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<div class="section" id="module-struct">
<h1>8.3. <tt class="xref docutils literal"><span class="pre">struct</span></tt> — Interpret strings as packed binary data<a class="headerlink" href="#module-struct" title="Permalink to this headline">¶</a></h1>
<p id="index-628">This module performs conversions between Python values and C structs represented
as Python strings. It uses <em>format strings</em> (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>
<p>The module defines the following exception and functions:</p>
<dl class="exception">
<dt id="struct.error">
<em class="property">exception </em><tt class="descclassname">struct.</tt><tt class="descname">error</tt><a class="headerlink" href="#struct.error" title="Permalink to this definition">¶</a></dt>
<dd>Exception raised on various occasions; argument is a string describing what is
wrong.</dd></dl>
<dl class="function">
<dt id="struct.pack">
<tt class="descclassname">struct.</tt><tt class="descname">pack</tt><big>(</big><em>fmt</em>, <em>v1</em>, <em>v2</em>, <em>...</em><big>)</big><a class="headerlink" href="#struct.pack" title="Permalink to this definition">¶</a></dt>
<dd>Return a string containing the values <tt class="docutils literal"><span class="pre">v1,</span> <span class="pre">v2,</span> <span class="pre">...</span></tt> packed according to the
given format. The arguments must match the values required by the format
exactly.</dd></dl>
<dl class="function">
<dt id="struct.pack_into">
<tt class="descclassname">struct.</tt><tt class="descname">pack_into</tt><big>(</big><em>fmt</em>, <em>buffer</em>, <em>offset</em>, <em>v1</em>, <em>v2</em>, <em>...</em><big>)</big><a class="headerlink" href="#struct.pack_into" title="Permalink to this definition">¶</a></dt>
<dd><p>Pack the values <tt class="docutils literal"><span class="pre">v1,</span> <span class="pre">v2,</span> <span class="pre">...</span></tt> according to the given format, write the packed
bytes into the writable <em>buffer</em> starting at <em>offset</em>. Note that the offset is
a required argument.</p>
<p class="versionadded">
<span class="versionmodified">New in version 2.5.</span></p>
</dd></dl>
<dl class="function">
<dt id="struct.unpack">
<tt class="descclassname">struct.</tt><tt class="descname">unpack</tt><big>(</big><em>fmt</em>, <em>string</em><big>)</big><a class="headerlink" href="#struct.unpack" title="Permalink to this definition">¶</a></dt>
<dd>Unpack the string (presumably packed by <tt class="docutils literal"><span class="pre">pack(fmt,</span> <span class="pre">...)</span></tt>) 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
(<tt class="docutils literal"><span class="pre">len(string)</span></tt> must equal <tt class="docutils literal"><span class="pre">calcsize(fmt)</span></tt>).</dd></dl>
<dl class="function">
<dt id="struct.unpack_from">
<tt class="descclassname">struct.</tt><tt class="descname">unpack_from</tt><big>(</big><em>fmt</em>, <em>buffer</em><span class="optional">[</span>, <em>offset=0</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#struct.unpack_from" title="Permalink to this definition">¶</a></dt>
<dd><p>Unpack the <em>buffer</em> according to the given format. The result is a tuple even
if it contains exactly one item. The <em>buffer</em> must contain at least the amount
of data required by the format (<tt class="docutils literal"><span class="pre">len(buffer[offset:])</span></tt> must be at least
<tt class="docutils literal"><span class="pre">calcsize(fmt)</span></tt>).</p>
<p class="versionadded">
<span class="versionmodified">New in version 2.5.</span></p>
</dd></dl>
<dl class="function">
<dt id="struct.calcsize">
<tt class="descclassname">struct.</tt><tt class="descname">calcsize</tt><big>(</big><em>fmt</em><big>)</big><a class="headerlink" href="#struct.calcsize" title="Permalink to this definition">¶</a></dt>
<dd>Return the size of the struct (and hence of the string) corresponding to the
given format.</dd></dl>
<p>Format characters have the following meaning; the conversion between C and
Python values should be obvious given their types:</p>
<table border="1" class="docutils">
<colgroup>
<col width="13%" />
<col width="42%" />
<col width="33%" />
<col width="12%" />
</colgroup>
<thead valign="bottom">
<tr><th class="head">Format</th>
<th class="head">C Type</th>
<th class="head">Python</th>
<th class="head">Notes</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="docutils literal"><span class="pre">x</span></tt></td>
<td>pad byte</td>
<td>no value</td>
<td> </td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">c</span></tt></td>
<td><tt class="xref docutils literal"><span class="pre">char</span></tt></td>
<td>string of length 1</td>
<td> </td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">b</span></tt></td>
<td><tt class="xref docutils literal"><span class="pre">signed</span> <span class="pre">char</span></tt></td>
<td>integer</td>
<td> </td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">B</span></tt></td>
<td><tt class="xref docutils literal"><span class="pre">unsigned</span> <span class="pre">char</span></tt></td>
<td>integer</td>
<td> </td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">?</span></tt></td>
<td><tt class="xref docutils literal"><span class="pre">_Bool</span></tt></td>
<td>bool</td>
<td>(1)</td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">h</span></tt></td>
<td><tt class="xref docutils literal"><span class="pre">short</span></tt></td>
<td>integer</td>
<td> </td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">H</span></tt></td>
<td><tt class="xref docutils literal"><span class="pre">unsigned</span> <span class="pre">short</span></tt></td>
<td>integer</td>
<td> </td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">i</span></tt></td>
<td><tt class="xref docutils literal"><span class="pre">int</span></tt></td>
<td>integer</td>
<td> </td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">I</span></tt></td>
<td><tt class="xref docutils literal"><span class="pre">unsigned</span> <span class="pre">int</span></tt></td>
<td>integer or long</td>
<td> </td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">l</span></tt></td>
<td><tt class="xref docutils literal"><span class="pre">long</span></tt></td>
<td>integer</td>
<td> </td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">L</span></tt></td>
<td><tt class="xref docutils literal"><span class="pre">unsigned</span> <span class="pre">long</span></tt></td>
<td>long</td>
<td> </td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">q</span></tt></td>
<td><tt class="xref docutils literal"><span class="pre">long</span> <span class="pre">long</span></tt></td>
<td>long</td>
<td>(2)</td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">Q</span></tt></td>
<td><tt class="xref docutils literal"><span class="pre">unsigned</span> <span class="pre">long</span>
<span class="pre">long</span></tt></td>
<td>long</td>
<td>(2)</td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">f</span></tt></td>
<td><tt class="xref docutils literal"><span class="pre">float</span></tt></td>
<td>float</td>
<td> </td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">d</span></tt></td>
<td><tt class="xref docutils literal"><span class="pre">double</span></tt></td>
<td>float</td>
<td> </td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">s</span></tt></td>
<td><tt class="xref docutils literal"><span class="pre">char[]</span></tt></td>
<td>string</td>
<td> </td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">p</span></tt></td>
<td><tt class="xref docutils literal"><span class="pre">char[]</span></tt></td>
<td>string</td>
<td> </td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">P</span></tt></td>
<td><tt class="xref docutils literal"><span class="pre">void</span> <span class="pre">*</span></tt></td>
<td>long</td>
<td> </td>
</tr>
</tbody>
</table>
<p>Notes:</p>
<ol class="arabic">
<li><p class="first">The <tt class="docutils literal"><span class="pre">'?'</span></tt> conversion code corresponds to the <tt class="xref docutils literal"><span class="pre">_Bool</span></tt> type defined by
C99. If this type is not available, it is simulated using a <tt class="xref docutils literal"><span class="pre">char</span></tt>. In
standard mode, it is always represented by one byte.</p>
<p class="versionadded">
<span class="versionmodified">New in version 2.6.</span></p>
</li>
<li><p class="first">The <tt class="docutils literal"><span class="pre">'q'</span></tt> and <tt class="docutils literal"><span class="pre">'Q'</span></tt> conversion codes are available in native mode only if
the platform C compiler supports C <tt class="xref docutils literal"><span class="pre">long</span> <span class="pre">long</span></tt>, or, on Windows,
<tt class="xref docutils literal"><span class="pre">__int64</span></tt>. They are always available in standard modes.</p>
<p class="versionadded">
<span class="versionmodified">New in version 2.2.</span></p>
</li>
</ol>
