init.rst

.. highlight:: c


.. _initialization:

*****************************************
Initialization, Finalization, and Threads
*****************************************

See :ref:`Python Initialization Configuration <init-config>` for details
on how to configure the interpreter prior to initialization.

.. _pre-init-safe:

Before Python Initialization
============================

In an application embedding  Python, the :c:func:`Py_Initialize` function must
be called before using any other Python/C API functions; with the exception of
a few functions and the :ref:`global configuration variables
<global-conf-vars>`.

The following functions can be safely called before Python is initialized:

* Functions that initialize the interpreter:

  * :c:func:`Py_Initialize`
  * :c:func:`Py_InitializeEx`
  * :c:func:`Py_InitializeFromConfig`
  * :c:func:`Py_BytesMain`
  * :c:func:`Py_Main`
  * the runtime pre-initialization functions covered in :ref:`init-config`

* Configuration functions:

  * :c:func:`PyImport_AppendInittab`
  * :c:func:`PyImport_ExtendInittab`
  * :c:func:`!PyInitFrozenExtensions`
  * :c:func:`PyMem_SetAllocator`
  * :c:func:`PyMem_SetupDebugHooks`
  * :c:func:`PyObject_SetArenaAllocator`
  * :c:func:`Py_SetProgramName`
  * :c:func:`Py_SetPythonHome`
  * :c:func:`PySys_ResetWarnOptions`
  * the configuration functions covered in :ref:`init-config`

* Informative functions:

  * :c:func:`Py_IsInitialized`
  * :c:func:`PyMem_GetAllocator`
  * :c:func:`PyObject_GetArenaAllocator`
  * :c:func:`Py_GetBuildInfo`
  * :c:func:`Py_GetCompiler`
  * :c:func:`Py_GetCopyright`
  * :c:func:`Py_GetPlatform`
  * :c:func:`Py_GetVersion`
  * :c:func:`Py_IsInitialized`

* Utilities:

  * :c:func:`Py_DecodeLocale`
  * the status reporting and utility functions covered in :ref:`init-config`

* Memory allocators:

  * :c:func:`PyMem_RawMalloc`
  * :c:func:`PyMem_RawRealloc`
  * :c:func:`PyMem_RawCalloc`
  * :c:func:`PyMem_RawFree`

* Synchronization:

  * :c:func:`PyMutex_Lock`
  * :c:func:`PyMutex_Unlock`

.. note::

   Despite their apparent similarity to some of the functions listed above,
   the following functions **should not be called** before the interpreter has
   been initialized: :c:func:`Py_EncodeLocale`, :c:func:`Py_GetPath`,
   :c:func:`Py_GetPrefix`, :c:func:`Py_GetExecPrefix`,
   :c:func:`Py_GetProgramFullPath`, :c:func:`Py_GetPythonHome`,
   :c:func:`Py_GetProgramName`, :c:func:`PyEval_InitThreads`, and
   :c:func:`Py_RunMain`.


.. _global-conf-vars:

Global configuration variables
==============================

Python has variables for the global configuration to control different features
and options. By default, these flags are controlled by :ref:`command line
options <using-on-interface-options>`.

When a flag is set by an option, the value of the flag is the number of times
that the option was set. For example, ``-b`` sets :c:data:`Py_BytesWarningFlag`
to 1 and ``-bb`` sets :c:data:`Py_BytesWarningFlag` to 2.

.. c:var:: int Py_BytesWarningFlag

   This API is kept for backward compatibility: setting
   :c:member:`PyConfig.bytes_warning` should be used instead, see :ref:`Python
   Initialization Configuration <init-config>`.

   Issue a warning when comparing :class:`bytes` or :class:`bytearray` with
   :class:`str` or :class:`bytes` with :class:`int`.  Issue an error if greater
   or equal to ``2``.

   Set by the :option:`-b` option.

   .. deprecated-removed:: 3.12 3.15

.. c:var:: int Py_DebugFlag

   This API is kept for backward compatibility: setting
   :c:member:`PyConfig.parser_debug` should be used instead, see :ref:`Python
   Initialization Configuration <init-config>`.

   Turn on parser debugging output (for expert only, depending on compilation
   options).

   Set by the :option:`-d` option and the :envvar:`PYTHONDEBUG` environment
   variable.

   .. deprecated-removed:: 3.12 3.15

.. c:var:: int Py_DontWriteBytecodeFlag

   This API is kept for backward compatibility: setting
   :c:member:`PyConfig.write_bytecode` should be used instead, see :ref:`Python
   Initialization Configuration <init-config>`.

   If set to non-zero, Python won't try to write ``.pyc`` files on the
   import of source modules.

   Set by the :option:`-B` option and the :envvar:`PYTHONDONTWRITEBYTECODE`
   environment variable.

   .. deprecated-removed:: 3.12 3.15

.. c:var:: int Py_FrozenFlag

   This API is kept for backward compatibility: setting
   :c:member:`PyConfig.pathconfig_warnings` should be used instead, see
   :ref:`Python Initialization Configuration <init-config>`.

   Suppress error messages when calculating the module search path in
   :c:func:`Py_GetPath`.

   Private flag used by ``_freeze_module`` and ``frozenmain`` programs.

   .. deprecated-removed:: 3.12 3.15

.. c:var:: int Py_HashRandomizationFlag

   This API is kept for backward compatibility: setting
   :c:member:`PyConfig.hash_seed` and :c:member:`PyConfig.use_hash_seed` should
   be used instead, see :ref:`Python Initialization Configuration
   <init-config>`.

   Set to ``1`` if the :envvar:`PYTHONHASHSEED` environment variable is set to
   a non-empty string.

   If the flag is non-zero, read the :envvar:`PYTHONHASHSEED` environment
   variable to initialize the secret hash seed.

   .. deprecated-removed:: 3.12 3.15

.. c:var:: int Py_IgnoreEnvironmentFlag

   This API is kept for backward compatibility: setting
   :c:member:`PyConfig.use_environment` should be used instead, see
   :ref:`Python Initialization Configuration <init-config>`.

   Ignore all :envvar:`!PYTHON*` environment variables, e.g.
   :envvar:`PYTHONPATH` and :envvar:`PYTHONHOME`, that might be set.

   Set by the :option:`-E` and :option:`-I` options.

   .. deprecated-removed:: 3.12 3.15

.. c:var:: int Py_InspectFlag

   This API is kept for backward compatibility: setting
   :c:member:`PyConfig.inspect` should be used instead, see
   :ref:`Python Initialization Configuration <init-config>`.

   When a script is passed as first argument or the :option:`-c` option is used,
   enter interactive mode after executing the script or the command, even when
   :data:`sys.stdin` does not appear to be a terminal.

   Set by the :option:`-i` option and the :envvar:`PYTHONINSPECT` environment
   variable.

   .. deprecated-removed:: 3.12 3.15

.. c:var:: int Py_InteractiveFlag

   This API is kept for backward compatibility: setting
   :c:member:`PyConfig.interactive` should be used instead, see
   :ref:`Python Initialization Configuration <init-config>`.

   Set by the :option:`-i` option.

   .. deprecated-removed:: 3.12 3.15

.. c:var:: int Py_IsolatedFlag

   This API is kept for backward compatibility: setting
   :c:member:`PyConfig.isolated` should be used instead, see
   :ref:`Python Initialization Configuration <init-config>`.

   Run Python in isolated mode. In isolated mode :data:`sys.path` contains
   neither the script's directory nor the user's site-packages directory.

   Set by the :option:`-I` option.

   .. versionadded:: 3.4

   .. deprecated-removed:: 3.12 3.15

.. c:var:: int Py_LegacyWindowsFSEncodingFlag

   This API is kept for backward compatibility: setting
   :c:member:`PyPreConfig.legacy_windows_fs_encoding` should be used instead, see
   :ref:`Python Initialization Configuration <init-config>`.

   If the flag is non-zero, use the ``mbcs`` encoding with ``replace`` error
   handler, instead of the UTF-8 encoding with ``surrogatepass`` error handler,
   for the :term:`filesystem encoding and error handler`.

   Set to ``1`` if the :envvar:`PYTHONLEGACYWINDOWSFSENCODING` environment
   variable is set to a non-empty string.

   See :pep:`529` for more details.

   .. availability:: Windows.

   .. deprecated-removed:: 3.12 3.15

.. c:var:: int Py_LegacyWindowsStdioFlag

   This API is kept for backward compatibility: setting
   :c:member:`PyConfig.legacy_windows_stdio` should be used instead, see
   :ref:`Python Initialization Configuration <init-config>`.

   If the flag is non-zero, use :class:`io.FileIO` instead of
   :class:`!io._WindowsConsoleIO` for :mod:`sys` standard streams.

   Set to ``1`` if the :envvar:`PYTHONLEGACYWINDOWSSTDIO` environment
   variable is set to a non-empty string.

   See :pep:`528` for more details.

   .. availability:: Windows.

   .. deprecated-removed:: 3.12 3.15

.. c:var:: int Py_NoSiteFlag

   This API is kept for backward compatibility: setting
   :c:member:`PyConfig.site_import` should be used instead, see
   :ref:`Python Initialization Configuration <init-config>`.

   Disable the import of the module :mod:`site` and the site-dependent
   manipulations of :data:`sys.path` that it entails.  Also disable these
   manipulations if :mod:`site` is explicitly imported later (call
   :func:`site.main` if you want them to be triggered).

   Set by the :option:`-S` option.

   .. deprecated-removed:: 3.12 3.15

.. c:var:: int Py_NoUserSiteDirectory

   This API is kept for backward compatibility: setting
   :c:member:`PyConfig.user_site_directory` should be used instead, see
   :ref:`Python Initialization Configuration <init-config>`.

   Don't add the :data:`user site-packages directory <site.USER_SITE>` to
   :data:`sys.path`.

   Set by the :option:`-s` and :option:`-I` options, and the
   :envvar:`PYTHONNOUSERSITE` environment variable.

   .. deprecated-removed:: 3.12 3.15

.. c:var:: int Py_OptimizeFlag

   This API is kept for backward compatibility: setting
   :c:member:`PyConfig.optimization_level` should be used instead, see
   :ref:`Python Initialization Configuration <init-config>`.

   Set by the :option:`-O` option and the :envvar:`PYTHONOPTIMIZE` environment
   variable.

   .. deprecated-removed:: 3.12 3.15

.. c:var:: int Py_QuietFlag

   This API is kept for backward compatibility: setting
   :c:member:`PyConfig.quiet` should be used instead, see :ref:`Python
   Initialization Configuration <init-config>`.

