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<div class="section" id="lo-leaf-object">
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<span id="pool-lo"></span><span id="index-0"></span><h1>8. LO (Leaf Object)<a class="headerlink" href="#lo-leaf-object" title="Permalink to this headline">¶</a></h1>
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<p><strong>LO</strong> is an <a class="reference internal" href="../glossary/a.html#term-automatic-memory-management"><em class="xref std std-term">automatically managed</em></a> <a class="reference internal" href="../glossary/p.html#term-pool-class"><em class="xref std std-term">pool class</em></a> for <a class="reference internal" href="../glossary/l.html#term-leaf-object"><em class="xref std std-term">leaf objects</em></a> (objects that
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contain no references). It does not move or protect its objects.</p>
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<p>This pool class is intended for unstructured data that needs to be
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accessed by <a class="reference internal" href="../glossary/f.html#term-foreign-code"><em class="xref std std-term">foreign code</em></a>. It’s ideal for allocating a buffer
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that needs to be passed to an operating system I/O function.</p>
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<div class="admonition-note admonition">
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<p class="first admonition-title">Note</p>
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<p>A thread that reads or writes from blocks allocated in this pool
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need not be <a class="reference internal" href="../topic/thread.html#topic-thread-register"><em>registered with the arena</em></a> so long as the <a class="reference internal" href="../glossary/l.html#term-live"><em class="xref std std-term">liveness</em></a> of
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the block is independent of that thread.</p>
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<p class="last">This means that you can launch a thread to read or write a buffer
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allocated in this pool, without having to register the thread, so
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long as you ensure that the buffer remains alive until the thread
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has finished (for example, by keeping a reference to the buffer in
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a <a class="reference internal" href="../glossary/r.html#term-root"><em class="xref std std-term">root</em></a> or a <a class="reference internal" href="../glossary/s.html#term-scan"><em class="xref std std-term">scanned</em></a> object).</p>
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</div>
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<p>If LO is used to allocate large numbers of small objects, the garbage
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collection performance will degrade. For leaf objects that can move
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and be protected, it is better to use <a class="reference internal" href="amcz.html#pool-amcz"><em>AMCZ (Automatic Mostly-Copying Zero-rank)</em></a> instead.</p>
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<div class="section" id="lo-properties">
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<span id="index-1"></span><h2>8.1. LO properties<a class="headerlink" href="#lo-properties" title="Permalink to this headline">¶</a></h2>
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<ul class="simple">
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<li>Does not support allocation via <a class="reference internal" href="../topic/allocation.html#mps_alloc" title="mps_alloc"><tt class="xref c c-func docutils literal"><span class="pre">mps_alloc()</span></tt></a> or deallocation
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via <a class="reference internal" href="../topic/allocation.html#mps_free" title="mps_free"><tt class="xref c c-func docutils literal"><span class="pre">mps_free()</span></tt></a>.</li>
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<li>Supports allocation via <a class="reference internal" href="../glossary/a.html#term-allocation-point"><em class="xref std std-term">allocation points</em></a>. If an allocation
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point is created in a LO pool, the call to <a class="reference internal" href="../topic/allocation.html#mps_ap_create" title="mps_ap_create"><tt class="xref c c-func docutils literal"><span class="pre">mps_ap_create()</span></tt></a>
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takes no additional parameters.</li>
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<li>Supports <a class="reference internal" href="../glossary/a.html#term-allocation-frame"><em class="xref std std-term">allocation frames</em></a> but does not use them to improve
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the efficiency of stack-like allocation.</li>
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<li>Does not support <a class="reference internal" href="../glossary/s.html#term-segregated-allocation-cache"><em class="xref std std-term">segregated allocation caches</em></a>.</li>
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<li>Garbage collections are scheduled automatically. See
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<a class="reference internal" href="../topic/collection.html#topic-collection-schedule"><em>Scheduling of collections</em></a>.</li>
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<li>Blocks may not contain <a class="reference internal" href="../glossary/r.html#term-reference"><em class="xref std std-term">references</em></a> to blocks in automatically
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managed pools.</li>
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<li>Allocations may be variable in size.</li>
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<li>The <a class="reference internal" href="../glossary/a.html#term-alignment"><em class="xref std std-term">alignment</em></a> of blocks is configurable.</li>
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<li>Blocks do not have <a class="reference internal" href="../glossary/d.html#term-dependent-object"><em class="xref std std-term">dependent objects</em></a>.</li>
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<li>Blocks that are not <a class="reference internal" href="../glossary/r.html#term-reachable"><em class="xref std std-term">reachable</em></a> from a <a class="reference internal" href="../glossary/r.html#term-root"><em class="xref std std-term">root</em></a> are
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automatically <a class="reference internal" href="../glossary/r.html#term-reclaim"><em class="xref std std-term">reclaimed</em></a>.</li>
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<li>Blocks are not <a class="reference internal" href="../glossary/s.html#term-scan"><em class="xref std std-term">scanned</em></a>. A consequence of this is that
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the pool’s <a class="reference internal" href="../glossary/o.html#term-object-format"><em class="xref std std-term">object format</em></a> need not provide a <a class="reference internal" href="../glossary/s.html#term-scan-method"><em class="xref std std-term">scan
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method</em></a>.</li>
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<li>Blocks may only be referenced by <a class="reference internal" href="../glossary/b.html#term-base-pointer"><em class="xref std std-term">base pointers</em></a> (unless they
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belong to an object format of variant auto-header).</li>
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<li>Blocks are not protected by <a class="reference internal" href="../glossary/b.html#term-barrier-1"><em class="xref std std-term">barriers<sup>(1)</sup></em></a>.</li>
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<li>Blocks do not <a class="reference internal" href="../glossary/m.html#term-moving-garbage-collector"><em class="xref std std-term">move</em></a>. A consequence
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of this is that the pool’s <a class="reference internal" href="../glossary/o.html#term-object-format"><em class="xref std std-term">object format</em></a> need not provide a
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<a class="reference internal" href="../glossary/f.html#term-forward-method"><em class="xref std std-term">forward method</em></a> or an <a class="reference internal" href="../glossary/i.html#term-is-forwarded-method"><em class="xref std std-term">is-forwarded method</em></a>. (It also
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does not need a <a class="reference internal" href="../glossary/p.html#term-padding-method"><em class="xref std std-term">padding method</em></a>.)</li>
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<li>Blocks may be registered for <a class="reference internal" href="../glossary/f.html#term-finalization"><em class="xref std std-term">finalization</em></a>.</li>
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<li>Blocks must belong to an <a class="reference internal" href="../glossary/o.html#term-object-format"><em class="xref std std-term">object format</em></a>.</li>
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</ul>
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</div>
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<div class="section" id="lo-interface">
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<span id="index-2"></span><h2>8.2. LO interface<a class="headerlink" href="#lo-interface" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-c"><div class="highlight"><pre><span class="cp">#include "mpsclo.h"</span>
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</pre></div>
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</div>
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<dl class="function">
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<dt id="mps_class_lo">
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<a class="reference internal" href="../topic/pool.html#mps_class_t" title="mps_class_t">mps_class_t</a> <tt class="descname">mps_class_lo</tt><big>(</big>void<big>)</big><a class="headerlink" href="#mps_class_lo" title="Permalink to this definition">¶</a></dt>
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<dd><p>Return the <a class="reference internal" href="../glossary/p.html#term-pool-class"><em class="xref std std-term">pool class</em></a> for an LO (Leaf Object)
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<a class="reference internal" href="../glossary/p.html#term-pool"><em class="xref std std-term">pool</em></a>.</p>
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<p>When creating an LO pool, <a class="reference internal" href="../topic/pool.html#mps_pool_create" title="mps_pool_create"><tt class="xref c c-func docutils literal"><span class="pre">mps_pool_create()</span></tt></a> takes one
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extra argument:</p>
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<div class="highlight-c"><div class="highlight"><pre><span class="n">mps_res_t</span> <span class="n">mps_pool_create</span><span class="p">(</span><span class="n">mps_pool_t</span> <span class="o">*</span><span class="n">pool_o</span><span class="p">,</span> <span class="n">mps_arena_t</span> <span class="n">arena</span><span class="p">,</span>
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<span class="n">mps_class_t</span> <span class="n">mps_class_lo</span><span class="p">(),</span>
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<span class="n">mps_fmt_t</span> <span class="n">fmt</span><span class="p">)</span>
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</pre></div>
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</div>
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<p><tt class="docutils literal"><span class="pre">fmt</span></tt> specifies the <a class="reference internal" href="../glossary/o.html#term-object-format"><em class="xref std std-term">object format</em></a> for the objects
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allocated in the pool. The format must provide a <a class="reference internal" href="../glossary/s.html#term-skip-method"><em class="xref std std-term">skip
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method</em></a>.</p>
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</dd></dl>
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<h3><a href="../index.html">Table Of Contents</a></h3>
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<ul>
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<li><a class="reference internal" href="#">8. LO (Leaf Object)</a><ul>
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<li><a class="reference internal" href="#lo-properties">8.1. LO properties</a></li>
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<li><a class="reference internal" href="#lo-interface">8.2. LO interface</a></li>
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