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gh-136520: Clarify docs for _pack_ & _align_ (GH-137036)

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Move docs to the reference section & reduce the “tutorial” part to a quick intro & link.

Clarify what values are accepted.

Add macro/attribute equivalents.

Discourage _align_ values that aren't powers of two.
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Petr Viktorin 2025-08-04 15:40:42 +02:00 • committed by GitHub
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Doc/library/ctypes.rst
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@ -700,14 +700,10 @@ compiler does it. It is possible to override this behavior entirely by specifyi
:attr:`~Structure._layout_` class attribute in the subclass definition; see
the attribute documentation for details.
It is possible to specify the maximum alignment for the fields by setting
the :attr:`~Structure._pack_` class attribute to a positive integer.
This matches what ``#pragma pack(n)`` does in MSVC.
It is also possible to set a minimum alignment for how the subclass itself is packed in the
same way ``#pragma align(n)`` works in MSVC.
This can be achieved by specifying a :attr:`~Structure._align_` class attribute
in the subclass definition.
It is possible to specify the maximum alignment for the fields and/or for the
structure itself by setting the class attributes :attr:`~Structure._pack_`
and/or :attr:`~Structure._align_`, respectively.
See the attribute documentation for details.
:mod:`ctypes` uses the native byte order for Structures and Unions. To build
structures with non-native byte order, you can use one of the
@ -2792,11 +2788,18 @@ fields, or any other data types containing pointer type fields.
.. attribute:: _pack_
An optional small integer that allows overriding the alignment of
structure fields in the instance. :attr:`_pack_` must already be defined
when :attr:`_fields_` is assigned, otherwise it will have no effect.
Setting this attribute to 0 is the same as not setting it at all.
structure fields in the instance.
This is only implemented for the MSVC-compatible memory layout.
This is only implemented for the MSVC-compatible memory layout
(see :attr:`_layout_`).
Setting :attr:`!_pack_` to 0 is the same as not setting it at all.
Otherwise, the value must be a positive power of two.
The effect is equivalent to ``#pragma pack(N)`` in C, except
:mod:`ctypes` may allow larger *n* than what the compiler accepts.
:attr:`!_pack_` must already be defined
when :attr:`_fields_` is assigned, otherwise it will have no effect.
.. deprecated-removed:: 3.14 3.19
@ -2809,9 +2812,22 @@ fields, or any other data types containing pointer type fields.
.. attribute:: _align_
An optional small integer that allows overriding the alignment of
An optional small integer that allows increasing the alignment of
the structure when being packed or unpacked to/from memory.
Setting this attribute to 0 is the same as not setting it at all.
The value must not be negative.
The effect is equivalent to ``__attribute__((aligned(N)))`` on GCC
or ``#pragma align(N)`` on MSVC, except :mod:`ctypes` may allow
values that the compiler would reject.
:attr:`!_align_` can only *increase* a structure's alignment
requirements. Setting it to 0 or 1 has no effect.
Using values that are not powers of two is discouraged and may lead to
surprising behavior.
:attr:`!_align_` must already be defined
when :attr:`_fields_` is assigned, otherwise it will have no effect.
.. versionadded:: 3.13