From cb532f13e3049d7ae76b7ea0bfe4fb4af0aebe7b Mon Sep 17 00:00:00 2001 From: Mark Dickinson Date: Tue, 15 Jun 2010 08:42:37 +0000 Subject: [PATCH] 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. ........ --- Doc/library/struct.rst | 63 ++++++++++++++++++++++-------------------- 1 file changed, 33 insertions(+), 30 deletions(-) diff --git a/Doc/library/struct.rst b/Doc/library/struct.rst index cc153b218ac..9ba3ce40d41 100644 --- a/Doc/library/struct.rst +++ b/Doc/library/struct.rst @@ -20,9 +20,9 @@ structs and the intended conversion to/from Python values. order to maintain proper alignment for the C types involved; similarly, alignment is taken into account when unpacking. This behavior is chosen so that the bytes of a packed struct correspond exactly to the layout in memory - of the corresponding C struct. To omit pad bytes, use `standard` size and - alignment instead of `native` size and alignment: see :ref:`struct-alignment` - for details. + of the corresponding C struct. To handle platform-independent data formats + or omit implicit pad bytes, use `standard` size and alignment instead of + `native` size and alignment: see :ref:`struct-alignment` for details. Functions and Exceptions ------------------------ @@ -95,19 +95,19 @@ Alternatively, the first character of the format string can be used to indicate the byte order, size and alignment of the packed data, according to the following table: -+-----------+------------------------+--------------------+ -| Character | Byte order | Size and alignment | -+===========+========================+====================+ -| ``@`` | native | native | -+-----------+------------------------+--------------------+ -| ``=`` | native | standard | -+-----------+------------------------+--------------------+ -| ``<`` | little-endian | standard | -+-----------+------------------------+--------------------+ -| ``>`` | big-endian | standard | -+-----------+------------------------+--------------------+ -| ``!`` | network (= big-endian) | standard | -+-----------+------------------------+--------------------+ ++-----------+------------------------+----------+-----------+ +| Character | Byte order | Size | Alignment | ++===========+========================+==========+===========+ +| ``@`` | native | native | native | ++-----------+------------------------+----------+-----------+ +| ``=`` | native | standard | none | ++-----------+------------------------+----------+-----------+ +| ``<`` | little-endian | standard | none | ++-----------+------------------------+----------+-----------+ +| ``>`` | big-endian | standard | none | ++-----------+------------------------+----------+-----------+ +| ``!`` | network (= big-endian) | standard | none | ++-----------+------------------------+----------+-----------+ If the first character is not one of these, ``'@'`` is assumed. @@ -120,11 +120,8 @@ endianness of your system. Native size and alignment are determined using the C compiler's ``sizeof`` expression. This is always combined with native byte order. -Standard size and alignment are as follows: no alignment is required for any -type (so you have to use pad bytes); :ctype:`short` is 2 bytes; :ctype:`int` and -:ctype:`long` are 4 bytes; :ctype:`long long` (:ctype:`__int64` on Windows) is 8 -bytes; :ctype:`float` and :ctype:`double` are 32-bit and 64-bit IEEE floating -point numbers, respectively. :ctype:`_Bool` is 1 byte. +Standard size depends only on the format character; see the table in +the :ref:`format-characters` section. Note the difference between ``'@'`` and ``'='``: both use native byte order, but the size and alignment of the latter is standardized. @@ -135,12 +132,6 @@ whether network byte order is big-endian or little-endian. There is no way to indicate non-native byte order (force byte-swapping); use the appropriate choice of ``'<'`` or ``'>'``. -The ``'P'`` format character is only available for the native byte ordering -(selected as the default or with the ``'@'`` byte order character). The byte -order character ``'='`` chooses to use little- or big-endian ordering based on -the host system. The struct module does not interpret this as native ordering, -so the ``'P'`` format is not available. - Notes: (1) Padding is only automatically added between successive structure members. @@ -192,15 +183,15 @@ Python values should be obvious given their types: | ``Q`` | :ctype:`unsigned long | integer | 8 | \(3), \(4) | | | long` | | | | +--------+-------------------------+--------------------+----------------+------------+ -| ``f`` | :ctype:`float` | float | 4 | | +| ``f`` | :ctype:`float` | float | 4 | \(5) | +--------+-------------------------+--------------------+----------------+------------+ -| ``d`` | :ctype:`double` | float | 8 | | +| ``d`` | :ctype:`double` | float | 8 | \(5) | +--------+-------------------------+--------------------+----------------+------------+ | ``s`` | :ctype:`char[]` | bytes | | \(1) | +--------+-------------------------+--------------------+----------------+------------+ | ``p`` | :ctype:`char[]` | bytes | | \(1) | +--------+-------------------------+--------------------+----------------+------------+ -| ``P`` | :ctype:`void \*` | integer | | | +| ``P`` | :ctype:`void \*` | integer | | \(6) | +--------+-------------------------+--------------------+----------------+------------+ Notes: @@ -228,6 +219,18 @@ Notes: .. versionchanged:: 3.2 Use of the :meth:`__index__` method for non-integers is new in 3.2. +(5) + For the ``'f'`` and ``'d'`` conversion codes, the packed representation uses + the IEEE 754 binary32 (for ``'f'``) or binary64 (for ``'d'``) format, + regardless of the floating-point format used by the platform. + +(6) + The ``'P'`` format character is only available for the native byte ordering + (selected as the default or with the ``'@'`` byte order character). The byte + order character ``'='`` chooses to use little- or big-endian ordering based + on the host system. The struct module does not interpret this as native + ordering, so the ``'P'`` format is not available. + A format character may be preceded by an integral repeat count. For example, the format string ``'4h'`` means exactly the same as ``'hhhh'``.