   Don't display the copyright and version messages even in interactive mode.

   Set by the :option:`-q` option.

   .. versionadded:: 3.2

   .. deprecated-removed:: 3.12 3.15

.. c:var:: int Py_UnbufferedStdioFlag

   This API is kept for backward compatibility: setting
   :c:member:`PyConfig.buffered_stdio` should be used instead, see :ref:`Python
   Initialization Configuration <init-config>`.

   Force the stdout and stderr streams to be unbuffered.

   Set by the :option:`-u` option and the :envvar:`PYTHONUNBUFFERED`
   environment variable.

   .. deprecated-removed:: 3.12 3.15

.. c:var:: int Py_VerboseFlag

   This API is kept for backward compatibility: setting
   :c:member:`PyConfig.verbose` should be used instead, see :ref:`Python
   Initialization Configuration <init-config>`.

   Print a message each time a module is initialized, showing the place
   (filename or built-in module) from which it is loaded.  If greater or equal
   to ``2``, print a message for each file that is checked for when
   searching for a module. Also provides information on module cleanup at exit.

   Set by the :option:`-v` option and the :envvar:`PYTHONVERBOSE` environment
   variable.

   .. deprecated-removed:: 3.12 3.15


Initializing and finalizing the interpreter
===========================================


.. c:function:: void Py_Initialize()

   .. index::
      single: PyEval_InitThreads()
      single: modules (in module sys)
      single: path (in module sys)
      pair: module; builtins
      pair: module; __main__
      pair: module; sys
      triple: module; search; path
      single: Py_FinalizeEx (C function)

   Initialize the Python interpreter.  In an application embedding  Python,
   this should be called before using any other Python/C API functions; see
   :ref:`Before Python Initialization <pre-init-safe>` for the few exceptions.

   This initializes the table of loaded modules (``sys.modules``), and creates
   the fundamental modules :mod:`builtins`, :mod:`__main__` and :mod:`sys`.
   It also initializes the module search path (``sys.path``). It does not set
   ``sys.argv``; use the :ref:`Python Initialization Configuration <init-config>`
   API for that. This is a no-op when called for a second time (without calling
   :c:func:`Py_FinalizeEx` first).  There is no return value; it is a fatal
   error if the initialization fails.

   Use :c:func:`Py_InitializeFromConfig` to customize the
   :ref:`Python Initialization Configuration <init-config>`.

   .. note::
      On Windows, changes the console mode from ``O_TEXT`` to ``O_BINARY``,
      which will also affect non-Python uses of the console using the C Runtime.


.. c:function:: void Py_InitializeEx(int initsigs)

   This function works like :c:func:`Py_Initialize` if *initsigs* is ``1``. If
   *initsigs* is ``0``, it skips initialization registration of signal handlers,
   which may be useful when CPython is embedded as part of a larger application.

   Use :c:func:`Py_InitializeFromConfig` to customize the
   :ref:`Python Initialization Configuration <init-config>`.


.. c:function:: PyStatus Py_InitializeFromConfig(const PyConfig *config)

   Initialize Python from *config* configuration, as described in
   :ref:`init-from-config`.

   See the :ref:`init-config` section for details on pre-initializing the
   interpreter, populating the runtime configuration structure, and querying
   the returned status structure.


.. c:function:: int Py_IsInitialized()

   Return true (nonzero) when the Python interpreter has been initialized, false
   (zero) if not.  After :c:func:`Py_FinalizeEx` is called, this returns false until
   :c:func:`Py_Initialize` is called again.


.. c:function:: int Py_IsFinalizing()

   Return true (non-zero) if the main Python interpreter is
   :term:`shutting down <interpreter shutdown>`. Return false (zero) otherwise.

   .. versionadded:: 3.13


.. c:function:: int Py_FinalizeEx()

   Undo all initializations made by :c:func:`Py_Initialize` and subsequent use of
   Python/C API functions, and destroy all sub-interpreters (see
   :c:func:`Py_NewInterpreter` below) that were created and not yet destroyed since
   the last call to :c:func:`Py_Initialize`.  This is a no-op when called for a second
   time (without calling :c:func:`Py_Initialize` again first).

   Since this is the reverse of :c:func:`Py_Initialize`, it should be called
   in the same thread with the same interpreter active.  That means
   the main thread and the main interpreter.
   This should never be called while :c:func:`Py_RunMain` is running.

   Normally the return value is ``0``.
   If there were errors during finalization (flushing buffered data),
   ``-1`` is returned.

   Note that Python will do a best effort at freeing all memory allocated by the Python
   interpreter.  Therefore, any C-Extension should make sure to correctly clean up all
   of the preveiously allocated PyObjects before using them in subsequent calls to
   :c:func:`Py_Initialize`.  Otherwise it could introduce vulnerabilities and incorrect
   behavior.

   This function is provided for a number of reasons.  An embedding application
   might want to restart Python without having to restart the application itself.
   An application that has loaded the Python interpreter from a dynamically
   loadable library (or DLL) might want to free all memory allocated by Python
   before unloading the DLL. During a hunt for memory leaks in an application a
   developer might want to free all memory allocated by Python before exiting from
   the application.

   **Bugs and caveats:** The destruction of modules and objects in modules is done
   in random order; this may cause destructors (:meth:`~object.__del__` methods) to fail
   when they depend on other objects (even functions) or modules.  Dynamically
   loaded extension modules loaded by Python are not unloaded.  Small amounts of
   memory allocated by the Python interpreter may not be freed (if you find a leak,
   please report it).  Memory tied up in circular references between objects is not
   freed.  Interned strings will all be deallocated regardless of their reference count.
   Some memory allocated by extension modules may not be freed.  Some extensions may not
   work properly if their initialization routine is called more than once; this can
   happen if an application calls :c:func:`Py_Initialize` and :c:func:`Py_FinalizeEx`
   more than once.  :c:func:`Py_FinalizeEx` must not be called recursively from
   within itself.  Therefore, it must not be called by any code that may be run
   as part of the interpreter shutdown process, such as :py:mod:`atexit`
   handlers, object finalizers, or any code that may be run while flushing the
   stdout and stderr files.

   .. audit-event:: cpython._PySys_ClearAuditHooks "" c.Py_FinalizeEx

   .. versionadded:: 3.6


.. c:function:: void Py_Finalize()

   This is a backwards-compatible version of :c:func:`Py_FinalizeEx` that
   disregards the return value.


.. c:function:: int Py_BytesMain(int argc, char **argv)

   Similar to :c:func:`Py_Main` but *argv* is an array of bytes strings,
   allowing the calling application to delegate the text decoding step to
   the CPython runtime.

   .. versionadded:: 3.8


.. c:function:: int Py_Main(int argc, wchar_t **argv)

   The main program for the standard interpreter, encapsulating a full
   initialization/finalization cycle, as well as additional
   behaviour to implement reading configurations settings from the environment
   and command line, and then executing ``__main__`` in accordance with
   :ref:`using-on-cmdline`.

   This is made available for programs which wish to support the full CPython
   command line interface, rather than just embedding a Python runtime in a
   larger application.

   The *argc* and *argv* parameters are similar to those which are passed to a
   C program's :c:func:`main` function, except that the *argv* entries are first
   converted to ``wchar_t`` using :c:func:`Py_DecodeLocale`. It is also
   important to note that the argument list entries may be modified to point to
   strings other than those passed in (however, the contents of the strings
   pointed to by the argument list are not modified).

   The return value is ``2`` if the argument list does not represent a valid
   Python command line, and otherwise the same as :c:func:`Py_RunMain`.

   In terms of the CPython runtime configuration APIs documented in the
   :ref:`runtime configuration <init-config>` section (and without accounting
   for error handling), ``Py_Main`` is approximately equivalent to::

      PyConfig config;
      PyConfig_InitPythonConfig(&config);
      PyConfig_SetArgv(&config, argc, argv);
      Py_InitializeFromConfig(&config);
      PyConfig_Clear(&config);

      Py_RunMain();

   In normal usage, an embedding application will call this function
   *instead* of calling :c:func:`Py_Initialize`, :c:func:`Py_InitializeEx` or
   :c:func:`Py_InitializeFromConfig` directly, and all settings will be applied
   as described elsewhere in this documentation. If this function is instead
   called *after* a preceding runtime initialization API call, then exactly
   which environmental and command line configuration settings will be updated
   is version dependent (as it depends on which settings correctly support
   being modified after they have already been set once when the runtime was
   first initialized).


.. c:function:: int Py_RunMain(void)

   Executes the main module in a fully configured CPython runtime.

   Executes the command (:c:member:`PyConfig.run_command`), the script
   (:c:member:`PyConfig.run_filename`) or the module
   (:c:member:`PyConfig.run_module`) specified on the command line or in the
   configuration. If none of these values are set, runs the interactive Python
   prompt (REPL) using the ``__main__`` module's global namespace.

   If :c:member:`PyConfig.inspect` is not set (the default), the return value
   will be ``0`` if the interpreter exits normally (that is, without raising
   an exception), the exit status of an unhandled :exc:`SystemExit`, or ``1``
   for any other unhandled exception.

   If :c:member:`PyConfig.inspect` is set (such as when the :option:`-i` option
   is used), rather than returning when the interpreter exits, execution will
   instead resume in an interactive Python prompt (REPL) using the ``__main__``
   module's global namespace. If the interpreter exited with an exception, it
   is immediately raised in the REPL session. The function return value is
   then determined by the way the *REPL session* terminates: ``0``, ``1``, or
   the status of a :exc:`SystemExit`, as specified above.

   This function always finalizes the Python interpreter before it returns.

   See :ref:`Python Configuration <init-python-config>` for an example of a
   customized Python that always runs in isolated mode using
   :c:func:`Py_RunMain`.

.. c:function:: int PyUnstable_AtExit(PyInterpreterState *interp, void (*func)(void *), void *data)

   Register an :mod:`atexit` callback for the target interpreter *interp*.
   This is similar to :c:func:`Py_AtExit`, but takes an explicit interpreter and
   data pointer for the callback.

   There must be an :term:`attached thread state` for *interp*.

   .. versionadded:: 3.13

Process-wide parameters
=======================


.. c:function:: void Py_SetProgramName(const wchar_t *name)

   .. index::
      single: Py_Initialize()
      single: main()
      single: Py_GetPath()

   This API is kept for backward compatibility: setting
   :c:member:`PyConfig.program_name` should be used instead, see :ref:`Python
   Initialization Configuration <init-config>`.

