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<div class="section" id="memory-management-glossary-o">
<span id="glossary-o"></span><h1>Memory Management Glossary: O<a class="headerlink" href="#memory-management-glossary-o" title="Permalink to this headline"></a></h1>
<p class="glossary-alphabet"><a class="reference internal" href="a.html#glossary-a"><em>A</em></a>
| <a class="reference internal" href="b.html#glossary-b"><em>B</em></a>
| <a class="reference internal" href="c.html#glossary-c"><em>C</em></a>
| <a class="reference internal" href="d.html#glossary-d"><em>D</em></a>
| <a class="reference internal" href="e.html#glossary-e"><em>E</em></a>
| <a class="reference internal" href="f.html#glossary-f"><em>F</em></a>
| <a class="reference internal" href="g.html#glossary-g"><em>G</em></a>
| <a class="reference internal" href="h.html#glossary-h"><em>H</em></a>
| <a class="reference internal" href="i.html#glossary-i"><em>I</em></a>
| J
| <a class="reference internal" href="k.html#glossary-k"><em>K</em></a>
| <a class="reference internal" href="l.html#glossary-l"><em>L</em></a>
| <a class="reference internal" href="m.html#glossary-m"><em>M</em></a>
| <a class="reference internal" href="n.html#glossary-n"><em>N</em></a>
| <a class="reference internal" href="#glossary-o"><em>O</em></a>
| <a class="reference internal" href="p.html#glossary-p"><em>P</em></a>
| <a class="reference internal" href="q.html#glossary-q"><em>Q</em></a>
| <a class="reference internal" href="r.html#glossary-r"><em>R</em></a>
| <a class="reference internal" href="s.html#glossary-s"><em>S</em></a>
| <a class="reference internal" href="t.html#glossary-t"><em>T</em></a>
| <a class="reference internal" href="u.html#glossary-u"><em>U</em></a>
| <a class="reference internal" href="v.html#glossary-v"><em>V</em></a>
| <a class="reference internal" href="w.html#glossary-w"><em>W</em></a>
| X
| Y
| <a class="reference internal" href="z.html#glossary-z"><em>Z</em></a></p>
<dl class="glossary docutils">
<dt id="term-object">object</dt>
<dd><div class="admonition-also-known-as first admonition">
<p class="first admonition-title">Also known as</p>
<p class="last"><em>cell</em>.</p>
</div>
<p>In <a class="reference internal" href="m.html#term-memory-management"><em class="xref std std-term">memory management</em></a>, we use the term <em>object</em> or
<em>cell</em> to mean a contiguous <a class="reference internal" href="b.html#term-block"><em class="xref std std-term">block</em></a> of <a class="reference internal" href="m.html#term-memory-2"><em class="xref std std-term">memory<sup>(2)</sup></em></a> forming a single logical structure.</p>
<p>Objects are the units of <a class="reference internal" href="a.html#term-allocate"><em class="xref std std-term">allocation</em></a>,
<a class="reference internal" href="f.html#term-free-1"><em class="xref std std-term">deallocation</em></a>, etc. No connection to an
object-oriented system is implied.</p>
<div class="admonition-in-the-mps last admonition">
<p class="first admonition-title">In the MPS</p>
<p class="last">The MPS documentation generally reserves the term <em>object</em>
for <a class="reference internal" href="f.html#term-formatted-object"><em class="xref std std-term">formatted objects</em></a>. For
units of allocation in general, it uses the term
<a class="reference internal" href="b.html#term-block"><em class="xref std std-term">block</em></a>.</p>
</div>
</dd>
<dt id="term-object-format">object format</dt>
<dd><div class="admonition-in-the-mps first last admonition">
<p class="first admonition-title">In the MPS</p>
<p class="last">A data structure provided by the <a class="reference internal" href="c.html#term-client-program"><em class="xref std std-term">client program</em></a>
which describes the format of <a class="reference internal" href="f.html#term-formatted-object"><em class="xref std std-term">objects</em></a> allocated in a <a class="reference internal" href="p.html#term-pool"><em class="xref std std-term">pool</em></a>. The MPS uses the
<a class="reference internal" href="f.html#term-format-method"><em class="xref std std-term">format methods</em></a> to find
<a class="reference internal" href="r.html#term-reference"><em class="xref std std-term">references</em></a> in an object, replace an
object with <a class="reference internal" href="p.html#term-padding"><em class="xref std std-term">padding</em></a>, replace an object with a
