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

See Python Initialization Configuration for details on how to
configure the interpreter prior to initialization.


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

In an application embedding  Python, the "Py_Initialize()" function
must be called before using any other Python/C API functions; with the
exception of a few functions and the global configuration variables.

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

* Functions that initialize the interpreter:

  * "Py_Initialize()"

  * "Py_InitializeEx()"

  * "Py_InitializeFromConfig()"

  * "Py_BytesMain()"

  * "Py_Main()"

  * the runtime pre-initialization functions covered in Python
    Initialization Configuration

* Configuration functions:

  * "PyImport_AppendInittab()"

  * "PyImport_ExtendInittab()"

  * "PyInitFrozenExtensions()"

  * "PyMem_SetAllocator()"

  * "PyMem_SetupDebugHooks()"

  * "PyObject_SetArenaAllocator()"

  * "Py_SetProgramName()"

  * "Py_SetPythonHome()"

  * "PySys_ResetWarnOptions()"

  * the configuration functions covered in Python Initialization
    Configuration

* Informative functions:

  * "Py_IsInitialized()"

  * "PyMem_GetAllocator()"

  * "PyObject_GetArenaAllocator()"

  * "Py_GetBuildInfo()"

  * "Py_GetCompiler()"

  * "Py_GetCopyright()"

  * "Py_GetPlatform()"

  * "Py_GetVersion()"

  * "Py_IsInitialized()"

* Utilities:

  * "Py_DecodeLocale()"

  * the status reporting and utility functions covered in Python
    Initialization Configuration

* Memory allocators:

  * "PyMem_RawMalloc()"

  * "PyMem_RawRealloc()"

  * "PyMem_RawCalloc()"

  * "PyMem_RawFree()"

* Synchronization:

  * "PyMutex_Lock()"

  * "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: "Py_EncodeLocale()",
  "Py_GetPath()", "Py_GetPrefix()", "Py_GetExecPrefix()",
  "Py_GetProgramFullPath()", "Py_GetPythonHome()",
  "Py_GetProgramName()", "PyEval_InitThreads()", and "Py_RunMain()".


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

Python has variables for the global configuration to control different
features and options. By default, these flags are controlled by
command line 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
"Py_BytesWarningFlag" to 1 and "-bb" sets "Py_BytesWarningFlag" to 2.

int Py_BytesWarningFlag

   This API is kept for backward compatibility: setting
   "PyConfig.bytes_warning" should be used instead, see Python
   Initialization Configuration.

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

   Set by the "-b" option.

   Deprecated since version 3.12, will be removed in version 3.15.

int Py_DebugFlag

   This API is kept for backward compatibility: setting
   "PyConfig.parser_debug" should be used instead, see Python
   Initialization Configuration.

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

   Set by the "-d" option and the "PYTHONDEBUG" environment variable.

   Deprecated since version 3.12, will be removed in version 3.15.

int Py_DontWriteBytecodeFlag

   This API is kept for backward compatibility: setting
   "PyConfig.write_bytecode" should be used instead, see Python
   Initialization Configuration.

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

   Set by the "-B" option and the "PYTHONDONTWRITEBYTECODE"
   environment variable.

   Deprecated since version 3.12, will be removed in version 3.15.

int Py_FrozenFlag

   This API is kept for backward compatibility: setting
   "PyConfig.pathconfig_warnings" should be used instead, see Python
   Initialization Configuration.

   Suppress error messages when calculating the module search path in
   "Py_GetPath()".

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

   Deprecated since version 3.12, will be removed in version 3.15.

int Py_HashRandomizationFlag

   This API is kept for backward compatibility: setting
   "PyConfig.hash_seed" and "PyConfig.use_hash_seed" should be used
   instead, see Python Initialization Configuration.

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

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

   Deprecated since version 3.12, will be removed in version 3.15.

int Py_IgnoreEnvironmentFlag

   This API is kept for backward compatibility: setting
   "PyConfig.use_environment" should be used instead, see Python
   Initialization Configuration.

   Ignore all "PYTHON*" environment variables, e.g. "PYTHONPATH" and
   "PYTHONHOME", that might be set.

   Set by the "-E" and "-I" options.

   Deprecated since version 3.12, will be removed in version 3.15.

int Py_InspectFlag

   This API is kept for backward compatibility: setting
   "PyConfig.inspect" should be used instead, see Python
   Initialization Configuration.

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

   Set by the "-i" option and the "PYTHONINSPECT" environment
   variable.

   Deprecated since version 3.12, will be removed in version 3.15.

int Py_InteractiveFlag

   This API is kept for backward compatibility: setting
   "PyConfig.interactive" should be used instead, see Python
   Initialization Configuration.

   Set by the "-i" option.

   Deprecated since version 3.12, will be removed in version 3.15.

int Py_IsolatedFlag

   This API is kept for backward compatibility: setting
   "PyConfig.isolated" should be used instead, see Python
   Initialization Configuration.

   Run Python in isolated mode. In isolated mode "sys.path" contains
   neither the script’s directory nor the user’s site-packages
   directory.

   Set by the "-I" option.

   Added in version 3.4.

   Deprecated since version 3.12, will be removed in version 3.15.

int Py_LegacyWindowsFSEncodingFlag

   This API is kept for backward compatibility: setting
   "PyPreConfig.legacy_windows_fs_encoding" should be used instead,
   see Python Initialization Configuration.

   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 *filesystem encoding and error handler*.

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

   See **PEP 529** for more details.

   Availability: Windows.

   Deprecated since version 3.12, will be removed in version 3.15.

int Py_LegacyWindowsStdioFlag

   This API is kept for backward compatibility: setting
   "PyConfig.legacy_windows_stdio" should be used instead, see Python
   Initialization Configuration.

   If the flag is non-zero, use "io.FileIO" instead of
   "io._WindowsConsoleIO" for "sys" standard streams.

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

   See **PEP 528** for more details.

   Availability: Windows.

   Deprecated since version 3.12, will be removed in version 3.15.

int Py_NoSiteFlag

   This API is kept for backward compatibility: setting
   "PyConfig.site_import" should be used instead, see Python
   Initialization Configuration.

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

   Set by the "-S" option.

   Deprecated since version 3.12, will be removed in version 3.15.

int Py_NoUserSiteDirectory

   This API is kept for backward compatibility: setting
   "PyConfig.user_site_directory" should be used instead, see Python
   Initialization Configuration.

   Don’t add the "user site-packages directory" to "sys.path".

   Set by the "-s" and "-I" options, and the "PYTHONNOUSERSITE"
   environment variable.

   Deprecated since version 3.12, will be removed in version 3.15.

int Py_OptimizeFlag

   This API is kept for backward compatibility: setting
   "PyConfig.optimization_level" should be used instead, see Python
   Initialization Configuration.

   Set by the "-O" option and the "PYTHONOPTIMIZE" environment
   variable.

   Deprecated since version 3.12, will be removed in version 3.15.

int Py_QuietFlag

   This API is kept for backward compatibility: setting
   "PyConfig.quiet" should be used instead, see Python Initialization
   Configuration.

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

   Set by the "-q" option.

   Added in version 3.2.

   Deprecated since version 3.12, will be removed in version 3.15.

int Py_UnbufferedStdioFlag

   This API is kept for backward compatibility: setting
   "PyConfig.buffered_stdio" should be used instead, see Python
   Initialization Configuration.

   Force the stdout and stderr streams to be unbuffered.

   Set by the "-u" option and the "PYTHONUNBUFFERED" environment
   variable.

   Deprecated since version 3.12, will be removed in version 3.15.

int Py_VerboseFlag

   This API is kept for backward compatibility: setting
   "PyConfig.verbose" should be used instead, see Python
   Initialization Configuration.

   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 "-v" option and the "PYTHONVERBOSE" environment
   variable.

   Deprecated since version 3.12, will be removed in version 3.15.


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

void Py_Initialize()
    * Part of the Stable ABI.*

   Initialize the Python interpreter.  In an application embedding
   Python, this should be called before using any other Python/C API
   functions; see Before Python Initialization for the few exceptions.

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

   Use "Py_InitializeFromConfig()" to customize the Python
   Initialization Configuration.

   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.

void Py_InitializeEx(int initsigs)
    * Part of the Stable ABI.*

   This function works like "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 "Py_InitializeFromConfig()" to customize the Python
   Initialization Configuration.

PyStatus Py_InitializeFromConfig(const PyConfig *config)

   Initialize Python from *config* configuration, as described in
   Initialization with PyConfig.

   See the Python Initialization Configuration section for details on
   pre-initializing the interpreter, populating the runtime
   configuration structure, and querying the returned status
   structure.

int Py_IsInitialized()
    * Part of the Stable ABI.*

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

int Py_IsFinalizing()
    * Part of the Stable ABI since version 3.13.*

   Return true (non-zero) if the main Python interpreter is *shutting
   down*. Return false (zero) otherwise.

   Added in version 3.13.

int Py_FinalizeEx()
    * Part of the Stable ABI since version 3.6.*

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

   Since this is the reverse of "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 "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
   "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
   ("__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
   "Py_Initialize()" and "Py_FinalizeEx()" more than once.
   "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 "atexit"
   handlers, object finalizers, or any code that may be run while
   flushing the stdout and stderr files.

   Raises an auditing event "cpython._PySys_ClearAuditHooks" with no
   arguments.

   Added in version 3.6.

void Py_Finalize()
    * Part of the Stable ABI.*

   This is a backwards-compatible version of "Py_FinalizeEx()" that
   disregards the return value.

int Py_BytesMain(int argc, char **argv)
    * Part of the Stable ABI since version 3.8.*

   Similar to "Py_Main()" but *argv* is an array of bytes strings,
   allowing the calling application to delegate the text decoding step
   to the CPython runtime.

   Added in version 3.8.

int Py_Main(int argc, wchar_t **argv)
    * Part of the Stable ABI.*

   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
   Command line.

   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 "main()" function, except that the *argv*
   entries are first converted to "wchar_t" using "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
   "Py_RunMain()".

   In terms of the CPython runtime configuration APIs documented in
   the runtime configuration 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 "Py_Initialize()", "Py_InitializeEx()" or
   "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).

int Py_RunMain(void)

   Executes the main module in a fully configured CPython runtime.

   Executes the command ("PyConfig.run_command"), the script
   ("PyConfig.run_filename") or the module ("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 "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
   "SystemExit", or "1" for any other unhandled exception.

   If "PyConfig.inspect" is set (such as when the "-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 "SystemExit", as
   specified above.

   This function always finalizes the Python interpreter before it
   returns.

   See Python Configuration for an example of a customized Python that
   always runs in isolated mode using "Py_RunMain()".

int PyUnstable_AtExit(PyInterpreterState *interp, void (*func)(void*), void *data)

   *This is Unstable API. It may change without warning in minor
   releases.*

   Register an "atexit" callback for the target interpreter *interp*.
   This is similar to "Py_AtExit()", but takes an explicit interpreter
   and data pointer for the callback.

   There must be an *attached thread state* for *interp*.

   Added in version 3.13.


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

void Py_SetProgramName(const wchar_t *name)
    * Part of the Stable ABI.*

   This API is kept for backward compatibility: setting
   "PyConfig.program_name" should be used instead, see Python
   Initialization Configuration.