   This function should be called before :c:func:`Py_Initialize` is called for
   the first time, if it is called at all.  It tells the interpreter the value
   of the ``argv[0]`` argument to the :c:func:`main` function of the program
   (converted to wide characters).
   This is used by :c:func:`Py_GetPath` and some other functions below to find
   the Python run-time libraries relative to the interpreter executable.  The
   default value is ``'python'``.  The argument should point to a
   zero-terminated wide character string in static storage whose contents will not
   change for the duration of the program's execution.  No code in the Python
   interpreter will change the contents of this storage.

   Use :c:func:`Py_DecodeLocale` to decode a bytes string to get a
   :c:expr:`wchar_t*` string.

   .. deprecated-removed:: 3.11 3.15


.. c:function:: wchar_t* Py_GetProgramName()

   Return the program name set with :c:member:`PyConfig.program_name`, or the default.
   The returned string points into static storage; the caller should not modify its
   value.

   This function should not be called before :c:func:`Py_Initialize`, otherwise
   it returns ``NULL``.

   .. versionchanged:: 3.10
      It now returns ``NULL`` if called before :c:func:`Py_Initialize`.

   .. deprecated-removed:: 3.13 3.15
      Use :c:func:`PyConfig_Get("executable") <PyConfig_Get>`
      (:data:`sys.executable`) instead.


.. c:function:: wchar_t* Py_GetPrefix()

   Return the *prefix* for installed platform-independent files. This is derived
   through a number of complicated rules from the program name set with
   :c:member:`PyConfig.program_name` and some environment variables; for example, if the
   program name is ``'/usr/local/bin/python'``, the prefix is ``'/usr/local'``. The
   returned string points into static storage; the caller should not modify its
   value.  This corresponds to the :makevar:`prefix` variable in the top-level
   :file:`Makefile` and the :option:`--prefix` argument to the :program:`configure`
   script at build time.  The value is available to Python code as ``sys.base_prefix``.
   It is only useful on Unix.  See also the next function.

   This function should not be called before :c:func:`Py_Initialize`, otherwise
   it returns ``NULL``.

   .. versionchanged:: 3.10
      It now returns ``NULL`` if called before :c:func:`Py_Initialize`.

   .. deprecated-removed:: 3.13 3.15
      Use :c:func:`PyConfig_Get("base_prefix") <PyConfig_Get>`
      (:data:`sys.base_prefix`) instead. Use :c:func:`PyConfig_Get("prefix")
      <PyConfig_Get>` (:data:`sys.prefix`) if :ref:`virtual environments
      <venv-def>` need to be handled.


.. c:function:: wchar_t* Py_GetExecPrefix()

   Return the *exec-prefix* for installed platform-*dependent* files.  This is
   derived through a number of complicated rules from the program name set with
   :c:member:`PyConfig.program_name` and some environment variables; for example, if the
   program name is ``'/usr/local/bin/python'``, the exec-prefix is
   ``'/usr/local'``.  The returned string points into static storage; the caller
   should not modify its value.  This corresponds to the :makevar:`exec_prefix`
   variable in the top-level :file:`Makefile` and the ``--exec-prefix``
   argument to the :program:`configure` script at build  time.  The value is
   available to Python code as ``sys.base_exec_prefix``.  It is only useful on
   Unix.

   Background: The exec-prefix differs from the prefix when platform dependent
   files (such as executables and shared libraries) are installed in a different
   directory tree.  In a typical installation, platform dependent files may be
   installed in the :file:`/usr/local/plat` subtree while platform independent may
   be installed in :file:`/usr/local`.

   Generally speaking, a platform is a combination of hardware and software
   families, e.g.  Sparc machines running the Solaris 2.x operating system are
   considered the same platform, but Intel machines running Solaris 2.x are another
   platform, and Intel machines running Linux are yet another platform.  Different
   major revisions of the same operating system generally also form different
   platforms.  Non-Unix operating systems are a different story; the installation
   strategies on those systems are so different that the prefix and exec-prefix are
   meaningless, and set to the empty string. Note that compiled Python bytecode
   files are platform independent (but not independent from the Python version by
   which they were compiled!).

   System administrators will know how to configure the :program:`mount` or
   :program:`automount` programs to share :file:`/usr/local` between platforms
   while having :file:`/usr/local/plat` be a different filesystem for each
   platform.

   This function should not be called before :c:func:`Py_Initialize`, otherwise
   it returns ``NULL``.

   .. versionchanged:: 3.10
      It now returns ``NULL`` if called before :c:func:`Py_Initialize`.

   .. deprecated-removed:: 3.13 3.15
      Use :c:func:`PyConfig_Get("base_exec_prefix") <PyConfig_Get>`
      (:data:`sys.base_exec_prefix`) instead. Use
      :c:func:`PyConfig_Get("exec_prefix") <PyConfig_Get>`
      (:data:`sys.exec_prefix`) if :ref:`virtual environments <venv-def>` need
      to be handled.

.. c:function:: wchar_t* Py_GetProgramFullPath()

   .. index::
      single: executable (in module sys)

   Return the full program name of the Python executable; this is  computed as a
   side-effect of deriving the default module search path  from the program name
   (set by :c:member:`PyConfig.program_name`). The returned string points into
   static storage; the caller should not modify its value.  The value is available
   to Python code as ``sys.executable``.

   This function should not be called before :c:func:`Py_Initialize`, otherwise
   it returns ``NULL``.

   .. versionchanged:: 3.10
      It now returns ``NULL`` if called before :c:func:`Py_Initialize`.

   .. deprecated-removed:: 3.13 3.15
      Use :c:func:`PyConfig_Get("executable") <PyConfig_Get>`
      (:data:`sys.executable`) instead.


.. c:function:: wchar_t* Py_GetPath()

   .. index::
      triple: module; search; path
      single: path (in module sys)

   Return the default module search path; this is computed from the program name
   (set by :c:member:`PyConfig.program_name`) and some environment variables.
   The returned string consists of a series of directory names separated by a
   platform dependent delimiter character.  The delimiter character is ``':'``
   on Unix and macOS, ``';'`` on Windows.  The returned string points into
   static storage; the caller should not modify its value.  The list
   :data:`sys.path` is initialized with this value on interpreter startup; it
   can be (and usually is) modified later to change the search path for loading
   modules.

   This function should not be called before :c:func:`Py_Initialize`, otherwise
   it returns ``NULL``.

   .. XXX should give the exact rules

   .. versionchanged:: 3.10
      It now returns ``NULL`` if called before :c:func:`Py_Initialize`.

   .. deprecated-removed:: 3.13 3.15
      Use :c:func:`PyConfig_Get("module_search_paths") <PyConfig_Get>`
      (:data:`sys.path`) instead.

.. c:function:: const char* Py_GetVersion()

   Return the version of this Python interpreter.  This is a string that looks
   something like ::

      "3.0a5+ (py3k:63103M, May 12 2008, 00:53:55) \n[GCC 4.2.3]"

   .. index:: single: version (in module sys)

   The first word (up to the first space character) is the current Python version;
   the first characters are the major and minor version separated by a
   period.  The returned string points into static storage; the caller should not
   modify its value.  The value is available to Python code as :data:`sys.version`.

   See also the :c:var:`Py_Version` constant.


.. c:function:: const char* Py_GetPlatform()

   .. index:: single: platform (in module sys)

   Return the platform identifier for the current platform.  On Unix, this is
   formed from the "official" name of the operating system, converted to lower
   case, followed by the major revision number; e.g., for Solaris 2.x, which is
   also known as SunOS 5.x, the value is ``'sunos5'``.  On macOS, it is
   ``'darwin'``.  On Windows, it is ``'win'``.  The returned string points into
   static storage; the caller should not modify its value.  The value is available
   to Python code as ``sys.platform``.


.. c:function:: const char* Py_GetCopyright()

   Return the official copyright string for the current Python version, for example

   ``'Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam'``

   .. index:: single: copyright (in module sys)

   The returned string points into static storage; the caller should not modify its
   value.  The value is available to Python code as ``sys.copyright``.


.. c:function:: const char* Py_GetCompiler()

   Return an indication of the compiler used to build the current Python version,
   in square brackets, for example::

      "[GCC 2.7.2.2]"

   .. index:: single: version (in module sys)

   The returned string points into static storage; the caller should not modify its
   value.  The value is available to Python code as part of the variable
   ``sys.version``.


.. c:function:: const char* Py_GetBuildInfo()

   Return information about the sequence number and build date and time  of the
   current Python interpreter instance, for example ::

      "#67, Aug  1 1997, 22:34:28"

   .. index:: single: version (in module sys)

   The returned string points into static storage; the caller should not modify its
   value.  The value is available to Python code as part of the variable
   ``sys.version``.


.. c:function:: void PySys_SetArgvEx(int argc, wchar_t **argv, int updatepath)

   .. index::
      single: main()
      single: Py_FatalError()
      single: argv (in module sys)

   This API is kept for backward compatibility: setting
   :c:member:`PyConfig.argv`, :c:member:`PyConfig.parse_argv` and
   :c:member:`PyConfig.safe_path` should be used instead, see :ref:`Python
   Initialization Configuration <init-config>`.

   Set :data:`sys.argv` based on *argc* and *argv*.  These parameters are
   similar to those passed to the program's :c:func:`main` function with the
   difference that the first entry should refer to the script file to be
   executed rather than the executable hosting the Python interpreter.  If there
   isn't a script that will be run, the first entry in *argv* can be an empty
   string.  If this function fails to initialize :data:`sys.argv`, a fatal
   condition is signalled using :c:func:`Py_FatalError`.

   If *updatepath* is zero, this is all the function does.  If *updatepath*
   is non-zero, the function also modifies :data:`sys.path` according to the
   following algorithm:

   - If the name of an existing script is passed in ``argv[0]``, the absolute
     path of the directory where the script is located is prepended to
     :data:`sys.path`.
   - Otherwise (that is, if *argc* is ``0`` or ``argv[0]`` doesn't point
     to an existing file name), an empty string is prepended to
     :data:`sys.path`, which is the same as prepending the current working
     directory (``"."``).

   Use :c:func:`Py_DecodeLocale` to decode a bytes string to get a
   :c:expr:`wchar_t*` string.

   See also :c:member:`PyConfig.orig_argv` and :c:member:`PyConfig.argv`
   members of the :ref:`Python Initialization Configuration <init-config>`.

   .. note::
      It is recommended that applications embedding the Python interpreter
      for purposes other than executing a single script pass ``0`` as *updatepath*,
      and update :data:`sys.path` themselves if desired.
      See :cve:`2008-5983`.

      On versions before 3.1.3, you can achieve the same effect by manually
      popping the first :data:`sys.path` element after having called
      :c:func:`PySys_SetArgv`, for example using::

         PyRun_SimpleString("import sys; sys.path.pop(0)\n");

   .. versionadded:: 3.1.3

   .. XXX impl. doesn't seem consistent in allowing ``0``/``NULL`` for the params;
      check w/ Guido.

   .. deprecated-removed:: 3.11 3.15


.. c:function:: void PySys_SetArgv(int argc, wchar_t **argv)

   This API is kept for backward compatibility: setting
   :c:member:`PyConfig.argv` and :c:member:`PyConfig.parse_argv` should be used
   instead, see :ref:`Python Initialization Configuration <init-config>`.

   This function works like :c:func:`PySys_SetArgvEx` with *updatepath* set
   to ``1`` unless the :program:`python` interpreter was started with the
   :option:`-I`.

   Use :c:func:`Py_DecodeLocale` to decode a bytes string to get a
   :c:expr:`wchar_t*` string.

   See also :c:member:`PyConfig.orig_argv` and :c:member:`PyConfig.argv`
   members of the :ref:`Python Initialization Configuration <init-config>`.

   .. versionchanged:: 3.4 The *updatepath* value depends on :option:`-I`.

   .. deprecated-removed:: 3.11 3.15


.. c:function:: void Py_SetPythonHome(const wchar_t *home)

   This API is kept for backward compatibility: setting
   :c:member:`PyConfig.home` should be used instead, see :ref:`Python
   Initialization Configuration <init-config>`.

   Set the default "home" directory, that is, the location of the standard
   Python libraries.  See :envvar:`PYTHONHOME` for the meaning of the
   argument string.

   The argument should point to a zero-terminated character string in static
   storage whose contents will not change for the duration of the program's
   execution.  No code in the Python interpreter will change the contents of
   this storage.

   Use :c:func:`Py_DecodeLocale` to decode a bytes string to get a
   :c:expr:`wchar_t*` string.

   .. deprecated-removed:: 3.11 3.15


.. c:function:: wchar_t* Py_GetPythonHome()

   Return the default "home", that is, the value set by
   :c:member:`PyConfig.home`, or the value of the :envvar:`PYTHONHOME`
   environment variable if it is set.

   This function should not be called before :c:func:`Py_Initialize`, otherwise
   it returns ``NULL``.

   .. versionchanged:: 3.10
      It now returns ``NULL`` if called before :c:func:`Py_Initialize`.

   .. deprecated-removed:: 3.13 3.15
      Use :c:func:`PyConfig_Get("home") <PyConfig_Get>` or the
      :envvar:`PYTHONHOME` environment variable instead.


.. _threads:

Thread State and the Global Interpreter Lock
============================================

.. index::
   single: global interpreter lock
   single: interpreter lock
   single: lock, interpreter

Unless on a :term:`free-threaded <free threading>` build of :term:`CPython`,
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.

.. index:: single: setswitchinterval (in module sys)

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 (C type)

The Python interpreter keeps some thread-specific bookkeeping information
inside a data structure called :c:type:`PyThreadState`, known as a :term:`thread state`.
Each OS thread has a thread-local pointer to a :c:type:`PyThreadState`; a thread state
referenced by this pointer is considered to be :term:`attached <attached thread state>`.

A thread can only have one :term:`attached thread state` at a time. An attached
thread state is typically analogous with holding the :term:`GIL`, except on
:term:`free-threaded <free threading>` builds.  On builds with the :term:`GIL` enabled,
:term:`attaching <attached thread state>` a thread state will block until the :term:`GIL`
can be acquired. However,  even on builds with the :term:`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 :term:`attached thread state` when using Python's C API.
Only in some specific cases (such as in a :c:macro:`Py_BEGIN_ALLOW_THREADS` block) will the
thread not have an attached thread state. If uncertain, check if :c:func:`PyThreadState_GetUnchecked` returns
``NULL``.

Detaching the thread state from extension code
----------------------------------------------

Most extension code manipulating the :term:`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

.. index::
   single: Py_BEGIN_ALLOW_THREADS (C macro)
   single: Py_END_ALLOW_THREADS (C macro)

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.

The block above expands to the following code::

   PyThreadState *_save;

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

.. index::
   single: PyEval_RestoreThread (C function)
   single: PyEval_SaveThread (C function)

Here is how these functions work:

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

For :term:`free-threaded <free threading>` builds, the :term:`GIL` is normally
out of the question, but detaching the :term:`thread state <attached thread state>` is still required
for blocking I/O and long operations. The difference is that threads don't have to wait for the :term:`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 :term:`thread state <attached 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 :mod:`zlib` and :mod:`hashlib` modules detach the
   :term:`thread state <attached thread state>` when compressing or hashing data.


.. _gilstate:

Non-Python created threads
--------------------------

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 :term:`GIL`, because they don't have an
:term:`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 :term:`attached thread state` before you can start using the Python/C
API.  When you are done, you should detach the :term:`thread state <attached thread state>`, and
finally free it.

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 ``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
``PyGILState_*`` API is unsupported. This is because :c:func:`PyGILState_Ensure`
and similar functions default to :term:`attaching <attached thread state>` a
:term:`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 :c:func:`PyThreadState_New`, which will ensure that
the :term:`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();

.. _fork-and-threads:

Cautions about fork()
---------------------

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.  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 :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_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 :func:`os.fork`
does.  This means finalizing all other :c:type:`PyThreadState` objects
belonging to the current interpreter and all other
:c:type:`PyInterpreterState` objects.  Due to this and the special
nature of the :ref:`"main" interpreter <sub-interpreter-support>`,
:c:func:`fork` should only be called in that interpreter's "main"
thread, where the CPython global runtime was originally initialized.
The only exception is if :c:func:`exec` will be called immediately
after.

.. _cautions-regarding-runtime-finalization:

Cautions regarding runtime finalization
---------------------------------------

In the late stage of :term:`interpreter shutdown`, after attempting to wait for
non-daemon threads to exit (though this can be interrupted by
:class:`KeyboardInterrupt`) and running the :mod:`atexit` functions, the runtime
is marked as *finalizing*: :c:func:`Py_IsFinalizing` and
:func:`sys.is_finalizing` return true.  At this point, only the *finalization
thread* that initiated finalization (typically the main thread) is allowed to
acquire the :term:`GIL`.

If any thread, other than the finalization thread, attempts to attach a :term:`thread state`
during finalization, either explicitly or
implicitly, the thread enters **a permanently blocked state**
where it remains until the program exits.  In most cases this is harmless, but this can result
in deadlock if a later stage of finalization attempts to acquire a lock owned by the
blocked thread, or otherwise waits on the blocked thread.

Gross? Yes. This prevents random crashes and/or unexpectedly skipped C++
finalizations further up the call stack when such threads were forcibly exited
here in CPython 3.13 and earlier. The CPython runtime :term:`thread state` C APIs
have never had any error reporting or handling expectations at :term:`thread state`
attachment time that would've allowed for graceful exit from this situation. Changing that
would require new stable C APIs and rewriting the majority of C code in the
CPython ecosystem to use those with error handling.


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
   threads.  Threads belonging to the same interpreter share their module
   administration and a few other internal items. There are no public members in
   this structure.

   Threads belonging to different interpreters initially share nothing, except
   process state like available memory, open file descriptors and such.  The global
   interpreter lock is also shared by all threads, regardless of to which
   interpreter they belong.

   .. versionchanged:: 3.12

      :pep:`684` introduced the possibility
      of a :ref:`per-interpreter GIL <per-interpreter-gil>`.
      See :c:func:`Py_NewInterpreterFromConfig`.


.. c:type:: PyThreadState

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

   .. c:member:: PyInterpreterState *interp

      This thread's interpreter state.


.. c:function:: void PyEval_InitThreads()

   .. index::
      single: PyEval_AcquireThread()
      single: PyEval_ReleaseThread()
      single: PyEval_SaveThread()
      single: PyEval_RestoreThread()

   Deprecated function which does nothing.

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

   .. versionchanged:: 3.9
      The function now does nothing.

   .. versionchanged:: 3.7
      This function is now called by :c:func:`Py_Initialize()`, so you don't
      have to call it yourself anymore.

   .. versionchanged:: 3.2
      This function cannot be called before :c:func:`Py_Initialize()` anymore.

   .. deprecated:: 3.9

   .. index:: pair: module; _thread


.. c:function:: PyThreadState* PyEval_SaveThread()

   Detach the :term:`attached thread state` and return it.
   The thread will have no :term:`thread state` upon returning.


.. c:function:: void PyEval_RestoreThread(PyThreadState *tstate)

   Set the :term:`attached thread state` to *tstate*.
   The passed :term:`thread state` **should not** be :term:`attached <attached thread state>`,
   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
      :ref:`cautions-regarding-runtime-finalization` for more details.

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

.. c:function:: PyThreadState* PyThreadState_Get()

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

   See also :c:func:`PyThreadState_GetUnchecked`.

.. c:function:: PyThreadState* PyThreadState_GetUnchecked()

   Similar to :c:func:`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.

   .. versionadded:: 3.13
      In Python 3.5 to 3.12, the function was private and known as
      ``_PyThreadState_UncheckedGet()``.


.. c:function:: PyThreadState* PyThreadState_Swap(PyThreadState *tstate)

   Set the :term:`attached thread state` to *tstate*, and return the
   :term:`thread state` that was attached prior to calling.

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

   .. seealso::
      :c:func:`PyEval_ReleaseThread`

   .. note::
      Similar to :c:func:`PyGILState_Ensure`, this function will hang the
      thread if the runtime is finalizing.


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

.. c:type:: PyGILState_STATE

   The type of the value returned by :c:func:`PyGILState_Ensure` and passed to
   :c:func:`PyGILState_Release`.

   .. c:enumerator:: PyGILState_LOCKED

      The GIL was already held when :c:func:`PyGILState_Ensure` was called.

   .. c:enumerator:: PyGILState_UNLOCKED

      The GIL was not held when :c:func:`PyGILState_Ensure` was called.

.. 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 :term:`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 :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 :term:`attached 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, there will be an :term:`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
      :ref:`cautions-regarding-runtime-finalization` for more details.

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

.. 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.

.. c:function:: PyThreadState* PyGILState_GetThisThreadState()

   Get the :term:`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 :term:`thread state`
      is detached.
      Prefer :c:func:`PyThreadState_Get` or :c:func:`PyThreadState_GetUnchecked`
      for most cases.

   .. seealso:: :c:func:`PyThreadState_Get`

.. c:function:: int PyGILState_Check()

   Return ``1`` if the current thread is holding the :term:`GIL` and ``0`` otherwise.
   This function can be called from any thread at any time.
   Only if it has had its :term:`thread state <attached thread state>` initialized
   via :c:func:`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 :term:`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 :c:func:`PyThreadState_GetUnchecked`
      for most cases.

   .. versionadded:: 3.4


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


.. c:macro:: Py_BEGIN_ALLOW_THREADS

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


.. c:macro:: Py_END_ALLOW_THREADS

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


.. c:macro:: Py_BLOCK_THREADS

   This macro expands to ``PyEval_RestoreThread(_save);``: it is equivalent to
   :c:macro:`Py_END_ALLOW_THREADS` without the closing brace.


.. c:macro:: Py_UNBLOCK_THREADS

   This macro expands to ``_save = PyEval_SaveThread();``: it is equivalent to
   :c:macro:`Py_BEGIN_ALLOW_THREADS` without the opening brace and variable
   declaration.


Low-level API
-------------

All of the following functions must be called after :c:func:`Py_Initialize`.

.. versionchanged:: 3.7
   :c:func:`Py_Initialize()` now initializes the :term:`GIL`
   and sets an :term:`attached thread state`.


.. c:function:: PyInterpreterState* PyInterpreterState_New()

   Create a new interpreter state object.  An :term:`attached thread state` is not needed,
   but may optionally exist if it is necessary to serialize calls to this
   function.

   .. audit-event:: cpython.PyInterpreterState_New "" c.PyInterpreterState_New


.. c:function:: void PyInterpreterState_Clear(PyInterpreterState *interp)

   Reset all information in an interpreter state object.  There must be
   an :term:`attached thread state` for the interpreter.

   .. audit-event:: cpython.PyInterpreterState_Clear "" c.PyInterpreterState_Clear


.. c:function:: void PyInterpreterState_Delete(PyInterpreterState *interp)

   Destroy an interpreter state object.  There **should not** be an
   :term:`attached thread state` for the target interpreter. The interpreter
   state must have been reset with a previous call to :c:func:`PyInterpreterState_Clear`.


.. c:function:: PyThreadState* PyThreadState_New(PyInterpreterState *interp)

   Create a new thread state object belonging to the given interpreter object.
   An :term:`attached thread state` is not needed.

.. c:function:: void PyThreadState_Clear(PyThreadState *tstate)

   Reset all information in a :term:`thread state` object.  *tstate*
   must be :term:`attached <attached thread state>`

   .. versionchanged:: 3.9
      This function now calls the :c:member:`!PyThreadState.on_delete` callback.
      Previously, that happened in :c:func:`PyThreadState_Delete`.

   .. versionchanged:: 3.13
      The :c:member:`!PyThreadState.on_delete` callback was removed.


.. c:function:: void PyThreadState_Delete(PyThreadState *tstate)

   Destroy a :term:`thread state` object.  *tstate* should not
   be :term:`attached <attached thread state>` to any thread.
   *tstate* must have been reset with a previous call to
   :c:func:`PyThreadState_Clear`.


.. c:function:: void PyThreadState_DeleteCurrent(void)

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

   No :term:`thread state` will be :term:`attached <attached thread state>` upon
   returning.

.. c:function:: PyFrameObject* PyThreadState_GetFrame(PyThreadState *tstate)

   Get the current frame of the Python thread state *tstate*.

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

   See also :c:func:`PyEval_GetFrame`.

   *tstate* must not be ``NULL``, and must be :term:`attached <attached thread state>`.

   .. versionadded:: 3.9


.. c:function:: uint64_t PyThreadState_GetID(PyThreadState *tstate)

   Get the unique :term:`thread state` identifier of the Python thread state *tstate*.

   *tstate* must not be ``NULL``, and must be :term:`attached <attached thread state>`.

   .. versionadded:: 3.9


.. c:function:: PyInterpreterState* PyThreadState_GetInterpreter(PyThreadState *tstate)

   Get the interpreter of the Python thread state *tstate*.

   *tstate* must not be ``NULL``, and must be :term:`attached <attached thread state>`.

   .. versionadded:: 3.9


.. c:function:: void PyThreadState_EnterTracing(PyThreadState *tstate)

   Suspend tracing and profiling in the Python thread state *tstate*.

   Resume them using the :c:func:`PyThreadState_LeaveTracing` function.

   .. versionadded:: 3.11


.. c:function:: void PyThreadState_LeaveTracing(PyThreadState *tstate)

   Resume tracing and profiling in the Python thread state *tstate* suspended
   by the :c:func:`PyThreadState_EnterTracing` function.

   See also :c:func:`PyEval_SetTrace` and :c:func:`PyEval_SetProfile`
   functions.

   .. versionadded:: 3.11


.. c:function:: int PyUnstable_ThreadState_SetStackProtection(PyThreadState *tstate, void *stack_start_addr, size_t stack_size)

   Set the stack protection start address and stack protection size
   of a Python thread state.

   On success, return ``0``.
   On failure, set an exception and return ``-1``.

   CPython implements :ref:`recursion control <recursion>` for C code by raising
   :py:exc:`RecursionError` when it notices that the machine execution stack is close
   to overflow. See for example the :c:func:`Py_EnterRecursiveCall` function.
   For this, it needs to know the location of the current thread's stack, which it
   normally gets from the operating system.
   When the stack is changed, for example using context switching techniques like the
   Boost library's ``boost::context``, you must call
   :c:func:`~PyUnstable_ThreadState_SetStackProtection` to inform CPython of the change.

   Call :c:func:`~PyUnstable_ThreadState_SetStackProtection` either before
   or after changing the stack.
   Do not call any other Python C API between the call and the stack
   change.

   See :c:func:`PyUnstable_ThreadState_ResetStackProtection` for undoing this operation.

   .. versionadded:: next


.. c:function:: void PyUnstable_ThreadState_ResetStackProtection(PyThreadState *tstate)

   Reset the stack protection start address and stack protection size
   of a Python thread state to the operating system defaults.

   See :c:func:`PyUnstable_ThreadState_SetStackProtection` for an explanation.

   .. versionadded:: next


.. c:function:: PyInterpreterState* PyInterpreterState_Get(void)

   Get the current interpreter.

   Issue a fatal error if there no :term:`attached thread state`.
   It cannot return NULL.

   .. versionadded:: 3.9


.. c:function:: int64_t PyInterpreterState_GetID(PyInterpreterState *interp)

   Return the interpreter's unique ID.  If there was any error in doing
   so then ``-1`` is returned and an error is set.

   The caller must have an :term:`attached thread state`.

   .. versionadded:: 3.7


.. c:function:: PyObject* PyInterpreterState_GetDict(PyInterpreterState *interp)

   Return a dictionary in which interpreter-specific data may be stored.
   If this function returns ``NULL`` then no exception has been raised and
   the caller should assume no interpreter-specific dict is available.

   This is not a replacement for :c:func:`PyModule_GetState()`, which
   extensions should use to store interpreter-specific state information.

   The returned dictionary is borrowed from the interpreter and is valid until
   interpreter shutdown.

   .. versionadded:: 3.8


.. c:type:: PyObject* (*_PyFrameEvalFunction)(PyThreadState *tstate, _PyInterpreterFrame *frame, int throwflag)

   Type of a frame evaluation function.

   The *throwflag* parameter is used by the ``throw()`` method of generators:
   if non-zero, handle the current exception.

   .. versionchanged:: 3.9
      The function now takes a *tstate* parameter.

   .. versionchanged:: 3.11
      The *frame* parameter changed from ``PyFrameObject*`` to ``_PyInterpreterFrame*``.

.. c:function:: _PyFrameEvalFunction _PyInterpreterState_GetEvalFrameFunc(PyInterpreterState *interp)

   Get the frame evaluation function.

   See the :pep:`523` "Adding a frame evaluation API to CPython".

   .. versionadded:: 3.9

.. c:function:: void _PyInterpreterState_SetEvalFrameFunc(PyInterpreterState *interp, _PyFrameEvalFunction eval_frame)

   Set the frame evaluation function.

   See the :pep:`523` "Adding a frame evaluation API to CPython".

   .. versionadded:: 3.9


.. c:function:: PyObject* PyThreadState_GetDict()

   Return a dictionary in which extensions can store thread-specific state
   information.  Each extension should use a unique key to use to store state in
   the dictionary.  It is okay to call this function when no :term:`thread state`
   is :term:`attached <attached thread state>`. If this function returns
   ``NULL``, no exception has been raised and the caller should assume no
   thread state is attached.


.. c:function:: int PyThreadState_SetAsyncExc(unsigned long id, PyObject *exc)

   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 :term:`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.

   .. versionchanged:: 3.7
      The type of the *id* parameter changed from :c:expr:`long` to
      :c:expr:`unsigned long`.

.. c:function:: void PyEval_AcquireThread(PyThreadState *tstate)

   :term:`Attach <attached thread state>` *tstate* to the current thread,
   which must not be ``NULL`` or already :term:`attached <attached thread state>`.

   The calling thread must not already have an :term:`attached thread state`.

   .. 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
      :ref:`cautions-regarding-runtime-finalization` for more details.

   .. versionchanged:: 3.8
      Updated to be consistent with :c:func:`PyEval_RestoreThread`,
      :c:func:`Py_END_ALLOW_THREADS`, and :c:func:`PyGILState_Ensure`,
      and terminate the current thread if called while the interpreter is finalizing.

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

   :c:func:`PyEval_RestoreThread` is a higher-level function which is always
   available (even when threads have not been initialized).


.. c:function:: void PyEval_ReleaseThread(PyThreadState *tstate)

   Detach the :term:`attached thread state`.
   The *tstate* argument, which must not be ``NULL``, is only used to check
   that it represents the :term:`attached thread state` --- if it isn't, a fatal error is
   reported.

   :c:func:`PyEval_SaveThread` is a higher-level function which is always
   available (even when threads have not been initialized).


.. _sub-interpreter-support:

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.

The "main" interpreter is the first one created when the runtime initializes.
It is usually the only Python interpreter in a process.  Unlike sub-interpreters,
the main interpreter has unique process-global responsibilities like signal
handling.  It is also responsible for execution during runtime initialization and
is usually the active interpreter during runtime finalization.  The
:c:func:`PyInterpreterState_Main` function returns a pointer to its state.

You can switch between sub-interpreters using the :c:func:`PyThreadState_Swap`
function. You can create and destroy them using the following functions:


.. c:type:: PyInterpreterConfig

   Structure containing most parameters to configure a sub-interpreter.
   Its values are used only in :c:func:`Py_NewInterpreterFromConfig` and
   never modified by the runtime.

