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Merged revisions 81957 via svnmerge from 

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  r81957 | mark.dickinson | 2010-06-12 19:50:34 +0100 (Sat, 12 Jun 2010) | 5 lines

  Issue #8469:  Add standard sizes to table in struct documentation; additional
  clarifications and documentation tweaks.

  Backport of revisions 81955-81956 from py3k.
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Doc/library/struct.rst
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@ -82,112 +82,9 @@ 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 | \(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:
(1)
The ``'?'`` conversion code corresponds to the :ctype:`_Bool` type defined by
C99. If this type is not available, it is simulated using a :ctype:`char`. In
standard mode, it is always represented by one byte.
.. versionadded:: 2.6
(2)
The ``'q'`` and ``'Q'`` conversion codes are available in native mode only if
the platform C compiler supports C :ctype:`long long`, or, on Windows,
:ctype:`__int64`. They are always available in standard modes.
.. versionadded:: 2.2
A format character may be preceded by an integral repeat count. For example,
the format string ``'4h'`` means exactly the same as ``'hhhh'``.
Whitespace characters between formats are ignored; a count and its format must
not contain whitespace though.
For the ``'s'`` format character, the count is interpreted as the size of the
string, not a repeat count like for the other format characters; for example,
``'10s'`` means a single 10-byte string, while ``'10c'`` means 10 characters.
For packing, the string is truncated or padded with null bytes as appropriate to
make it fit. For unpacking, the resulting string always has exactly the
specified number of bytes. As a special case, ``'0s'`` means a single, empty
string (while ``'0c'`` means 0 characters).
The ``'p'`` format character encodes a "Pascal string", meaning a short
variable-length string stored in a fixed number of bytes. The count is the total
number of bytes stored. The first byte stored is the length of the string, or
255, whichever is smaller. The bytes of the string follow. If the string
passed in to :func:`pack` is too long (longer than the count minus 1), only the
leading count-1 bytes of the string are stored. If the string is shorter than
count-1, it is padded with null bytes so that exactly count bytes in all are
used. Note that for :func:`unpack`, the ``'p'`` format character consumes count
bytes, but that the string returned can never contain more than 255 characters.
For the ``'I'``, ``'L'``, ``'q'`` and ``'Q'`` format characters, the return
value is a Python long integer.
For the ``'P'`` format character, the return value is a Python integer or long
integer, depending on the size needed to hold a pointer when it has been cast to
an integer type. A *NULL* pointer will always be returned as the Python integer
``0``. When packing pointer-sized values, Python integer or long integer objects
may be used. For example, the Alpha and Merced processors use 64-bit pointer
values, meaning a Python long integer will be used to hold the pointer; other
platforms use 32-bit pointers and will use a Python integer.
For the ``'?'`` format character, the return value is either :const:`True` or
:const:`False`. When packing, the truth value of the argument object is used.
Either 0 or 1 in the native or standard bool representation will be packed, and
any non-zero value will be True when unpacking.
packing and unpacking data. They are built up from :ref:`format-characters`,
which specify the type of data being packed/unpacked. In addition, there are
special characters for controlling the :ref:`struct-alignment`.
.. _struct-alignment:
@ -262,6 +159,110 @@ Notes:
count of zero. See :ref:`struct-examples`.
.. _format-characters:
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 type | Standard size | Notes |
+========+=========================+====================+================+============+
| ``x`` | pad byte | no value | | |
+--------+-------------------------+--------------------+----------------+------------+
| ``c`` | :ctype:`char` | string of length 1 | 1 | |
+--------+-------------------------+--------------------+----------------+------------+
| ``b`` | :ctype:`signed char` | integer | 1 | \(3) |
+--------+-------------------------+--------------------+----------------+------------+
| ``B`` | :ctype:`unsigned char` | integer | 1 | \(3) |
+--------+-------------------------+--------------------+----------------+------------+
| ``?`` | :ctype:`_Bool` | bool | 1 | \(1) |
+--------+-------------------------+--------------------+----------------+------------+
