 bytecode instructions
-(this can be changed with :func:`sys.setcheckinterval`). The lock is also
-released and reacquired around potentially blocking I/O operations like reading
-or writing a file, so that other threads can run while the thread that requests
-the I/O is waiting for the I/O operation to complete.
+Therefore, the rule exists that only the thread that has acquired the
+:term:`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 :func:`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.
.. index::
single: PyThreadState
single: PyThreadState
-The Python interpreter needs to keep some bookkeeping information separate per
-thread --- for this it uses a data structure called :c:type:`PyThreadState`.
-There's one global variable, however: the pointer to the current
-:c:type:`PyThreadState` structure. Before the addition of :dfn:`thread-local
-storage` (:dfn:`TLS`) the current thread state had to be manipulated
-explicitly.
+The Python interpreter keeps some thread-specific bookkeeping information
+inside a data structure called :c:type:`PyThreadState`. There's also one
+global variable pointing to the current :c:type:`PyThreadState`: it can
+be retrieved using :c:func:`PyThreadState_Get`.
-This is easy enough in most cases. Most code manipulating the global
-interpreter lock has the following simple structure::
+Releasing the GIL from extension code
+-------------------------------------
+
+Most extension code manipulating the :term:`GIL` has the following simple
+structure::
Save the thread state in a local variable.
Release the global interpreter lock.
- ...Do some blocking I/O operation...
+ ... Do some blocking I/O operation ...
Reacquire the global interpreter lock.
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...
+ ... Do some blocking I/O operation ...
Py_END_ALLOW_THREADS
.. index::
@@ -416,9 +415,8 @@
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. Another advantage of using these two macros is that when Python is
-compiled without thread support, they are defined empty, thus saving the thread
-state and GIL manipulations.
+block. These two macros are still available when Python is compiled without
+thread support (they simply have an empty expansion).
When thread support is enabled, the block above expands to the following code::
@@ -428,65 +426,60 @@
...Do some blocking I/O operation...
PyEval_RestoreThread(_save);
-Using even lower level primitives, we can get roughly the same effect as
-follows::
-
- PyThreadState *_save;
-
- _save = PyThreadState_Swap(NULL);
- PyEval_ReleaseLock();
- ...Do some blocking I/O operation...
- PyEval_AcquireLock();
- PyThreadState_Swap(_save);
-
.. index::
single: PyEval_RestoreThread()
- single: errno
single: PyEval_SaveThread()
- single: PyEval_ReleaseLock()
- single: PyEval_AcquireLock()
-There are some subtle differences; in particular, :c:func:`PyEval_RestoreThread`
-saves and restores the value of the global variable :c:data:`errno`, since the
-lock manipulation does not guarantee that :c:data:`errno` is left alone. Also,
-when thread support is disabled, :c:func:`PyEval_SaveThread` and
-:c:func:`PyEval_RestoreThread` don't manipulate the GIL; in this case,
-:c:func:`PyEval_ReleaseLock` and :c:func:`PyEval_AcquireLock` are not available.
-This is done so that dynamically loaded extensions compiled with thread support
-enabled can be loaded by an interpreter that was compiled with disabled thread
-support.
+Here is how these functions work: the global interpreter lock is used to protect the pointer to the
+current thread state. When releasing the lock and saving the thread state,
+the current thread state pointer must be retrieved before the lock is released
+(since another thread could immediately acquire the lock and store its own thread
+state in the global variable). Conversely, when acquiring the lock and restoring
+the thread state, the lock must be acquired before storing the thread state
+pointer.
-The global interpreter lock is used to protect the pointer to the current thread
-state. When releasing the lock and saving the thread state, the current thread
-state pointer must be retrieved before the lock is released (since another
-thread could immediately acquire the lock and store its own thread state in the
-global variable). Conversely, when acquiring the lock and restoring the thread
-state, the lock must be acquired before storing the thread state pointer.
+.. note::
+ Calling system I/O functions is the most common use case for releasing
+ the GIL, 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 :mod:`zlib` and :mod:`hashlib` modules release the GIL when
+ compressing or hashing data.
-It is important to note that when threads are created from C, they don't have
-the global interpreter lock, nor is there a thread state data structure for
-them. Such threads must bootstrap themselves into existence, by first
-creating a thread state data structure, then acquiring the lock, and finally
-storing their thread state pointer, before they can start using the Python/C
-API. When they are done, they should reset the thread state pointer, release
-the lock, and finally free their thread state data structure.