<p>A format character may be preceded by an integral repeat count. For example,
the format string <tt class="docutils literal"><span class="pre">'4h'</span></tt> means exactly the same as <tt class="docutils literal"><span class="pre">'hhhh'</span></tt>.</p>
<p>Whitespace characters between formats are ignored; a count and its format must
not contain whitespace though.</p>
<p>For the <tt class="docutils literal"><span class="pre">'s'</span></tt> format character, the count is interpreted as the size of the
string, not a repeat count like for the other format characters; for example,
<tt class="docutils literal"><span class="pre">'10s'</span></tt> means a single 10-byte string, while <tt class="docutils literal"><span class="pre">'10c'</span></tt> 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, <tt class="docutils literal"><span class="pre">'0s'</span></tt> means a single, empty
string (while <tt class="docutils literal"><span class="pre">'0c'</span></tt> means 0 characters).</p>
<p>The <tt class="docutils literal"><span class="pre">'p'</span></tt> format character encodes a “Pascal string”, meaning a short
variable-length string stored in a fixed number of bytes. The count is the total
number of bytes stored. The first byte stored is the length of the string, or
255, whichever is smaller. The bytes of the string follow. If the string
passed in to <a title="struct.pack" class="reference internal" href="#struct.pack"><tt class="xref docutils literal"><span class="pre">pack()</span></tt></a> is too long (longer than the count minus 1), only the
leading count-1 bytes of the string are stored. If the string is shorter than
count-1, it is padded with null bytes so that exactly count bytes in all are
used. Note that for <a title="struct.unpack" class="reference internal" href="#struct.unpack"><tt class="xref docutils literal"><span class="pre">unpack()</span></tt></a>, the <tt class="docutils literal"><span class="pre">'p'</span></tt> format character consumes count
bytes, but that the string returned can never contain more than 255 characters.</p>
<p>For the <tt class="docutils literal"><span class="pre">'I'</span></tt>, <tt class="docutils literal"><span class="pre">'L'</span></tt>, <tt class="docutils literal"><span class="pre">'q'</span></tt> and <tt class="docutils literal"><span class="pre">'Q'</span></tt> format characters, the return
value is a Python long integer.</p>
<p>For the <tt class="docutils literal"><span class="pre">'P'</span></tt> 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 <em>NULL</em> pointer will always be returned as the Python integer
<tt class="docutils literal"><span class="pre">0</span></tt>. 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.</p>
<p>For the <tt class="docutils literal"><span class="pre">'?'</span></tt> format character, the return value is either <a title="True" class="reference external" href="constants.html#True"><tt class="xref xref docutils literal"><span class="pre">True</span></tt></a> or
<a title="False" class="reference external" href="constants.html#False"><tt class="xref xref docutils literal"><span class="pre">False</span></tt></a>. When packing, the truth value of the argument object is used.
Either 0 or 1 in the native or standard bool representation will be packed, and
any non-zero value will be True when unpacking.</p>
<p>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).</p>
<p>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:</p>
<table border="1" class="docutils">
<colgroup>
<col width="20%" />
<col width="44%" />
<col width="36%" />
</colgroup>
<thead valign="bottom">
<tr><th class="head">Character</th>
<th class="head">Byte order</th>
<th class="head">Size and alignment</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="docutils literal"><span class="pre">@</span></tt></td>
<td>native</td>
<td>native</td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">=</span></tt></td>
<td>native</td>
<td>standard</td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre"><</span></tt></td>
<td>little-endian</td>
<td>standard</td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">></span></tt></td>
<td>big-endian</td>
<td>standard</td>
</tr>
<tr><td><tt class="docutils literal"><span class="pre">!</span></tt></td>
<td>network (= big-endian)</td>
<td>standard</td>
</tr>
</tbody>
</table>
<p>If the first character is not one of these, <tt class="docutils literal"><span class="pre">'@'</span></tt> is assumed.</p>
<p>Native byte order is big-endian or little-endian, depending on the host system.
