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annotate Doc/libstruct.tex @ 4566:6403c94de50d legacy-trunk

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Added safeguard against failure in __del__.
author Guido van Rossum <guido@python.org>
date Wed, 27 Nov 1996 19:39:58 +0000
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1 \section{Built-in Module \sectcode{struct}}
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2 \bimodindex{struct}
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3 \indexii{C}{structures}
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4
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5 This module performs conversions between Python values and C
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6 structs represented as Python strings. It uses \dfn{format strings}
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7 (explained below) as compact descriptions of the lay-out of the C
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8 structs and the intended conversion to/from Python values.
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9
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10 See also built-in module \code{array}.
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11 \bimodindex{array}
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12
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13 The module defines the following exception and functions:
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14
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15 \renewcommand{\indexsubitem}{(in module struct)}
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16 \begin{excdesc}{error}
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17 Exception raised on various occasions; argument is a string
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18 describing what is wrong.
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19 \end{excdesc}
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20
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21 \begin{funcdesc}{pack}{fmt\, v1\, v2\, {\rm \ldots}}
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22 Return a string containing the values
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23 \code{\var{v1}, \var{v2}, {\rm \ldots}} packed according to the given
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24 format. The arguments must match the values required by the format
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25 exactly.
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26 \end{funcdesc}
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27
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28 \begin{funcdesc}{unpack}{fmt\, string}
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29 Unpack the string (presumably packed by \code{pack(\var{fmt}, {\rm \ldots})})
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30 according to the given format. The result is a tuple even if it
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31 contains exactly one item. The string must contain exactly the
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32 amount of data required by the format (i.e. \code{len(\var{string})} must
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33 equal \code{calcsize(\var{fmt})}).
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34 \end{funcdesc}
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35
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36 \begin{funcdesc}{calcsize}{fmt}
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37 Return the size of the struct (and hence of the string)
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38 corresponding to the given format.
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39 \end{funcdesc}
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40
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41 Format characters have the following meaning; the conversion between C
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42 and Python values should be obvious given their types:
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43
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44 \begin{tableiii}{|c|l|l|}{samp}{Format}{C}{Python}
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45 \lineiii{x}{pad byte}{no value}
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46 \lineiii{c}{char}{string of length 1}
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47 \lineiii{b}{signed char}{integer}
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48 \lineiii{h}{short}{integer}
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49 \lineiii{i}{int}{integer}
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50 \lineiii{l}{long}{integer}
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51 \lineiii{f}{float}{float}
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52 \lineiii{d}{double}{float}
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53 \end{tableiii}
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54
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55 A format character may be preceded by an integral repeat count; e.g.\
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56 the format string \code{'4h'} means exactly the same as \code{'hhhh'}.
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57
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58 C numbers are represented in the machine's native format and byte
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59 order, and properly aligned by skipping pad bytes if necessary
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60 (according to the rules used by the C compiler).
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61
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62 Examples (all on a big-endian machine):
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63
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64 \bcode\begin{verbatim}
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65 pack('hhl', 1, 2, 3) == '\000\001\000\002\000\000\000\003'
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66 unpack('hhl', '\000\001\000\002\000\000\000\003') == (1, 2, 3)
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67 calcsize('hhl') == 8
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68 \end{verbatim}\ecode
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69
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70 Hint: to align the end of a structure to the alignment requirement of
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71 a particular type, end the format with the code for that type with a
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72 repeat count of zero, e.g.\ the format \code{'llh0l'} specifies two
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73 pad bytes at the end, assuming longs are aligned on 4-byte boundaries.
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74
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75 (More format characters are planned, e.g.\ \code{'s'} for character
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76 arrays, upper case for unsigned variants, and a way to specify the
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77 byte order, which is useful for [de]constructing network packets and
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78 reading/writing portable binary file formats like TIFF and AIFF.)