<a class="reference internal" href="f.html#term-forwarding-marker"><em class="xref std std-term">forwarding marker</em></a>, and other essential
<a class="reference internal" href="g.html#term-garbage-collection"><em class="xref std std-term">garbage collection</em></a> tasks. See <a class="reference internal" href="../topic/format.html#topic-format"><em>Object formats</em></a>.</p>
</div>
</dd>
<dt id="term-object-pointer">object pointer</dt>
<dd><p class="first">In the <a class="reference internal" href="../mmref/lang.html#term-c"><em class="xref std std-term">C</em></a> programming language, a <a class="reference internal" href="p.html#term-pointer"><em class="xref std std-term">pointer</em></a> to an
<a class="reference internal" href="#term-object"><em class="xref std std-term">object</em></a>, as distinct from a <a class="reference internal" href="f.html#term-function-pointer"><em class="xref std std-term">function pointer</em></a>.
The C programming language guarantees that you can cast any
object pointer to <tt class="docutils literal"><span class="pre">void</span> <span class="pre">*</span></tt> and back without losing
information.</p>
<div class="admonition-opposite-term last admonition">
<p class="first admonition-title">Opposite term</p>
<p class="last"><a class="reference internal" href="f.html#term-function-pointer"><em class="xref std std-term">function pointer</em></a>.</p>
</div>
</dd>
<dt id="term-off-white">off-white</dt>
<dd><div class="admonition-also-known-as first admonition">
<p class="first admonition-title">Also known as</p>
<p class="last"><em>ecru</em>.</p>
</div>
<p>In a <a class="reference internal" href="t.html#term-treadmill"><em class="xref std std-term">treadmill</em></a> <a class="reference internal" href="g.html#term-garbage-collector"><em class="xref std std-term">garbage collector</em></a>, the
<a class="reference internal" href="c.html#term-color"><em class="xref std std-term">color</em></a> off-white is used to describe <a class="reference internal" href="#term-object"><em class="xref std std-term">objects</em></a>
which are <a class="reference internal" href="f.html#term-free-3"><em class="xref std std-term">free<sup>(3)</sup></em></a>. <a class="reference internal" href="../mmref/bib.html#baker92c"><em>Baker (1992c)</em></a>
used the term <em>ecru</em>.</p>
<div class="admonition-opposite-term admonition">
<p class="first admonition-title">Opposite terms</p>
<p class="last"><a class="reference internal" href="w.html#term-white"><em class="xref std std-term">white</em></a>, <a class="reference internal" href="g.html#term-gray"><em class="xref std std-term">gray</em></a>, <a class="reference internal" href="b.html#term-black"><em class="xref std std-term">black</em></a>.</p>
</div>
<div class="admonition-see-also last admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference internal" href="t.html#term-treadmill"><em class="xref std std-term">treadmill</em></a>, <a class="reference internal" href="c.html#term-color"><em class="xref std std-term">color</em></a>.</p>
</div>
</dd>
<dt id="term-old-space"><span id="term-oldspace"></span>old space<br />oldspace</dt>
<dd><div class="admonition-see first last admonition">
<p class="first admonition-title">See</p>
<p class="last"><a class="reference internal" href="f.html#term-fromspace"><em class="xref std std-term">fromspace</em></a>.</p>
</div>
</dd>
<dt id="term-one-bit-reference-count">one-bit reference count</dt>
<dd><p class="first">The one-bit <a class="reference internal" href="r.html#term-reference-counting"><em class="xref std std-term">reference count</em></a> is a
heuristic mechanism that lets a program test, at low cost,
whether an <a class="reference internal" href="#term-object"><em class="xref std std-term">object</em></a> is <a class="reference internal" href="d.html#term-dead"><em class="xref std std-term">dead</em></a>.</p>
<p>The one-bit reference count is a special case of the
<a class="reference internal" href="l.html#term-limited-field-reference-count"><em class="xref std std-term">limited-field reference count</em></a>. A single bit in an
object, called the MRB (Multiple Reference Bit), is cleared
when the object is <a class="reference internal" href="a.html#term-allocate"><em class="xref std std-term">allocated</em></a>. Whenever another
<a class="reference internal" href="r.html#term-reference"><em class="xref std std-term">reference</em></a> to the object is created, the bit is set.
Thus, MRB=0 indicates that there is exactly one reference to
the object, and MRB=1 indicates that there may be more than
one reference to the object.</p>
<p>The MRB can be stored in the reference rather than in the
object; doing so reduces the number of memory accesses due to
MRB checking and setting. When a reference is copied, the
copy&#8217;s MRB is set. If the MRB in the old reference is 0, it
also needs to be set. Setting the MRB in the old reference
requires that the program knows the location the old reference
came from, and that it can prove that location has not since
been overwritten with other data.</p>
<p>The one-bit reference count is used by a compiler to augment
an object lifetime analysis. When compile-time analysis
predicts that a particular object may be dead (typically
because the variable that references the object is dead), the
compiler can generate code that will check the object&#8217;s MRB at
run-time. If the MRB is 0, then the object is dead.</p>
<p>Using a one-bit reference count does have a cost: the MRB uses
space that could sometimes be put to other use, and the MRB
must be set every time the number of references to the object
increases. The one-bit reference count is cheaper than other
kinds of reference counting, however, since the space cost is
only one bit and the reference count is not adjusted when
references are destroyed.</p>
<div class="admonition-historical-note admonition">
<p class="first admonition-title">Historical note</p>
<p class="last">The one-bit reference count was suggested by
<a class="reference internal" href="../mmref/bib.html#fw77"><em>Friedman &amp; Wise (1977)</em></a>. Storing the MRB in
the reference was suggested by <a class="reference internal" href="../mmref/bib.html#scn84"><em>Stoye, Clarke, and
Norman (1984)</em></a>.</p>
</div>
<div class="admonition-related-publication last admonition">
<p class="first admonition-title">Related publication</p>
<p class="last"><a class="reference internal" href="../mmref/bib.html#jones12"><em>Jones et al. (2012)</em></a>.</p>
</div>
</dd>
<dt id="term-opaque-type">opaque type</dt>
<dd><div class="admonition-in-the-mps first admonition">
<p class="first admonition-title">In the MPS</p>
<p class="last">In the MPS interface, an <em>opaque type</em> is a pointer to an
incomplete structure type. The client programs must not
rely on the details of its implementation. For example,
the type <a class="reference internal" href="../topic/arena.html#mps_arena_t" title="mps_arena_t"><tt class="xref c c-type docutils literal"><span class="pre">mps_arena_t</span></tt></a> is an alias for <tt class="docutils literal"><span class="pre">struct</span>
<span class="pre">mps_arena_s</span> <span class="pre">*</span></tt>, but the implementation of <tt class="docutils literal"><span class="pre">struct</span>
<span class="pre">mps_arena_s</span></tt> is not public. See <a class="reference internal" href="../topic/interface.html#topic-interface"><em>Interface conventions</em></a>.</p>
</div>
<div class="admonition-opposite-term last admonition">
<p class="first admonition-title">Opposite terms</p>
<p class="last"><a class="reference internal" href="d.html#term-derived-type"><em class="xref std std-term">derived type</em></a>, <a class="reference internal" href="t.html#term-transparent-type"><em class="xref std std-term">transparent type</em></a>.</p>
</div>
</dd>
<dt id="term-out-parameter">out parameter</dt>
<dd><p class="first">A function parameter that points to a location for the caller
to receive data from the function.</p>
<div class="admonition-opposite-term admonition">
<p class="first admonition-title">Opposite term</p>
