diff --git a/Doc/library/ctypes.rst b/Doc/library/ctypes.rst index 09f596101b4d1e..d8dac24c8ab532 100644 --- a/Doc/library/ctypes.rst +++ b/Doc/library/ctypes.rst @@ -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 + (see :attr:`_layout_`). - This is only implemented for the MSVC-compatible memory 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 pFad - Phonifier reborn

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