   .. versionadded:: 3.12

   Structure fields:

   .. c:member:: int use_main_obmalloc

      If this is ``0`` then the sub-interpreter will use its own
      "object" allocator state.
      Otherwise it will use (share) the main interpreter's.

      If this is ``0`` then
      :c:member:`~PyInterpreterConfig.check_multi_interp_extensions`
      must be ``1`` (non-zero).
      If this is ``1`` then :c:member:`~PyInterpreterConfig.gil`
      must not be :c:macro:`PyInterpreterConfig_OWN_GIL`.

   .. c:member:: int allow_fork

      If this is ``0`` then the runtime will not support forking the
      process in any thread where the sub-interpreter is currently active.
      Otherwise fork is unrestricted.

      Note that the :mod:`subprocess` module still works
      when fork is disallowed.

   .. c:member:: int allow_exec

      If this is ``0`` then the runtime will not support replacing the
      current process via exec (e.g. :func:`os.execv`) in any thread
      where the sub-interpreter is currently active.
      Otherwise exec is unrestricted.

      Note that the :mod:`subprocess` module still works
      when exec is disallowed.

   .. c:member:: int allow_threads

      If this is ``0`` then the sub-interpreter's :mod:`threading` module
      won't create threads.
      Otherwise threads are allowed.

   .. c:member:: int allow_daemon_threads

      If this is ``0`` then the sub-interpreter's :mod:`threading` module
      won't create daemon threads.
      Otherwise daemon threads are allowed (as long as
      :c:member:`~PyInterpreterConfig.allow_threads` is non-zero).

   .. c:member:: int check_multi_interp_extensions

      If this is ``0`` then all extension modules may be imported,
      including legacy (single-phase init) modules,
      in any thread where the sub-interpreter is currently active.
      Otherwise only multi-phase init extension modules
      (see :pep:`489`) may be imported.
      (Also see :c:macro:`Py_mod_multiple_interpreters`.)

      This must be ``1`` (non-zero) if
      :c:member:`~PyInterpreterConfig.use_main_obmalloc` is ``0``.

   .. c:member:: int gil

      This determines the operation of the GIL for the sub-interpreter.
      It may be one of the following:

      .. c:namespace:: NULL

      .. c:macro:: PyInterpreterConfig_DEFAULT_GIL

         Use the default selection (:c:macro:`PyInterpreterConfig_SHARED_GIL`).

      .. c:macro:: PyInterpreterConfig_SHARED_GIL

         Use (share) the main interpreter's GIL.

      .. c:macro:: PyInterpreterConfig_OWN_GIL

         Use the sub-interpreter's own GIL.

      If this is :c:macro:`PyInterpreterConfig_OWN_GIL` then
      :c:member:`PyInterpreterConfig.use_main_obmalloc` must be ``0``.


.. c:function:: PyStatus Py_NewInterpreterFromConfig(PyThreadState **tstate_p, const PyInterpreterConfig *config)

   .. index::
      pair: module; builtins
      pair: module; __main__
      pair: module; sys
      single: stdout (in module sys)
      single: stderr (in module sys)
      single: stdin (in module sys)

   Create a new sub-interpreter.  This is an (almost) totally separate environment
   for the execution of Python code.  In particular, the new interpreter has
   separate, independent versions of all imported modules, including the
   fundamental modules :mod:`builtins`, :mod:`__main__` and :mod:`sys`.  The
   table of loaded modules (``sys.modules``) and the module search path
   (``sys.path``) are also separate.  The new environment has no ``sys.argv``
   variable.  It has new standard I/O stream file objects ``sys.stdin``,
   ``sys.stdout`` and ``sys.stderr`` (however these refer to the same underlying
   file descriptors).

   The given *config* controls the options with which the interpreter
   is initialized.

   Upon success, *tstate_p* will be set to the first :term:`thread state`
   created in the new sub-interpreter.  This thread state is
   :term:`attached <attached thread state>`.
   Note that no actual thread is created; see the discussion of thread states
   below.  If creation of the new interpreter is unsuccessful,
   *tstate_p* is set to ``NULL``;
   no exception is set since the exception state is stored in the
   :term:`attached thread state`, which might not exist.

   Like all other Python/C API functions, an :term:`attached thread state`
   must be present before calling this function, but it might be detached upon
   returning. On success, the returned thread state will be :term:`attached <attached thread state>`.
   If the sub-interpreter is created with its own :term:`GIL` then the
   :term:`attached thread state` of the calling interpreter will be detached.
   When the function returns, the new interpreter's :term:`thread state`
   will be :term:`attached <attached thread state>` to the current thread and
   the previous interpreter's :term:`attached thread state` will remain detached.

   .. versionadded:: 3.12

   Sub-interpreters are most effective when isolated from each other,
   with certain functionality restricted::

      PyInterpreterConfig config = {
          .use_main_obmalloc = 0,
          .allow_fork = 0,
          .allow_exec = 0,
          .allow_threads = 1,
          .allow_daemon_threads = 0,
          .check_multi_interp_extensions = 1,
          .gil = PyInterpreterConfig_OWN_GIL,
      };
      PyThreadState *tstate = NULL;
      PyStatus status = Py_NewInterpreterFromConfig(&tstate, &config);
      if (PyStatus_Exception(status)) {
          Py_ExitStatusException(status);
      }

   Note that the config is used only briefly and does not get modified.
   During initialization the config's values are converted into various
   :c:type:`PyInterpreterState` values.  A read-only copy of the config
   may be stored internally on the :c:type:`PyInterpreterState`.

   .. index::
      single: Py_FinalizeEx (C function)
      single: Py_Initialize (C function)

   Extension modules are shared between (sub-)interpreters as follows:

   *  For modules using multi-phase initialization,
      e.g. :c:func:`PyModule_FromDefAndSpec`, a separate module object is
      created and initialized for each interpreter.
      Only C-level static and global variables are shared between these
      module objects.

   *  For modules using single-phase initialization,
      e.g. :c:func:`PyModule_Create`, the first time a particular extension
      is imported, it is initialized normally, and a (shallow) copy of its
      module's dictionary is squirreled away.
      When the same extension is imported by another (sub-)interpreter, a new
      module is initialized and filled with the contents of this copy; the
      extension's ``init`` function is not called.
      Objects in the module's dictionary thus end up shared across
      (sub-)interpreters, which might cause unwanted behavior (see
      `Bugs and caveats`_ below).

      Note that this is different from what happens when an extension is
      imported after the interpreter has been completely re-initialized by
      calling :c:func:`Py_FinalizeEx` and :c:func:`Py_Initialize`; in that
      case, the extension's ``initmodule`` function *is* called again.
      As with multi-phase initialization, this means that only C-level static
      and global variables are shared between these modules.

   .. index:: single: close (in module os)


.. c:function:: PyThreadState* Py_NewInterpreter(void)

   .. index::
      pair: module; builtins
      pair: module; __main__
      pair: module; sys
      single: stdout (in module sys)
      single: stderr (in module sys)
      single: stdin (in module sys)

   Create a new sub-interpreter.  This is essentially just a wrapper
   around :c:func:`Py_NewInterpreterFromConfig` with a config that
   preserves the existing behavior.  The result is an unisolated
   sub-interpreter that shares the main interpreter's GIL, allows
   fork/exec, allows daemon threads, and allows single-phase init
   modules.


.. c:function:: void Py_EndInterpreter(PyThreadState *tstate)

   .. index:: single: Py_FinalizeEx (C function)

   Destroy the (sub-)interpreter represented by the given :term:`thread state`.
   The given thread state must be :term:`attached <attached thread state>`.
   When the call returns, there will be no :term:`attached thread state`.
   All thread states associated with this interpreter are destroyed.

   :c:func:`Py_FinalizeEx` will destroy all sub-interpreters that
   haven't been explicitly destroyed at that point.


.. _per-interpreter-gil:

A Per-Interpreter GIL
---------------------

Using :c:func:`Py_NewInterpreterFromConfig` you can create
a sub-interpreter that is completely isolated from other interpreters,
including having its own GIL.  The most important benefit of this
isolation is that such an interpreter can execute Python code without
being blocked by other interpreters or blocking any others.  Thus a
single Python process can truly take advantage of multiple CPU cores
when running Python code.  The isolation also encourages a different
approach to concurrency than that of just using threads.
(See :pep:`554` and :pep:`684`.)

Using an isolated interpreter requires vigilance in preserving that
isolation.  That especially means not sharing any objects or mutable
state without guarantees about thread-safety.  Even objects that are
otherwise immutable (e.g. ``None``, ``(1, 5)``) can't normally be shared
because of the refcount.  One simple but less-efficient approach around
this is to use a global lock around all use of some state (or object).
Alternately, effectively immutable objects (like integers or strings)
can be made safe in spite of their refcounts by making them :term:`immortal`.
In fact, this has been done for the builtin singletons, small integers,
and a number of other builtin objects.

If you preserve isolation then you will have access to proper multi-core
computing without the complications that come with free-threading.
Failure to preserve isolation will expose you to the full consequences
of free-threading, including races and hard-to-debug crashes.

Aside from that, one of the main challenges of using multiple isolated
interpreters is how to communicate between them safely (not break
isolation) and efficiently.  The runtime and stdlib do not provide
any standard approach to this yet.  A future stdlib module would help
mitigate the effort of preserving isolation and expose effective tools
for communicating (and sharing) data between interpreters.

.. versionadded:: 3.12


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 using single-phase initialization
or (static) global variables.
It is possible to insert objects created in one sub-interpreter into
a namespace of another (sub-)interpreter; this should be avoided if possible.

Special care should be taken 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. It is equally important to avoid sharing
objects from which the above are reachable.

Also note that combining this functionality with ``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.


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.


.. c:function:: int Py_AddPendingCall(int (*func)(void *), void *arg)

   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 :term:`bytecode` boundary;
   * with the main thread holding an :term:`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 :term:`thread state <attached thread state>` is detached.

   This function doesn't need an :term:`attached thread state`. However, to call this
   function in a subinterpreter, the caller must have an :term:`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 :ref:`PyGILState API<gilstate>`.

   .. versionadded:: 3.1

   .. versionchanged:: 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.

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


.. c:function:: int Py_MakePendingCalls(void)

   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 :term:`attached thread state`.

   .. versionadded:: 3.1

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


.. _profiling:

Profiling and Tracing
=====================

.. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org>


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.


.. c:type:: int (*Py_tracefunc)(PyObject *obj, PyFrameObject *frame, int what, PyObject *arg)

   The type of the trace function registered using :c:func:`PyEval_SetProfile` and
   :c:func:`PyEval_SetTrace`. The first parameter is the object passed to the
   registration function as *obj*, *frame* is the frame object to which the event
   pertains, *what* is one of the constants :c:data:`PyTrace_CALL`,
   :c:data:`PyTrace_EXCEPTION`, :c:data:`PyTrace_LINE`, :c:data:`PyTrace_RETURN`,
   :c:data:`PyTrace_C_CALL`, :c:data:`PyTrace_C_EXCEPTION`, :c:data:`PyTrace_C_RETURN`,
   or :c:data:`PyTrace_OPCODE`, and *arg* depends on the value of *what*:

   +-------------------------------+----------------------------------------+
   | Value of *what*               | Meaning of *arg*                       |
   +===============================+========================================+
   | :c:data:`PyTrace_CALL`        | Always :c:data:`Py_None`.              |
   +-------------------------------+----------------------------------------+
   | :c:data:`PyTrace_EXCEPTION`   | Exception information as returned by   |
   |                               | :func:`sys.exc_info`.                  |
   +-------------------------------+----------------------------------------+
   | :c:data:`PyTrace_LINE`        | Always :c:data:`Py_None`.              |
   +-------------------------------+----------------------------------------+
   | :c:data:`PyTrace_RETURN`      | Value being returned to the caller,    |
   |                               | or ``NULL`` if caused by an exception. |
   +-------------------------------+----------------------------------------+
   | :c:data:`PyTrace_C_CALL`      | Function object being called.          |
   +-------------------------------+----------------------------------------+
   | :c:data:`PyTrace_C_EXCEPTION` | Function object being called.          |
   +-------------------------------+----------------------------------------+
   | :c:data:`PyTrace_C_RETURN`    | Function object being called.          |
   +-------------------------------+----------------------------------------+
   | :c:data:`PyTrace_OPCODE`      | Always :c:data:`Py_None`.              |
   +-------------------------------+----------------------------------------+

.. c:var:: int PyTrace_CALL

   The value of the *what* parameter to a :c:type:`Py_tracefunc` function when a new
   call to a function or method is being reported, or a new entry into a generator.
   Note that the creation of the iterator for a generator function is not reported
   as there is no control transfer to the Python bytecode in the corresponding
   frame.


.. c:var:: int PyTrace_EXCEPTION

   The value of the *what* parameter to a :c:type:`Py_tracefunc` function when an
   exception has been raised.  The callback function is called with this value for
   *what* when after any bytecode is processed after which the exception becomes
   set within the frame being executed.  The effect of this is that as exception
   propagation causes the Python stack to unwind, the callback is called upon
   return to each frame as the exception propagates.  Only trace functions receives
   these events; they are not needed by the profiler.


.. c:var:: int PyTrace_LINE

   The value passed as the *what* parameter to a :c:type:`Py_tracefunc` function
   (but not a profiling function) when a line-number event is being reported.
   It may be disabled for a frame by setting :attr:`~frame.f_trace_lines` to
   *0* on that frame.


.. c:var:: int PyTrace_RETURN

   The value for the *what* parameter to :c:type:`Py_tracefunc` functions when a
   call is about to return.


.. c:var:: int PyTrace_C_CALL

   The value for the *what* parameter to :c:type:`Py_tracefunc` functions when a C
   function is about to be called.


.. c:var:: int PyTrace_C_EXCEPTION

   The value for the *what* parameter to :c:type:`Py_tracefunc` functions when a C
   function has raised an exception.


.. c:var:: int PyTrace_C_RETURN

   The value for the *what* parameter to :c:type:`Py_tracefunc` functions when a C
   function has returned.


.. c:var:: int PyTrace_OPCODE

   The value for the *what* parameter to :c:type:`Py_tracefunc` functions (but not
   profiling functions) when a new opcode is about to be executed.  This event is
   not emitted by default: it must be explicitly requested by setting
   :attr:`~frame.f_trace_opcodes` to *1* on the frame.


.. c:function:: void PyEval_SetProfile(Py_tracefunc func, PyObject *obj)

   Set the profiler function to *func*.  The *obj* parameter is passed to the
   function as its first parameter, and may be any Python object, or ``NULL``.  If
   the profile function needs to maintain state, using a different value for *obj*
   for each thread provides a convenient and thread-safe place to store it.  The
   profile function is called for all monitored events except :c:data:`PyTrace_LINE`
   :c:data:`PyTrace_OPCODE` and :c:data:`PyTrace_EXCEPTION`.

   See also the :func:`sys.setprofile` function.

   The caller must have an :term:`attached thread state`.

.. c:function:: void PyEval_SetProfileAllThreads(Py_tracefunc func, PyObject *obj)

   Like :c:func:`PyEval_SetProfile` but sets the profile function in all running threads
   belonging to the current interpreter instead of the setting it only on the current thread.

   The caller must have an :term:`attached thread state`.

   As :c:func:`PyEval_SetProfile`, this function ignores any exceptions raised while
   setting the profile functions in all threads.

.. versionadded:: 3.12


.. c:function:: void PyEval_SetTrace(Py_tracefunc func, PyObject *obj)

   Set the tracing function to *func*.  This is similar to
   :c:func:`PyEval_SetProfile`, except the tracing function does receive line-number
   events and per-opcode events, but does not receive any event related to C function
   objects being called.  Any trace function registered using :c:func:`PyEval_SetTrace`
   will not receive :c:data:`PyTrace_C_CALL`, :c:data:`PyTrace_C_EXCEPTION` or
   :c:data:`PyTrace_C_RETURN` as a value for the *what* parameter.

   See also the :func:`sys.settrace` function.

   The caller must have an :term:`attached thread state`.

.. c:function:: void PyEval_SetTraceAllThreads(Py_tracefunc func, PyObject *obj)

   Like :c:func:`PyEval_SetTrace` but sets the tracing function in all running threads
   belonging to the current interpreter instead of the setting it only on the current thread.

   The caller must have an :term:`attached thread state`.

   As :c:func:`PyEval_SetTrace`, this function ignores any exceptions raised while
   setting the trace functions in all threads.

.. versionadded:: 3.12

Reference tracing
=================

.. versionadded:: 3.13

.. c:type:: int (*PyRefTracer)(PyObject *, int event, void* data)

   The type of the trace function registered using :c:func:`PyRefTracer_SetTracer`.
   The first parameter is a Python object that has been just created (when **event**
   is set to :c:data:`PyRefTracer_CREATE`) or about to be destroyed (when **event**
   is set to :c:data:`PyRefTracer_DESTROY`). The **data** argument is the opaque pointer
   that was provided when :c:func:`PyRefTracer_SetTracer` was called.

.. versionadded:: 3.13

.. c:var:: int PyRefTracer_CREATE

   The value for the *event* parameter to :c:type:`PyRefTracer` functions when a Python
   object has been created.

.. c:var:: int PyRefTracer_DESTROY

   The value for the *event* parameter to :c:type:`PyRefTracer` functions when a Python
   object has been destroyed.

.. c:function:: int PyRefTracer_SetTracer(PyRefTracer tracer, void *data)

   Register a reference tracer function. The function will be called when a new
   Python has been created or when an object is going to be destroyed. If
   **data** is provided it must be an opaque pointer that will be provided when
   the tracer function is called. Return ``0`` on success. Set an exception and
   return ``-1`` on error.

   Not that tracer functions **must not** create Python objects inside or
   otherwise the call will be re-entrant. The tracer also **must not** clear
   any existing exception or set an exception.  A :term:`thread state` will be active
   every time the tracer function is called.

   There must be an :term:`attached thread state` when calling this function.

.. versionadded:: 3.13

.. c:function:: PyRefTracer PyRefTracer_GetTracer(void** data)

   Get the registered reference tracer function and the value of the opaque data
   pointer that was registered when :c:func:`PyRefTracer_SetTracer` was called.
   If no tracer was registered this function will return NULL and will set the
   **data** pointer to NULL.

   There must be an :term:`attached thread state` when calling this function.

.. versionadded:: 3.13

.. _advanced-debugging:

Advanced Debugger Support
=========================

.. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org>


These functions are only intended to be used by advanced debugging tools.


.. c:function:: PyInterpreterState* PyInterpreterState_Head()

   Return the interpreter state object at the head of the list of all such objects.


.. c:function:: PyInterpreterState* PyInterpreterState_Main()

   Return the main interpreter state object.


.. c:function:: PyInterpreterState* PyInterpreterState_Next(PyInterpreterState *interp)

   Return the next interpreter state object after *interp* from the list of all
   such objects.


.. c:function:: PyThreadState * PyInterpreterState_ThreadHead(PyInterpreterState *interp)

   Return the pointer to the first :c:type:`PyThreadState` object in the list of
   threads associated with the interpreter *interp*.


.. c:function:: PyThreadState* PyThreadState_Next(PyThreadState *tstate)

   Return the next thread state object after *tstate* from the list of all such
   objects belonging to the same :c:type:`PyInterpreterState` object.


.. _thread-local-storage:

Thread Local Storage Support
============================

.. sectionauthor:: Masayuki Yamamoto <ma3yuki.8mamo10@gmail.com>

The Python interpreter provides low-level support for thread-local storage
(TLS) which wraps the underlying native TLS implementation to support the
Python-level thread local storage API (:class:`threading.local`).  The
CPython C level APIs are similar to those offered by pthreads and Windows:
use a thread key and functions to associate a :c:expr:`void*` value per
thread.

A :term:`thread state` does *not* need to be :term:`attached <attached thread state>`
when calling these functions; they suppl their own locking.

Note that :file:`Python.h` does not include the declaration of the TLS APIs,
you need to include :file:`pythread.h` to use thread-local storage.

.. note::
   None of these API functions handle memory management on behalf of the
   :c:expr:`void*` values.  You need to allocate and deallocate them yourself.
   If the :c:expr:`void*` values happen to be :c:expr:`PyObject*`, these
   functions don't do refcount operations on them either.

.. _thread-specific-storage-api:

Thread Specific Storage (TSS) API
---------------------------------

TSS API is introduced to supersede the use of the existing TLS API within the
CPython interpreter.  This API uses a new type :c:type:`Py_tss_t` instead of
:c:expr:`int` to represent thread keys.

.. versionadded:: 3.7

.. seealso:: "A New C-API for Thread-Local Storage in CPython" (:pep:`539`)


.. c:type:: Py_tss_t

   This data structure represents the state of a thread key, the definition of
   which may depend on the underlying TLS implementation, and it has an
   internal field representing the key's initialization state.  There are no
   public members in this structure.

