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76 lines
2.8 KiB
TeX
76 lines
2.8 KiB
TeX
\section{Built-in module \sectcode{struct}}
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\bimodindex{struct}
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\indexii{C}{structures}
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This module performs conversions between Python values and C
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structs represented as Python strings. It uses \dfn{format strings}
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(explained below) as compact descriptions of the lay-out of the C
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structs and the intended conversion to/from Python values.
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The module defines the following exception and functions:
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\renewcommand{\indexsubitem}{(in module struct)}
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\begin{excdesc}{error}
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Exception raised on various occasions; argument is a string
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describing what is wrong.
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\end{excdesc}
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\begin{funcdesc}{pack}{fmt\, v1\, v2\, {\rm \ldots}}
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Return a string containing the values
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\code{\var{v1}, \var{v2}, {\rm \ldots}} packed according to the given
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format. The arguments must match the values required by the format
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exactly.
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\end{funcdesc}
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\begin{funcdesc}{unpack}{fmt\, string}
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Unpack the string (presumably packed by \code{pack(\var{fmt}, {\rm \ldots})})
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according to the given format. The result is a tuple even if it
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contains exactly one item. The string must contain exactly the
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amount of data required by the format (i.e. \code{len(\var{string})} must
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equal \code{calcsize(\var{fmt})}).
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\end{funcdesc}
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\begin{funcdesc}{calcsize}{fmt}
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Return the size of the struct (and hence of the string)
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corresponding to the given format.
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\end{funcdesc}
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Format characters have the following meaning; the conversion between C
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and Python values should be obvious given their types:
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\begin{tableiii}{|c|l|l|}{samp}{Format}{C}{Python}
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\lineiii{x}{pad byte}{no value}
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\lineiii{c}{char}{string of length 1}
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\lineiii{b}{signed char}{integer}
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\lineiii{h}{short}{integer}
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\lineiii{i}{int}{integer}
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\lineiii{l}{long}{integer}
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\lineiii{f}{float}{float}
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\lineiii{d}{double}{float}
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\end{tableiii}
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A format character may be preceded by an integral repeat count; e.g.
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the format string \code{'4h'} means exactly the same as \code{'hhhh'}.
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C numbers are represented in the machine's native format and byte
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order, and properly aligned by skipping pad bytes if necessary
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(according to the rules used by the C compiler).
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Examples (all on a big-endian machine):
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\bcode\begin{verbatim}
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pack('hhl', 1, 2, 3) == '\000\001\000\002\000\000\000\003'
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unpack('hhl', '\000\001\000\002\000\000\000\003') == (1, 2, 3)
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calcsize('hhl') == 8
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\end{verbatim}\ecode
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Hint: to align the end of a structure to the alignment requirement of
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a particular type, end the format with the code for that type with a
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repeat count of zero, e.g. the format \code{'llh0l'} specifies two
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pad bytes at the end, assuming longs are aligned on 4-byte boundaries.
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(More format characters are planned, e.g. \code{'s'} for character
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arrays, upper case for unsigned variants, and a way to specify the
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byte order, which is useful for [de]constructing network packets and
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reading/writing portable binary file formats like TIFF and AIFF.)
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