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<title>Platform-Specific Implementation Notes</title>
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<div class="titlepage"><div><div><h3 class="title">
<a name="asio.overview.implementation"></a><a class="link" href="implementation.html" title="Platform-Specific Implementation Notes">Platform-Specific Implementation
      Notes</a>
</h3></div></div></div>
<p>
        This section lists platform-specific implementation details, such as the
        default demultiplexing mechanism, the number of threads created internally,
        and when threads are created.
      </p>
<h5>
<a name="asio.overview.implementation.h0"></a>
        <span><a name="asio.overview.implementation.linux_kernel_2_4"></a></span><a class="link" href="implementation.html#asio.overview.implementation.linux_kernel_2_4">Linux
        Kernel 2.4</a>
      </h5>
<p>
        Demultiplexing mechanism:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
            Uses <code class="computeroutput"><span class="identifier">select</span></code> for demultiplexing.
            This means that the number of file descriptors in the process cannot
            be permitted to exceed <code class="computeroutput"><span class="identifier">FD_SETSIZE</span></code>.
          </li></ul></div>
<p>
        Threads:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
            Demultiplexing using <code class="computeroutput"><span class="identifier">select</span></code>
            is performed in one of the threads that calls <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run_one</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">poll</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">poll_one</span><span class="special">()</span></code>.
          </li>
<li class="listitem">
            An additional thread per <code class="computeroutput"><span class="identifier">io_service</span></code>
            is used to emulate asynchronous host resolution. This thread is created
            on the first call to either <code class="computeroutput"><span class="identifier">ip</span><span class="special">::</span><span class="identifier">tcp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">async_resolve</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">ip</span><span class="special">::</span><span class="identifier">udp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">async_resolve</span><span class="special">()</span></code>.
          </li>
</ul></div>
<p>
        Scatter-Gather:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
            At most <code class="computeroutput"><span class="identifier">min</span><span class="special">(</span><span class="number">64</span><span class="special">,</span><span class="identifier">IOV_MAX</span><span class="special">)</span></code> buffers may be transferred in a single
            operation.
          </li></ul></div>
<h5>
<a name="asio.overview.implementation.h1"></a>
        <span><a name="asio.overview.implementation.linux_kernel_2_6"></a></span><a class="link" href="implementation.html#asio.overview.implementation.linux_kernel_2_6">Linux
        Kernel 2.6</a>
      </h5>
<p>
        Demultiplexing mechanism:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
            Uses <code class="computeroutput"><span class="identifier">epoll</span></code> for demultiplexing.
          </li></ul></div>
<p>
        Threads:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
            Demultiplexing using <code class="computeroutput"><span class="identifier">epoll</span></code>
            is performed in one of the threads that calls <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run_one</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">poll</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">poll_one</span><span class="special">()</span></code>.
          </li>
<li class="listitem">
            An additional thread per <code class="computeroutput"><span class="identifier">io_service</span></code>
            is used to emulate asynchronous host resolution. This thread is created
            on the first call to either <code class="computeroutput"><span class="identifier">ip</span><span class="special">::</span><span class="identifier">tcp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">async_resolve</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">ip</span><span class="special">::</span><span class="identifier">udp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">async_resolve</span><span class="special">()</span></code>.
          </li>
</ul></div>
<p>
        Scatter-Gather:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
            At most <code class="computeroutput"><span class="identifier">min</span><span class="special">(</span><span class="number">64</span><span class="special">,</span><span class="identifier">IOV_MAX</span><span class="special">)</span></code> buffers may be transferred in a single
            operation.
          </li></ul></div>
<h5>
<a name="asio.overview.implementation.h2"></a>
        <span><a name="asio.overview.implementation.solaris"></a></span><a class="link" href="implementation.html#asio.overview.implementation.solaris">Solaris</a>
      </h5>
<p>
        Demultiplexing mechanism:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
            Uses <code class="literal">/dev/poll</code> for demultiplexing.
          </li></ul></div>
<p>
        Threads:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
            Demultiplexing using <code class="literal">/dev/poll</code> is performed in one
            of the threads that calls <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run_one</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">poll</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">poll_one</span><span class="special">()</span></code>.
