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+
+:mod:`struct` --- Interpret strings as packed binary data
+=========================================================
+
+.. module:: struct
+   :synopsis: Interpret strings as packed binary data.
+
+.. index::
+   pair: C; structures
+   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.
+
+The module defines the following exception and functions:
+
+
+.. exception:: error
+
+   Exception raised on various occasions; argument is a string describing what is
+   wrong.
+
+
+.. function:: pack(fmt, v1, v2, ...)
+
+   Return a string containing the values ``v1, v2, ...`` packed according to the
+   given format.  The arguments must match the values required by the format
+   exactly.
+
+
+.. function:: pack_into(fmt, buffer, offset, v1, v2, ...)
+
+   Pack the values ``v1, v2, ...`` according to the given format, write the packed
+   bytes into the writable *buffer* starting at *offset*. Note that the offset is
+   a required argument.
+
+   .. versionadded:: 2.5
+
+
+.. function:: unpack(fmt, string)
+
+   Unpack the string (presumably packed by ``pack(fmt, ...)``) according to the
+   given format.  The result is a tuple even if it contains exactly one item.  The
+   string must contain exactly the amount of data required by the format
+   (``len(string)`` must equal ``calcsize(fmt)``).
+
+
+.. function:: unpack_from(fmt, buffer[,offset=0])
+
+   Unpack the *buffer* according to tthe given format. The result is a tuple even
+   if it contains exactly one item. The *buffer* must contain at least the amount
+   of data required by the format (``len(buffer[offset:])`` must be at least
+   ``calcsize(fmt)``).
+
+   .. versionadded:: 2.5
+
+
+.. function:: calcsize(fmt)
+
+   Return the size of the struct (and hence of the string) corresponding to the
+   given format.
+
+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            |       |
++--------+-------------------------+--------------------+-------+
+| ``t``  | :ctype:`_Bool`          | bool               | \(1)  |
++--------+-------------------------+--------------------+-------+
+| ``h``  | :ctype:`short`          | integer            |       |
++--------+-------------------------+--------------------+-------+
+| ``H``  | :ctype:`unsigned short` | integer            |       |
++--------+-------------------------+--------------------+-------+
+| ``i``  | :ctype:`int`            | integer            |       |
++--------+-------------------------+--------------------+-------+
+| ``I``  | :ctype:`unsigned int`   | 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 \*`        | integer            |       |
++--------+-------------------------+--------------------+-------+
+
+Notes:
+
+(1)
+   The ``'t'`` 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 ``'t'`` 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.
+
+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).
+
+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           |
++-----------+------------------------+--------------------+
+
+If the first character is not one of these, ``'@'`` is assumed.
+
+Native byte order is big-endian or little-endian, depending on the host system.
+For example, Motorola and Sun processors are big-endian; Intel and DEC
+processors are little-endian.
+
+Native size and alignment are determined using the C compiler's
+:keyword:`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.
+
+Note the difference between ``'@'`` and ``'='``: both use native byte order, but
+the size and alignment of the latter is standardized.
+
+The form ``'!'`` is available for those poor souls who claim they can't remember
+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.
+
+Examples (all using native byte order, size and alignment, on a big-endian
+machine)::
+
+   >>> from struct import *
+   >>> pack('hhl', 1, 2, 3)
+   '\x00\x01\x00\x02\x00\x00\x00\x03'
+   >>> unpack('hhl', '\x00\x01\x00\x02\x00\x00\x00\x03')
+   (1, 2, 3)
+   >>> 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.
+
+
+.. seealso::
+
+   Module :mod:`array`
+      Packed binary storage of homogeneous data.
+
+   Module :mod:`xdrlib`
+      Packing and unpacking of XDR data.
+
+
+.. _struct-objects:
+
+Struct Objects
+--------------
+
+The :mod:`struct` module also defines the following type:
+
+
+.. class:: Struct(format)
+
+   Return a new Struct object which writes and reads binary data according to the
+   format string *format*.  Creating a Struct object once and calling its methods
+   is more efficient than calling the :mod:`struct` functions with the same format
+   since the format string only needs to be compiled once.
+
+   .. versionadded:: 2.5
+
+Compiled Struct objects support the following methods and attributes:
+
+
+.. method:: Struct.pack(v1, v2, ...)
+
+   Identical to the :func:`pack` function, using the compiled format.
+   (``len(result)`` will equal :attr:`self.size`.)
+
+
+.. method:: Struct.pack_into(buffer, offset, v1, v2, ...)
+
+   Identical to the :func:`pack_into` function, using the compiled format.
+
+
+.. method:: Struct.unpack(string)
+
+   Identical to the :func:`unpack` function, using the compiled format.
+   (``len(string)`` must equal :attr:`self.size`).
+
+
+.. method:: Struct.unpack_from(buffer[, offset=0])
+
+   Identical to the :func:`unpack_from` function, using the compiled format.
+   (``len(buffer[offset:])`` must be at least :attr:`self.size`).
+
+
+.. attribute:: Struct.format
+
+   The format string used to construct this Struct object.
+