Index: Doc/library/struct.rst =================================================================== --- Doc/library/struct.rst (revision 79941) +++ Doc/library/struct.rst (working copy) @@ -10,11 +10,19 @@ 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). @@ -234,9 +258,30 @@ 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) '\x00\x01\x00\x02\x00\x00\x00\x03' @@ -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: