# python-clinic / Doc / c-api / init.rst

## Thread State and the Global Interpreter Lock

The Python interpreter is not fully 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 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 :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.

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.

### 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 ...
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 ...


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. 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:

PyThreadState *_save;

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


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.

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.

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. */
result = CallSomeFunction();
/* 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 supports the creation of additional interpreters (using :c:func:Py_NewInterpreter), but mixing multiple interpreters and the :c:func:PyGILState_\* API is unsupported.

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. That also 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 tries to reset the necessary locks, but is not 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:

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

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

### 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.

## Sub-interpreter support

While in most uses, you will only embed a single Python interpreter, there are cases where you need to create several independent interpreters in the same process and perhaps even in the same thread. Sub-interpreters allow you to do that. You can switch between sub-interpreters using the :c:func:PyThreadState_Swap function. You can create and destroy them using the following functions:

### Bugs and caveats

Because sub-interpreters (and the main interpreter) are part of the same process, the insulation between them isn't perfect --- for example, using low-level file operations like :func:os.close they can (accidentally or maliciously) affect each other's open files. Because of the way extensions are shared between (sub-)interpreters, some extensions may not work properly; this is especially likely when the extension makes use of (static) global variables, or when the extension manipulates its module's dictionary after its initialization. It is possible to insert objects created in one sub-interpreter into a namespace of another sub-interpreter; this should be done with great care to avoid sharing user-defined functions, methods, instances or classes between sub-interpreters, since import operations executed by such objects may affect the wrong (sub-)interpreter's dictionary of loaded modules.

Also note that combining this functionality with :c:func:PyGILState_\* APIs is delicate, because these APIs assume a bijection between Python thread states and OS-level threads, an assumption broken by the presence of sub-interpreters. It is highly recommended that you don't switch sub-interpreters between a pair of matching :c:func:PyGILState_Ensure and :c:func:PyGILState_Release calls. Furthermore, extensions (such as :mod:ctypes) using these APIs to allow calling of Python code from non-Python created threads will probably be broken when using sub-interpreters.

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 argument.

Every check interval, when the global interpreter lock is released and reacquired, Python will also call any such provided functions. This can be used for example by asynchronous IO handlers. The notification can be scheduled from a worker thread and the actual call than made at the earliest convenience by the main thread where it has possession of the global interpreter lock and can perform any Python API calls.

## Profiling and Tracing

The Python interpreter provides some low-level support for attaching profiling and execution tracing facilities. These are used for profiling, debugging, and coverage analysis tools.

This C interface allows the profiling or tracing code to avoid the overhead of calling through Python-level callable objects, making a direct C function call instead. The essential attributes of the facility have not changed; the interface allows trace functions to be installed per-thread, and the basic events reported to the trace function are the same as had been reported to the Python-level trace functions in previous versions.