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

On some platforms, and in particular macOS/arm64, the calling
convention for variadic arguments is different from the regular
calling convention. Add a section to the documentation to document
this.

1 files changed, 20 insertions, 0 deletions

diff --git a/Doc/library/ctypes.rst b/Doc/library/ctypes.rst
index e85a6cb..71e5545 100644
--- a/Doc/library/ctypes.rst
+++ b/Doc/library/ctypes.rst
@@ -373,6 +373,26 @@ 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: