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<H1><A NAME="SECTION006300000000000000000">
4.3 <tt class="module">struct</tt> --
Interpret strings as packed binary data</A>
</H1>
<P>
<P>
<P>
This module performs conversions between Python values and C
structs represented as Python strings. It uses <i class="dfn">format strings</i>
(explained below) as compact descriptions of the lay-out of the C
structs and the intended conversion to/from Python values. This can
be used in handling binary data stored in files or from network
connections, among other sources.
<P>
The module defines the following exception and functions:
<P>
<dl><dt><b><a name='l2h-632'><tt class='exception'>error</tt></a></b>
<dd>
Exception raised on various occasions; argument is a string
describing what is wrong.
</dl>
<P>
<dl><dt><b><a name='l2h-633'><tt class='function'>pack</tt></a></b> (<var>fmt, v1, v2, ...</var>)
<dd>
Return a string containing the values
<code><var>v1</var>, <var>v2</var>, ...</code> packed according to the given
format. The arguments must match the values required by the format
exactly.
</dl>
<P>
<dl><dt><b><a name='l2h-634'><tt class='function'>unpack</tt></a></b> (<var>fmt, string</var>)
<dd>
Unpack the string (presumably packed by <code>pack(<var>fmt</var>,
...)</code>) 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</var>)</code> must equal <code>calcsize(<var>fmt</var>)</code>).
</dl>
<P>
<dl><dt><b><a name='l2h-635'><tt class='function'>calcsize</tt></a></b> (<var>fmt</var>)
<dd>
Return the size of the struct (and hence of the string)
corresponding to the given format.
</dl>
<P>
Format characters have the following meaning; the conversion between
C and Python values should be obvious given their types:
<P>
<table border align="center" style="border-collapse: collapse">
<thead>
<tr>
<th align="center"><b>Format</b> </th>
<th align="left"><b>C Type</b> </th>
<th align="left"><b>Python</b> </th>
<th align="center"><b>Notes</b> </th>
</thead>
<tbody valign='baseline'>
<tr><td align="center" valign="baseline"><samp>x</samp></td>
<td align="left">pad byte</td>
<td align="left">no value</td>
<td align="center"> </td>
<tr><td align="center" valign="baseline"><samp>c</samp></td>
<td align="left"><tt class="ctype">char</tt></td>
<td align="left">string of length 1</td>
<td align="center"> </td>
<tr><td align="center" valign="baseline"><samp>b</samp></td>
<td align="left"><tt class="ctype">signed char</tt></td>
<td align="left">integer</td>
<td align="center"> </td>
<tr><td align="center" valign="baseline"><samp>B</samp></td>
<td align="left"><tt class="ctype">unsigned char</tt></td>
<td align="left">integer</td>
<td align="center"> </td>
<tr><td align="center" valign="baseline"><samp>h</samp></td>
<td align="left"><tt class="ctype">short</tt></td>
<td align="left">integer</td>
<td align="center"> </td>
<tr><td align="center" valign="baseline"><samp>H</samp></td>
<td align="left"><tt class="ctype">unsigned short</tt></td>
<td align="left">integer</td>
<td align="center"> </td>
<tr><td align="center" valign="baseline"><samp>i</samp></td>
<td align="left"><tt class="ctype">int</tt></td>
<td align="left">integer</td>
<td align="center"> </td>
<tr><td align="center" valign="baseline"><samp>I</samp></td>
<td align="left"><tt class="ctype">unsigned int</tt></td>
<td align="left">long</td>
<td align="center">(1)</td>
<tr><td align="center" valign="baseline"><samp>l</samp></td>
<td align="left"><tt class="ctype">long</tt></td>
<td align="left">integer</td>
<td align="center"> </td>
<tr><td align="center" valign="baseline"><samp>L</samp></td>
<td align="left"><tt class="ctype">unsigned long</tt></td>
<td align="left">long</td>
<td align="center"> </td>
<tr><td align="center" valign="baseline"><samp>f</samp></td>
<td align="left"><tt class="ctype">float</tt></td>
<td align="left">float</td>
<td align="center"> </td>
<tr><td align="center" valign="baseline"><samp>d</samp></td>
<td align="left"><tt class="ctype">double</tt></td>
<td align="left">float</td>
<td align="center"> </td>
<tr><td align="center" valign="baseline"><samp>s</samp></td>
<td align="left"><tt class="ctype">char[]</tt></td>
<td align="left">string</td>
<td align="center"> </td>
<tr><td align="center" valign="baseline"><samp>p</samp></td>
<td align="left"><tt class="ctype">char[]</tt></td>
<td align="left">string</td>
<td align="center"> </td>
<tr><td align="center" valign="baseline"><samp>P</samp></td>
<td align="left"><tt class="ctype">void *</tt></td>
<td align="left">integer</td>
<td align="center"> </td></tbody>
</table>
<P>
Notes:
<P>
<DL>
<DT><STRONG>(1)</STRONG></DT>
<DD>The "<tt class="character">I</tt>" conversion code will convert to a Python long if
the C <tt class="ctype">int</tt> is the same size as a C <tt class="ctype">long</tt>, which is
typical on most modern systems. If a C <tt class="ctype">int</tt> is smaller than
a C <tt class="ctype">long</tt>, an Python integer will be created instead.
</DD>
</DL>
<P>
A format character may be preceded by an integral repeat count;
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