          </li>
<li class="listitem">
            An additional thread per <code class="computeroutput"><span class="identifier">io_service</span></code>
            is used to emulate asynchronous host resolution. This thread is created
            on the first call to either <code class="computeroutput"><span class="identifier">ip</span><span class="special">::</span><span class="identifier">tcp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">async_resolve</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">ip</span><span class="special">::</span><span class="identifier">udp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">async_resolve</span><span class="special">()</span></code>.
          </li>
</ul></div>
<p>
        Scatter-Gather:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
            At most <code class="computeroutput"><span class="identifier">min</span><span class="special">(</span><span class="number">64</span><span class="special">,</span><span class="identifier">IOV_MAX</span><span class="special">)</span></code> buffers may be transferred in a single
            operation.
          </li></ul></div>
<h5>
<a name="asio.overview.implementation.h3"></a>
        <span><a name="asio.overview.implementation.qnx_neutrino"></a></span><a class="link" href="implementation.html#asio.overview.implementation.qnx_neutrino">QNX
        Neutrino</a>
      </h5>
<p>
        Demultiplexing mechanism:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
            Uses <code class="computeroutput"><span class="identifier">select</span></code> for demultiplexing.
            This means that the number of file descriptors in the process cannot
            be permitted to exceed <code class="computeroutput"><span class="identifier">FD_SETSIZE</span></code>.
          </li></ul></div>
<p>
        Threads:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
            Demultiplexing using <code class="computeroutput"><span class="identifier">select</span></code>
            is performed in one of the threads that calls <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run_one</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">poll</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">poll_one</span><span class="special">()</span></code>.
          </li>
<li class="listitem">
            An additional thread per <code class="computeroutput"><span class="identifier">io_service</span></code>
            is used to emulate asynchronous host resolution. This thread is created
            on the first call to either <code class="computeroutput"><span class="identifier">ip</span><span class="special">::</span><span class="identifier">tcp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">async_resolve</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">ip</span><span class="special">::</span><span class="identifier">udp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">async_resolve</span><span class="special">()</span></code>.
          </li>
</ul></div>
<p>
        Scatter-Gather:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
            At most <code class="computeroutput"><span class="identifier">min</span><span class="special">(</span><span class="number">64</span><span class="special">,</span><span class="identifier">IOV_MAX</span><span class="special">)</span></code> buffers may be transferred in a single
            operation.
          </li></ul></div>
<h5>
<a name="asio.overview.implementation.h4"></a>
        <span><a name="asio.overview.implementation.mac_os_x"></a></span><a class="link" href="implementation.html#asio.overview.implementation.mac_os_x">Mac
        OS X</a>
      </h5>
<p>
        Demultiplexing mechanism:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
            Uses <code class="computeroutput"><span class="identifier">kqueue</span></code> for demultiplexing.
          </li></ul></div>
<p>
        Threads:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
            Demultiplexing using <code class="computeroutput"><span class="identifier">kqueue</span></code>
            is performed in one of the threads that calls <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run_one</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">poll</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">poll_one</span><span class="special">()</span></code>.
          </li>
<li class="listitem">
            An additional thread per <code class="computeroutput"><span class="identifier">io_service</span></code>
            is used to emulate asynchronous host resolution. This thread is created
            on the first call to either <code class="computeroutput"><span class="identifier">ip</span><span class="special">::</span><span class="identifier">tcp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">async_resolve</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">ip</span><span class="special">::</span><span class="identifier">udp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">async_resolve</span><span class="special">()</span></code>.
          </li>
</ul></div>
<p>
        Scatter-Gather:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
            At most <code class="computeroutput"><span class="identifier">min</span><span class="special">(</span><span class="number">64</span><span class="special">,</span><span class="identifier">IOV_MAX</span><span class="special">)</span></code> buffers may be transferred in a single
            operation.
          </li></ul></div>
<h5>
<a name="asio.overview.implementation.h5"></a>
        <span><a name="asio.overview.implementation.freebsd"></a></span><a class="link" href="implementation.html#asio.overview.implementation.freebsd">FreeBSD</a>
      </h5>
<p>
        Demultiplexing mechanism:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
            Uses <code class="computeroutput"><span class="identifier">kqueue</span></code> for demultiplexing.
          </li></ul></div>
<p>
        Threads:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
            Demultiplexing using <code class="computeroutput"><span class="identifier">kqueue</span></code>
            is performed in one of the threads that calls <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run_one</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">poll</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">poll_one</span><span class="special">()</span></code>.
          </li>
<li class="listitem">
            An additional thread per <code class="computeroutput"><span class="identifier">io_service</span></code>
            is used to emulate asynchronous host resolution. This thread is created
            on the first call to either <code class="computeroutput"><span class="identifier">ip</span><span class="special">::</span><span class="identifier">tcp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">async_resolve</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">ip</span><span class="special">::</span><span class="identifier">udp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">async_resolve</span><span class="special">()</span></code>.
          </li>
</ul></div>
<p>
        Scatter-Gather:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
            At most <code class="computeroutput"><span class="identifier">min</span><span class="special">(</span><span class="number">64</span><span class="special">,</span><span class="identifier">IOV_MAX</span><span class="special">)</span></code> buffers may be transferred in a single
            operation.
          </li></ul></div>
<h5>
<a name="asio.overview.implementation.h6"></a>
        <span><a name="asio.overview.implementation.aix"></a></span><a class="link" href="implementation.html#asio.overview.implementation.aix">AIX</a>
      </h5>
<p>
        Demultiplexing mechanism:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
            Uses <code class="computeroutput"><span class="identifier">select</span></code> for demultiplexing.
            This means that the number of file descriptors in the process cannot
            be permitted to exceed <code class="computeroutput"><span class="identifier">FD_SETSIZE</span></code>.
          </li></ul></div>
<p>
        Threads:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
            Demultiplexing using <code class="computeroutput"><span class="identifier">select</span></code>
            is performed in one of the threads that calls <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run_one</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">poll</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">poll_one</span><span class="special">()</span></code>.
          </li>
<li class="listitem">
            An additional thread per <code class="computeroutput"><span class="identifier">io_service</span></code>
            is used to emulate asynchronous host resolution. This thread is created
            on the first call to either <code class="computeroutput"><span class="identifier">ip</span><span class="special">::</span><span class="identifier">tcp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">async_resolve</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">ip</span><span class="special">::</span><span class="identifier">udp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">async_resolve</span><span class="special">()</span></code>.
          </li>
</ul></div>
<p>
        Scatter-Gather:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
            At most <code class="computeroutput"><span class="identifier">min</span><span class="special">(</span><span class="number">64</span><span class="special">,</span><span class="identifier">IOV_MAX</span><span class="special">)</span></code> buffers may be transferred in a single
            operation.
          </li></ul></div>
<h5>
<a name="asio.overview.implementation.h7"></a>
        <span><a name="asio.overview.implementation.hp_ux"></a></span><a class="link" href="implementation.html#asio.overview.implementation.hp_ux">HP-UX</a>
      </h5>
<p>
        Demultiplexing mechanism:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
            Uses <code class="computeroutput"><span class="identifier">select</span></code> for demultiplexing.
            This means that the number of file descriptors in the process cannot
            be permitted to exceed <code class="computeroutput"><span class="identifier">FD_SETSIZE</span></code>.
          </li></ul></div>
<p>
        Threads:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
            Demultiplexing using <code class="computeroutput"><span class="identifier">select</span></code>
            is performed in one of the threads that calls <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run_one</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">poll</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">poll_one</span><span class="special">()</span></code>.
          </li>
<li class="listitem">
            An additional thread per <code class="computeroutput"><span class="identifier">io_service</span></code>
            is used to emulate asynchronous host resolution. This thread is created
            on the first call to either <code class="computeroutput"><span class="identifier">ip</span><span class="special">::</span><span class="identifier">tcp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">async_resolve</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">ip</span><span class="special">::</span><span class="identifier">udp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">async_resolve</span><span class="special">()</span></code>.
          </li>
</ul></div>
<p>
        Scatter-Gather:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
            At most <code class="computeroutput"><span class="identifier">min</span><span class="special">(</span><span class="number">64</span><span class="special">,</span><span class="identifier">IOV_MAX</span><span class="special">)</span></code> buffers may be transferred in a single
            operation.
          </li></ul></div>
<h5>
<a name="asio.overview.implementation.h8"></a>
        <span><a name="asio.overview.implementation.tru64"></a></span><a class="link" href="implementation.html#asio.overview.implementation.tru64">Tru64</a>
      </h5>
<p>
        Demultiplexing mechanism:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
            Uses <code class="computeroutput"><span class="identifier">select</span></code> for demultiplexing.
            This means that the number of file descriptors in the process cannot
            be permitted to exceed <code class="computeroutput"><span class="identifier">FD_SETSIZE</span></code>.
          </li></ul></div>
<p>
        Threads:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
            Demultiplexing using <code class="computeroutput"><span class="identifier">select</span></code>
            is performed in one of the threads that calls <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run_one</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">poll</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">poll_one</span><span class="special">()</span></code>.
          </li>
<li class="listitem">
            An additional thread per <code class="computeroutput"><span class="identifier">io_service</span></code>
            is used to emulate asynchronous host resolution. This thread is created
            on the first call to either <code class="computeroutput"><span class="identifier">ip</span><span class="special">::</span><span class="identifier">tcp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">async_resolve</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">ip</span><span class="special">::</span><span class="identifier">udp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">async_resolve</span><span class="special">()</span></code>.
          </li>
</ul></div>
<p>
        Scatter-Gather:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
            At most <code class="computeroutput"><span class="identifier">min</span><span class="special">(</span><span class="number">64</span><span class="special">,</span><span class="identifier">IOV_MAX</span><span class="special">)</span></code> buffers may be transferred in a single
            operation.
          </li></ul></div>
<h5>
<a name="asio.overview.implementation.h9"></a>
        <span><a name="asio.overview.implementation.windows_95__98_and_me"></a></span><a class="link" href="implementation.html#asio.overview.implementation.windows_95__98_and_me">Windows
        95, 98 and Me</a>
      </h5>
<p>
        Demultiplexing mechanism:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
            Uses <code class="computeroutput"><span class="identifier">select</span></code> for demultiplexing.
          </li></ul></div>
<p>
        Threads:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
            Demultiplexing using <code class="computeroutput"><span class="identifier">select</span></code>
            is performed in one of the threads that calls <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run_one</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">poll</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">poll_one</span><span class="special">()</span></code>.
          </li>
<li class="listitem">
            An additional thread per <code class="computeroutput"><span class="identifier">io_service</span></code>
            is used to emulate asynchronous host resolution. This thread is created
            on the first call to either <code class="computeroutput"><span class="identifier">ip</span><span class="special">::</span><span class="identifier">tcp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">async_resolve</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">ip</span><span class="special">::</span><span class="identifier">udp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">async_resolve</span><span class="special">()</span></code>.
          </li>
</ul></div>
<p>
        Scatter-Gather:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
            For sockets, at most 16 buffers may be transferred in a single operation.
          </li></ul></div>
<h5>
<a name="asio.overview.implementation.h10"></a>
        <span><a name="asio.overview.implementation.windows_nt__2000__xp__2003__vista__7_and_8"></a></span><a class="link" href="implementation.html#asio.overview.implementation.windows_nt__2000__xp__2003__vista__7_and_8">Windows
        NT, 2000, XP, 2003, Vista, 7 and 8</a>
      </h5>
<p>
        Demultiplexing mechanism:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
            Uses overlapped I/O and I/O completion ports for all asynchronous socket
            operations except for asynchronous connect.
          </li>
<li class="listitem">
            Uses <code class="computeroutput"><span class="identifier">select</span></code> for emulating
            asynchronous connect.
          </li>
</ul></div>
<p>
        Threads:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
            Demultiplexing using I/O completion ports is performed in all threads
            that call <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">run_one</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">poll</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">poll_one</span><span class="special">()</span></code>.