   This function should be called before "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 "main()"
   function of the program (converted to wide characters). This is
   used by "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 "Py_DecodeLocale()" to decode a bytes string to get a wchar_t*
   string.

   Deprecated since version 3.11, will be removed in version 3.15.

wchar_t *Py_GetProgramName()
    * Part of the Stable ABI.*

   Return the program name set with "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 "Py_Initialize()",
   otherwise it returns "NULL".

   Changed in version 3.10: It now returns "NULL" if called before
   "Py_Initialize()".

   Deprecated since version 3.13, will be removed in version 3.15: Use
   "PyConfig_Get("executable")" ("sys.executable") instead.

wchar_t *Py_GetPrefix()
    * Part of the Stable ABI.*

   Return the *prefix* for installed platform-independent files. This
   is derived through a number of complicated rules from the program
   name set with "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 **prefix** variable in
   the top-level "Makefile" and the "--prefix" argument to the
   **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 "Py_Initialize()",
   otherwise it returns "NULL".

   Changed in version 3.10: It now returns "NULL" if called before
   "Py_Initialize()".

   Deprecated since version 3.13, will be removed in version 3.15: Use
   "PyConfig_Get("base_prefix")" ("sys.base_prefix") instead. Use
   "PyConfig_Get("prefix")" ("sys.prefix") if virtual environments
   need to be handled.

wchar_t *Py_GetExecPrefix()
    * Part of the Stable ABI.*

   Return the *exec-prefix* for installed platform-*dependent* files.
   This is derived through a number of complicated rules from the
   program name set with "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 **exec_prefix** variable
   in the top-level "Makefile" and the "--exec-prefix" argument to the
   **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
   "/usr/local/plat" subtree while platform independent may be
   installed in "/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 **mount** or
   **automount** programs to share "/usr/local" between platforms
   while having "/usr/local/plat" be a different filesystem for each
   platform.

   This function should not be called before "Py_Initialize()",
   otherwise it returns "NULL".

   Changed in version 3.10: It now returns "NULL" if called before
   "Py_Initialize()".

   Deprecated since version 3.13, will be removed in version 3.15: Use
   "PyConfig_Get("base_exec_prefix")" ("sys.base_exec_prefix")
   instead. Use "PyConfig_Get("exec_prefix")" ("sys.exec_prefix") if
   virtual environments need to be handled.

wchar_t *Py_GetProgramFullPath()
    * Part of the Stable ABI.*

   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 "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 "Py_Initialize()",
   otherwise it returns "NULL".

   Changed in version 3.10: It now returns "NULL" if called before
   "Py_Initialize()".

   Deprecated since version 3.13, will be removed in version 3.15: Use
   "PyConfig_Get("executable")" ("sys.executable") instead.

wchar_t *Py_GetPath()
    * Part of the Stable ABI.*

   Return the default module search path; this is computed from the
   program name (set by "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 "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 "Py_Initialize()",
   otherwise it returns "NULL".

   Changed in version 3.10: It now returns "NULL" if called before
   "Py_Initialize()".

   Deprecated since version 3.13, will be removed in version 3.15: Use
   "PyConfig_Get("module_search_paths")" ("sys.path") instead.

const char *Py_GetVersion()
    * Part of the Stable ABI.*

   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]"

   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 "sys.version".

   See also the "Py_Version" constant.

const char *Py_GetPlatform()
    * Part of the Stable ABI.*

   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".

const char *Py_GetCopyright()
    * Part of the Stable ABI.*

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

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

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

const char *Py_GetCompiler()
    * Part of the Stable ABI.*

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

      "[GCC 2.7.2.2]"

   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".

const char *Py_GetBuildInfo()
    * Part of the Stable ABI.*

   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"

   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".

void PySys_SetArgvEx(int argc, wchar_t **argv, int updatepath)
    * Part of the Stable ABI.*

   This API is kept for backward compatibility: setting
   "PyConfig.argv", "PyConfig.parse_argv" and "PyConfig.safe_path"
   should be used instead, see Python Initialization Configuration.

   Set "sys.argv" based on *argc* and *argv*.  These parameters are
   similar to those passed to the program’s "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 "sys.argv", a fatal condition is signalled using
   "Py_FatalError()".

   If *updatepath* is zero, this is all the function does.  If
   *updatepath* is non-zero, the function also modifies "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 "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
     "sys.path", which is the same as prepending the current working
     directory (""."").

   Use "Py_DecodeLocale()" to decode a bytes string to get a wchar_t*
   string.

   See also "PyConfig.orig_argv" and "PyConfig.argv" members of the
   Python Initialization Configuration.

   Note:

     It is recommended that applications embedding the Python
     interpreter for purposes other than executing a single script
     pass "0" as *updatepath*, and update "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 "sys.path"
     element after having called "PySys_SetArgv()", for example using:

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

   Added in version 3.1.3.

   Deprecated since version 3.11, will be removed in version 3.15.

void PySys_SetArgv(int argc, wchar_t **argv)
    * Part of the Stable ABI.*

   This API is kept for backward compatibility: setting
   "PyConfig.argv" and "PyConfig.parse_argv" should be used instead,
   see Python Initialization Configuration.

   This function works like "PySys_SetArgvEx()" with *updatepath* set
   to "1" unless the **python** interpreter was started with the "-I".

   Use "Py_DecodeLocale()" to decode a bytes string to get a wchar_t*
   string.

   See also "PyConfig.orig_argv" and "PyConfig.argv" members of the
   Python Initialization Configuration.

   Changed in version 3.4: The *updatepath* value depends on "-I".

   Deprecated since version 3.11, will be removed in version 3.15.

void Py_SetPythonHome(const wchar_t *home)
    * Part of the Stable ABI.*

   This API is kept for backward compatibility: setting
   "PyConfig.home" should be used instead, see Python Initialization
   Configuration.

