[Python-checkins] r80013 - python/trunk/Doc/library/struct.rst
mark.dickinson
python-checkins at python.org
Mon Apr 12 21:25:32 CEST 2010
Author: mark.dickinson
Date: Mon Apr 12 21:25:32 2010
New Revision: 80013
Log:
Issue #7355: Various improvements to struct module documentation.
- note early on that the result of struct.pack includes padding
bytes by default
- add examples showing how order of struct fields can affect size
(due to padding)
- better headers and references; introduction to format strings
- integrate packing notes into table
Many thanks to Meador Inge for the patch.
Modified:
python/trunk/Doc/library/struct.rst
Modified: python/trunk/Doc/library/struct.rst
==============================================================================
--- python/trunk/Doc/library/struct.rst (original)
+++ python/trunk/Doc/library/struct.rst Mon Apr 12 21:25:32 2010
@@ -10,10 +10,18 @@
triple: packing; binary; data
This module performs conversions between Python values and C structs represented
-as Python strings. It uses :dfn:`format strings` (explained below) as compact
-descriptions of the lay-out of the C structs and the intended conversion to/from
-Python values. This can be used in handling binary data stored in files or from
-network connections, among other sources.
+as Python strings. This can be used in handling binary data stored in files or from network connections, among other sources. It uses
+:ref:`struct-format-strings` as compact descriptions of the layout of the C
+structs and the intended conversion to/from Python values.
+
+.. note::
+
+ The string representation of a given C struct includes padding where
+ necessary by default. This is the same behavior as provided by most
+ C compilers. The padding may be disabled if desired.
+
+Functions and Exceptions
+------------------------
The module defines the following exception and functions:
@@ -63,49 +71,62 @@
Return the size of the struct (and hence of the string) corresponding to the
given format.
+.. _struct-format-strings:
+
+Format Strings
+--------------
+
+Format strings are the mechanism used to specify the expected layout when
+packing and unpacking data. They are built up from format characters, which
+specify the type of data being packed/unpacked. In addition, there are
+special characters for controlling the byte order, size, and alignment.
+
+Format Characters
+^^^^^^^^^^^^^^^^^
+
Format characters have the following meaning; the conversion between C and
Python values should be obvious given their types:
-+--------+-------------------------+--------------------+-------+
-| Format | C Type | Python | Notes |
-+========+=========================+====================+=======+
-| ``x`` | pad byte | no value | |
-+--------+-------------------------+--------------------+-------+
-| ``c`` | :ctype:`char` | string of length 1 | |
-+--------+-------------------------+--------------------+-------+
-| ``b`` | :ctype:`signed char` | integer | |
-+--------+-------------------------+--------------------+-------+
-| ``B`` | :ctype:`unsigned char` | integer | |
-+--------+-------------------------+--------------------+-------+
-| ``?`` | :ctype:`_Bool` | bool | \(1) |
-+--------+-------------------------+--------------------+-------+
-| ``h`` | :ctype:`short` | integer | |
-+--------+-------------------------+--------------------+-------+
-| ``H`` | :ctype:`unsigned short` | integer | |
-+--------+-------------------------+--------------------+-------+
-| ``i`` | :ctype:`int` | integer | |
-+--------+-------------------------+--------------------+-------+
-| ``I`` | :ctype:`unsigned int` | integer or long | |
-+--------+-------------------------+--------------------+-------+
-| ``l`` | :ctype:`long` | integer | |
-+--------+-------------------------+--------------------+-------+
-| ``L`` | :ctype:`unsigned long` | long | |
-+--------+-------------------------+--------------------+-------+
-| ``q`` | :ctype:`long long` | long | \(2) |
-+--------+-------------------------+--------------------+-------+
-| ``Q`` | :ctype:`unsigned long | long | \(2) |
-| | long` | | |
-+--------+-------------------------+--------------------+-------+
-| ``f`` | :ctype:`float` | float | |
-+--------+-------------------------+--------------------+-------+
-| ``d`` | :ctype:`double` | float | |
-+--------+-------------------------+--------------------+-------+
-| ``s`` | :ctype:`char[]` | string | |
-+--------+-------------------------+--------------------+-------+
-| ``p`` | :ctype:`char[]` | string | |
-+--------+-------------------------+--------------------+-------+
-| ``P`` | :ctype:`void \*` | long | |
-+--------+-------------------------+--------------------+-------+
++--------+-------------------------+--------------------+------------+
+| Format | C Type | Python | Notes |
++========+=========================+====================+============+
+| ``x`` | pad byte | no value | |
++--------+-------------------------+--------------------+------------+
