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<div class="section" id="object-formats">
<span id="topic-format"></span><span id="index-0"></span><h1>7. Object formats<a class="headerlink" href="#object-formats" title="Permalink to this headline"></a></h1>
<p>The need for some means of describing objects in the <a class="reference internal" href="../glossary/c.html#term-client-program"><em class="xref std std-term">client
program</em></a> comes from <a class="reference internal" href="../glossary/t.html#term-trace"><em class="xref std std-term">tracing</em></a> and <a class="reference internal" href="../glossary/m.html#term-moving-memory-manager"><em class="xref std std-term">moving</em></a>. During tracing, when an object is <a class="reference internal" href="../glossary/s.html#term-scan"><em class="xref std std-term">scanned</em></a>, all the <a class="reference internal" href="../glossary/r.html#term-reference"><em class="xref std std-term">references</em></a> in the object must be
identified so that the objects they point to can be scanned in their
turn. When an object has moved, references to that object must be
identified so that they can be updated to point to the new location of
the object.</p>
<p>In general, only the client program can say which fields in an object
are references, and only the client program knows how references are
represented (for example, are they tagged?). <em>Object formats</em> provide
the means by which the client program communicates this information to
the MPS.</p>
<p>An object format is a collection of <a class="reference internal" href="../glossary/f.html#term-format-method"><em class="xref std std-term">format methods</em></a> and other
(usually scalar) values which together describe programatically the
layout of objects belonging to the format. Format methods include the
<a class="reference internal" href="../glossary/s.html#term-skip-method"><em class="xref std std-term">skip method</em></a> (which calculates an object&#8217;s size), the
<a class="reference internal" href="../glossary/s.html#term-scan-method"><em class="xref std std-term">scan method</em></a> (which <a class="reference internal" href="../glossary/f.html#term-fix"><em class="xref std std-term">fixes</em></a> references in the
object), and the <a class="reference internal" href="../glossary/f.html#term-forward-method"><em class="xref std std-term">forward method</em></a> (which replaces an object that
has moved with a <a class="reference internal" href="../glossary/f.html#term-forwarding-object"><em class="xref std std-term">forwarding object</em></a>).</p>
<p>Not every <a class="reference internal" href="../glossary/p.html#term-pool-class"><em class="xref std std-term">pool class</em></a> supports <a class="reference internal" href="../glossary/f.html#term-formatted-object"><em class="xref std std-term">formatted objects</em></a>.</p>
<dl class="type">
<dt id="mps_fmt_t">
<tt class="descname">mps_fmt_t</tt><a class="headerlink" href="#mps_fmt_t" title="Permalink to this definition"></a></dt>
<dd><p>The type of an <a class="reference internal" href="../glossary/o.html#term-object-format"><em class="xref std std-term">object format</em></a>.</p>
</dd></dl>
<div class="section" id="creating-an-object-format">
<span id="index-1"></span><h2>7.1. Creating an object format<a class="headerlink" href="#creating-an-object-format" title="Permalink to this headline"></a></h2>
<p>Different <a class="reference internal" href="../glossary/p.html#term-pool-class"><em class="xref std std-term">pool classes</em></a> use different sets of format methods
and values (for example, a non-moving pool does not need forwarding
objects, so its object formats do not need to contain a forward
method). To accommodate this variance, it is possible to construct
object formats from different collections of format methods and
values. Such a collection is called a <em>format variant</em>.</p>
<p>There are three supported format variants. All are suitable for
copying and moving pools.</p>
<ul class="simple">
<li>Variant A (<a class="reference internal" href="#mps_fmt_A_s" title="mps_fmt_A_s"><tt class="xref c c-type docutils literal"><span class="pre">mps_fmt_A_s</span></tt></a>): for objects without
<a class="reference internal" href="../glossary/i.html#term-in-band-header"><em class="xref std std-term">headers</em></a>.</li>
<li>Variant B (<a class="reference internal" href="#mps_fmt_B_s" title="mps_fmt_B_s"><tt class="xref c c-type docutils literal"><span class="pre">mps_fmt_B_s</span></tt></a>): as variant A, but with the
addition of a class method.</li>
<li>Variant auto-header (<a class="reference internal" href="#mps_fmt_auto_header_s" title="mps_fmt_auto_header_s"><tt class="xref c c-type docutils literal"><span class="pre">mps_fmt_auto_header_s</span></tt></a>): for objects
