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   1.. highlightlang:: c
   2
   3
   4.. _initialization:
   5
   6*****************************************
   7Initialization, Finalization, and Threads
   8*****************************************
   9
  10
  11.. cfunction:: void Py_Initialize()
  12
  13   .. index::
  14      single: Py_SetProgramName()
  15      single: PyEval_InitThreads()
  16      single: PyEval_ReleaseLock()
  17      single: PyEval_AcquireLock()
  18      single: modules (in module sys)
  19      single: path (in module sys)
  20      module: __builtin__
  21      module: __main__
  22      module: sys
  23      triple: module; search; path
  24      single: PySys_SetArgv()
  25      single: Py_Finalize()
  26
  27   Initialize the Python interpreter.  In an application embedding  Python, this
  28   should be called before using any other Python/C API functions; with the
  29   exception of :cfunc:`Py_SetProgramName`, :cfunc:`PyEval_InitThreads`,
  30   :cfunc:`PyEval_ReleaseLock`, and :cfunc:`PyEval_AcquireLock`. This initializes
  31   the table of loaded modules (``sys.modules``), and creates the fundamental
  32   modules :mod:`__builtin__`, :mod:`__main__` and :mod:`sys`.  It also initializes
  33   the module search path (``sys.path``). It does not set ``sys.argv``; use
  34   :cfunc:`PySys_SetArgv` for that.  This is a no-op when called for a second time
  35   (without calling :cfunc:`Py_Finalize` first).  There is no return value; it is a
  36   fatal error if the initialization fails.
  37
  38
  39.. cfunction:: void Py_InitializeEx(int initsigs)
  40
  41   This function works like :cfunc:`Py_Initialize` if *initsigs* is 1. If
  42   *initsigs* is 0, it skips initialization registration of signal handlers, which
  43   might be useful when Python is embedded.
  44
  45   .. versionadded:: 2.4
  46
  47
  48.. cfunction:: int Py_IsInitialized()
  49
  50   Return true (nonzero) when the Python interpreter has been initialized, false
  51   (zero) if not.  After :cfunc:`Py_Finalize` is called, this returns false until
  52   :cfunc:`Py_Initialize` is called again.
  53
  54
  55.. cfunction:: void Py_Finalize()
  56
  57   Undo all initializations made by :cfunc:`Py_Initialize` and subsequent use of
  58   Python/C API functions, and destroy all sub-interpreters (see
  59   :cfunc:`Py_NewInterpreter` below) that were created and not yet destroyed since
  60   the last call to :cfunc:`Py_Initialize`.  Ideally, this frees all memory
  61   allocated by the Python interpreter.  This is a no-op when called for a second
  62   time (without calling :cfunc:`Py_Initialize` again first).  There is no return
  63   value; errors during finalization are ignored.
  64
  65   This function is provided for a number of reasons.  An embedding application
  66   might want to restart Python without having to restart the application itself.
  67   An application that has loaded the Python interpreter from a dynamically
  68   loadable library (or DLL) might want to free all memory allocated by Python
  69   before unloading the DLL. During a hunt for memory leaks in an application a
  70   developer might want to free all memory allocated by Python before exiting from
  71   the application.
  72
  73   **Bugs and caveats:** The destruction of modules and objects in modules is done
  74   in random order; this may cause destructors (:meth:`__del__` methods) to fail
  75   when they depend on other objects (even functions) or modules.  Dynamically
  76   loaded extension modules loaded by Python are not unloaded.  Small amounts of
  77   memory allocated by the Python interpreter may not be freed (if you find a leak,
  78   please report it).  Memory tied up in circular references between objects is not
  79   freed.  Some memory allocated by extension modules may not be freed.  Some
  80   extensions may not work properly if their initialization routine is called more
  81   than once; this can happen if an application calls :cfunc:`Py_Initialize` and
  82   :cfunc:`Py_Finalize` more than once.
  83
  84
  85.. cfunction:: PyThreadState* Py_NewInterpreter()
  86
  87   .. index::
  88      module: __builtin__
  89      module: __main__
  90      module: sys
  91      single: stdout (in module sys)
  92      single: stderr (in module sys)
  93      single: stdin (in module sys)
  94
  95   Create a new sub-interpreter.  This is an (almost) totally separate environment
  96   for the execution of Python code.  In particular, the new interpreter has
  97   separate, independent versions of all imported modules, including the
  98   fundamental modules :mod:`__builtin__`, :mod:`__main__` and :mod:`sys`.  The
  99   table of loaded modules (``sys.modules``) and the module search path
 100   (``sys.path``) are also separate.  The new environment has no ``sys.argv``
 101   variable.  It has new standard I/O stream file objects ``sys.stdin``,
 102   ``sys.stdout`` and ``sys.stderr`` (however these refer to the same underlying
 103   :ctype:`FILE` structures in the C library).
 104
 105   The return value points to the first thread state created in the new
 106   sub-interpreter.  This thread state is made in the current thread state.
 107   Note that no actual thread is created; see the discussion of thread states
 108   below.  If creation of the new interpreter is unsuccessful, *NULL* is
 109   returned; no exception is set since the exception state is stored in the
 110   current thread state and there may not be a current thread state.  (Like all
 111   other Python/C API functions, the global interpreter lock must be held before
 112   calling this function and is still held when it returns; however, unlike most
 113   other Python/C API functions, there needn't be a current thread state on
 114   entry.)