   Set the default “home” directory, that is, the location of the
   standard Python libraries.  See "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 "Py_DecodeLocale()" to decode a bytes string to get a wchar_t*
   string.

   Deprecated since version 3.11, will be removed in version 3.15.

wchar_t *Py_GetPythonHome()
    * Part of the Stable ABI.*

   Return the default “home”, that is, the value set by
   "PyConfig.home", or the value of the "PYTHONHOME" environment
   variable if it is set.

   This function should not be called before "Py_Initialize()",
   otherwise it returns "NULL".

   Changed in version 3.10: It now returns "NULL" if called before
   "Py_Initialize()".

   Deprecated since version 3.13, will be removed in version 3.15: Use
   "PyConfig_Get("home")" or the "PYTHONHOME" environment variable
   instead.


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

Unless on a *free-threaded* build of *CPython*, the Python interpreter
is not fully thread-safe.  In order to support multi-threaded Python
programs, there’s a global lock, called the *global interpreter lock*
or *GIL*, that must be held by the current thread before it can safely
access Python objects. Without the lock, even the simplest operations
could cause problems in a multi-threaded program: for example, when
two threads simultaneously increment the reference count of the same
object, the reference count could end up being incremented only once
instead of twice.

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

The Python interpreter keeps some thread-specific bookkeeping
information inside a data structure called "PyThreadState", known as a
*thread state*. Each OS thread has a thread-local pointer to a
"PyThreadState"; a thread state referenced by this pointer is
considered to be *attached*.

A thread can only have one *attached thread state* at a time. An
attached thread state is typically analogous with holding the *GIL*,
except on *free-threaded* builds.  On builds with the *GIL* enabled,
*attaching* a thread state will block until the *GIL* can be acquired.
However,  even on builds with the *GIL* disabled, it is still required
to have an attached thread state to call most of the C API.

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


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

Most extension code manipulating the *thread state* has the following
simple structure:

   Save the thread state in a local variable.
   ... Do some blocking I/O operation ...
   Restore the thread state from the local variable.

This is so common that a pair of macros exists to simplify it:

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

The "Py_BEGIN_ALLOW_THREADS" macro opens a new block and declares a
hidden local variable; the "Py_END_ALLOW_THREADS" macro closes the
block.

The block above expands to the following code:

   PyThreadState *_save;

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

Here is how these functions work:

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

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

Note:

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


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

When threads are created using the dedicated Python APIs (such as the
"threading" module), a thread state is automatically associated to
them and the code showed above is therefore correct.  However, when
threads are created from C (for example by a third-party library with
its own thread management), they don’t hold the *GIL*, because they
don’t have an *attached thread state*.

If you need to call Python code from these threads (often this will be
part of a callback API provided by the aforementioned third-party
library), you must first register these threads with the interpreter
by creating an *attached thread state* before you can start using the
Python/C API.  When you are done, you should detach the *thread
state*, and finally free it.

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

   PyGILState_STATE gstate;
   gstate = PyGILState_Ensure();

   /* Perform Python actions here. */
   result = CallSomeFunction();
   /* evaluate result or handle exception */

   /* Release the thread. No Python API allowed beyond this point. */
   PyGILState_Release(gstate);

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


Supporting subinterpreters in non-Python threads
------------------------------------------------

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

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

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

   /* GIL of the subinterpreter is now held.
      Perform Python actions here. */
   result = CallSomeFunction();
   /* evaluate result or handle exception */

   /* Destroy the thread state. No Python API allowed beyond this point. */
   PyThreadState_Clear(tstate);
   PyThreadState_DeleteCurrent();


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

Another important thing to note about threads is their behaviour in
the face of the C "fork()" call. On most systems with "fork()", after
a process forks only the thread that issued the fork will exist.  This
has a concrete impact both on how locks must be handled and on all
stored state in CPython’s runtime.

The fact that only the “current” thread remains means any locks held
by other threads will never be released. Python solves this for
"os.fork()" by acquiring the locks it uses internally before the fork,
and releasing them afterwards. In addition, it resets any Lock objects
in the child. When extending or embedding Python, there is no way to
inform Python of additional (non-Python) locks that need to be
acquired before or reset after a fork. OS facilities such as
"pthread_atfork()" would need to be used to accomplish the same thing.
Additionally, when extending or embedding Python, calling "fork()"
directly rather than through "os.fork()" (and returning to or calling
into Python) may result in a deadlock by one of Python’s internal
locks being held by a thread that is defunct after the fork.
"PyOS_AfterFork_Child()" tries to reset the necessary locks, but is
not always able to.

The fact that all other threads go away also means that CPython’s
runtime state there must be cleaned up properly, which "os.fork()"
does.  This means finalizing all other "PyThreadState" objects
belonging to the current interpreter and all other
"PyInterpreterState" objects.  Due to this and the special nature of
the “main” interpreter, "fork()" should only be called in that
interpreter’s “main” thread, where the CPython global runtime was
originally initialized. The only exception is if "exec()" will be
called immediately after.


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

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

If any thread, other than the finalization thread, attempts to attach
a *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
*thread state* C APIs have never had any error reporting or handling
expectations at *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:

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

   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.

   Changed in version 3.12: **PEP 684** introduced the possibility of
   a per-interpreter GIL. See "Py_NewInterpreterFromConfig()".

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

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

   PyInterpreterState *interp

      This thread’s interpreter state.

void PyEval_InitThreads()
    * Part of the Stable ABI.*

   Deprecated function which does nothing.

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

   Changed in version 3.9: The function now does nothing.

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

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

   Deprecated since version 3.9.

