mirror of https://github.com/python/cpython
174 lines
6.9 KiB
TeX
174 lines
6.9 KiB
TeX
\section{\module{struct} ---
|
|
Interpret strings as packed binary data.}
|
|
\declaremodule{builtin}{struct}
|
|
|
|
\modulesynopsis{Interpret strings as packed binary data.}
|
|
|
|
\indexii{C@\C{}}{structures}
|
|
|
|
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.
|
|
|
|
The module defines the following exception and functions:
|
|
|
|
|
|
\begin{excdesc}{error}
|
|
Exception raised on various occasions; argument is a string
|
|
describing what is wrong.
|
|
\end{excdesc}
|
|
|
|
\begin{funcdesc}{pack}{fmt, v1, v2, \textrm{\ldots}}
|
|
Return a string containing the values
|
|
\code{\var{v1}, \var{v2}, \textrm{\ldots}} packed according to the given
|
|
format. The arguments must match the values required by the format
|
|
exactly.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{unpack}{fmt, string}
|
|
Unpack the string (presumably packed by \code{pack(\var{fmt},
|
|
\textrm{\ldots})}) 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 (i.e.
|
|
\code{len(\var{string})} must equal \code{calcsize(\var{fmt})}).
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{calcsize}{fmt}
|
|
Return the size of the struct (and hence of the string)
|
|
corresponding to the given format.
|
|
\end{funcdesc}
|
|
|
|
Format characters have the following meaning; the conversion between
|
|
\C{} and Python values should be obvious given their types:
|
|
|
|
\begin{tableiii}{c|l|l}{samp}{Format}{C Type}{Python}
|
|
\lineiii{x}{pad byte}{no value}
|
|
\lineiii{c}{\ctype{char}}{string of length 1}
|
|
\lineiii{b}{\ctype{signed char}}{integer}
|
|
\lineiii{B}{\ctype{unsigned char}}{integer}
|
|
\lineiii{h}{\ctype{short}}{integer}
|
|
\lineiii{H}{\ctype{unsigned short}}{integer}
|
|
\lineiii{i}{\ctype{int}}{integer}
|
|
\lineiii{I}{\ctype{unsigned int}}{integer}
|
|
\lineiii{l}{\ctype{long}}{integer}
|
|
\lineiii{L}{\ctype{unsigned long}}{integer}
|
|
\lineiii{f}{\ctype{float}}{float}
|
|
\lineiii{d}{\ctype{double}}{float}
|
|
\lineiii{s}{\ctype{char[]}}{string}
|
|
\lineiii{p}{\ctype{char[]}}{string}
|
|
\lineiii{P}{\ctype{void *}}{integer}
|
|
\end{tableiii}
|
|
|
|
A format character may be preceded by an integral repeat count;
|
|
e.g.\ the format string \code{'4h'} means exactly the same as
|
|
\code{'hhhh'}.
|
|
|
|
Whitespace characters between formats are ignored; a count and its
|
|
format must not contain whitespace though.
|
|
|
|
For the \character{s} format character, the count is interpreted as the
|
|
size of the string, not a repeat count like for the other format
|
|
characters; e.g. \code{'10s'} means a single 10-byte string, while
|
|
\code{'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, \code{'0s'} means a single, empty
|
|
string (while \code{'0c'} means 0 characters).
|
|
|
|
The \character{p} format character can be used to encode a Pascal
|
|
string. The first byte is the length of the stored string, with the
|
|
bytes of the string following. If count is given, it is used as the
|
|
total number of bytes used, including the length byte. If the string
|
|
passed in to \function{pack()} is too long, the stored representation
|
|
is truncated. If the string is too short, padding is used to ensure
|
|
that exactly enough bytes are used to satisfy the count.
|
|
|
|
For the \character{I} and \character{L} format characters, the return
|
|
value is a Python long integer.
|
|
|
|
For the \character{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 \code{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.
|
|
|
|
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:
|
|
|
|
\begin{tableiii}{c|l|l}{samp}{Character}{Byte order}{Size and alignment}
|
|
\lineiii{@}{native}{native}
|
|
\lineiii{=}{native}{standard}
|
|
\lineiii{<}{little-endian}{standard}
|
|
\lineiii{>}{big-endian}{standard}
|
|
\lineiii{!}{network (= big-endian)}{standard}
|
|
\end{tableiii}
|
|
|
|
If the first character is not one of these, \character{@} is assumed.
|
|
|
|
Native byte order is big-endian or little-endian, depending on the
|
|
host system (e.g. Motorola and Sun are big-endian; Intel and DEC 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{float} and
|
|
\ctype{double} are 32-bit and 64-bit IEEE floating point numbers,
|
|
respectively.
|
|
|
|
Note the difference between \character{@} and \character{=}: both use
|
|
native byte order, but the size and alignment of the latter is
|
|
standardized.
|
|
|
|
The form \character{!} 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 (i.e. force
|
|
byte-swapping); use the appropriate choice of \character{<} or
|
|
\character{>}.
|
|
|
|
The \character{P} format character is only available for the native
|
|
byte ordering (selected as the default or with the \character{@} byte
|
|
order character). The byte order character \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
|
|
\character{P} format is not available.
|
|
|
|
Examples (all using native byte order, size and alignment, on a
|
|
big-endian machine):
|
|
|
|
\begin{verbatim}
|
|
>>> from struct import *
|
|
>>> pack('hhl', 1, 2, 3)
|
|
'\000\001\000\002\000\000\000\003'
|
|
>>> unpack('hhl', '\000\001\000\002\000\000\000\003')
|
|
(1, 2, 3)
|
|
>>> calcsize('hhl')
|
|
8
|
|
\end{verbatim}
|
|
|
|
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, e.g.\ the format \code{'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.
|
|
|
|
\begin{seealso}
|
|
\seemodule{array}{packed binary storage of homogeneous data}
|
|
\seemodule{xdrlib}{packing and unpacking of XDR data}
|
|
\end{seealso}
|