 115
 116   .. index::
 117      single: Py_Finalize()
 118      single: Py_Initialize()
 119
 120   Extension modules are shared between (sub-)interpreters as follows: the first
 121   time a particular extension is imported, it is initialized normally, and a
 122   (shallow) copy of its module's dictionary is squirreled away.  When the same
 123   extension is imported by another (sub-)interpreter, a new module is initialized
 124   and filled with the contents of this copy; the extension's ``init`` function is
 125   not called.  Note that this is different from what happens when an extension is
 126   imported after the interpreter has been completely re-initialized by calling
 127   :cfunc:`Py_Finalize` and :cfunc:`Py_Initialize`; in that case, the extension's
 128   ``initmodule`` function *is* called again.
 129
 130   .. index:: single: close() (in module os)
 131
 132   **Bugs and caveats:** Because sub-interpreters (and the main interpreter) are
 133   part of the same process, the insulation between them isn't perfect --- for
 134   example, using low-level file operations like  :func:`os.close` they can
 135   (accidentally or maliciously) affect each other's open files.  Because of the
 136   way extensions are shared between (sub-)interpreters, some extensions may not
 137   work properly; this is especially likely when the extension makes use of
 138   (static) global variables, or when the extension manipulates its module's
 139   dictionary after its initialization.  It is possible to insert objects created
 140   in one sub-interpreter into a namespace of another sub-interpreter; this should
 141   be done with great care to avoid sharing user-defined functions, methods,
 142   instances or classes between sub-interpreters, since import operations executed
 143   by such objects may affect the wrong (sub-)interpreter's dictionary of loaded
 144   modules.  (XXX This is a hard-to-fix bug that will be addressed in a future
 145   release.)
 146
 147   Also note that the use of this functionality is incompatible with extension
 148   modules such as PyObjC and ctypes that use the :cfunc:`PyGILState_\*` APIs (and
 149   this is inherent in the way the :cfunc:`PyGILState_\*` functions work).  Simple
 150   things may work, but confusing behavior will always be near.
 151
 152
 153.. cfunction:: void Py_EndInterpreter(PyThreadState *tstate)
 154
 155   .. index:: single: Py_Finalize()
 156
 157   Destroy the (sub-)interpreter represented by the given thread state. The given
 158   thread state must be the current thread state.  See the discussion of thread
 159   states below.  When the call returns, the current thread state is *NULL*.  All
 160   thread states associated with this interpreter are destroyed.  (The global
 161   interpreter lock must be held before calling this function and is still held
 162   when it returns.)  :cfunc:`Py_Finalize` will destroy all sub-interpreters that
 163   haven't been explicitly destroyed at that point.
 164
 165
 166.. cfunction:: void Py_SetProgramName(char *name)
 167
 168   .. index::
 169      single: Py_Initialize()
 170      single: main()
 171      single: Py_GetPath()
 172
 173   This function should be called before :cfunc:`Py_Initialize` is called for
 174   the first time, if it is called at all.  It tells the interpreter the value
 175   of the ``argv[0]`` argument to the :cfunc:`main` function of the program.
 176   This is used by :cfunc:`Py_GetPath` and some other functions below to find
 177   the Python run-time libraries relative to the interpreter executable.  The
 178   default value is ``'python'``.  The argument should point to a
 179   zero-terminated character string in static storage whose contents will not
 180   change for the duration of the program's execution.  No code in the Python
 181   interpreter will change the contents of this storage.
 182
 183
 184.. cfunction:: char* Py_GetProgramName()
 185
 186   .. index:: single: Py_SetProgramName()
 187
 188   Return the program name set with :cfunc:`Py_SetProgramName`, or the default.
 189   The returned string points into static storage; the caller should not modify its
 190   value.
 191
 192
 193.. cfunction:: char* Py_GetPrefix()
 194
 195   Return the *prefix* for installed platform-independent files. This is derived
 196   through a number of complicated rules from the program name set with
 197   :cfunc:`Py_SetProgramName` and some environment variables; for example, if the
 198   program name is ``'/usr/local/bin/python'``, the prefix is ``'/usr/local'``. The
 199   returned string points into static storage; the caller should not modify its
 200   value.  This corresponds to the :makevar:`prefix` variable in the top-level
 201   :file:`Makefile` and the :option:`--prefix` argument to the :program:`configure`
 202   script at build time.  The value is available to Python code as ``sys.prefix``.
 203   It is only useful on Unix.  See also the next function.
 204
 205
 206.. cfunction:: char* Py_GetExecPrefix()
 207
 208   Return the *exec-prefix* for installed platform-*dependent* files.  This is
 209   derived through a number of complicated rules from the program name set with
 210   :cfunc:`Py_SetProgramName` and some environment variables; for example, if the
 211   program name is ``'/usr/local/bin/python'``, the exec-prefix is
 212   ``'/usr/local'``.  The returned string points into static storage; the caller
 213   should not modify its value.  This corresponds to the :makevar:`exec_prefix`
 214   variable in the top-level :file:`Makefile` and the :option:`--exec-prefix`
 215   argument to the :program:`configure` script at build  time.  The value is
 216   available to Python code as ``sys.exec_prefix``.  It is only useful on Unix.
 217
 218   Background: The exec-prefix differs from the prefix when platform dependent
 219   files (such as executables and shared libraries) are installed in a different
 220   directory tree.  In a typical installation, platform dependent files may be
 221   installed in the :file:`/usr/local/plat` subtree while platform independent may
 222   be installed in :file:`/usr/local`.
 223
 224   Generally speaking, a platform is a combination of hardware and software
 225   families, e.g.  Sparc machines running the Solaris 2.x operating system are
 226   considered the same platform, but Intel machines running Solaris 2.x are another
 227   platform, and Intel machines running Linux are yet another platform.  Different
 228   major revisions of the same operating system generally also form different
 229   platforms.  Non-Unix operating systems are a different story; the installation
 230   strategies on those systems are so different that the prefix and exec-prefix are
 231   meaningless, and set to the empty string. Note that compiled Python bytecode
 232   files are platform independent (but not independent from the Python version by
 233   which they were compiled!).
 234
 235   System administrators will know how to configure the :program:`mount` or
 236   :program:`automount` programs to share :file:`/usr/local` between platforms
 237   while having :file:`/usr/local/plat` be a different filesystem for each
 238   platform.
 239
 240
 241.. cfunction:: char* Py_GetProgramFullPath()
 242
 243   .. index::
 244      single: Py_SetProgramName()
 245      single: executable (in module sys)
 246
 247   Return the full program name of the Python executable; this is  computed as a
 248   side-effect of deriving the default module search path  from the program name
 249   (set by :cfunc:`Py_SetProgramName` above). The returned string points into
 250   static storage; the caller should not modify its value.  The value is available
 251   to Python code as ``sys.executable``.
 252
 253
 254.. cfunction:: char* Py_GetPath()
 255
 256   .. index::
 257      triple: module; search; path
 258      single: path (in module sys)
 259
 260   Return the default module search path; this is computed from the  program name
 261   (set by :cfunc:`Py_SetProgramName` above) and some environment variables.  The
 262   returned string consists of a series of directory names separated by a platform
 263   dependent delimiter character.  The delimiter character is ``':'`` on Unix and
 264   Mac OS X, ``';'`` on Windows.  The returned string points into static storage;
 265   the caller should not modify its value.  The value is available to Python code
 266   as the list ``sys.path``, which may be modified to change the future search path
 267   for loaded modules.
 268
 269   .. XXX should give the exact rules
 270
 271
 272.. cfunction:: const char* Py_GetVersion()
 273
 274   Return the version of this Python interpreter.  This is a string that looks
 275   something like ::
 276
 277      "1.5 (#67, Dec 31 1997, 22:34:28) [GCC 2.7.2.2]"
 278
 279   .. index:: single: version (in module sys)
 280
 281   The first word (up to the first space character) is the current Python version;
 282   the first three characters are the major and minor version separated by a
 283   period.  The returned string points into static storage; the caller should not
 284   modify its value.  The value is available to Python code as ``sys.version``.
 285
 286
 287.. cfunction:: const char* Py_GetBuildNumber()
 288
 289   Return a string representing the Subversion revision that this Python executable
 290   was built from.  This number is a string because it may contain a trailing 'M'
 291   if Python was built from a mixed revision source tree.
 292
 293   .. versionadded:: 2.5
 294
 295
 296.. cfunction:: const char* Py_GetPlatform()
 297
 298   .. index:: single: platform (in module sys)
 299
 300   Return the platform identifier for the current platform.  On Unix, this is
 301   formed from the "official" name of the operating system, converted to lower
 302   case, followed by the major revision number; e.g., for Solaris 2.x, which is
 303   also known as SunOS 5.x, the value is ``'sunos5'``.  On Mac OS X, it is
 304   ``'darwin'``.  On Windows, it is ``'win'``.  The returned string points into
 305   static storage; the caller should not modify its value.  The value is available
 306   to Python code as ``sys.platform``.
 307
 308
 309.. cfunction:: const char* Py_GetCopyright()
 310
 311   Return the official copyright string for the current Python version, for example
 312
 313   ``'Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam'``
 314
 315   .. index:: single: copyright (in module sys)
 316
 317   The returned string points into static storage; the caller should not modify its
 318   value.  The value is available to Python code as ``sys.copyright``.
 319
 320
 321.. cfunction:: const char* Py_GetCompiler()
 322
 323   Return an indication of the compiler used to build the current Python version,
 324   in square brackets, for example::
 325
 326      "[GCC 2.7.2.2]"
 327
 328   .. index:: single: version (in module sys)
 329
 330   The returned string points into static storage; the caller should not modify its
 331   value.  The value is available to Python code as part of the variable
 332   ``sys.version``.
 333
 334
 335.. cfunction:: const char* Py_GetBuildInfo()
 336
 337   Return information about the sequence number and build date and time  of the
 338   current Python interpreter instance, for example ::
 339
 340      "#67, Aug  1 1997, 22:34:28"
 341
 342   .. index:: single: version (in module sys)
 343
 344   The returned string points into static storage; the caller should not modify its
 345   value.  The value is available to Python code as part of the variable
 346   ``sys.version``.
 347
 348
 349.. cfunction:: void PySys_SetArgv(int argc, char **argv)
 350
 351   .. index::
 352      single: main()
 353      single: Py_FatalError()
 354      single: argv (in module sys)
 355
 356   Set :data:`sys.argv` based on *argc* and *argv*.  These parameters are
 357   similar to those passed to the program's :cfunc:`main` function with the
 358   difference that the first entry should refer to the script file to be
 359   executed rather than the executable hosting the Python interpreter.  If there
 360   isn't a script that will be run, the first entry in *argv* can be an empty
 361   string.  If this function fails to initialize :data:`sys.argv`, a fatal
 362   condition is signalled using :cfunc:`Py_FatalError`.
 363
 364   This function also prepends the executed script's path to :data:`sys.path`.
 365   If no script is executed (in the case of calling ``python -c`` or just the
 366   interactive interpreter), the empty string is used instead.
 367
 368   .. XXX impl. doesn't seem consistent in allowing 0/NULL for the params;
 369      check w/ Guido.
 370
 371
 372.. cfunction:: void Py_SetPythonHome(char *home)
 373
 374   Set the default "home" directory, that is, the location of the standard
 375   Python libraries.  The libraries are searched in
 376   :file:`{home}/lib/python{version}` and :file:`{home}/lib/python{version}`.