+Non-Python created threads
+--------------------------
-Threads can take advantage of the :c:func:`PyGILState_\*` functions to do all of
-the above automatically. The typical idiom for calling into Python from a C
-thread is now::
+When threads are created using the dedicated Python APIs (such as the
+:mod:`threading` module), a thread state is automatically associated to them
+and the code showed 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, nor is there a thread state structure
+for them.
+
+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 thread state data structure, then acquiring the GIL, and finally
+storing their thread state pointer, before you can start using the Python/C
+API. When you are done, you should reset the thread state pointer, release
+the GIL, and finally free the thread state data structure.
+
+The :c:func:`PyGILState_Ensure` and :c:func:`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. */
+ /* Perform Python actions here. */
result = CallSomeFunction();
- /* evaluate result */
+ /* evaluate result or handle exception */
/* Release the thread. No Python API allowed beyond this point. */
PyGILState_Release(gstate);
Note that the :c:func:`PyGILState_\*` functions assume there is only one global
-interpreter (created automatically by :c:func:`Py_Initialize`). Python still
+interpreter (created automatically by :c:func:`Py_Initialize`). Python
supports the creation of additional interpreters (using
:c:func:`Py_NewInterpreter`), but mixing multiple interpreters and the
:c:func:`PyGILState_\*` API is unsupported.
@@ -509,6 +502,12 @@
always able to.
+High-level API
+--------------
+
+These are the most commonly used types and functions when writing C extension
+code, or when embedding the Python interpreter:
+
.. c:type:: PyInterpreterState
This data structure represents the state shared by a number of cooperating
@@ -550,21 +549,22 @@
.. index:: module: _thread
- When only the main thread exists, no GIL operations are needed. This is a
- common situation (most Python programs do not use threads), and the lock
- operations slow the interpreter down a bit. Therefore, the lock is not
- created initially. This situation is equivalent to having acquired the lock:
- when there is only a single thread, all object accesses are safe. Therefore,
- when this function initializes the global interpreter lock, it also acquires
- it. Before the Python :mod:`_thread` module creates a new thread, knowing
- that either it has the lock or the lock hasn't been created yet, it calls
- :c:func:`PyEval_InitThreads`. When this call returns, it is guaranteed that
- the lock has been created and that the calling thread has acquired it.
+ .. note::
+ When only the main thread exists, no GIL operations are needed. This is a
+ common situation (most Python programs do not use threads), and the lock
+ operations slow the interpreter down a bit. Therefore, the lock is not
+ created initially. This situation is equivalent to having acquired the lock:
+ when there is only a single thread, all object accesses are safe. Therefore,
+ when this function initializes the global interpreter lock, it also acquires
+ it. Before the Python :mod:`_thread` module creates a new thread, knowing
+ that either it has the lock or the lock hasn't been created yet, it calls
+ :c:func:`PyEval_InitThreads`. When this call returns, it is guaranteed that
+ the lock has been created and that the calling thread has acquired it.
- It is **not** safe to call this function when it is unknown which thread (if
- any) currently has the global interpreter lock.
+ It is **not** safe to call this function when it is unknown which thread (if
+ any) currently has the global interpreter lock.
- This function is not available when thread support is disabled at compile time.
+ This function is not available when thread support is disabled at compile time.
.. c:function:: int PyEval_ThreadsInitialized()
@@ -575,37 +575,6 @@
not available when thread support is disabled at compile time.
-.. c:function:: void PyEval_AcquireLock()
-
- Acquire the global interpreter lock. The lock must have been created earlier.
- If this thread already has the lock, a deadlock ensues. This function is not
- available when thread support is disabled at compile time.
-
-
-.. c:function:: void PyEval_ReleaseLock()
-
- Release the global interpreter lock. The lock must have been created earlier.
- This function is not available when thread support is disabled at compile time.
-
-
-.. c:function:: void PyEval_AcquireThread(PyThreadState *tstate)
-
- Acquire the global interpreter lock and set the current thread state to
- *tstate*, which should not be *NULL*. The lock must have been created earlier.
- If this thread already has the lock, deadlock ensues. This function is not
- available when thread support is disabled at compile time.
