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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="fiber.rationale"></a><a class="link" href="rationale.html" title="Rationale">Rationale</a>
</h2></div></div></div>
<h4>
<a name="fiber.rationale.h0"></a>
<span><a name="fiber.rationale.distinction_between_coroutines_and_fibers"></a></span><a class="link" href="rationale.html#fiber.rationale.distinction_between_coroutines_and_fibers">distinction
between coroutines and fibers</a>
</h4>
<p>
The fiber library extends the coroutine library by adding a scheduler and synchronization
mechanisms.
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
a coroutine yields
</li>
<li class="listitem">
a fiber blocks
</li>
</ul></div>
<p>
When a coroutine yields, it passes control directly to its caller (or, in the
case of symmetric coroutines, a designated other coroutine). When a fiber blocks,
it implicitly passes control to the fiber scheduler. Coroutines have no scheduler
because they need no scheduler. <sup>[<a name="fiber.rationale.f0" href="#ftn.fiber.rationale.f0" class="footnote">1</a>]</sup>.
</p>
<h4>
<a name="fiber.rationale.h1"></a>
<span><a name="fiber.rationale.what_about_transactional_memory"></a></span><a class="link" href="rationale.html#fiber.rationale.what_about_transactional_memory">what
about transactional memory</a>
</h4>
<p>
GCC supports transactional memory since version 4.7. Unfortunately tests show
that transactional memory is slower (ca. 4x) than spinlocks using atomics.
Once transactional memory is improved (supporting hybrid tm), spinlocks will
be replaced by __transaction_atomic{} statements surrounding the critical sections.
</p>
<h4>
<a name="fiber.rationale.h2"></a>
<span><a name="fiber.rationale.synchronization_between_fibers_running_in_different_threads"></a></span><a class="link" href="rationale.html#fiber.rationale.synchronization_between_fibers_running_in_different_threads">synchronization
between fibers running in different threads</a>
</h4>
<p>
Synchronization classes from <a href="http://www.boost.org/doc/libs/release/libs/thread/index.html" target="_top">Boost.Thread</a>
block the entire thread. In contrast, the synchronization classes from <span class="bold"><strong>Boost.Fiber</strong></span> block only specific fibers, so that the
scheduler can still keep the thread busy running other fibers in the meantime.
The synchronization classes from <span class="bold"><strong>Boost.Fiber</strong></span>
are designed to be thread-safe, i.e. it is possible to synchronize fibers running
in different threads as well as fibers running in the same thread. (However,
there is a build option to disable cross-thread fiber synchronization support;
see <a class="link" href="overview.html#cross_thread_sync">this description</a>.)
</p>
<h4>
<a name="fiber.rationale.h3"></a>
<span><a name="fiber.rationale.migrating_fibers_between_threads"></a></span><a class="link" href="rationale.html#fiber.rationale.migrating_fibers_between_threads">migrating
fibers between threads</a>
</h4>
<p>
Support for migrating fibers between threads has been integrated. The users-defined
scheduler has to call <code class="computeroutput"><span class="identifier">context</span><span class="special">::</span><span class="identifier">migrate</span><span class="special">()</span></code> with the migrated fiber-context as argument
(see <a class="link" href="custom.html#custom">Customization</a>; examles <code class="computeroutput"> <span class="identifier">work_sharing</span></code>
and <code class="computeroutput"><span class="identifier">work_stealing</span></code> in directory
<code class="computeroutput"><span class="identifier">examples</span></code> might be used as a
blue-print).
</p>
<h4>
<a name="fiber.rationale.h4"></a>
<span><a name="fiber.rationale.support_for_boost_asio"></a></span><a class="link" href="rationale.html#fiber.rationale.support_for_boost_asio">support
for Boost.Asio</a>
</h4>
<p>
Support for <a href="http://www.boost.org/doc/libs/release/libs/asio/index.html" target="_top">Boost.Asio</a>'s
<span class="emphasis"><em>async-result</em></span> is not part of the official API. However,
to integrate with a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_service</span></code>,
see <a class="link" href="integration.html#integration">Sharing a Thread with Another Main Loop</a>.
To interface smoothly with an arbitrary Asio async I/O operation, see <a class="link" href="callbacks.html#callbacks_asio">Then There's <a href="http://www.boost.org/doc/libs/release/libs/asio/index.html" target="_top">Boost.Asio</a></a>.
</p>
<h4>
<a name="fiber.rationale.h5"></a>
<span><a name="fiber.rationale.tested_compilers"></a></span><a class="link" href="rationale.html#fiber.rationale.tested_compilers">tested
compilers</a>
</h4>
<p>
The library was tested with GCC-4.9.2, GCC-5.1.1, Clang-3.6.0 and MSVC-14.0
in c++14-mode.
</p>
<h4>
<a name="fiber.rationale.h6"></a>
<span><a name="fiber.rationale.supported_architectures"></a></span><a class="link" href="rationale.html#fiber.rationale.supported_architectures">supported
architectures</a>
</h4>
<p>
<span class="bold"><strong>Boost.Fiber</strong></span> depends on <a href="http://www.boost.org/doc/libs/release/libs/context/index.html" target="_top">Boost.Context</a>
- the list of supported architectures can be found <a href="http://www.boost.org/doc/libs/release/libs/context/doc/html/context/architectures.html" target="_top">here</a>.
</p>
<div class="footnotes">
<br><hr width="100" align="left">
<div class="footnote"><p><sup>[<a name="ftn.fiber.rationale.f0" href="#fiber.rationale.f0" class="para">1</a>] </sup>
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4024.pdf" target="_top">'N4024:
Distinguishing coroutines and fibers'</a>
</p></div>
</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2013 Oliver Kowalke<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>
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