diff --git a/Doc/library/struct.rst b/Doc/library/struct.rst index 620f50376beb62..50d70731f77523 100644 --- a/Doc/library/struct.rst +++ b/Doc/library/struct.rst @@ -12,21 +12,25 @@ -------------- -This module performs conversions between Python values and C structs represented -as Python :class:`bytes` objects. 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. +This module converts between Python values and C structs represented +as Python :class:`bytes` objects. Compact :ref:`format strings ` +describe the intended conversions to/from Python values. +The module's functions and objects can be used for two largely +distinct applications, data exchange with external sources (files or +network connections), or data transfer between the Python application +and the C layer. .. note:: - By default, the result of packing a given C struct includes pad bytes in - order to maintain proper alignment for the C types involved; similarly, - alignment is taken into account when unpacking. This behavior is chosen so - that the bytes of a packed struct correspond exactly to the layout in memory - of the corresponding C struct. To handle platform-independent data formats - or omit implicit pad bytes, use ``standard`` size and alignment instead of - ``native`` size and alignment: see :ref:`struct-alignment` for details. + When no prefix character is given, native mode is the default. It + packs or unpacks data based on the platform and compiler on which + the Python interpreter was built. + The result of packing a given C struct includes pad bytes which + maintain proper alignment for the C types involved; similarly, + alignment is taken into account when unpacking. In contrast, when + communicating data between external sources, the programmer is + responsible for defining byte ordering and padding between elements. + See :ref:`struct-alignment` for details. Several :mod:`struct` functions (and methods of :class:`Struct`) take a *buffer* argument. This refers to objects that implement the :ref:`bufferobjects` and @@ -102,10 +106,13 @@ The module defines the following exception and functions: Format Strings -------------- -Format strings are the mechanism used to specify the expected layout when -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`. +Format strings describe the data layout when +packing and unpacking data. They are built up from :ref:`format characters`, +which specify the type of data being packed/unpacked. In addition, +special characters control the :ref:`byte order, size and alignment`. +Each format string consists of an optional prefix character which +describes the overall properties of the data and one or more format +characters which describe the actual data values and padding. .. _struct-alignment: @@ -116,6 +123,11 @@ Byte Order, Size, and Alignment By default, C types 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). +This behavior is chosen so +that the bytes of a packed struct correspond exactly to the memory layout +of the corresponding C struct. +Whether to use native byte ordering +and padding or standard formats depends on the application. .. index:: single: @ (at); in struct format strings @@ -144,12 +156,10 @@ following table: 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, Intel x86 and AMD64 (x86-64) are little-endian; -IBM z and most legacy architectures are big-endian; -and ARM, RISC-V and IBM Power feature switchable endianness -(bi-endian, though the former two are nearly always little-endian in practice). -Use ``sys.byteorder`` to check the endianness of your system. +Native byte order is big-endian or little-endian, depending on the +host system. For example, Intel x86, AMD64 (x86-64), and Apple M1 are +little-endian; IBM z and many legacy architectures are big-endian. +Use :data:`sys.byteorder` to check the endianness of your system. Native size and alignment are determined using the C compiler's ``sizeof`` expression. This is always combined with native byte order. @@ -231,9 +241,9 @@ platform-dependent. +--------+--------------------------+--------------------+----------------+------------+ | ``d`` | :c:expr:`double` | float | 8 | \(4) | +--------+--------------------------+--------------------+----------------+------------+ -| ``s`` | :c:expr:`char[]` | bytes | | | +| ``s`` | :c:expr:`char[]` | bytes | | \(9) | +--------+--------------------------+--------------------+----------------+------------+ -| ``p`` | :c:expr:`char[]` | bytes | | | +| ``p`` | :c:expr:`char[]` | bytes | | \(8) | +--------+--------------------------+--------------------+----------------+------------+ | ``P`` | :c:expr:`void \*` | integer | | \(5) | +--------+--------------------------+--------------------+----------------+------------+ @@ -292,8 +302,33 @@ Notes: format `_ for more information. (7) - For padding, ``x`` inserts null bytes. - + When packing, ``'x'`` inserts one NUL byte. + +(8) + The ``'p'`` format character encodes a "Pascal string", meaning a short + variable-length string stored in a *fixed number of bytes*, given by the count. + 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 + bytes. + +(9) + For the ``'s'`` format character, the count is interpreted as the length of the + bytes, not a repeat count like for the other format characters; for example, + ``'10s'`` means a single 10-byte string mapping to or from a single + Python byte string, while ``'10c'`` means 10 + separate one byte character elements (e.g., ``cccccccccc``) mapping + to or from ten different Python byte objects. (See :ref:`struct-examples` + for a concrete demonstration of the difference.) + If a count is not given, it defaults to 1. For packing, the string is + truncated or padded with null bytes as appropriate to make it fit. For + unpacking, the resulting bytes object always has exactly the specified number + of bytes. As a special case, ``'0s'`` means a single, empty string (while + ``'0c'`` means 0 characters). A format character may be preceded by an integral repeat count. For example, the format string ``'4h'`` means exactly the same as ``'hhhh'``. @@ -301,15 +336,6 @@ 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 length of the -bytes, not a repeat count like for the other format characters; for example, -``'10s'`` means a single 10-byte string, while ``'10c'`` means 10 characters. -If a count is not given, it defaults to 1. For packing, the string is -truncated or padded with null bytes as appropriate to make it fit. For -unpacking, the resulting bytes object always has exactly the specified number -of bytes. As a special case, ``'0s'`` means a single, empty string (while -``'0c'`` means 0 characters). - When packing a value ``x`` using one of the integer formats (``'b'``, ``'B'``, ``'h'``, ``'H'``, ``'i'``, ``'I'``, ``'l'``, ``'L'``, ``'q'``, ``'Q'``), if ``x`` is outside the valid range for that format @@ -319,17 +345,6 @@ then :exc:`struct.error` is raised. Previously, some of the integer formats wrapped out-of-range values and raised :exc:`DeprecationWarning` instead of :exc:`struct.error`. -The ``'p'`` format character encodes a "Pascal string", meaning a short -variable-length string stored in a *fixed number of bytes*, given by the count. -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 -bytes. - .. index:: single: ? (question mark); in struct format strings For the ``'?'`` format character, the return value is either :const:`True` or @@ -345,18 +360,36 @@ Examples ^^^^^^^^ .. note:: - All examples assume a native byte order, size, and alignment with a - big-endian machine. + Native byte order examples (designated by the ``'@'`` format prefix or + lack of any prefix character) may not match what the reader's + machine produces as + that depends on the platform and compiler. + +Pack and unpack integers of three different sizes, using big endian +ordering:: -A basic example of packing/unpacking three integers:: + >>> from struct import * + >>> pack(">bhl", 1, 2, 3) + b'\x01\x00\x02\x00\x00\x00\x03' + >>> unpack('>bhl', b'\x01\x00\x02\x00\x00\x00\x03' + (1, 2, 3) + >>> calcsize('>bhl') + 7 - >>> from struct import * - >>> pack('hhl', 1, 2, 3) - b'\x00\x01\x00\x02\x00\x00\x00\x03' - >>> unpack('hhl', b'\x00\x01\x00\x02\x00\x00\x00\x03') - (1, 2, 3) - >>> calcsize('hhl') - 8 +Attempt to pack an integer which is too large for the defined field:: + + >>> pack(">h", 99999) + Traceback (most recent call last): + File "", line 1, in + struct.error: 'h' format requires -32768 <= number <= 32767 + +Demonstrate the difference between ``'s'`` and ``'c'`` format +characters:: + + >>> pack("@ccc", b'1', b'2', b'3') + b'123' + >>> pack("@3s", b'123') + b'123' Unpacked fields can be named by assigning them to variables or by wrapping the result in a named tuple:: @@ -369,35 +402,132 @@ the result in a named tuple:: >>> Student._make(unpack('<10sHHb', record)) Student(name=b'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', b'*', 0x12131415) - b'*\x00\x00\x00\x12\x13\x14\x15' - >>> pack('ic', 0x12131415, b'*') - b'\x12\x13\x14\x15*' - >>> calcsize('ci') +The ordering of format characters may have an impact on size in native +mode since padding is implicit. In standard mode, the user is +responsible for inserting any desired padding. +Note in +the first ``pack`` call below that three NUL bytes were added after the +packed ``'#'`` to align the following integer on a four-byte boundary. +In this example, the output was produced on a little endian machine:: + + >>> pack('@ci', b'#', 0x12131415) + b'#\x00\x00\x00\x15\x14\x13\x12' + >>> pack('@ic', 0x12131415, b'#') + b'\x15\x14\x13\x12#' + >>> calcsize('@ci') 8 - >>> calcsize('ic') + >>> calcsize('@ic') 5 -The following format ``'llh0l'`` specifies two pad bytes at the end, assuming -longs are aligned on 4-byte boundaries:: +The following format ``'llh0l'`` results in two pad bytes being added +at the end, assuming the platform's longs are aligned on 4-byte boundaries:: - >>> pack('llh0l', 1, 2, 3) + >>> pack('@llh0l', 1, 2, 3) b'\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` Packed binary storage of homogeneous data. - Module :mod:`xdrlib` - Packing and unpacking of XDR data. + Module :mod:`json` + JSON encoder and decoder. + + Module :mod:`pickle` + Python object serialization. + + +.. _applications: + +Applications +------------ + +Two main applications for the :mod:`struct` module exist, data +interchange between Python and C code within an application or another +application compiled using the same compiler (:ref:`native formats`), and +data interchange between applications using agreed upon data layout +(:ref:`standard formats`). Generally speaking, the format strings +constructed for these two domains are distinct. + + +.. _struct-native-formats: + +Native Formats +^^^^^^^^^^^^^^ + +When constructing format strings which mimic native layouts, the +compiler and machine architecture determine byte ordering and padding. +In such cases, the ``@`` format character should be used to specify +native byte ordering and data sizes. Internal pad bytes are normally inserted +automatically. It is possible that a zero-repeat format code will be +needed at the end of a format string to round up to the correct +byte boundary for proper alignment of consective chunks of data. + +Consider these two simple examples (on a 64-bit, little-endian +machine):: + + >>> calcsize('@lhl') + 24 + >>> calcsize('@llh') + 18 + +Data is not padded to an 8-byte boundary at the end of the second +format string without the use of extra padding. A zero-repeat format +code solves that problem:: + + >>> calcsize('@llh0l') + 24 + +The ``'x'`` format code can be used to specify the repeat, but for +native formats it is better to use a zero-repeat format like ``'0l'``. + +By default, native byte ordering and alignment is used, but it is +better to be explicit and use the ``'@'`` prefix character. + + +.. _struct-standard-formats: + +Standard Formats +^^^^^^^^^^^^^^^^ + +When exchanging data beyond your process such as networking or storage, +be precise. Specify the exact byte order, size, and alignment. Do +not assume they match the native order of a particular machine. +For example, network byte order is big-endian, while many popular CPUs +are little-endian. By defining this explicitly, the user need not +care about the specifics of the platform their code is running on. +The first character should typically be ``<`` or ``>`` +(or ``!``). Padding is the responsibility of the programmer. The +zero-repeat format character won't work. Instead, the user must +explicitly add ``'x'`` pad bytes where needed. Revisiting the +examples from the previous section, we have:: + + >>> calcsize('>> pack('>> calcsize('@llh') + 18 + >>> pack('@llh', 1, 2, 3) == pack('>> calcsize('>> calcsize('@llh0l') + 24 + >>> pack('@llh0l', 1, 2, 3) == pack('>> calcsize('>> calcsize('@llh0l') + 12 + >>> pack('@llh0l', 1, 2, 3) == pack(' pFad - Phonifier reborn

Pfad - The Proxy pFad of © 2024 Garber Painting. All rights reserved.

Note: This service is not intended for secure transactions such as banking, social media, email, or purchasing. Use at your own risk. We assume no liability whatsoever for broken pages.


Alternative Proxies:

Alternative Proxy

pFad Proxy

pFad v3 Proxy

pFad v4 Proxy