with <a class="reference internal" href="../glossary/i.html#term-in-band-header"><em class="xref std std-term">headers</em></a>.</li>
</ul>
<p>The client program creates an object format by construct a format
variant structure and then calling the appropriate <tt class="docutils literal"><span class="pre">mps_fmt_create_</span></tt>
function for the variant. The variant structure can then be disposed
of.</p>
<p>For example:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="k">struct</span> <span class="n">mps_fmt_A_s</span> <span class="n">obj_fmt_s</span> <span class="o">=</span> <span class="p">{</span>
<span class="n">ALIGNMENT</span><span class="p">,</span>
<span class="n">obj_scan</span><span class="p">,</span>
<span class="n">obj_skip</span><span class="p">,</span>
<span class="nb">NULL</span><span class="p">,</span> <span class="cm">/* Obsolete copy method */</span>
<span class="n">obj_fwd</span><span class="p">,</span>
<span class="n">obj_isfwd</span><span class="p">,</span>
<span class="n">obj_pad</span>
<span class="p">};</span>
<span class="n">mps_pool_t</span> <span class="n">obj_pool</span><span class="p">;</span>
<span class="n">mps_fmt_t</span> <span class="n">obj_fmt</span><span class="p">;</span>
<span class="n">mps_res_t</span> <span class="n">res</span><span class="p">;</span>
<span class="n">res</span> <span class="o">=</span> <span class="n">mps_fmt_create_A</span><span class="p">(</span><span class="o">&amp;</span><span class="n">obj_fmt</span><span class="p">,</span> <span class="n">arena</span><span class="p">,</span> <span class="o">&amp;</span><span class="n">obj_fmt_s</span><span class="p">);</span>
<span class="k">if</span> <span class="p">(</span><span class="n">res</span> <span class="o">!=</span> <span class="n">MPS_RES_OK</span><span class="p">)</span> <span class="n">error</span><span class="p">(</span><span class="s">&quot;Couldn&#39;t create obj format&quot;</span><span class="p">);</span>
<span class="cm">/* obj_fmt created successfully */</span>
<span class="n">MPS_ARGS_BEGIN</span><span class="p">(</span><span class="n">args</span><span class="p">)</span> <span class="p">{</span>
<span class="n">MPS_ARGS_ADD</span><span class="p">(</span><span class="n">args</span><span class="p">,</span> <span class="n">MPS_KEY_FORMAT</span><span class="p">,</span> <span class="n">obj_fmt</span><span class="p">);</span>
<span class="n">MPS_ARGS_DONE</span><span class="p">(</span><span class="n">args</span><span class="p">);</span>
<span class="n">res</span> <span class="o">=</span> <span class="n">mps_pool_create_k</span><span class="p">(</span><span class="o">&amp;</span><span class="n">obj_pool</span><span class="p">,</span> <span class="n">arena</span><span class="p">,</span> <span class="n">pool_class</span><span class="p">,</span> <span class="n">args</span><span class="p">);</span>
<span class="p">}</span> <span class="n">MPS_ARGS_END</span><span class="p">(</span><span class="n">args</span><span class="p">);</span>
<span class="k">if</span> <span class="p">(</span><span class="n">res</span> <span class="o">!=</span> <span class="n">MPS_RES_OK</span><span class="p">)</span> <span class="n">error</span><span class="p">(</span><span class="s">&quot;Couldn&#39;t create obj pool&quot;</span><span class="p">);</span>
</pre></div>
</div>
<dl class="type">
<dt id="mps_fmt_A_s">
<tt class="descname">mps_fmt_A_s</tt><a class="headerlink" href="#mps_fmt_A_s" title="Permalink to this definition"></a></dt>
<dd><p>The type of the structure used to create an <a class="reference internal" href="../glossary/o.html#term-object-format"><em class="xref std std-term">object format</em></a>
of variant A.</p>
<div class="highlight-c"><div class="highlight"><pre><span class="k">typedef</span> <span class="k">struct</span> <span class="n">mps_fmt_A_s</span> <span class="p">{</span>
<span class="n">mps_align_t</span> <span class="n">align</span><span class="p">;</span>
<span class="n">mps_fmt_scan_t</span> <span class="n">scan</span><span class="p">;</span>
<span class="n">mps_fmt_skip_t</span> <span class="n">skip</span><span class="p">;</span>
<span class="n">mps_fmt_copy_t</span> <span class="n">copy</span><span class="p">;</span>
<span class="n">mps_fmt_fwd_t</span> <span class="n">fwd</span><span class="p">;</span>
<span class="n">mps_fmt_isfwd_t</span> <span class="n">isfwd</span><span class="p">;</span>
<span class="n">mps_fmt_pad_t</span> <span class="n">pad</span><span class="p">;</span>
<span class="p">}</span> <span class="n">mps_fmt_A_s</span><span class="p">;</span>
</pre></div>
</div>
<p>Broadly speaking, object formats of variant A are suitable for use