PyThreadState *PyEval_SaveThread()
    * Part of the Stable ABI.*

   Detach the *attached thread state* and return it. The thread will
   have no *thread state* upon returning.

void PyEval_RestoreThread(PyThreadState *tstate)
    * Part of the Stable ABI.*

   Set the *attached thread state* to *tstate*. The passed *thread
   state* **should not** be *attached*, otherwise deadlock ensues.
   *tstate* will be attached upon returning.

   Note:

     Calling this function from a thread when the runtime is
     finalizing will hang the thread until the program exits, even if
     the thread was not created by Python.  Refer to Cautions
     regarding runtime finalization for more details.

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

PyThreadState *PyThreadState_Get()
    * Part of the Stable ABI.*

   Return the *attached thread state*. If the thread has no attached
   thread state, (such as when inside of "Py_BEGIN_ALLOW_THREADS"
   block), then this issues a fatal error (so that the caller needn’t
   check for "NULL").

   See also "PyThreadState_GetUnchecked()".

PyThreadState *PyThreadState_GetUnchecked()

   Similar to "PyThreadState_Get()", but don’t kill the process with a
   fatal error if it is NULL. The caller is responsible to check if
   the result is NULL.

   Added in version 3.13: In Python 3.5 to 3.12, the function was
   private and known as "_PyThreadState_UncheckedGet()".

PyThreadState *PyThreadState_Swap(PyThreadState *tstate)
    * Part of the Stable ABI.*

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

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

   Note:

     Similar to "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:

PyGILState_STATE PyGILState_Ensure()
    * Part of the Stable ABI.*

   Ensure that the current thread is ready to call the Python C API
   regardless of the current state of Python, or of the *attached
   thread state*. This may be called as many times as desired by a
   thread as long as each call is matched with a call to
   "PyGILState_Release()". In general, other thread-related APIs may
   be used between "PyGILState_Ensure()" and "PyGILState_Release()"
   calls as long as the thread state is restored to its previous state
   before the Release().  For example, normal usage of the
   "Py_BEGIN_ALLOW_THREADS" and "Py_END_ALLOW_THREADS" macros is
   acceptable.

   The return value is an opaque “handle” to the *attached thread
   state* when "PyGILState_Ensure()" was called, and must be passed to
   "PyGILState_Release()" to ensure Python is left in the same state.
   Even though recursive calls are allowed, these handles *cannot* be
   shared - each unique call to "PyGILState_Ensure()" must save the
   handle for its call to "PyGILState_Release()".

   When the function returns, there will be an *attached thread state*
   and the thread will be able to call arbitrary Python code.  Failure
   is a fatal error.

   Warning:

     Calling this function when the runtime is finalizing is unsafe.
     Doing so will either hang the thread until the program ends, or
     fully crash the interpreter in rare cases. Refer to Cautions
     regarding runtime finalization for more details.

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

void PyGILState_Release(PyGILState_STATE)
    * Part of the Stable ABI.*

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

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

PyThreadState *PyGILState_GetThisThreadState()
    * Part of the Stable ABI.*

   Get the *attached thread state* for this thread.  May return "NULL"
   if no GILState API has been used on the current thread.  Note that
   the main thread always has such a thread-state, even if no auto-
   thread-state call has been made on the main thread.  This is mainly
   a helper/diagnostic function.

   Note:

     This function does not account for *thread states* created by
     something other than "PyGILState_Ensure()" (such as
     "PyThreadState_New()"). Prefer "PyThreadState_Get()" or
     "PyThreadState_GetUnchecked()" for most cases.

int PyGILState_Check()

   Return "1" if the current thread is holding the *GIL* and "0"
   otherwise. This function can be called from any thread at any time.
   Only if it has had its *thread state* initialized via
   "PyGILState_Ensure()" will it return "1". This is mainly a
   helper/diagnostic function.  It can be useful for example in
   callback contexts or memory allocation functions when knowing that
   the *GIL* is locked can allow the caller to perform sensitive
   actions or otherwise behave differently.

   Note:

     If the current Python process has ever created a subinterpreter,
     this function will *always* return "1". Prefer
     "PyThreadState_GetUnchecked()" for most cases.

   Added in version 3.4.

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

Py_BEGIN_ALLOW_THREADS
    * Part of the Stable ABI.*

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

Py_END_ALLOW_THREADS
    * Part of the Stable ABI.*

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

Py_BLOCK_THREADS
    * Part of the Stable ABI.*

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

Py_UNBLOCK_THREADS
    * Part of the Stable ABI.*

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


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

All of the following functions must be called after "Py_Initialize()".

Changed in version 3.7: "Py_Initialize()" now initializes the *GIL*
and sets an *attached thread state*.

PyInterpreterState *PyInterpreterState_New()
    * Part of the Stable ABI.*

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

   Raises an auditing event "cpython.PyInterpreterState_New" with no
   arguments.

void PyInterpreterState_Clear(PyInterpreterState *interp)
    * Part of the Stable ABI.*

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

   Raises an auditing event "cpython.PyInterpreterState_Clear" with no
   arguments.

void PyInterpreterState_Delete(PyInterpreterState *interp)
    * Part of the Stable ABI.*

   Destroy an interpreter state object.  There **should not** be an
   *attached thread state* for the target interpreter. The interpreter
   state must have been reset with a previous call to
   "PyInterpreterState_Clear()".

PyThreadState *PyThreadState_New(PyInterpreterState *interp)
    * Part of the Stable ABI.*

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

void PyThreadState_Clear(PyThreadState *tstate)
    * Part of the Stable ABI.*

   Reset all information in a *thread state* object.  *tstate* must be
   *attached*

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

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

void PyThreadState_Delete(PyThreadState *tstate)
    * Part of the Stable ABI.*

   Destroy a *thread state* object.  *tstate* should not be *attached*
   to any thread. *tstate* must have been reset with a previous call
   to "PyThreadState_Clear()".

void PyThreadState_DeleteCurrent(void)

   Detach the *attached thread state* (which must have been reset with
   a previous call to "PyThreadState_Clear()") and then destroy it.