| ``h`` | :ctype:`short` | integer | 2 | \(3) |
+--------+-------------------------+--------------------+----------------+------------+
| ``H`` | :ctype:`unsigned short` | integer | 2 | \(3) |
+--------+-------------------------+--------------------+----------------+------------+
| ``i`` | :ctype:`int` | integer | 4 | \(3) |
+--------+-------------------------+--------------------+----------------+------------+
| ``I`` | :ctype:`unsigned int` | integer | 4 | \(3) |
+--------+-------------------------+--------------------+----------------+------------+
| ``l`` | :ctype:`long` | integer | 4 | \(3) |
+--------+-------------------------+--------------------+----------------+------------+
| ``L`` | :ctype:`unsigned long` | integer | 4 | \(3) |
+--------+-------------------------+--------------------+----------------+------------+
| ``q`` | :ctype:`long long` | integer | 8 | \(2), \(3) |
+--------+-------------------------+--------------------+----------------+------------+
| ``Q`` | :ctype:`unsigned long | integer | 8 | \(2), \(3) |
| | long` | | | |
+--------+-------------------------+--------------------+----------------+------------+
| ``f`` | :ctype:`float` | float | 4 | |
+--------+-------------------------+--------------------+----------------+------------+
| ``d`` | :ctype:`double` | float | 8 | |
+--------+-------------------------+--------------------+----------------+------------+
| ``s`` | :ctype:`char[]` | string | | |
+--------+-------------------------+--------------------+----------------+------------+
| ``p`` | :ctype:`char[]` | string | | |
+--------+-------------------------+--------------------+----------------+------------+
| ``P`` | :ctype:`void \*` | integer | | \(3) |
+--------+-------------------------+--------------------+----------------+------------+
Notes:
(1)
The ``'?'`` conversion code corresponds to the :ctype:`_Bool` type defined by
C99. If this type is not available, it is simulated using a :ctype:`char`. In
standard mode, it is always represented by one byte.
.. versionadded:: 2.6
(2)
The ``'q'`` and ``'Q'`` conversion codes are available in native mode only if
the platform C compiler supports C :ctype:`long long`, or, on Windows,
:ctype:`__int64`. They are always available in standard modes.
.. versionadded:: 2.2
A format character may be preceded by an integral repeat count. For example,
the format string ``'4h'`` means exactly the same as ``'hhhh'``.
Whitespace characters between formats are ignored; a count and its format must
not contain whitespace though.
For the ``'s'`` format character, the count is interpreted as the size of the
string, not a repeat count like for the other format characters; for example,
``'10s'`` means a single 10-byte string, while ``'10c'`` means 10 characters.
For packing, the string is truncated or padded with null bytes as appropriate to
make it fit. For unpacking, the resulting string always has exactly the
specified number of bytes. As a special case, ``'0s'`` means a single, empty
string (while ``'0c'`` means 0 characters).
The ``'p'`` format character encodes a "Pascal string", meaning a short
variable-length string stored in a fixed number of bytes. The count is the total
number of bytes stored. The first byte stored is the length of the string, or
255, whichever is smaller. The bytes of the string follow. If the string
passed in to :func:`pack` is too long (longer than the count minus 1), only the
leading count-1 bytes of the string are stored. If the string is shorter than
count-1, it is padded with null bytes so that exactly count bytes in all are
used. Note that for :func:`unpack`, the ``'p'`` format character consumes count
bytes, but that the string returned can never contain more than 255 characters.
For the ``'P'`` format character, the return value is a Python integer or long
integer, depending on the size needed to hold a pointer when it has been cast to
an integer type. A *NULL* pointer will always be returned as the Python integer
``0``. When packing pointer-sized values, Python integer or long integer objects
may be used. For example, the Alpha and Merced processors use 64-bit pointer
values, meaning a Python long integer will be used to hold the pointer; other
platforms use 32-bit pointers and will use a Python integer.
For the ``'?'`` format character, the return value is either :const:`True` or
:const:`False`. When packing, the truth value of the argument object is used.
Either 0 or 1 in the native or standard bool representation will be packed, and
any non-zero value will be True when unpacking.
.. _struct-examples:
Examples
@ -325,7 +326,7 @@ alignment does not enforce any alignment.
.. _struct-objects:
Objects
Classes
-------
The :mod:`struct` module also defines the following type: