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matthiasgoergensgpsheadencukou
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gh-97588: Align ctypes struct layout to GCC/MSVC (GH-97702)
Structure layout, and especially bitfields, sometimes resulted in clearly wrong behaviour like overlapping fields. This fixes Co-authored-by: Gregory P. Smith <[email protected]> Co-authored-by: Petr Viktorin <[email protected]>
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‎Doc/library/ctypes.rst‎

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@@ -661,14 +661,18 @@ for debugging because they can provide useful information::
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guaranteed by the library to work in the general case. Unions and
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structures with bit-fields should always be passed to functions by pointer.
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Structure/union alignment and byte order
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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By default, Structure and Union fields are aligned in the same way the C
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compiler does it. It is possible to override this behavior by specifying a
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:attr:`~Structure._pack_` class attribute in the subclass definition.
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This must be set to a positive integer and specifies the maximum alignment for the fields.
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This is what ``#pragma pack(n)`` also does in MSVC.
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Structure/union layout, alignment and byte order
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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By default, Structure and Union fields are laid out in the same way the C
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compiler does it. It is possible to override this behavior entirely by specifying a
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:attr:`~Structure._layout_` class attribute in the subclass definition; see
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the attribute documentation for details.
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It is possible to specify the maximum alignment for the fields by setting
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the :attr:`~Structure._pack_` class attribute to a positive integer.
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This matches what ``#pragma pack(n)`` does in MSVC.
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It is also possible to set a minimum alignment for how the subclass itself is packed in the
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same way ``#pragma align(n)`` works in MSVC.
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This can be achieved by specifying a ::attr:`~Structure._align_` class attribute
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the structure when being packed or unpacked to/from memory.
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Setting this attribute to 0 is the same as not setting it at all.
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.. attribute:: _layout_
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An optional string naming the struct/union layout. It can currently
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be set to:
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- ``"ms"``: the layout used by the Microsoft compiler (MSVC).
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On GCC and Clang, this layout can be selected with
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``__attribute__((ms_struct))``.
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- ``"gcc-sysv"``: the layout used by GCC with the System V or “SysV-like”
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data model, as used on Linux and macOS.
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With this layout, :attr:`~Structure._pack_` must be unset or zero.
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If not set explicitly, ``ctypes`` will use a default that
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matches the platform conventions. This default may change in future
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Python releases (for example, when a new platform gains official support,
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or when a difference between similar platforms is found).
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Currently the default will be:
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- On Windows: ``"ms"``
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- When :attr:`~Structure._pack_` is specified: ``"ms"``
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- Otherwise: ``"gcc-sysv"``
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:attr:`!_layout_` must already be defined when
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:attr:`~Structure._fields_` is assigned, otherwise it will have no effect.
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.. attribute:: _anonymous_
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An optional sequence that lists the names of unnamed (anonymous) fields.

‎Doc/whatsnew/3.13.rst‎

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any user classes which define the :meth:`!__replace__` method.
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(Contributed by Serhiy Storchaka in :gh:`108751`.)
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ctypes
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------
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* The layout of :ref:`bit fields <ctypes-bit-fields-in-structures-unions>` in
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:class:`~ctypes.Structure` and :class:`~ctypes.Union` was improved to better
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match platform defaults (GCC/Clang or MSC). In particular, fields no longer
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overlap.
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(Contributed by Matthias Görgens in :gh:`97702`.)
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* A :attr:`ctypes.Structure._layout_` class attribute can be set
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to help match a non-default ABI.
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(Contributed by Petr Viktorin in :gh:`97702`.)
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dbm
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---
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‎Include/internal/pycore_global_objects_fini_generated.h‎

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‎Include/internal/pycore_global_strings.h‎

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STRUCT_FOR_ID(_initializing)
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STRUCT_FOR_ID(_io)
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STRUCT_FOR_ID(_is_text_encoding)
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STRUCT_FOR_ID(_layout_)
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STRUCT_FOR_ID(_length_)
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STRUCT_FOR_ID(_limbo)
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STRUCT_FOR_ID(_lock_unlock_module)

‎Include/internal/pycore_runtime_init_generated.h‎

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‎Include/internal/pycore_unicodeobject_generated.h‎

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