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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 | |
++--------+-------------------------+--------------------+-------+
+| ``?`` | :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 | |
++--------+-------------------------+--------------------+-------+
+
+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.
+
+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
+``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.
+
+Unpacked fields can be named by assigning them to variables or by wrapping
+the result in a named tuple::
+
+ >>> record = 'raymond \x32\x12\x08\x01\x08'
+ >>> name, serialnum, school, gradelevel = unpack('<10sHHb', record)
+
+ >>> from collections import namedtuple
+ >>> Student = namedtuple('Student', 'name serialnum school gradelevel')
+ >>> Student._make(unpack('<10sHHb', s))
+ Student(name='raymond ', serialnum=4658, school=264, gradelevel=8)
+
+.. 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:: pack(v1, v2, ...)
+
+ Identical to the :func:`pack` function, using the compiled format.
+ (``len(result)`` will equal :attr:`self.size`.)
+
+
+ .. method:: pack_into(buffer, offset, v1, v2, ...)
+
+ Identical to the :func:`pack_into` function, using the compiled format.
+
+
+ .. method:: unpack(string)
+
+ Identical to the :func:`unpack` function, using the compiled format.
+ (``len(string)`` must equal :attr:`self.size`).
+
+
+ .. method:: 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:: format
+
+ The format string used to construct this Struct object.
+
+ .. attribute:: size
+
+ The calculated size of the struct (and hence of the string) corresponding
+ to :attr:`format`.
+