 377   The argument should point to a zero-terminated character string in static
 378   storage whose contents will not change for the duration of the program's
 379   execution.  No code in the Python interpreter will change the contents of
 380   this storage.
 381
 382
 383.. cfunction:: char* Py_GetPythonHome()
 384
 385   Return the default "home", that is, the value set by a previous call to
 386   :cfunc:`Py_SetPythonHome`, or the value of the :envvar:`PYTHONHOME`
 387   environment variable if it is set.
 388
 389
 390.. _threads:
 391
 392Thread State and the Global Interpreter Lock
 393============================================
 394
 395.. index::
 396   single: global interpreter lock
 397   single: interpreter lock
 398   single: lock, interpreter
 399
 400The Python interpreter is not fully thread safe.  In order to support
 401multi-threaded Python programs, there's a global lock, called the :dfn:`global
 402interpreter lock` or :dfn:`GIL`, that must be held by the current thread before
 403it can safely access Python objects. Without the lock, even the simplest
 404operations could cause problems in a multi-threaded program: for example, when
 405two threads simultaneously increment the reference count of the same object, the
 406reference count could end up being incremented only once instead of twice.
 407
 408.. index:: single: setcheckinterval() (in module sys)
 409
 410Therefore, the rule exists that only the thread that has acquired the global
 411interpreter lock may operate on Python objects or call Python/C API functions.
 412In order to support multi-threaded Python programs, the interpreter regularly
 413releases and reacquires the lock --- by default, every 100 bytecode instructions
 414(this can be changed with  :func:`sys.setcheckinterval`).  The lock is also
 415released and reacquired around potentially blocking I/O operations like reading
 416or writing a file, so that other threads can run while the thread that requests
 417the I/O is waiting for the I/O operation to complete.
 418
 419.. index::
 420   single: PyThreadState
 421   single: PyThreadState
 422
 423The Python interpreter needs to keep some bookkeeping information separate per
 424thread --- for this it uses a data structure called :ctype:`PyThreadState`.
 425There's one global variable, however: the pointer to the current
 426:ctype:`PyThreadState` structure.  Before the addition of :dfn:`thread-local
 427storage` (:dfn:`TLS`) the current thread state had to be manipulated
 428explicitly.
 429
 430This is easy enough in most cases.  Most code manipulating the global
 431interpreter lock has the following simple structure::
 432
 433   Save the thread state in a local variable.
 434   Release the global interpreter lock.
 435   ...Do some blocking I/O operation...
 436   Reacquire the global interpreter lock.
 437   Restore the thread state from the local variable.
 438
 439This is so common that a pair of macros exists to simplify it::
 440
 441   Py_BEGIN_ALLOW_THREADS
 442   ...Do some blocking I/O operation...
 443   Py_END_ALLOW_THREADS
 444
 445.. index::
 446   single: Py_BEGIN_ALLOW_THREADS
 447   single: Py_END_ALLOW_THREADS
 448
 449The :cmacro:`Py_BEGIN_ALLOW_THREADS` macro opens a new block and declares a
 450hidden local variable; the :cmacro:`Py_END_ALLOW_THREADS` macro closes the
 451block.  Another advantage of using these two macros is that when Python is
 452compiled without thread support, they are defined empty, thus saving the thread
 453state and GIL manipulations.
 454
 455When thread support is enabled, the block above expands to the following code::
 456
 457   PyThreadState *_save;
 458
 459   _save = PyEval_SaveThread();
 460   ...Do some blocking I/O operation...
 461   PyEval_RestoreThread(_save);
 462
 463Using even lower level primitives, we can get roughly the same effect as
 464follows::
 465
 466   PyThreadState *_save;
 467
 468   _save = PyThreadState_Swap(NULL);
 469   PyEval_ReleaseLock();
 470   ...Do some blocking I/O operation...
 471   PyEval_AcquireLock();
 472   PyThreadState_Swap(_save);
 473
 474.. index::
 475   single: PyEval_RestoreThread()
 476   single: errno
 477   single: PyEval_SaveThread()
 478   single: PyEval_ReleaseLock()
 479   single: PyEval_AcquireLock()
 480
 481There are some subtle differences; in particular, :cfunc:`PyEval_RestoreThread`
 482saves and restores the value of the  global variable :cdata:`errno`, since the
 483lock manipulation does not guarantee that :cdata:`errno` is left alone.  Also,
 484when thread support is disabled, :cfunc:`PyEval_SaveThread` and
 485:cfunc:`PyEval_RestoreThread` don't manipulate the GIL; in this case,
 486:cfunc:`PyEval_ReleaseLock` and :cfunc:`PyEval_AcquireLock` are not available.
 487This is done so that dynamically loaded extensions compiled with thread support
 488enabled can be loaded by an interpreter that was compiled with disabled thread
 489support.
 490
 491The global interpreter lock is used to protect the pointer to the current thread
 492state.  When releasing the lock and saving the thread state, the current thread
 493state pointer must be retrieved before the lock is released (since another
 494thread could immediately acquire the lock and store its own thread state in the
 495global variable). Conversely, when acquiring the lock and restoring the thread
 496state, the lock must be acquired before storing the thread state pointer.
 497
 498It is important to note that when threads are created from C, they don't have
 499the global interpreter lock, nor is there a thread state data structure for
 500them.  Such threads must bootstrap themselves into existence, by first
 501creating a thread state data structure, then acquiring the lock, and finally
 502storing their thread state pointer, before they can start using the Python/C
 503API.  When they are done, they should reset the thread state pointer, release
 504the lock, and finally free their thread state data structure.
