Merged revisions 81994 via svnmerge from
svn+ssh://pythondev@svn.python.org/python/branches/py3k ................ r81994 | mark.dickinson | 2010-06-15 09:42:37 +0100 (Tue, 15 Jun 2010) | 10 lines Merged revisions 81992 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r81992 | mark.dickinson | 2010-06-15 09:33:03 +0100 (Tue, 15 Jun 2010) | 3 lines Issue #8469: Further clarifications and improvements to struct module documentation. Thanks Mads Kiilerich. ........ ................
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@ -20,9 +20,9 @@ structs and the intended conversion to/from Python values.
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order to maintain proper alignment for the C types involved; similarly,
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alignment is taken into account when unpacking. This behavior is chosen so
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that the bytes of a packed struct correspond exactly to the layout in memory
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of the corresponding C struct. To omit pad bytes, use `standard` size and
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alignment instead of `native` size and alignment: see :ref:`struct-alignment`
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for details.
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of the corresponding C struct. To handle platform-independent data formats
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or omit implicit pad bytes, use `standard` size and alignment instead of
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`native` size and alignment: see :ref:`struct-alignment` for details.
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Functions and Exceptions
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------------------------
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@ -95,19 +95,19 @@ Alternatively, the first character of the format string can be used to indicate
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the byte order, size and alignment of the packed data, according to the
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following table:
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+-----------+------------------------+--------------------+
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| Character | Byte order | Size and alignment |
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+===========+========================+====================+
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| ``@`` | native | native |
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+-----------+------------------------+--------------------+
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| ``=`` | native | standard |
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+-----------+------------------------+--------------------+
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| ``<`` | little-endian | standard |
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+-----------+------------------------+--------------------+
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| ``>`` | big-endian | standard |
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+-----------+------------------------+--------------------+
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| ``!`` | network (= big-endian) | standard |
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+-----------+------------------------+--------------------+
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+-----------+------------------------+----------+-----------+
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| Character | Byte order | Size | Alignment |
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+===========+========================+==========+===========+
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| ``@`` | native | native | native |
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+-----------+------------------------+----------+-----------+
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| ``=`` | native | standard | none |
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+-----------+------------------------+----------+-----------+
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| ``<`` | little-endian | standard | none |
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+-----------+------------------------+----------+-----------+
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| ``>`` | big-endian | standard | none |
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+-----------+------------------------+----------+-----------+
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| ``!`` | network (= big-endian) | standard | none |
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+-----------+------------------------+----------+-----------+
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If the first character is not one of these, ``'@'`` is assumed.
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@ -120,11 +120,8 @@ endianness of your system.
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Native size and alignment are determined using the C compiler's
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``sizeof`` expression. This is always combined with native byte order.
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Standard size and alignment are as follows: no alignment is required for any
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type (so you have to use pad bytes); :ctype:`short` is 2 bytes; :ctype:`int` and
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:ctype:`long` are 4 bytes; :ctype:`long long` (:ctype:`__int64` on Windows) is 8
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bytes; :ctype:`float` and :ctype:`double` are 32-bit and 64-bit IEEE floating
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point numbers, respectively. :ctype:`_Bool` is 1 byte.
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Standard size depends only on the format character; see the table in
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the :ref:`format-characters` section.
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Note the difference between ``'@'`` and ``'='``: both use native byte order, but
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the size and alignment of the latter is standardized.
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@ -135,12 +132,6 @@ whether network byte order is big-endian or little-endian.
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There is no way to indicate non-native byte order (force byte-swapping); use the
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appropriate choice of ``'<'`` or ``'>'``.
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The ``'P'`` format character is only available for the native byte ordering
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(selected as the default or with the ``'@'`` byte order character). The byte
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order character ``'='`` chooses to use little- or big-endian ordering based on
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the host system. The struct module does not interpret this as native ordering,
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so the ``'P'`` format is not available.
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Notes:
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(1) Padding is only automatically added between successive structure members.
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@ -192,15 +183,15 @@ Python values should be obvious given their types:
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| ``Q`` | :ctype:`unsigned long | integer | 8 | \(3), \(4) |
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| | long` | | | |
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+--------+-------------------------+--------------------+----------------+------------+
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| ``f`` | :ctype:`float` | float | 4 | |
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| ``f`` | :ctype:`float` | float | 4 | \(5) |
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+--------+-------------------------+--------------------+----------------+------------+
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| ``d`` | :ctype:`double` | float | 8 | |
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| ``d`` | :ctype:`double` | float | 8 | \(5) |
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+--------+-------------------------+--------------------+----------------+------------+
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| ``s`` | :ctype:`char[]` | bytes | | \(1) |
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+--------+-------------------------+--------------------+----------------+------------+
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| ``p`` | :ctype:`char[]` | bytes | | \(1) |
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+--------+-------------------------+--------------------+----------------+------------+
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| ``P`` | :ctype:`void \*` | integer | | |
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| ``P`` | :ctype:`void \*` | integer | | \(6) |
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+--------+-------------------------+--------------------+----------------+------------+
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Notes:
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@ -220,6 +211,27 @@ Notes:
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the platform C compiler supports C :ctype:`long long`, or, on Windows,
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:ctype:`__int64`. They are always available in standard modes.
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(4)
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When attempting to pack a non-integer using any of the integer conversion
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codes, if the non-integer has a :meth:`__index__` method then that method is
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called to convert the argument to an integer before packing.
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.. versionchanged:: 3.2
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Use of the :meth:`__index__` method for non-integers is new in 3.2.
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(5)
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For the ``'f'`` and ``'d'`` conversion codes, the packed representation uses
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the IEEE 754 binary32 (for ``'f'``) or binary64 (for ``'d'``) format,
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regardless of the floating-point format used by the platform.
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(6)
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The ``'P'`` format character is only available for the native byte ordering
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(selected as the default or with the ``'@'`` byte order character). The byte
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order character ``'='`` chooses to use little- or big-endian ordering based
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on the host system. The struct module does not interpret this as native
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ordering, so the ``'P'`` format is not available.
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A format character may be preceded by an integral repeat count. For example,
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the format string ``'4h'`` means exactly the same as ``'hhhh'``.
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