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      1 .. highlightlang:: c
      2 
      3 .. _stable:
      4 
      5 ***********************************
      6 Stable Application Binary Interface
      7 ***********************************
      8 
      9 Traditionally, the C API of Python will change with every release.  Most changes
     10 will be source-compatible, typically by only adding API, rather than changing
     11 existing API or removing API (although some interfaces do get removed after
     12 being deprecated first).
     13 
     14 Unfortunately, the API compatibility does not extend to binary compatibility
     15 (the ABI). The reason is primarily the evolution of struct definitions, where
     16 addition of a new field, or changing the type of a field, might not break the
     17 API, but can break the ABI.  As a consequence, extension modules need to be
     18 recompiled for every Python release (although an exception is possible on Unix
     19 when none of the affected interfaces are used). In addition, on Windows,
     20 extension modules link with a specific pythonXY.dll and need to be recompiled to
     21 link with a newer one.
     22 
     23 Since Python 3.2, a subset of the API has been declared to guarantee a stable
     24 ABI. Extension modules wishing to use this API (called "limited API") need to
     25 define ``Py_LIMITED_API``. A number of interpreter details then become hidden
     26 from the extension module; in return, a module is built that works on any 3.x
     27 version (x>=2) without recompilation.
     28 
     29 In some cases, the stable ABI needs to be extended with new functions.
     30 Extension modules wishing to use these new APIs need to set ``Py_LIMITED_API``
     31 to the ``PY_VERSION_HEX`` value (see :ref:`apiabiversion`) of the minimum Python
     32 version they want to support (e.g. ``0x03030000`` for Python 3.3). Such modules
     33 will work on all subsequent Python releases, but fail to load (because of
     34 missing symbols) on the older releases.
     35 
     36 As of Python 3.2, the set of functions available to the limited API is
     37 documented in :pep:`384`.  In the C API documentation, API elements that are not
     38 part of the limited API are marked as "Not part of the limited API."
     39