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<div class="section" id="sending-binary-data">
<h1>Sending Binary Data<a class="headerlink" href="#sending-binary-data" title="Permalink to this headline">¶</a></h1>
<p>Sockets transmit streams of bytes. Those bytes can contain text
messages, as in the previous examples, or they can be made up of
binary data that has been encoded for transmission. To prepare binary
data values for transmission, pack them into a buffer with
<a class="reference internal" href="../struct/index.html#module-struct" title="struct: Convert between strings and binary data."><tt class="xref py py-mod docutils literal"><span class="pre">struct</span></tt></a>.</p>
<p>This client program encodes an integer, a string of two characters,
and a floating point value into a sequence of bytes that can be passed
to the socket for transmission.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">binascii</span>
<span class="kn">import</span> <span class="nn">socket</span>
<span class="kn">import</span> <span class="nn">struct</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="c"># Create a TCP/IP socket</span>
<span class="n">sock</span> <span class="o">=</span> <span class="n">socket</span><span class="o">.</span><span class="n">socket</span><span class="p">(</span><span class="n">socket</span><span class="o">.</span><span class="n">AF_INET</span><span class="p">,</span> <span class="n">socket</span><span class="o">.</span><span class="n">SOCK_STREAM</span><span class="p">)</span>
<span class="n">server_address</span> <span class="o">=</span> <span class="p">(</span><span class="s">'localhost'</span><span class="p">,</span> <span class="mi">10000</span><span class="p">)</span>
<span class="n">sock</span><span class="o">.</span><span class="n">connect</span><span class="p">(</span><span class="n">server_address</span><span class="p">)</span>
<span class="n">values</span> <span class="o">=</span> <span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="s">'ab'</span><span class="p">,</span> <span class="mf">2.7</span><span class="p">)</span>
<span class="n">packer</span> <span class="o">=</span> <span class="n">struct</span><span class="o">.</span><span class="n">Struct</span><span class="p">(</span><span class="s">'I 2s f'</span><span class="p">)</span>
<span class="n">packed_data</span> <span class="o">=</span> <span class="n">packer</span><span class="o">.</span><span class="n">pack</span><span class="p">(</span><span class="o">*</span><span class="n">values</span><span class="p">)</span>
<span class="k">try</span><span class="p">:</span>
<span class="c"># Send data</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'sending "</span><span class="si">%s</span><span class="s">"'</span> <span class="o">%</span> <span class="n">binascii</span><span class="o">.</span><span class="n">hexlify</span><span class="p">(</span><span class="n">packed_data</span><span class="p">),</span> <span class="n">values</span>
<span class="n">sock</span><span class="o">.</span><span class="n">sendall</span><span class="p">(</span><span class="n">packed_data</span><span class="p">)</span>
<span class="k">finally</span><span class="p">:</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'closing socket'</span>
<span class="n">sock</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
</pre></div>
</div>
<p>The server program uses the same <tt class="xref py py-class docutils literal"><span class="pre">Struct</span></tt> specifier to unpack
the bytes it receives.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">binascii</span>
<span class="kn">import</span> <span class="nn">socket</span>
<span class="kn">import</span> <span class="nn">struct</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="c"># Create a TCP/IP socket</span>
<span class="n">sock</span> <span class="o">=</span> <span class="n">socket</span><span class="o">.</span><span class="n">socket</span><span class="p">(</span><span class="n">socket</span><span class="o">.</span><span class="n">AF_INET</span><span class="p">,</span> <span class="n">socket</span><span class="o">.</span><span class="n">SOCK_STREAM</span><span class="p">)</span>
<span class="n">server_address</span> <span class="o">=</span> <span class="p">(</span><span class="s">'localhost'</span><span class="p">,</span> <span class="mi">10000</span><span class="p">)</span>
<span class="n">sock</span><span class="o">.</span><span class="n">bind</span><span class="p">(</span><span class="n">server_address</span><span class="p">)</span>
<span class="n">sock</span><span class="o">.</span><span class="n">listen</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="n">unpacker</span> <span class="o">=</span> <span class="n">struct</span><span class="o">.</span><span class="n">Struct</span><span class="p">(</span><span class="s">'I 2s f'</span><span class="p">)</span>
<span class="k">while</span> <span class="bp">True</span><span class="p">:</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'</span><span class="se">\n</span><span class="s">waiting for a connection'</span>
<span class="n">connection</span><span class="p">,</span> <span class="n">client_address</span> <span class="o">=</span> <span class="n">sock</span><span class="o">.</span><span class="n">accept</span><span class="p">()</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">connection</span><span class="o">.</span><span class="n">recv</span><span class="p">(</span><span class="n">unpacker</span><span class="o">.</span><span class="n">size</span><span class="p">)</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'received "</span><span class="si">%s</span><span class="s">"'</span> <span class="o">%</span> <span class="n">binascii</span><span class="o">.</span><span class="n">hexlify</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>
<span class="n">unpacked_data</span> <span class="o">=</span> <span class="n">unpacker</span><span class="o">.</span><span class="n">unpack</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>
<span class="k">print</span> <span class="o">>></span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">,</span> <span class="s">'unpacked:'</span><span class="p">,</span> <span class="n">unpacked_data</span>
<span class="k">finally</span><span class="p">:</span>
<span class="n">connection</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
</pre></div>
</div>
<p>Running the client produces:</p>
<div class="highlight-python"><pre>$ python ./socket_binary_client.py
sending "0100000061620000cdcc2c40" (1, 'ab', 2.7)
closing socket</pre>
</div>
<p>And the server shows the values it receives:</p>
<div class="highlight-python"><pre>$ python ./socket_binary_server.py
waiting for a connection
received "0100000061620000cdcc2c40"
unpacked: (1, 'ab', 2.700000047683716)
waiting for a connection</pre>
</div>
<p>The floating point value loses some precision as it is packed and
unpacked, but otherwise the data is transmitted as expected. One
thing to keep in mind is that depending on the value of the integer,
it may be more efficient to convert it to text and then transmit,
instead of using <a class="reference internal" href="../struct/index.html#module-struct" title="struct: Convert between strings and binary data."><tt class="xref py py-mod docutils literal"><span class="pre">struct</span></tt></a>. The integer <tt class="docutils literal"><span class="pre">1</span></tt> uses one byte when
represented as a string, but four when packed into the structure.</p>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt><a class="reference internal" href="../struct/index.html#module-struct" title="struct: Convert between strings and binary data."><tt class="xref py py-mod docutils literal"><span class="pre">struct</span></tt></a></dt>
<dd>Converting between strings and other data types.</dd>
</dl>
</div>
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