<p class="last"><a class="reference internal" href="i.html#term-in-parameter"><em class="xref std std-term">in parameter</em></a>.</p>
</div>
<div class="admonition-in-the-mps last admonition">
<p class="first admonition-title">In the MPS</p>
<p class="last">Out parameters are given names ending with <tt class="docutils literal"><span class="pre">_o</span></tt>. See
<a class="reference internal" href="../topic/interface.html#topic-interface"><em>Interface conventions</em></a>.</p>
</div>
</dd>
<dt id="term-out-of-band-header">out-of-band header</dt>
<dd><p class="first">In some <a class="reference internal" href="m.html#term-memory-manager"><em class="xref std std-term">memory managers</em></a>, each <a class="reference internal" href="a.html#term-allocate"><em class="xref std std-term">allocated</em></a>
<a class="reference internal" href="b.html#term-block"><em class="xref std std-term">block</em></a> has additional information (such as the size of
the block or a <a class="reference internal" href="t.html#term-tag"><em class="xref std std-term">tag</em></a>) stored in a separate block; this
is called <em>an out-of-band header</em>.</p>
<div class="admonition-opposite-term last admonition">
<p class="first admonition-title">Opposite term</p>
<p class="last"><a class="reference internal" href="i.html#term-in-band-header"><em class="xref std std-term">in-band header</em></a>.</p>
</div>
</dd>
<dt id="term-overcommit">overcommit</dt>
<dd><p class="first">In some circumstances, although a range of <a class="reference internal" href="v.html#term-virtual-address"><em class="xref std std-term">virtual
addresses</em></a> has been <a class="reference internal" href="m.html#term-mapped"><em class="xref std std-term">mapped</em></a> as far as the user program
is concerned, the <a class="reference internal" href="p.html#term-physical-storage"><em class="xref std std-term">physical storage</em></a> might not be
allocated until it is accessed. This is called
<em>overcommitting</em>.</p>
<p>Overcommitting shares <a class="reference internal" href="s.html#term-swap-space"><em class="xref std std-term">swap space</em></a> resources more
flexibly, especially when crude <a class="reference internal" href="s.html#term-suballocator"><em class="xref std std-term">suballocators</em></a> are
involved, but it can lead to an out-of-resource error during a
<a class="reference internal" href="m.html#term-memory-2"><em class="xref std std-term">memory<sup>(2)</sup></em></a> access; few environments deal with this
situation gracefully.</p>
<p class="last">Unix systems such as IRIX and AIX can do this on <a class="reference internal" href="s.html#term-sbrk"><em class="xref std std-term">sbrk</em></a>
and <a class="reference internal" href="m.html#term-mmap"><em class="xref std std-term">mmap</em></a> calls.</p>
</dd>
<dt id="term-overwriting-error">overwriting error</dt>
<dd><div class="admonition-also-known-as first admonition">
<p class="first admonition-title">Also known as</p>
<p class="last"><em>bounds error</em>.</p>
</div>
<p>An overwriting or bounds error occurs when the programmer
intends his program to write to a particular <a class="reference internal" href="b.html#term-block"><em class="xref std std-term">block</em></a> of
<a class="reference internal" href="m.html#term-memory-1"><em class="xref std std-term">memory<sup>(1)</sup></em></a>, but a program error causes the program to
write outside the bounds of that block.</p>
<div class="admonition-see-also last admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference internal" href="f.html#term-fencepost"><em class="xref std std-term">fencepost</em></a>.</p>
</div>
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