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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Implementation</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="ext_concurrency.html" title="Chapter 29. Concurrency" /><link rel="prev" href="ext_concurrency.html" title="Chapter 29. Concurrency" /><link rel="next" href="ext_concurrency_use.html" title="Use" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Implementation</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ext_concurrency.html">Prev</a> </td><th width="60%" align="center">Chapter 29. Concurrency</th><td width="20%" align="right"> <a accesskey="n" href="ext_concurrency_use.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.ext.concurrency.impl"></a>Implementation</h2></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="manual.ext.concurrency.impl.atomic_fallbacks"></a>Using Built-in Atomic Functions</h3></div></div></div><p>The functions for atomic operations described above are either
implemented via compiler intrinsics (if the underlying host is
capable) or by library fallbacks.</p><p>Compiler intrinsics (builtins) are always preferred.  However, as
the compiler builtins for atomics are not universally implemented,
using them directly is problematic, and can result in undefined
function calls.
</p><p>Prior to GCC 4.7 the older <code class="code">__sync</code> intrinsics were used.
An example of an undefined symbol from the use
of <code class="code">__sync_fetch_and_add</code> on an unsupported host is a
missing reference to <code class="code">__sync_fetch_and_add_4</code>.
</p><p>Current releases use the newer <code class="code">__atomic</code> intrinsics,
which are implemented by library calls if the hardware doesn't support them.
Undefined references to functions like
<code class="code">__atomic_is_lock_free</code> should be resolved by linking to
<code class="filename">libatomic</code>, which is usually
installed alongside libstdc++.
</p><p>In addition, on some hosts the compiler intrinsics are enabled
conditionally, via the <code class="code">-march</code> command line flag. This makes
usage vary depending on the target hardware and the flags used during
compile.
</p><p>
<em><span class="remark">
Incomplete/inconsistent. This is only C++11.
</span></em>
</p><p>
If builtins are possible for bool-sized integral types,
<code class="code">ATOMIC_BOOL_LOCK_FREE</code> will be defined.
If builtins are possible for int-sized integral types,
<code class="code">ATOMIC_INT_LOCK_FREE</code> will be defined.
</p><p>For the following hosts, intrinsics are enabled by default.
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>alpha</p></li><li class="listitem"><p>ia64</p></li><li class="listitem"><p>powerpc</p></li><li class="listitem"><p>s390</p></li></ul></div><p>For others, some form of <code class="code">-march</code> may work. On
non-ancient x86 hardware, <code class="code">-march=native</code> usually does the
trick.</p><p> For hosts without compiler intrinsics, but with capable
hardware, hand-crafted assembly is selected. This is the case for the following hosts:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>cris</p></li><li class="listitem"><p>hppa</p></li><li class="listitem"><p>i386</p></li><li class="listitem"><p>i486</p></li><li class="listitem"><p>m48k</p></li><li class="listitem"><p>mips</p></li><li class="listitem"><p>sparc</p></li></ul></div><p>And for the rest, a simulated atomic lock via pthreads.
</p><p> Detailed information about compiler intrinsics for atomic operations can be found in the GCC <a class="link" href="http://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html" target="_top"> documentation</a>.
</p><p> More details on the library fallbacks from the porting <a class="link" href="internals.html#internals.thread_safety" title="Thread Safety">section</a>.
</p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="manual.ext.concurrency.impl.thread"></a>Thread Abstraction</h3></div></div></div><p>A thin layer above IEEE 1003.1 (i.e. pthreads) is used to abstract
the thread interface for GCC. This layer is called "gthread," and is
comprised of one header file that wraps the host's default thread layer with
a POSIX-like interface.
</p><p> The file &lt;gthr-default.h&gt; points to the deduced wrapper for
the current host. In libstdc++ implementation files,
&lt;bits/gthr.h&gt; is used to select the proper gthreads file.
</p><p>Within libstdc++ sources, all calls to underlying thread functionality
use this layer. More detail as to the specific interface can be found in the source <a class="link" href="http://gcc.gnu.org/onlinedocs/libstdc++/latest-doxygen/index.html" target="_top">documentation</a>.
</p><p>By design, the gthread layer is interoperable with the types,
functions, and usage found in the usual &lt;pthread.h&gt; file,
including <code class="code">pthread_t</code>, <code class="code">pthread_once_t</code>, <code class="code">pthread_create</code>,
etc.
</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ext_concurrency.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ext_concurrency.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ext_concurrency_use.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 29. Concurrency </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Use</td></tr></table></div></body></html>