mirror of
https://github.com/python/cpython.git
synced 2024-11-28 16:45:42 +01:00
76 lines
2.8 KiB
TeX
76 lines
2.8 KiB
TeX
\section{Built-in Module \sectcode{struct}}
|
|
\bimodindex{struct}
|
|
\indexii{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:
|
|
|
|
\renewcommand{\indexsubitem}{(in module struct)}
|
|
\begin{excdesc}{error}
|
|
Exception raised on various occasions; argument is a string
|
|
describing what is wrong.
|
|
\end{excdesc}
|
|
|
|
\begin{funcdesc}{pack}{fmt\, v1\, v2\, {\rm \ldots}}
|
|
Return a string containing the values
|
|
\code{\var{v1}, \var{v2}, {\rm \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}, {\rm \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}{Python}
|
|
\lineiii{x}{pad byte}{no value}
|
|
\lineiii{c}{char}{string of length 1}
|
|
\lineiii{b}{signed char}{integer}
|
|
\lineiii{h}{short}{integer}
|
|
\lineiii{i}{int}{integer}
|
|
\lineiii{l}{long}{integer}
|
|
\lineiii{f}{float}{float}
|
|
\lineiii{d}{double}{float}
|
|
\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'}.
|
|
|
|
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).
|
|
|
|
Examples (all on a big-endian machine):
|
|
|
|
\bcode\begin{verbatim}
|
|
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}\ecode
|
|
|
|
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.
|
|
|
|
(More format characters are planned, e.g.\ \code{'s'} for character
|
|
arrays, upper case for unsigned variants, and a way to specify the
|
|
byte order, which is useful for [de]constructing network packets and
|
|
reading/writing portable binary file formats like TIFF and AIFF.)
|