          </li>
<li class="listitem">
            An additional thread per <code class="computeroutput"><span class="identifier">io_service</span></code>
            is used to trigger timers. This thread is created on construction of
            the first <code class="computeroutput"><span class="identifier">deadline_timer</span></code>
            or <code class="computeroutput"><span class="identifier">deadline_timer_service</span></code>
            objects.
          </li>
<li class="listitem">
            An additional thread per <code class="computeroutput"><span class="identifier">io_service</span></code>
            is used for the <code class="computeroutput"><span class="identifier">select</span></code>
            demultiplexing. This thread is created on the first call to <code class="computeroutput"><span class="identifier">async_connect</span><span class="special">()</span></code>.
          </li>
<li class="listitem">
            An additional thread per <code class="computeroutput"><span class="identifier">io_service</span></code>
            is used to emulate asynchronous host resolution. This thread is created
            on the first call to either <code class="computeroutput"><span class="identifier">ip</span><span class="special">::</span><span class="identifier">tcp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">async_resolve</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">ip</span><span class="special">::</span><span class="identifier">udp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">async_resolve</span><span class="special">()</span></code>.
          </li>
</ul></div>
<p>
        Scatter-Gather:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
            For sockets, at most 64 buffers may be transferred in a single operation.
          </li>
<li class="listitem">
            For stream-oriented handles, only one buffer may be transferred in a
            single operation.
          </li>
</ul></div>
<h5>
<a name="asio.overview.implementation.h11"></a>
        <span><a name="asio.overview.implementation.windows_runtime"></a></span><a class="link" href="implementation.html#asio.overview.implementation.windows_runtime">Windows
        Runtime</a>
      </h5>
<p>
        Asio provides limited support for the Windows Runtime. It requires that the
        language extensions be enabled. Due to the restricted facilities exposed
        by the Windows Runtime API, the support comes with the following caveats:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
            The core facilities such as the <code class="computeroutput"><span class="identifier">io_service</span></code>,
            <code class="computeroutput"><span class="identifier">strand</span></code>, buffers, composed
            operations, timers, etc., should all work as normal.
          </li>
<li class="listitem">
            For sockets, only client-side TCP is supported.
          </li>
<li class="listitem">
            Explicit binding of a client-side TCP socket is not supported.
          </li>
<li class="listitem">
            The <code class="computeroutput"><span class="identifier">cancel</span><span class="special">()</span></code>
            function is not supported for sockets. Asynchronous operations may only
            be cancelled by closing the socket.
          </li>
<li class="listitem">
            Operations that use <code class="computeroutput"><span class="identifier">null_buffers</span></code>
            are not supported.
          </li>
<li class="listitem">
            Only <code class="computeroutput"><span class="identifier">tcp</span><span class="special">::</span><span class="identifier">no_delay</span></code> and <code class="computeroutput"><span class="identifier">socket_base</span><span class="special">::</span><span class="identifier">keep_alive</span></code>
            options are supported.
          </li>
<li class="listitem">
            Resolvers do not support service names, only numbers. I.e. you must use
            "80" rather than "http".
          </li>
<li class="listitem">
            Most resolver query flags have no effect.
          </li>
</ul></div>
<p>
        Demultiplexing mechanism:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
            Uses the <code class="computeroutput"><span class="identifier">Windows</span><span class="special">::</span><span class="identifier">Networking</span><span class="special">::</span><span class="identifier">Sockets</span><span class="special">::</span><span class="identifier">StreamSocket</span></code> class to implement asynchronous
            TCP socket operations.
          </li></ul></div>
<p>
        Threads:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
            Event completions are delivered to the Windows thread pool and posted
            to the <code class="computeroutput"><span class="identifier">io_service</span></code> for
            the handler to be executed.
          </li>
<li class="listitem">
            An additional thread per <code class="computeroutput"><span class="identifier">io_service</span></code>
            is used to trigger timers. This thread is created on construction of
            the first timer objects.
          </li>
</ul></div>
<p>
        Scatter-Gather:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
            For sockets, at most one buffer may be transferred in a single operation.
          </li></ul></div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2014 Christopher M. Kohlhoff<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
</div></td>
</tr></table>
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