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138 lines
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<title>Atomics (Guile Reference Manual)</title>
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<div class="subsection-level-extent" id="Atomics">
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<p>
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Next: <a href="Mutexes-and-Condition-Variables.html" accesskey="n" rel="next">Mutexes and Condition Variables</a>, Previous: <a href="Asyncs.html" accesskey="p" rel="prev">Asynchronous Interrupts</a>, Up: <a href="Scheduling.html" accesskey="u" rel="up">Threads, Mutexes, Asyncs and Dynamic Roots</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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<hr>
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<h4 class="subsection" id="Atomics-1"><span>6.22.4 Atomics<a class="copiable-link" href="#Atomics-1"> ¶</a></span></h4>
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<p>When accessing data in parallel from multiple threads, updates made by
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one thread are not generally guaranteed to be visible by another thread.
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It could be that your hardware requires special instructions to be
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emitted to propagate a change from one CPU core to another. Or, it
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could be that your hardware updates values with a sequence of
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instructions, and a parallel thread could see a value that is in the
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process of being updated but not fully updated.
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</p>
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<p>Atomic references solve this problem. Atomics are a standard, primitive
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facility to allow for concurrent access and update of mutable variables
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from multiple threads with guaranteed forward-progress and well-defined
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intermediate states.
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</p>
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<p>Atomic references serve not only as a hardware memory barrier but also
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as a compiler barrier. Normally a compiler might choose to reorder or
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elide certain memory accesses due to optimizations like common
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subexpression elimination. Atomic accesses however will not be
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reordered relative to each other, and normal memory accesses will not be
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reordered across atomic accesses.
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</p>
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<p>As an implementation detail, currently all atomic accesses and updates
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use the sequential consistency memory model from C11. We may relax this
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in the future to the acquire/release semantics, which still issues a
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memory barrier so that non-atomic updates are not reordered across
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atomic accesses or updates.
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</p>
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<p>To use Guile’s atomic operations, load the <code class="code">(ice-9 atomic)</code> module:
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</p>
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<div class="example">
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<pre class="example-preformatted">(use-modules (ice-9 atomic))
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</pre></div>
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<dl class="first-deffn">
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<dt class="deffn" id="index-make_002datomic_002dbox"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">make-atomic-box</strong> <var class="def-var-arguments">init</var><a class="copiable-link" href="#index-make_002datomic_002dbox"> ¶</a></span></dt>
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<dd><p>Return an atomic box initialized to value <var class="var">init</var>.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-atomic_002dbox_003f"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">atomic-box?</strong> <var class="def-var-arguments">obj</var><a class="copiable-link" href="#index-atomic_002dbox_003f"> ¶</a></span></dt>
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<dd><p>Return <code class="code">#t</code> if <var class="var">obj</var> is an atomic-box object, else
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return <code class="code">#f</code>.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-atomic_002dbox_002dref"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">atomic-box-ref</strong> <var class="def-var-arguments">box</var><a class="copiable-link" href="#index-atomic_002dbox_002dref"> ¶</a></span></dt>
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<dd><p>Fetch the value stored in the atomic box <var class="var">box</var> and return it.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-atomic_002dbox_002dset_0021"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">atomic-box-set!</strong> <var class="def-var-arguments">box val</var><a class="copiable-link" href="#index-atomic_002dbox_002dset_0021"> ¶</a></span></dt>
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<dd><p>Store <var class="var">val</var> into the atomic box <var class="var">box</var>.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-atomic_002dbox_002dswap_0021"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">atomic-box-swap!</strong> <var class="def-var-arguments">box val</var><a class="copiable-link" href="#index-atomic_002dbox_002dswap_0021"> ¶</a></span></dt>
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<dd><p>Store <var class="var">val</var> into the atomic box <var class="var">box</var>, and return the value that
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was previously stored in the box.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-atomic_002dbox_002dcompare_002dand_002dswap_0021"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">atomic-box-compare-and-swap!</strong> <var class="def-var-arguments">box expected desired</var><a class="copiable-link" href="#index-atomic_002dbox_002dcompare_002dand_002dswap_0021"> ¶</a></span></dt>
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<dd><p>If the value of the atomic box <var class="var">box</var> is the same as, <var class="var">expected</var>
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(in the sense of <code class="code">eq?</code>), replace the contents of the box with
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<var class="var">desired</var>. Otherwise does not update the box. Returns the previous
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value of the box in either case, so you can know if the swap worked by
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checking if the return value is <code class="code">eq?</code> to <var class="var">expected</var>.
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</p></dd></dl>
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</div>
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<hr>
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<div class="nav-panel">
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<p>
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Next: <a href="Mutexes-and-Condition-Variables.html">Mutexes and Condition Variables</a>, Previous: <a href="Asyncs.html">Asynchronous Interrupts</a>, Up: <a href="Scheduling.html">Threads, Mutexes, Asyncs and Dynamic Roots</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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