For example, Motorola and Sun processors are big-endian; Intel and DEC
processors are little-endian.</p>
<p>Native size and alignment are determined using the C compiler’s
<tt class="docutils literal"><span class="pre">sizeof</span></tt> expression. This is always combined with native byte order.</p>
<p>Standard size and alignment are as follows: no alignment is required for any
type (so you have to use pad bytes); <tt class="xref docutils literal"><span class="pre">short</span></tt> is 2 bytes; <tt class="xref docutils literal"><span class="pre">int</span></tt> and
<tt class="xref docutils literal"><span class="pre">long</span></tt> are 4 bytes; <tt class="xref docutils literal"><span class="pre">long</span> <span class="pre">long</span></tt> (<tt class="xref docutils literal"><span class="pre">__int64</span></tt> on Windows) is 8
bytes; <tt class="xref docutils literal"><span class="pre">float</span></tt> and <tt class="xref docutils literal"><span class="pre">double</span></tt> are 32-bit and 64-bit IEEE floating
point numbers, respectively. <tt class="xref docutils literal"><span class="pre">_Bool</span></tt> is 1 byte.</p>
<p>Note the difference between <tt class="docutils literal"><span class="pre">'@'</span></tt> and <tt class="docutils literal"><span class="pre">'='</span></tt>: both use native byte order, but
the size and alignment of the latter is standardized.</p>
<p>The form <tt class="docutils literal"><span class="pre">'!'</span></tt> is available for those poor souls who claim they can’t remember
whether network byte order is big-endian or little-endian.</p>
<p>There is no way to indicate non-native byte order (force byte-swapping); use the
appropriate choice of <tt class="docutils literal"><span class="pre">'<'</span></tt> or <tt class="docutils literal"><span class="pre">'>'</span></tt>.</p>
<p>The <tt class="docutils literal"><span class="pre">'P'</span></tt> format character is only available for the native byte ordering
(selected as the default or with the <tt class="docutils literal"><span class="pre">'@'</span></tt> byte order character). The byte
order character <tt class="docutils literal"><span class="pre">'='</span></tt> 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 <tt class="docutils literal"><span class="pre">'P'</span></tt> format is not available.</p>
<p>Examples (all using native byte order, size and alignment, on a big-endian
machine):</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">struct</span> <span class="kn">import</span> <span class="o">*</span>
<span class="gp">>>> </span><span class="n">pack</span><span class="p">(</span><span class="s">'hhl'</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span>
<span class="go">'\x00\x01\x00\x02\x00\x00\x00\x03'</span>
<span class="gp">>>> </span><span class="n">unpack</span><span class="p">(</span><span class="s">'hhl'</span><span class="p">,</span> <span class="s">'</span><span class="se">\x00\x01\x00\x02\x00\x00\x00\x03</span><span class="s">'</span><span class="p">)</span>
<span class="go">(1, 2, 3)</span>
<span class="gp">>>> </span><span class="n">calcsize</span><span class="p">(</span><span class="s">'hhl'</span><span class="p">)</span>
<span class="go">8</span>
</pre></div>
</div>
<p>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. For example, the format <tt class="docutils literal"><span class="pre">'llh0l'</span></tt> 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.</p>
<p>Unpacked fields can be named by assigning them to variables or by wrapping
the result in a named tuple:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">record</span> <span class="o">=</span> <span class="s">'raymond </span><span class="se">\x32\x12\x08\x01\x08</span><span class="s">'</span>
<span class="gp">>>> </span><span class="n">name</span><span class="p">,</span> <span class="n">serialnum</span><span class="p">,</span> <span class="n">school</span><span class="p">,</span> <span class="n">gradelevel</span> <span class="o">=</span> <span class="n">unpack</span><span class="p">(</span><span class="s">'<10sHHb'</span><span class="p">,</span> <span class="n">record</span><span class="p">)</span>
<span class="gp">>>> </span><span class="kn">from</span> <span class="nn">collections</span> <span class="kn">import</span> <span class="n">namedtuple</span>
<span class="gp">>>> </span><span class="n">Student</span> <span class="o">=</span> <span class="n">namedtuple</span><span class="p">(</span><span class="s">'Student'</span><span class="p">,</span> <span class="s">'name serialnum school gradelevel'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">Student</span><span class="o">.</span><span class="n">_make</span><span class="p">(</span><span class="n">unpack</span><span class="p">(</span><span class="s">'<10sHHb'</span><span class="p">,</span> <span class="n">s</span><span class="p">))</span>
<span class="go">Student(name='raymond ', serialnum=4658, school=264, gradelevel=8)</span>
</pre></div>