 505
 506Beginning with version 2.3, threads can now take advantage of the
 507:cfunc:`PyGILState_\*` functions to do all of the above automatically.  The
 508typical idiom for calling into Python from a C thread is now::
 509
 510   PyGILState_STATE gstate;
 511   gstate = PyGILState_Ensure();
 512
 513   /* Perform Python actions here.  */
 514   result = CallSomeFunction();
 515   /* evaluate result */
 516
 517   /* Release the thread. No Python API allowed beyond this point. */
 518   PyGILState_Release(gstate);
 519
 520Note that the :cfunc:`PyGILState_\*` functions assume there is only one global
 521interpreter (created automatically by :cfunc:`Py_Initialize`).  Python still
 522supports the creation of additional interpreters (using
 523:cfunc:`Py_NewInterpreter`), but mixing multiple interpreters and the
 524:cfunc:`PyGILState_\*` API is unsupported.
 525
 526Another important thing to note about threads is their behaviour in the face
 527of the C :cfunc:`fork` call. On most systems with :cfunc:`fork`, after a
 528process forks only the thread that issued the fork will exist. That also
 529means any locks held by other threads will never be released. Python solves
 530this for :func:`os.fork` by acquiring the locks it uses internally before
 531the fork, and releasing them afterwards. In addition, it resets any
 532:ref:`lock-objects` in the child. When extending or embedding Python, there
 533is no way to inform Python of additional (non-Python) locks that need to be
 534acquired before or reset after a fork. OS facilities such as
 535:cfunc:`posix_atfork` would need to be used to accomplish the same thing.
 536Additionally, when extending or embedding Python, calling :cfunc:`fork`
 537directly rather than through :func:`os.fork` (and returning to or calling
 538into Python) may result in a deadlock by one of Python's internal locks
 539being held by a thread that is defunct after the fork.
 540:cfunc:`PyOS_AfterFork` tries to reset the necessary locks, but is not
 541always able to.
 542
 543.. ctype:: PyInterpreterState
 544
 545   This data structure represents the state shared by a number of cooperating
 546   threads.  Threads belonging to the same interpreter share their module
 547   administration and a few other internal items. There are no public members in
 548   this structure.
 549
 550   Threads belonging to different interpreters initially share nothing, except
 551   process state like available memory, open file descriptors and such.  The global
 552   interpreter lock is also shared by all threads, regardless of to which
 553   interpreter they belong.
 554
 555
 556.. ctype:: PyThreadState
 557
 558   This data structure represents the state of a single thread.  The only public
 559   data member is :ctype:`PyInterpreterState \*`:attr:`interp`, which points to
 560   this thread's interpreter state.
 561
 562
 563.. cfunction:: void PyEval_InitThreads()
 564
 565   .. index::
 566      single: PyEval_ReleaseLock()
 567      single: PyEval_ReleaseThread()
 568      single: PyEval_SaveThread()
 569      single: PyEval_RestoreThread()
 570
 571   Initialize and acquire the global interpreter lock.  It should be called in the
 572   main thread before creating a second thread or engaging in any other thread
 573   operations such as :cfunc:`PyEval_ReleaseLock` or
 574   ``PyEval_ReleaseThread(tstate)``. It is not needed before calling
 575   :cfunc:`PyEval_SaveThread` or :cfunc:`PyEval_RestoreThread`.
 576
 577   .. index:: single: Py_Initialize()
 578
 579   This is a no-op when called for a second time.  It is safe to call this function
 580   before calling :cfunc:`Py_Initialize`.
 581
 582   .. index:: module: thread
 583
 584   When only the main thread exists, no GIL operations are needed. This is a
 585   common situation (most Python programs do not use threads), and the lock
 586   operations slow the interpreter down a bit. Therefore, the lock is not
 587   created initially.  This situation is equivalent to having acquired the lock:
 588   when there is only a single thread, all object accesses are safe.  Therefore,
 589   when this function initializes the global interpreter lock, it also acquires
 590   it.  Before the Python :mod:`thread` module creates a new thread, knowing
 591   that either it has the lock or the lock hasn't been created yet, it calls
 592   :cfunc:`PyEval_InitThreads`.  When this call returns, it is guaranteed that
 593   the lock has been created and that the calling thread has acquired it.
 594
 595   It is **not** safe to call this function when it is unknown which thread (if
 596   any) currently has the global interpreter lock.
 597
 598   This function is not available when thread support is disabled at compile time.
 599
 600
 601.. cfunction:: int PyEval_ThreadsInitialized()
 602
 603   Returns a non-zero value if :cfunc:`PyEval_InitThreads` has been called.  This
 604   function can be called without holding the GIL, and therefore can be used to
 605   avoid calls to the locking API when running single-threaded.  This function is
 606   not available when thread support is disabled at compile time.
 607
 608   .. versionadded:: 2.4
 609
 610
 611.. cfunction:: void PyEval_AcquireLock()
 612
 613   Acquire the global interpreter lock.  The lock must have been created earlier.
 614   If this thread already has the lock, a deadlock ensues.  This function is not
 615   available when thread support is disabled at compile time.
 616
 617
 618.. cfunction:: void PyEval_ReleaseLock()
 619
 620   Release the global interpreter lock.  The lock must have been created earlier.
 621   This function is not available when thread support is disabled at compile time.
 622
 623
 624.. cfunction:: void PyEval_AcquireThread(PyThreadState *tstate)
 625
 626   Acquire the global interpreter lock and set the current thread state to
 627   *tstate*, which should not be *NULL*.  The lock must have been created earlier.
 628   If this thread already has the lock, deadlock ensues.  This function is not
 629   available when thread support is disabled at compile time.
