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[3.11] GH-92892: Add section about variadic functions to ctypes documentation (GH-99529) #99682

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20 changes: 20 additions & 0 deletions Doc/library/ctypes.rst
Original file line number Diff line number Diff line change
Expand Up @@ -370,6 +370,26 @@ that they can be converted to the required C data type::
31
>>>

.. _ctypes-calling-variadic-functions:

Calling varadic functions
^^^^^^^^^^^^^^^^^^^^^^^^^

On a lot of platforms calling variadic functions through ctypes is exactly the same
as calling functions with a fixed number of parameters. On some platforms, and in
particular ARM64 for Apple Platforms, the calling convention for variadic functions
is different than that for regular functions.

On those platforms it is required to specify the *argtypes* attribute for the
regular, non-variadic, function arguments:

.. code-block:: python3

libc.printf.argtypes = [ctypes.c_char_p]

Because specifying the attribute does inhibit portability it is adviced to always
specify ``argtypes`` for all variadic functions.


.. _ctypes-calling-functions-with-own-custom-data-types:

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@@ -0,0 +1 @@
Document that calling variadic functions with ctypes requires special care on macOS/arm64 (and possibly other platforms).