mirror of https://github.com/python/cpython
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.
This commit is contained in:
Mark Dickinson
parent
4e20ab24cf
commit
bbacb838c9
|
|
@ -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 @@ The module defines the following exception and functions:
|
|||
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 | |
|
||||
+--------+-------------------------+--------------------+-------+
|
||||
+--------+-------------------------+--------------------+------------+
|
||||
| ``b`` | :ctype:`signed char` | integer | \(3) |
|
||||
+--------+-------------------------+--------------------+------------+
|
||||
| ``B`` | :ctype:`unsigned char` | integer | \(3) |
|
||||
+--------+-------------------------+--------------------+------------+
|
||||
| ``?`` | :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) |
|
||||
+--------+-------------------------+--------------------+------------+
|
||||
| ``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 | |
|
||||
+--------+-------------------------+--------------------+-------+
|
||||
+--------+-------------------------+--------------------+------------+
|
||||
| ``P`` | :ctype:`void \*` | long | \(3) |
|
||||
+--------+-------------------------+--------------------+------------+
|
||||
|
||||
Notes:
|
||||
|
||||
|
|
@ -180,6 +201,9 @@ For the ``'?'`` format character, the return value is either :const:`True` or
|
|||
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 @@ 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.
|
||||
|
||||
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 @@ machine)::
|
|||
>>> 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 @@ the result in a named tuple::
|
|||
>>> 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 @@ the result in a named tuple::
|
|||
|
||||
.. _struct-objects:
|
||||
|
||||
Struct Objects
|
||||
--------------
|
||||
Objects
|
||||
-------
|
||||
|
||||
The :mod:`struct` module also defines the following type:
|
||||
|
||||
|
|
|
|||
Loading…
Reference in New Issue