 630
 631
 632.. cfunction:: void PyEval_ReleaseThread(PyThreadState *tstate)
 633
 634   Reset the current thread state to *NULL* and release the global interpreter
 635   lock.  The lock must have been created earlier and must be held by the current
 636   thread.  The *tstate* argument, which must not be *NULL*, is only used to check
 637   that it represents the current thread state --- if it isn't, a fatal error is
 638   reported. This function is not available when thread support is disabled at
 639   compile time.
 640
 641
 642.. cfunction:: PyThreadState* PyEval_SaveThread()
 643
 644   Release the global interpreter lock (if it has been created and thread
 645   support is enabled) and reset the thread state to *NULL*, returning the
 646   previous thread state (which is not *NULL*).  If the lock has been created,
 647   the current thread must have acquired it.  (This function is available even
 648   when thread support is disabled at compile time.)
 649
 650
 651.. cfunction:: void PyEval_RestoreThread(PyThreadState *tstate)
 652
 653   Acquire the global interpreter lock (if it has been created and thread
 654   support is enabled) and set the thread state to *tstate*, which must not be
 655   *NULL*.  If the lock has been created, the current thread must not have
 656   acquired it, otherwise deadlock ensues.  (This function is available even
 657   when thread support is disabled at compile time.)
 658
 659
 660.. cfunction:: void PyEval_ReInitThreads()
 661
 662   This function is called from :cfunc:`PyOS_AfterFork` to ensure that newly
 663   created child processes don't hold locks referring to threads which
 664   are not running in the child process.
 665
 666
 667The following macros are normally used without a trailing semicolon; look for
 668example usage in the Python source distribution.
 669
 670
 671.. cmacro:: Py_BEGIN_ALLOW_THREADS
 672
 673   This macro expands to ``{ PyThreadState *_save; _save = PyEval_SaveThread();``.
 674   Note that it contains an opening brace; it must be matched with a following
 675   :cmacro:`Py_END_ALLOW_THREADS` macro.  See above for further discussion of this
 676   macro.  It is a no-op when thread support is disabled at compile time.
 677
 678
 679.. cmacro:: Py_END_ALLOW_THREADS
 680
 681   This macro expands to ``PyEval_RestoreThread(_save); }``. Note that it contains
 682   a closing brace; it must be matched with an earlier
 683   :cmacro:`Py_BEGIN_ALLOW_THREADS` macro.  See above for further discussion of
 684   this macro.  It is a no-op when thread support is disabled at compile time.
 685
 686
 687.. cmacro:: Py_BLOCK_THREADS
 688
 689   This macro expands to ``PyEval_RestoreThread(_save);``: it is equivalent to
 690   :cmacro:`Py_END_ALLOW_THREADS` without the closing brace.  It is a no-op when
 691   thread support is disabled at compile time.
 692
 693
 694.. cmacro:: Py_UNBLOCK_THREADS
 695
 696   This macro expands to ``_save = PyEval_SaveThread();``: it is equivalent to
 697   :cmacro:`Py_BEGIN_ALLOW_THREADS` without the opening brace and variable
 698   declaration.  It is a no-op when thread support is disabled at compile time.
 699
 700All of the following functions are only available when thread support is enabled
 701at compile time, and must be called only when the global interpreter lock has
 702been created.
 703
 704
 705.. cfunction:: PyInterpreterState* PyInterpreterState_New()
 706
 707   Create a new interpreter state object.  The global interpreter lock need not
 708   be held, but may be held if it is necessary to serialize calls to this
 709   function.
 710
 711
 712.. cfunction:: void PyInterpreterState_Clear(PyInterpreterState *interp)
 713
 714   Reset all information in an interpreter state object.  The global interpreter
 715   lock must be held.
 716
 717
 718.. cfunction:: void PyInterpreterState_Delete(PyInterpreterState *interp)
 719
 720   Destroy an interpreter state object.  The global interpreter lock need not be
 721   held.  The interpreter state must have been reset with a previous call to
 722   :cfunc:`PyInterpreterState_Clear`.
 723
 724
 725.. cfunction:: PyThreadState* PyThreadState_New(PyInterpreterState *interp)
 726
 727   Create a new thread state object belonging to the given interpreter object.
 728   The global interpreter lock need not be held, but may be held if it is
 729   necessary to serialize calls to this function.
 730
 731
 732.. cfunction:: void PyThreadState_Clear(PyThreadState *tstate)
 733
 734   Reset all information in a thread state object.  The global interpreter lock
 735   must be held.
 736
 737
 738.. cfunction:: void PyThreadState_Delete(PyThreadState *tstate)
 739
 740   Destroy a thread state object.  The global interpreter lock need not be held.
 741   The thread state must have been reset with a previous call to
 742   :cfunc:`PyThreadState_Clear`.
 743
 744
 745.. cfunction:: PyThreadState* PyThreadState_Get()
 746
 747   Return the current thread state.  The global interpreter lock must be held.
 748   When the current thread state is *NULL*, this issues a fatal error (so that
 749   the caller needn't check for *NULL*).
 750
 751
 752.. cfunction:: PyThreadState* PyThreadState_Swap(PyThreadState *tstate)
 753
 754   Swap the current thread state with the thread state given by the argument
 755   *tstate*, which may be *NULL*.  The global interpreter lock must be held.
 756
 757
 758.. cfunction:: PyObject* PyThreadState_GetDict()
 759
 760   Return a dictionary in which extensions can store thread-specific state
 761   information.  Each extension should use a unique key to use to store state in
 762   the dictionary.  It is okay to call this function when no current thread state
 763   is available. If this function returns *NULL*, no exception has been raised and
 764   the caller should assume no current thread state is available.