   When :ref:`Py_LIMITED_API <stable>` is not defined, static allocation of
   this type by :c:macro:`Py_tss_NEEDS_INIT` is allowed.


.. c:macro:: Py_tss_NEEDS_INIT

   This macro expands to the initializer for :c:type:`Py_tss_t` variables.
   Note that this macro won't be defined with :ref:`Py_LIMITED_API <stable>`.


Dynamic Allocation
~~~~~~~~~~~~~~~~~~

Dynamic allocation of the :c:type:`Py_tss_t`, required in extension modules
built with :ref:`Py_LIMITED_API <stable>`, where static allocation of this type
is not possible due to its implementation being opaque at build time.


.. c:function:: Py_tss_t* PyThread_tss_alloc()

   Return a value which is the same state as a value initialized with
   :c:macro:`Py_tss_NEEDS_INIT`, or ``NULL`` in the case of dynamic allocation
   failure.


.. c:function:: void PyThread_tss_free(Py_tss_t *key)

   Free the given *key* allocated by :c:func:`PyThread_tss_alloc`, after
   first calling :c:func:`PyThread_tss_delete` to ensure any associated
   thread locals have been unassigned. This is a no-op if the *key*
   argument is ``NULL``.

   .. note::
      A freed key becomes a dangling pointer. You should reset the key to
      ``NULL``.


Methods
~~~~~~~

The parameter *key* of these functions must not be ``NULL``.  Moreover, the
behaviors of :c:func:`PyThread_tss_set` and :c:func:`PyThread_tss_get` are
undefined if the given :c:type:`Py_tss_t` has not been initialized by
:c:func:`PyThread_tss_create`.


.. c:function:: int PyThread_tss_is_created(Py_tss_t *key)

   Return a non-zero value if the given :c:type:`Py_tss_t` has been initialized
   by :c:func:`PyThread_tss_create`.


.. c:function:: int PyThread_tss_create(Py_tss_t *key)

   Return a zero value on successful initialization of a TSS key.  The behavior
   is undefined if the value pointed to by the *key* argument is not
   initialized by :c:macro:`Py_tss_NEEDS_INIT`.  This function can be called
   repeatedly on the same key -- calling it on an already initialized key is a
   no-op and immediately returns success.


.. c:function:: void PyThread_tss_delete(Py_tss_t *key)

   Destroy a TSS key to forget the values associated with the key across all
   threads, and change the key's initialization state to uninitialized.  A
   destroyed key is able to be initialized again by
   :c:func:`PyThread_tss_create`. This function can be called repeatedly on
   the same key -- calling it on an already destroyed key is a no-op.


.. c:function:: int PyThread_tss_set(Py_tss_t *key, void *value)

   Return a zero value to indicate successfully associating a :c:expr:`void*`
   value with a TSS key in the current thread.  Each thread has a distinct
   mapping of the key to a :c:expr:`void*` value.


.. c:function:: void* PyThread_tss_get(Py_tss_t *key)

   Return the :c:expr:`void*` value associated with a TSS key in the current
   thread.  This returns ``NULL`` if no value is associated with the key in the
   current thread.


.. _thread-local-storage-api:

Thread Local Storage (TLS) API
------------------------------

.. deprecated:: 3.7
   This API is superseded by
   :ref:`Thread Specific Storage (TSS) API <thread-specific-storage-api>`.

.. note::
   This version of the API does not support platforms where the native TLS key
   is defined in a way that cannot be safely cast to ``int``.  On such platforms,
   :c:func:`PyThread_create_key` will return immediately with a failure status,
   and the other TLS functions will all be no-ops on such platforms.

Due to the compatibility problem noted above, this version of the API should not
be used in new code.

.. c:function:: int PyThread_create_key()
.. c:function:: void PyThread_delete_key(int key)
.. c:function:: int PyThread_set_key_value(int key, void *value)
.. c:function:: void* PyThread_get_key_value(int key)
.. c:function:: void PyThread_delete_key_value(int key)
.. c:function:: void PyThread_ReInitTLS()

Synchronization Primitives
==========================

The C-API provides a basic mutual exclusion lock.

.. c:type:: PyMutex

   A mutual exclusion lock.  The :c:type:`!PyMutex` should be initialized to
   zero to represent the unlocked state.  For example::

      PyMutex mutex = {0};

   Instances of :c:type:`!PyMutex` should not be copied or moved.  Both the
   contents and address of a :c:type:`!PyMutex` are meaningful, and it must
   remain at a fixed, writable location in memory.

   .. note::

      A :c:type:`!PyMutex` currently occupies one byte, but the size should be
      considered unstable.  The size may change in future Python releases
      without a deprecation period.

   .. versionadded:: 3.13

.. c:function:: void PyMutex_Lock(PyMutex *m)

   Lock mutex *m*.  If another thread has already locked it, the calling
   thread will block until the mutex is unlocked.  While blocked, the thread
   will temporarily detach the :term:`thread state <attached thread state>` if one exists.

   .. versionadded:: 3.13

.. c:function:: void PyMutex_Unlock(PyMutex *m)

   Unlock mutex *m*. The mutex must be locked --- otherwise, the function will
   issue a fatal error.

   .. versionadded:: 3.13

.. c:function:: int PyMutex_IsLocked(PyMutex *m)

   Returns non-zero if the mutex *m* is currently locked, zero otherwise.

   .. note::

      This function is intended for use in assertions and debugging only and
      should not be used to make concurrency control decisions, as the lock
      state may change immediately after the check.

   .. versionadded:: 3.14

.. _python-critical-section-api:

Python Critical Section API
---------------------------

The critical section API provides a deadlock avoidance layer on top of
per-object locks for :term:`free-threaded <free threading>` CPython.  They are
intended to replace reliance on the :term:`global interpreter lock`, and are
no-ops in versions of Python with the global interpreter lock.

Critical sections are intended to be used for custom types implemented
in C-API extensions. They should generally not be used with built-in types like
:class:`list` and :class:`dict` because their public C-APIs
already use critical sections internally, with the notable
exception of :c:func:`PyDict_Next`, which requires critical section
to be acquired externally.

Critical sections avoid deadlocks by implicitly suspending active critical
sections, hence, they do not provide exclusive access such as provided by
traditional locks like :c:type:`PyMutex`.  When a critical section is started,
the per-object lock for the object is acquired. If the code executed inside the
critical section calls C-API functions then it can suspend the critical section thereby
releasing the per-object lock, so other threads can acquire the per-object lock
for the same object.

Variants that accept :c:type:`PyMutex` pointers rather than Python objects are also
available. Use these variants to start a critical section in a situation where
there is no :c:type:`PyObject` -- for example, when working with a C type that
does not extend or wrap :c:type:`PyObject` but still needs to call into the C
API in a manner that might lead to deadlocks.

The functions and structs used by the macros are exposed for cases
where C macros are not available. They should only be used as in the
given macro expansions. Note that the sizes and contents of the structures may
change in future Python versions.

.. note::

   Operations that need to lock two objects at once must use
   :c:macro:`Py_BEGIN_CRITICAL_SECTION2`.  You *cannot* use nested critical
   sections to lock more than one object at once, because the inner critical
   section may suspend the outer critical sections.  This API does not provide
   a way to lock more than two objects at once.

Example usage::

   static PyObject *
   set_field(MyObject *self, PyObject *value)
   {
      Py_BEGIN_CRITICAL_SECTION(self);
      Py_SETREF(self->field, Py_XNewRef(value));
      Py_END_CRITICAL_SECTION();
      Py_RETURN_NONE;
   }

In the above example, :c:macro:`Py_SETREF` calls :c:macro:`Py_DECREF`, which
can call arbitrary code through an object's deallocation function.  The critical
section API avoids potential deadlocks due to reentrancy and lock ordering
by allowing the runtime to temporarily suspend the critical section if the
code triggered by the finalizer blocks and calls :c:func:`PyEval_SaveThread`.

.. c:macro:: Py_BEGIN_CRITICAL_SECTION(op)

   Acquires the per-object lock for the object *op* and begins a
   critical section.

   In the free-threaded build, this macro expands to::

      {
          PyCriticalSection _py_cs;
          PyCriticalSection_Begin(&_py_cs, (PyObject*)(op))

   In the default build, this macro expands to ``{``.

   .. versionadded:: 3.13

.. c:macro:: Py_BEGIN_CRITICAL_SECTION_MUTEX(m)

   Locks the mutex *m* and begins a critical section.

   In the free-threaded build, this macro expands to::

     {
          PyCriticalSection _py_cs;
          PyCriticalSection_BeginMutex(&_py_cs, m)

   Note that unlike :c:macro:`Py_BEGIN_CRITICAL_SECTION`, there is no cast for
   the argument of the macro - it must be a :c:type:`PyMutex` pointer.

   On the default build, this macro expands to ``{``.

   .. versionadded:: 3.14

.. c:macro:: Py_END_CRITICAL_SECTION()

   Ends the critical section and releases the per-object lock.

   In the free-threaded build, this macro expands to::

          PyCriticalSection_End(&_py_cs);
      }

   In the default build, this macro expands to ``}``.

   .. versionadded:: 3.13

.. c:macro:: Py_BEGIN_CRITICAL_SECTION2(a, b)

   Acquires the per-objects locks for the objects *a* and *b* and begins a
   critical section.  The locks are acquired in a consistent order (lowest
   address first) to avoid lock ordering deadlocks.

   In the free-threaded build, this macro expands to::

      {
          PyCriticalSection2 _py_cs2;
          PyCriticalSection2_Begin(&_py_cs2, (PyObject*)(a), (PyObject*)(b))

   In the default build, this macro expands to ``{``.

   .. versionadded:: 3.13

.. c:macro:: Py_BEGIN_CRITICAL_SECTION2_MUTEX(m1, m2)

   Locks the mutexes *m1* and *m2* and begins a critical section.

   In the free-threaded build, this macro expands to::

     {
          PyCriticalSection2 _py_cs2;
          PyCriticalSection2_BeginMutex(&_py_cs2, m1, m2)

   Note that unlike :c:macro:`Py_BEGIN_CRITICAL_SECTION2`, there is no cast for
   the arguments of the macro - they must be :c:type:`PyMutex` pointers.

   On the default build, this macro expands to ``{``.

   .. versionadded:: 3.14

.. c:macro:: Py_END_CRITICAL_SECTION2()

   Ends the critical section and releases the per-object locks.

   In the free-threaded build, this macro expands to::

          PyCriticalSection2_End(&_py_cs2);
      }

   In the default build, this macro expands to ``}``.

   .. versionadded:: 3.13