-
-
-.. c:function:: void PyEval_ReleaseThread(PyThreadState *tstate)
-
- Reset the current thread state to *NULL* and release the global interpreter
- lock. The lock must have been created earlier and must be held by the current
- thread. The *tstate* argument, which must not be *NULL*, is only used to check
- that it represents the current thread state --- if it isn't, a fatal error is
- reported. This function is not available when thread support is disabled at
- compile time.
-
-
.. c:function:: PyThreadState* PyEval_SaveThread()
Release the global interpreter lock (if it has been created and thread
@@ -624,6 +593,20 @@
when thread support is disabled at compile time.)
+.. c:function:: PyThreadState* PyThreadState_Get()
+
+ Return the current thread state. The global interpreter lock must be held.
+ When the current thread state is *NULL*, this issues a fatal error (so that
+ the caller needn't check for *NULL*).
+
+
+.. c:function:: PyThreadState* PyThreadState_Swap(PyThreadState *tstate)
+
+ Swap the current thread state with the thread state given by the argument
+ *tstate*, which may be *NULL*. The global interpreter lock must be held
+ and is not released.
+
+
.. c:function:: void PyEval_ReInitThreads()
This function is called from :c:func:`PyOS_AfterFork` to ensure that newly
@@ -631,6 +614,43 @@
are not running in the child process.
+The following functions use thread-local storage, and are not compatible
+with sub-interpreters:
+
+.. 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 global interpreter lock. 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 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, the current thread will hold the GIL and be able
+ to call arbitrary Python code. Failure is a fatal error.
+
+
+.. 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.
+
+
The following macros are normally used without a trailing semicolon; look for
example usage in the Python source distribution.
@@ -664,6 +684,10 @@
:c:macro:`Py_BEGIN_ALLOW_THREADS` without the opening brace and variable
declaration. It is a no-op when thread support is disabled at compile time.
+
+Low-level API
+-------------
+
All of the following functions are only available when thread support is enabled
at compile time, and must be called only when the global interpreter lock has
been created.
@@ -709,19 +733,6 @@
:c:func:`PyThreadState_Clear`.
-.. c:function:: PyThreadState* PyThreadState_Get()
-
- Return the current thread state. The global interpreter lock must be held.
- When the current thread state is *NULL*, this issues a fatal error (so that
- the caller needn't check for *NULL*).
-
-
-.. c:function:: PyThreadState* PyThreadState_Swap(PyThreadState *tstate)
-
- Swap the current thread state with the thread state given by the argument
- *tstate*, which may be *NULL*. The global interpreter lock must be held.
-
-
.. c:function:: PyObject* PyThreadState_GetDict()
Return a dictionary in which extensions can store thread-specific state
@@ -742,38 +753,31 @@
exception (if any) for the thread is cleared. This raises no exceptions.
-.. c:function:: PyGILState_STATE PyGILState_Ensure()
+.. c:function:: void PyEval_AcquireThread(PyThreadState *tstate)
- Ensure that the current thread is ready to call the Python C API regardless
- of the current state of Python, or of the global interpreter lock. 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 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, the current thread will hold the GIL. Failure is a
- fatal error.
+ Acquire the global interpreter lock and set the current thread state to
+ *tstate*, which should not be *NULL*. The lock must have been created earlier.
+ If this thread already has the lock, deadlock ensues.
-.. c:function:: void PyGILState_Release(PyGILState_STATE)
+.. c:function:: void PyEval_ReleaseThread(PyThreadState *tstate)
- 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.)
+ Reset the current thread state to *NULL* and release the global interpreter
+ lock. The lock must have been created earlier and must be held by the current
+ thread. The *tstate* argument, which must not be *NULL*, is only used to check
+ that it represents the current thread state --- if it isn't, a fatal error is
+ reported.
- Every call to :c:func:`PyGILState_Ensure` must be matched by a call to
- :c:func:`PyGILState_Release` on the same thread.
+
+.. c:function:: void PyEval_AcquireLock()
+
+ Acquire the global interpreter lock. The lock must have been created earlier.
+ If this thread already has the lock, a deadlock ensues.
+
+
+.. c:function:: void PyEval_ReleaseLock()
+
+ Release the global interpreter lock. The lock must have been created earlier.
Sub-interpreter support