 765
 766   .. versionchanged:: 2.3
 767      Previously this could only be called when a current thread is active, and *NULL*
 768      meant that an exception was raised.
 769
 770
 771.. cfunction:: int PyThreadState_SetAsyncExc(long id, PyObject *exc)
 772
 773   Asynchronously raise an exception in a thread. The *id* argument is the thread
 774   id of the target thread; *exc* is the exception object to be raised. This
 775   function does not steal any references to *exc*. To prevent naive misuse, you
 776   must write your own C extension to call this.  Must be called with the GIL held.
 777   Returns the number of thread states modified; this is normally one, but will be
 778   zero if the thread id isn't found.  If *exc* is :const:`NULL`, the pending
 779   exception (if any) for the thread is cleared. This raises no exceptions.
 780
 781   .. versionadded:: 2.3
 782
 783
 784.. cfunction:: PyGILState_STATE PyGILState_Ensure()
 785
 786   Ensure that the current thread is ready to call the Python C API regardless
 787   of the current state of Python, or of the global interpreter lock. This may
 788   be called as many times as desired by a thread as long as each call is
 789   matched with a call to :cfunc:`PyGILState_Release`. In general, other
 790   thread-related APIs may be used between :cfunc:`PyGILState_Ensure` and
 791   :cfunc:`PyGILState_Release` calls as long as the thread state is restored to
 792   its previous state before the Release().  For example, normal usage of the
 793   :cmacro:`Py_BEGIN_ALLOW_THREADS` and :cmacro:`Py_END_ALLOW_THREADS` macros is
 794   acceptable.
 795
 796   The return value is an opaque "handle" to the thread state when
 797   :cfunc:`PyGILState_Ensure` was called, and must be passed to
 798   :cfunc:`PyGILState_Release` to ensure Python is left in the same state. Even
 799   though recursive calls are allowed, these handles *cannot* be shared - each
 800   unique call to :cfunc:`PyGILState_Ensure` must save the handle for its call
 801   to :cfunc:`PyGILState_Release`.
 802
 803   When the function returns, the current thread will hold the GIL. Failure is a
 804   fatal error.
 805
 806   .. versionadded:: 2.3
 807
 808
 809.. cfunction:: void PyGILState_Release(PyGILState_STATE)
 810
 811   Release any resources previously acquired.  After this call, Python's state will
 812   be the same as it was prior to the corresponding :cfunc:`PyGILState_Ensure` call
 813   (but generally this state will be unknown to the caller, hence the use of the
 814   GILState API.)
 815
 816   Every call to :cfunc:`PyGILState_Ensure` must be matched by a call to
 817   :cfunc:`PyGILState_Release` on the same thread.
 818
 819   .. versionadded:: 2.3
 820
 821
 822.. _profiling:
 823
 824Profiling and Tracing
 825=====================
 826
 827.. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org>
 828
 829
 830The Python interpreter provides some low-level support for attaching profiling
 831and execution tracing facilities.  These are used for profiling, debugging, and
 832coverage analysis tools.
 833
 834Starting with Python 2.2, the implementation of this facility was substantially
 835revised, and an interface from C was added.  This C interface allows the
 836profiling or tracing code to avoid the overhead of calling through Python-level
 837callable objects, making a direct C function call instead.  The essential
 838attributes of the facility have not changed; the interface allows trace
 839functions to be installed per-thread, and the basic events reported to the trace
 840function are the same as had been reported to the Python-level trace functions
 841in previous versions.
 842
 843
 844.. ctype:: int (*Py_tracefunc)(PyObject *obj, PyFrameObject *frame, int what, PyObject *arg)
 845
 846   The type of the trace function registered using :cfunc:`PyEval_SetProfile` and
 847   :cfunc:`PyEval_SetTrace`. The first parameter is the object passed to the
 848   registration function as *obj*, *frame* is the frame object to which the event
 849   pertains, *what* is one of the constants :const:`PyTrace_CALL`,
 850   :const:`PyTrace_EXCEPTION`, :const:`PyTrace_LINE`, :const:`PyTrace_RETURN`,
 851   :const:`PyTrace_C_CALL`, :const:`PyTrace_C_EXCEPTION`, or
 852   :const:`PyTrace_C_RETURN`, and *arg* depends on the value of *what*:
 853
 854   +------------------------------+--------------------------------------+
 855   | Value of *what*              | Meaning of *arg*                     |
 856   +==============================+======================================+
 857   | :const:`PyTrace_CALL`        | Always *NULL*.                       |
 858   +------------------------------+--------------------------------------+
 859   | :const:`PyTrace_EXCEPTION`   | Exception information as returned by |
 860   |                              | :func:`sys.exc_info`.                |
 861   +------------------------------+--------------------------------------+
 862   | :const:`PyTrace_LINE`        | Always *NULL*.                       |
 863   +------------------------------+--------------------------------------+
 864   | :const:`PyTrace_RETURN`      | Value being returned to the caller.  |
 865   +------------------------------+--------------------------------------+
 866   | :const:`PyTrace_C_CALL`      | Name of function being called.       |
 867   +------------------------------+--------------------------------------+
 868   | :const:`PyTrace_C_EXCEPTION` | Always *NULL*.                       |
 869   +------------------------------+--------------------------------------+
 870   | :const:`PyTrace_C_RETURN`    | Always *NULL*.                       |
 871   +------------------------------+--------------------------------------+
 872
 873
 874.. cvar:: int PyTrace_CALL
 875
 876   The value of the *what* parameter to a :ctype:`Py_tracefunc` function when a new
 877   call to a function or method is being reported, or a new entry into a generator.