</div>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt>Module <a title="Space efficient arrays of uniformly typed numeric values." class="reference external" href="array.html#module-array"><tt class="xref docutils literal"><span class="pre">array</span></tt></a></dt>
<dd>Packed binary storage of homogeneous data.</dd>
<dt>Module <a title="Encoders and decoders for the External Data Representation (XDR)." class="reference external" href="xdrlib.html#module-xdrlib"><tt class="xref docutils literal"><span class="pre">xdrlib</span></tt></a></dt>
<dd>Packing and unpacking of XDR data.</dd>
</dl>
</div>
<div class="section" id="struct-objects">
<span id="id1"></span><h2>8.3.1. Struct Objects<a class="headerlink" href="#struct-objects" title="Permalink to this headline">¶</a></h2>
<p>The <tt class="xref docutils literal"><span class="pre">struct</span></tt> module also defines the following type:</p>
<dl class="class">
<dt id="struct.Struct">
<em class="property">class </em><tt class="descclassname">struct.</tt><tt class="descname">Struct</tt><big>(</big><em>format</em><big>)</big><a class="headerlink" href="#struct.Struct" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a new Struct object which writes and reads binary data according to the
format string <em>format</em>. Creating a Struct object once and calling its methods
is more efficient than calling the <tt class="xref docutils literal"><span class="pre">struct</span></tt> functions with the same format
since the format string only needs to be compiled once.</p>
<p class="versionadded">
<span class="versionmodified">New in version 2.5.</span></p>
<p>Compiled Struct objects support the following methods and attributes:</p>
<dl class="method">
<dt id="struct.Struct.pack">
<tt class="descname">pack</tt><big>(</big><em>v1</em>, <em>v2</em>, <em>...</em><big>)</big><a class="headerlink" href="#struct.Struct.pack" title="Permalink to this definition">¶</a></dt>
<dd>Identical to the <a title="struct.pack" class="reference internal" href="#struct.pack"><tt class="xref docutils literal"><span class="pre">pack()</span></tt></a> function, using the compiled format.
(<tt class="docutils literal"><span class="pre">len(result)</span></tt> will equal <tt class="xref docutils literal"><span class="pre">self.size</span></tt>.)</dd></dl>
<dl class="method">
<dt id="struct.Struct.pack_into">
<tt class="descname">pack_into</tt><big>(</big><em>buffer</em>, <em>offset</em>, <em>v1</em>, <em>v2</em>, <em>...</em><big>)</big><a class="headerlink" href="#struct.Struct.pack_into" title="Permalink to this definition">¶</a></dt>
<dd>Identical to the <a title="struct.pack_into" class="reference internal" href="#struct.pack_into"><tt class="xref docutils literal"><span class="pre">pack_into()</span></tt></a> function, using the compiled format.</dd></dl>
<dl class="method">
<dt id="struct.Struct.unpack">
<tt class="descname">unpack</tt><big>(</big><em>string</em><big>)</big><a class="headerlink" href="#struct.Struct.unpack" title="Permalink to this definition">¶</a></dt>
<dd>Identical to the <a title="struct.unpack" class="reference internal" href="#struct.unpack"><tt class="xref docutils literal"><span class="pre">unpack()</span></tt></a> function, using the compiled format.
(<tt class="docutils literal"><span class="pre">len(string)</span></tt> must equal <tt class="xref docutils literal"><span class="pre">self.size</span></tt>).</dd></dl>
<dl class="method">
<dt id="struct.Struct.unpack_from">
<tt class="descname">unpack_from</tt><big>(</big><em>buffer</em><span class="optional">[</span>, <em>offset=0</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#struct.Struct.unpack_from" title="Permalink to this definition">¶</a></dt>
<dd>Identical to the <a title="struct.unpack_from" class="reference internal" href="#struct.unpack_from"><tt class="xref docutils literal"><span class="pre">unpack_from()</span></tt></a> function, using the compiled format.
(<tt class="docutils literal"><span class="pre">len(buffer[offset:])</span></tt> must be at least <tt class="xref docutils literal"><span class="pre">self.size</span></tt>).</dd></dl>
<dl class="attribute">
<dt id="struct.Struct.format">
<tt class="descname">format</tt><a class="headerlink" href="#struct.Struct.format" title="Permalink to this definition">¶</a></dt>
<dd>The format string used to construct this Struct object.</dd></dl>
<dl class="attribute">
<dt id="struct.Struct.size">
<tt class="descname">size</tt><a class="headerlink" href="#struct.Struct.size" title="Permalink to this definition">¶</a></dt>
<dd>The calculated size of the struct (and hence of the string) corresponding
to <a title="format" class="reference external" href="functions.html#format"><tt class="xref docutils literal"><span class="pre">format</span></tt></a>.</dd></dl>
</dd></dl>
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<li><a class="reference external" href="#struct-objects">8.3.1. Struct Objects</a></li>
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