+| ``c`` | :ctype:`char` | string of length 1 | |
++--------+-------------------------+--------------------+------------+
+| ``b`` | :ctype:`signed char` | integer | \(3) |
++--------+-------------------------+--------------------+------------+
+| ``B`` | :ctype:`unsigned char` | integer | \(3) |
++--------+-------------------------+--------------------+------------+
+| ``?`` | :ctype:`_Bool` | bool | \(1) |
++--------+-------------------------+--------------------+------------+
+| ``h`` | :ctype:`short` | integer | \(3) |
++--------+-------------------------+--------------------+------------+
+| ``H`` | :ctype:`unsigned short` | integer | \(3) |
++--------+-------------------------+--------------------+------------+
+| ``i`` | :ctype:`int` | integer | \(3) |
++--------+-------------------------+--------------------+------------+
+| ``I`` | :ctype:`unsigned int` | integer or long | \(3) |
++--------+-------------------------+--------------------+------------+
+| ``l`` | :ctype:`long` | integer | \(3) |
++--------+-------------------------+--------------------+------------+
+| ``L`` | :ctype:`unsigned long` | long | \(3) |
++--------+-------------------------+--------------------+------------+
+| ``q`` | :ctype:`long long` | long | \(2),\(3) |
++--------+-------------------------+--------------------+------------+
+| ``Q`` | :ctype:`unsigned long | long | \(2),\(3) |
+| | long` | | |
++--------+-------------------------+--------------------+------------+
+| ``f`` | :ctype:`float` | float | |
++--------+-------------------------+--------------------+------------+
+| ``d`` | :ctype:`double` | float | |
++--------+-------------------------+--------------------+------------+
+| ``s`` | :ctype:`char[]` | string | |
++--------+-------------------------+--------------------+------------+
+| ``p`` | :ctype:`char[]` | string | |
++--------+-------------------------+--------------------+------------+
+| ``P`` | :ctype:`void \*` | long | \(3) |
++--------+-------------------------+--------------------+------------+
Notes:
@@ -180,6 +201,9 @@
Either 0 or 1 in the native or standard bool representation will be packed, and
any non-zero value will be True when unpacking.
+Byte Order, Size, and Alignment
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
By default, C numbers are represented in the machine's native format and byte
order, and properly aligned by skipping pad bytes if necessary (according to the
rules used by the C compiler).
@@ -234,8 +258,29 @@
the host system. The struct module does not interpret this as native ordering,
so the ``'P'`` format is not available.
-Examples (all using native byte order, size and alignment, on a big-endian
-machine)::
+Notes:
+
+(1) Padding is only automatically added between successive structure members.
+ Never at the beginning of the string encoding and never at the end.
+
+(2) Padding is disabled when using non-native size and alignment, e.g.
+ with '<', '>', '=', and '!'.
+
+(3) To align the end of a structure to the alignment requirement of a
+ particular type, end the format with the code for that type with a repeat
+ count of zero. See :ref:`struct-examples`.
+
+
+.. _struct-examples:
+
+Examples
+^^^^^^^^
+
+.. note::
+ All examples assume a native byte order, size, and alignment with a
+ big-endian machine.
+
+A basic example of packing/unpacking three integers::
>>> from struct import *
>>> pack('hhl', 1, 2, 3)
@@ -245,13 +290,6 @@
>>> calcsize('hhl')
8
-Hint: to align the end of a structure to the alignment requirement of a
-particular type, end the format with the code for that type with a repeat count
-of zero. For example, the format ``'llh0l'`` specifies two pad bytes at the
-end, assuming longs are aligned on 4-byte boundaries. This only works when
-native size and alignment are in effect; standard size and alignment does not
-enforce any alignment.
-
Unpacked fields can be named by assigning them to variables or by wrapping
the result in a named tuple::
@@ -263,6 +301,27 @@
>>> Student._make(unpack('<10sHHb', s))
Student(name='raymond ', serialnum=4658, school=264, gradelevel=8)
+The ordering of format characters may have an impact on size since the padding
+needed to satisfy alignment requirements is different::
+
+ >>> pack('ci', '*', 0x12131415)
+ '*\x00\x00\x00\x12\x13\x14\x15'
+ >>> pack('ic', 0x12131415, '*')
+ '\x12\x13\x14\x15*'
+ >>> calcsize('ci')
+ 8
+ >>> calcsize('ic')
+ 5
+
+The following format ``'llh0l'`` specifies two pad bytes at the end, assuming longs are aligned on 4-byte boundaries::
+
+ >>> pack('llh0l', 1, 2, 3)
+ '\x00\x00\x00\x01\x00\x00\x00\x02\x00\x03\x00\x00'
+
+This only works when native size and alignment are in effect; standard size and
+alignment does not enforce any alignment.
+
+
.. seealso::
Module :mod:`array`
@@ -274,8 +333,8 @@
.. _struct-objects:
-Struct Objects
---------------
+Objects
+-------
The :mod:`struct` module also defines the following type:
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