   No *thread state* will be *attached* upon returning.

PyFrameObject *PyThreadState_GetFrame(PyThreadState *tstate)
    * Part of the Stable ABI since version 3.10.*

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

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

   See also "PyEval_GetFrame()".

   *tstate* must not be "NULL", and must be *attached*.

   Added in version 3.9.

uint64_t PyThreadState_GetID(PyThreadState *tstate)
    * Part of the Stable ABI since version 3.10.*

   Get the unique *thread state* identifier of the Python thread state
   *tstate*.

   *tstate* must not be "NULL", and must be *attached*.

   Added in version 3.9.

PyInterpreterState *PyThreadState_GetInterpreter(PyThreadState *tstate)
    * Part of the Stable ABI since version 3.10.*

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

   *tstate* must not be "NULL", and must be *attached*.

   Added in version 3.9.

void PyThreadState_EnterTracing(PyThreadState *tstate)

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

   Resume them using the "PyThreadState_LeaveTracing()" function.

   Added in version 3.11.

void PyThreadState_LeaveTracing(PyThreadState *tstate)

   Resume tracing and profiling in the Python thread state *tstate*
   suspended by the "PyThreadState_EnterTracing()" function.

   See also "PyEval_SetTrace()" and "PyEval_SetProfile()" functions.

   Added in version 3.11.

PyInterpreterState *PyInterpreterState_Get(void)
    * Part of the Stable ABI since version 3.9.*

   Get the current interpreter.

   Issue a fatal error if there no *attached thread state*. It cannot
   return NULL.

   Added in version 3.9.

int64_t PyInterpreterState_GetID(PyInterpreterState *interp)
    * Part of the Stable ABI since version 3.7.*

   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 *attached thread state*.

   Added in version 3.7.

PyObject *PyInterpreterState_GetDict(PyInterpreterState *interp)
    *Return value: Borrowed reference.** Part of the Stable ABI since
   version 3.8.*

   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 "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.

   Added in version 3.8.

typedef 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.

   Changed in version 3.9: The function now takes a *tstate*
   parameter.

   Changed in version 3.11: The *frame* parameter changed from
   "PyFrameObject*" to "_PyInterpreterFrame*".

_PyFrameEvalFunction _PyInterpreterState_GetEvalFrameFunc(PyInterpreterState *interp)

   Get the frame evaluation function.

   See the **PEP 523** “Adding a frame evaluation API to CPython”.

   Added in version 3.9.

void _PyInterpreterState_SetEvalFrameFunc(PyInterpreterState *interp, _PyFrameEvalFunction eval_frame)

   Set the frame evaluation function.

   See the **PEP 523** “Adding a frame evaluation API to CPython”.

   Added in version 3.9.

PyObject *PyThreadState_GetDict()
    *Return value: Borrowed reference.** Part of the Stable ABI.*

   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 *thread state* is *attached*. If this function returns
   "NULL", no exception has been raised and the caller should assume
   no thread state is attached.

int PyThreadState_SetAsyncExc(unsigned long id, PyObject *exc)
    * Part of the Stable ABI.*

   Asynchronously raise an exception in a thread. The *id* argument is
   the thread id of the target thread; *exc* is the exception object
   to be raised. This function does not steal any references to *exc*.
   To prevent naive misuse, you must write your own C extension to
   call this.  Must be called with an *attached thread state*. Returns
   the number of thread states modified; this is normally one, but
   will be zero if the thread id isn’t found.  If *exc* is "NULL", the
   pending exception (if any) for the thread is cleared. This raises
   no exceptions.

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

void PyEval_AcquireThread(PyThreadState *tstate)
    * Part of the Stable ABI.*

   *Attach* *tstate* to the current thread, which must not be "NULL"
   or already *attached*.

   The calling thread must not already have an *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 Cautions
     regarding runtime finalization for more details.

   Changed in version 3.8: Updated to be consistent with
   "PyEval_RestoreThread()", "Py_END_ALLOW_THREADS()", and
   "PyGILState_Ensure()", and terminate the current thread if called
   while the interpreter is finalizing.

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

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

void PyEval_ReleaseThread(PyThreadState *tstate)
    * Part of the Stable ABI.*

   Detach the *attached thread state*. The *tstate* argument, which
   must not be "NULL", is only used to check that it represents the
   *attached thread state* — if it isn’t, a fatal error is reported.

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


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
"PyInterpreterState_Main()" function returns a pointer to its state.

You can switch between sub-interpreters using the
"PyThreadState_Swap()" function. You can create and destroy them using
the following functions:

type PyInterpreterConfig

   Structure containing most parameters to configure a sub-
   interpreter. Its values are used only in
   "Py_NewInterpreterFromConfig()" and never modified by the runtime.

   Added in version 3.12.

   Structure fields:

   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 "check_multi_interp_extensions" must be "1"
      (non-zero). If this is "1" then "gil" must not be
      "PyInterpreterConfig_OWN_GIL".

   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 "subprocess" module still works when fork is
      disallowed.

   int allow_exec

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

      Note that the "subprocess" module still works when exec is
      disallowed.

   int allow_threads

      If this is "0" then the sub-interpreter’s "threading" module
      won’t create threads. Otherwise threads are allowed.

   int allow_daemon_threads

      If this is "0" then the sub-interpreter’s "threading" module
      won’t create daemon threads. Otherwise daemon threads are
      allowed (as long as "allow_threads" is non-zero).

   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 "Py_mod_multiple_interpreters".)

      This must be "1" (non-zero) if "use_main_obmalloc" is "0".

   int gil

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

      PyInterpreterConfig_DEFAULT_GIL

         Use the default selection ("PyInterpreterConfig_SHARED_GIL").

      PyInterpreterConfig_SHARED_GIL

         Use (share) the main interpreter’s GIL.

      PyInterpreterConfig_OWN_GIL

         Use the sub-interpreter’s own GIL.

      If this is "PyInterpreterConfig_OWN_GIL" then
      "PyInterpreterConfig.use_main_obmalloc" must be "0".

PyStatus Py_NewInterpreterFromConfig(PyThreadState **tstate_p, const PyInterpreterConfig *config)

   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 "builtins", "__main__"
   and "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 *thread state*
   created in the new sub-interpreter.  This thread state is
   *attached*. 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
   *attached thread state*, which might not exist.

   Like all other Python/C API functions, an *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 *attached*. If the sub-interpreter is created with its own *GIL*
   then the *attached thread state* of the calling interpreter will be
   detached. When the function returns, the new interpreter’s *thread
   state* will be *attached* to the current thread and the previous
   interpreter’s *attached thread state* will remain detached.

   Added in version 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 "PyInterpreterState" values.  A read-only copy of the
   config may be stored internally on the "PyInterpreterState".

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

   * For modules using multi-phase initialization, e.g.
     "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.
     "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 "Py_FinalizeEx()" and "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.

PyThreadState *Py_NewInterpreter(void)
    * Part of the Stable ABI.*

   Create a new sub-interpreter.  This is essentially just a wrapper
   around "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.

void Py_EndInterpreter(PyThreadState *tstate)
    * Part of the Stable ABI.*

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

   "Py_FinalizeEx()" will destroy all sub-interpreters that haven’t
   been explicitly destroyed at that point.


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

Using "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
*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.

Added in version 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  "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
"PyGILState_Ensure()" and "PyGILState_Release()" calls. Furthermore,
extensions (such as "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.

int Py_AddPendingCall(int (*func)(void*), void *arg)
    * Part of the Stable ABI.*

   Schedule a function to be called from the main interpreter thread.
   On success, "0" is returned and *func* is queued for being called
   in the main thread.  On failure, "-1" is returned without setting
   any exception.

   When successfully queued, *func* will be *eventually* called from
   the main interpreter thread with the argument *arg*.  It will be
   called asynchronously with respect to normally running Python code,
   but with both these conditions met:

   * on a *bytecode* boundary;

   * with the main thread holding an *attached thread state* (*func*
     can therefore use the full C API).

   *func* must return "0" on success, or "-1" on failure with an
   exception set.  *func* won’t be interrupted to perform another
   asynchronous notification recursively, but it can still be
   interrupted to switch threads if the *thread state* is detached.

   This function doesn’t need an *attached thread state*. However, to
   call this function in a subinterpreter, the caller must have an
   *attached thread state*. Otherwise, the function *func* can be
   scheduled to be called from the wrong interpreter.

   Warning:

     This is a low-level function, only useful for very special cases.
     There is no guarantee that *func* will be called as quick as
     possible.  If the main thread is busy executing a system call,
     *func* won’t be called before the system call returns.  This
     function is generally **not** suitable for calling Python code
     from arbitrary C threads.  Instead, use the PyGILState API.

   Added in version 3.1.

   Changed in version 3.9: If this function is called in a
   subinterpreter, the function *func* is now scheduled to be called
   from the subinterpreter, rather than being called from the main
   interpreter. Each subinterpreter now has its own list of scheduled
   calls.

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


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

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

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

typedef int (*Py_tracefunc)(PyObject *obj, PyFrameObject *frame, int what, PyObject *arg)

   The type of the trace function registered using
   "PyEval_SetProfile()" and "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 "PyTrace_CALL", "PyTrace_EXCEPTION", "PyTrace_LINE",
   "PyTrace_RETURN", "PyTrace_C_CALL", "PyTrace_C_EXCEPTION",
   "PyTrace_C_RETURN", or "PyTrace_OPCODE", and *arg* depends on the
   value of *what*:

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

int PyTrace_CALL

   The value of the *what* parameter to a "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.

int PyTrace_EXCEPTION

   The value of the *what* parameter to a "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.

int PyTrace_LINE

   The value passed as the *what* parameter to a "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
   "f_trace_lines" to *0* on that frame.

int PyTrace_RETURN

   The value for the *what* parameter to "Py_tracefunc" functions when
   a call is about to return.

int PyTrace_C_CALL

   The value for the *what* parameter to "Py_tracefunc" functions when
   a C function is about to be called.

int PyTrace_C_EXCEPTION

   The value for the *what* parameter to "Py_tracefunc" functions when
   a C function has raised an exception.

int PyTrace_C_RETURN

   The value for the *what* parameter to "Py_tracefunc" functions when
   a C function has returned.

int PyTrace_OPCODE

   The value for the *what* parameter to "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 "f_trace_opcodes" to *1* on the frame.

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 "PyTrace_LINE"
   "PyTrace_OPCODE" and "PyTrace_EXCEPTION".

   See also the "sys.setprofile()" function.

   The caller must have an *attached thread state*.

void PyEval_SetProfileAllThreads(Py_tracefunc func, PyObject *obj)

   Like "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 *attached thread state*.

   As "PyEval_SetProfile()", this function ignores any exceptions
   raised while setting the profile functions in all threads.

Added in version 3.12.

void PyEval_SetTrace(Py_tracefunc func, PyObject *obj)

   Set the tracing function to *func*.  This is similar to
   "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 "PyEval_SetTrace()" will not receive
   "PyTrace_C_CALL", "PyTrace_C_EXCEPTION" or "PyTrace_C_RETURN" as a
   value for the *what* parameter.

   See also the "sys.settrace()" function.

   The caller must have an *attached thread state*.

void PyEval_SetTraceAllThreads(Py_tracefunc func, PyObject *obj)

   Like "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 *attached thread state*.

   As "PyEval_SetTrace()", this function ignores any exceptions raised
   while setting the trace functions in all threads.

Added in version 3.12.


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

Added in version 3.13.

typedef int (*PyRefTracer)(PyObject*, int event, void *data)

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

Added in version 3.13.

int PyRefTracer_CREATE

   The value for the *event* parameter to "PyRefTracer" functions when
   a Python object has been created.

int PyRefTracer_DESTROY

   The value for the *event* parameter to "PyRefTracer" functions when
   a Python object has been destroyed.

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 *thread
   state* will be active every time the tracer function is called.

   There must be an *attached thread state* when calling this
   function.

Added in version 3.13.

PyRefTracer PyRefTracer_GetTracer(void **data)

   Get the registered reference tracer function and the value of the
   opaque data pointer that was registered when
   "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 *attached thread state* when calling this
   function.

Added in version 3.13.


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

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

PyInterpreterState *PyInterpreterState_Head()

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

PyInterpreterState *PyInterpreterState_Main()

   Return the main interpreter state object.

PyInterpreterState *PyInterpreterState_Next(PyInterpreterState *interp)

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

PyThreadState *PyInterpreterState_ThreadHead(PyInterpreterState *interp)

   Return the pointer to the first "PyThreadState" object in the list
   of threads associated with the interpreter *interp*.

PyThreadState *PyThreadState_Next(PyThreadState *tstate)

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


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

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 ("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 void* value per
thread.

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

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

Note:

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


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 "Py_tss_t"
instead of int to represent thread keys.

Added in version 3.7.

See also:

  “A New C-API for Thread-Local Storage in CPython” (**PEP 539**)

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 Py_LIMITED_API is not defined, static allocation of this type
   by "Py_tss_NEEDS_INIT" is allowed.

Py_tss_NEEDS_INIT

   This macro expands to the initializer for "Py_tss_t" variables.
   Note that this macro won’t be defined with Py_LIMITED_API.


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

Dynamic allocation of the "Py_tss_t", required in extension modules
built with Py_LIMITED_API, where static allocation of this type is not
possible due to its implementation being opaque at build time.

Py_tss_t *PyThread_tss_alloc()
    * Part of the Stable ABI since version 3.7.*

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

void PyThread_tss_free(Py_tss_t *key)
    * Part of the Stable ABI since version 3.7.*

   Free the given *key* allocated by "PyThread_tss_alloc()", after
   first calling "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 "PyThread_tss_set()" and "PyThread_tss_get()" are
undefined if the given "Py_tss_t" has not been initialized by
"PyThread_tss_create()".

int PyThread_tss_is_created(Py_tss_t *key)
    * Part of the Stable ABI since version 3.7.*

   Return a non-zero value if the given "Py_tss_t" has been
   initialized by "PyThread_tss_create()".

int PyThread_tss_create(Py_tss_t *key)
    * Part of the Stable ABI since version 3.7.*

   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 "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.

void PyThread_tss_delete(Py_tss_t *key)
    * Part of the Stable ABI since version 3.7.*

   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
   "PyThread_tss_create()". This function can be called repeatedly on
   the same key – calling it on an already destroyed key is a no-op.

int PyThread_tss_set(Py_tss_t *key, void *value)
    * Part of the Stable ABI since version 3.7.*

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

void *PyThread_tss_get(Py_tss_t *key)
    * Part of the Stable ABI since version 3.7.*

   Return the 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 (TLS) API
------------------------------

Deprecated since version 3.7: This API is superseded by Thread
Specific Storage (TSS) 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, "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.

int PyThread_create_key()
    * Part of the Stable ABI.*

void PyThread_delete_key(int key)
    * Part of the Stable ABI.*

int PyThread_set_key_value(int key, void *value)
    * Part of the Stable ABI.*

void *PyThread_get_key_value(int key)
    * Part of the Stable ABI.*

void PyThread_delete_key_value(int key)
    * Part of the Stable ABI.*

void PyThread_ReInitTLS()
    * Part of the Stable ABI.*


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

The C-API provides a basic mutual exclusion lock.

type PyMutex

   A mutual exclusion lock.  The "PyMutex" should be initialized to
   zero to represent the unlocked state.  For example:

      PyMutex mutex = {0};

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

   Note:

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

   Added in version 3.13.

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 *thread state* if
   one exists.

   Added in version 3.13.

void PyMutex_Unlock(PyMutex *m)

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

   Added in version 3.13.

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.

   Added in version 3.14.


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

The critical section API provides a deadlock avoidance layer on top of
per-object locks for *free-threaded* CPython.  They are intended to
replace reliance on the *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 "list" and "dict" because their public C-APIs already use
critical sections internally, with the notable exception of
"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 "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 "PyMutex" pointers rather than Python objects are
also available. Use these variants to start a critical section in a
situation where there is no "PyObject" – for example, when working
with a C type that does not extend or wrap "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
  "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, "Py_SETREF" calls "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 "PyEval_SaveThread()".

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 "{".

   Added in version 3.13.

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 "Py_BEGIN_CRITICAL_SECTION", there is no cast for
   the argument of the macro - it must be a "PyMutex" pointer.

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

   Added in version 3.14.

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 "}".

   Added in version 3.13.

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 "{".

   Added in version 3.13.

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 "Py_BEGIN_CRITICAL_SECTION2", there is no cast for
   the arguments of the macro - they must be "PyMutex" pointers.

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

   Added in version 3.14.

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 "}".

   Added in version 3.13.