 878   Note that the creation of the iterator for a generator function is not reported
 879   as there is no control transfer to the Python bytecode in the corresponding
 880   frame.
 881
 882
 883.. cvar:: int PyTrace_EXCEPTION
 884
 885   The value of the *what* parameter to a :ctype:`Py_tracefunc` function when an
 886   exception has been raised.  The callback function is called with this value for
 887   *what* when after any bytecode is processed after which the exception becomes
 888   set within the frame being executed.  The effect of this is that as exception
 889   propagation causes the Python stack to unwind, the callback is called upon
 890   return to each frame as the exception propagates.  Only trace functions receives
 891   these events; they are not needed by the profiler.
 892
 893
 894.. cvar:: int PyTrace_LINE
 895
 896   The value passed as the *what* parameter to a trace function (but not a
 897   profiling function) when a line-number event is being reported.
 898
 899
 900.. cvar:: int PyTrace_RETURN
 901
 902   The value for the *what* parameter to :ctype:`Py_tracefunc` functions when a
 903   call is returning without propagating an exception.
 904
 905
 906.. cvar:: int PyTrace_C_CALL
 907
 908   The value for the *what* parameter to :ctype:`Py_tracefunc` functions when a C
 909   function is about to be called.
 910
 911
 912.. cvar:: int PyTrace_C_EXCEPTION
 913
 914   The value for the *what* parameter to :ctype:`Py_tracefunc` functions when a C
 915   function has thrown an exception.
 916
 917
 918.. cvar:: int PyTrace_C_RETURN
 919
 920   The value for the *what* parameter to :ctype:`Py_tracefunc` functions when a C
 921   function has returned.
 922
 923
 924.. cfunction:: void PyEval_SetProfile(Py_tracefunc func, PyObject *obj)
 925
 926   Set the profiler function to *func*.  The *obj* parameter is passed to the
 927   function as its first parameter, and may be any Python object, or *NULL*.  If
 928   the profile function needs to maintain state, using a different value for *obj*
 929   for each thread provides a convenient and thread-safe place to store it.  The
 930   profile function is called for all monitored events except the line-number
 931   events.
 932
 933
 934.. cfunction:: void PyEval_SetTrace(Py_tracefunc func, PyObject *obj)
 935
 936   Set the tracing function to *func*.  This is similar to
 937   :cfunc:`PyEval_SetProfile`, except the tracing function does receive line-number
 938   events.
 939
 940.. cfunction:: PyObject* PyEval_GetCallStats(PyObject *self)
 941
 942   Return a tuple of function call counts.  There are constants defined for the
 943   positions within the tuple:
 944
 945   +-------------------------------+-------+
 946   | Name                          | Value |
 947   +===============================+=======+
 948   | :const:`PCALL_ALL`            | 0     |
 949   +-------------------------------+-------+
 950   | :const:`PCALL_FUNCTION`       | 1     |
 951   +-------------------------------+-------+
 952   | :const:`PCALL_FAST_FUNCTION`  | 2     |
 953   +-------------------------------+-------+
 954   | :const:`PCALL_FASTER_FUNCTION`| 3     |
 955   +-------------------------------+-------+
 956   | :const:`PCALL_METHOD`         | 4     |
 957   +-------------------------------+-------+
 958   | :const:`PCALL_BOUND_METHOD`   | 5     |
 959   +-------------------------------+-------+
 960   | :const:`PCALL_CFUNCTION`      | 6     |
 961   +-------------------------------+-------+
 962   | :const:`PCALL_TYPE`           | 7     |
 963   +-------------------------------+-------+
 964   | :const:`PCALL_GENERATOR`      | 8     |
 965   +-------------------------------+-------+
 966   | :const:`PCALL_OTHER`          | 9     |
 967   +-------------------------------+-------+
 968   | :const:`PCALL_POP`            | 10    |
 969   +-------------------------------+-------+
 970
 971   :const:`PCALL_FAST_FUNCTION` means no argument tuple needs to be created.
 972   :const:`PCALL_FASTER_FUNCTION` means that the fast-path frame setup code is used.
 973
 974   If there is a method call where the call can be optimized by changing
 975   the argument tuple and calling the function directly, it gets recorded
 976   twice.
 977
 978   This function is only present if Python is compiled with :const:`CALL_PROFILE`
 979   defined.
 980
 981.. _advanced-debugging:
 982
 983Advanced Debugger Support
 984=========================
 985
 986.. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org>
 987
 988
 989These functions are only intended to be used by advanced debugging tools.
 990
 991
 992.. cfunction:: PyInterpreterState* PyInterpreterState_Head()
 993
 994   Return the interpreter state object at the head of the list of all such objects.
 995
 996   .. versionadded:: 2.2
 997
 998
 999.. cfunction:: PyInterpreterState* PyInterpreterState_Next(PyInterpreterState *interp)
1000
1001   Return the next interpreter state object after *interp* from the list of all
1002   such objects.
1003
1004   .. versionadded:: 2.2
1005
1006
1007.. cfunction:: PyThreadState * PyInterpreterState_ThreadHead(PyInterpreterState *interp)
1008
1009   Return the a pointer to the first :ctype:`PyThreadState` object in the list of
1010   threads associated with the interpreter *interp*.
1011
1012   .. versionadded:: 2.2
1013
1014
1015.. cfunction:: PyThreadState* PyThreadState_Next(PyThreadState *tstate)
1016
1017   Return the next thread state object after *tstate* from the list of all such
1018   objects belonging to the same :ctype:`PyInterpreterState` object.
1019
1020   .. versionadded:: 2.2
1021