in <a class="reference internal" href="../glossary/c.html#term-copying-garbage-collection"><em class="xref std std-term">copying</em></a> or <a class="reference internal" href="../glossary/m.html#term-moving-garbage-collector"><em class="xref std std-term">moving</em></a> <a class="reference internal" href="../glossary/p.html#term-pool"><em class="xref std std-term">pools</em></a>.</p>
<p><tt class="docutils literal"><span class="pre">align</span></tt> is an integer value specifying the alignment of objects
allocated with this format. It should be large enough to satisfy
the alignment requirements of any field in the objects, and it
must not be larger than the pool alignment.</p>
<p><tt class="docutils literal"><span class="pre">scan</span></tt> is a <a class="reference internal" href="../glossary/s.html#term-scan-method"><em class="xref std std-term">scan method</em></a> that identifies references
within objects belonging to this format. See
<a class="reference internal" href="#mps_fmt_scan_t" title="mps_fmt_scan_t"><tt class="xref c c-type docutils literal"><span class="pre">mps_fmt_scan_t</span></tt></a>.</p>
<p><tt class="docutils literal"><span class="pre">skip</span></tt> is a <a class="reference internal" href="../glossary/s.html#term-skip-method"><em class="xref std std-term">skip method</em></a> that skips over objects
belonging to this format. See <a class="reference internal" href="#mps_fmt_skip_t" title="mps_fmt_skip_t"><tt class="xref c c-type docutils literal"><span class="pre">mps_fmt_skip_t</span></tt></a>.</p>
<p><tt class="docutils literal"><span class="pre">copy</span></tt> is not used. (In older versions of the MPS it was a
<a class="reference internal" href="../glossary/c.html#term-copy-method"><em class="xref std std-term">copy method</em></a> that copied objects belonging to this
format.)</p>
<p><tt class="docutils literal"><span class="pre">fwd</span></tt> is a <a class="reference internal" href="../glossary/f.html#term-forward-method"><em class="xref std std-term">forward method</em></a> that stores relocation
information for an object belonging to this format that has moved.
See <a class="reference internal" href="#mps_fmt_fwd_t" title="mps_fmt_fwd_t"><tt class="xref c c-type docutils literal"><span class="pre">mps_fmt_fwd_t</span></tt></a>.</p>
<p><tt class="docutils literal"><span class="pre">isfwd</span></tt> is a <a class="reference internal" href="../glossary/i.html#term-is-forwarded-method"><em class="xref std std-term">is-forwarded method</em></a> that determines if an
object belonging to this format has been moved. See
<a class="reference internal" href="#mps_fmt_isfwd_t" title="mps_fmt_isfwd_t"><tt class="xref c c-type docutils literal"><span class="pre">mps_fmt_isfwd_t</span></tt></a>.</p>
<p><tt class="docutils literal"><span class="pre">pad</span></tt> is a <a class="reference internal" href="../glossary/p.html#term-padding-method"><em class="xref std std-term">padding method</em></a> that creates <a class="reference internal" href="../glossary/p.html#term-padding-object"><em class="xref std std-term">padding
objects</em></a> belonging to this format. See <a class="reference internal" href="#mps_fmt_pad_t" title="mps_fmt_pad_t"><tt class="xref c c-type docutils literal"><span class="pre">mps_fmt_pad_t</span></tt></a>.</p>
</dd></dl>
<dl class="function">
<dt id="mps_fmt_create_A">
<a class="reference internal" href="error.html#mps_res_t" title="mps_res_t">mps_res_t</a> <tt class="descname">mps_fmt_create_A</tt><big>(</big><a class="reference internal" href="#mps_fmt_t" title="mps_fmt_t">mps_fmt_t</a><em>&nbsp;*fmt_o</em>, <a class="reference internal" href="arena.html#mps_arena_t" title="mps_arena_t">mps_arena_t</a><em>&nbsp;arena</em>, <a class="reference internal" href="#mps_fmt_A_s" title="mps_fmt_A_s">mps_fmt_A_s</a><em>&nbsp;*fmt_A</em><big>)</big><a class="headerlink" href="#mps_fmt_create_A" title="Permalink to this definition"></a></dt>
<dd><p>Create an <a class="reference internal" href="../glossary/o.html#term-object-format"><em class="xref std std-term">object format</em></a> of variant A.</p>
<p><tt class="docutils literal"><span class="pre">fmt_o</span></tt> points to a location that will hold the address of the new
object format.</p>
<p><tt class="docutils literal"><span class="pre">arena</span></tt> is the arena in which to create the format.</p>
<p><tt class="docutils literal"><span class="pre">fmt_A</span></tt> points to a description of an object format of variant A.</p>
<p>Returns <a class="reference internal" href="error.html#MPS_RES_OK" title="MPS_RES_OK"><tt class="xref c c-macro docutils literal"><span class="pre">MPS_RES_OK</span></tt></a> if successful. The MPS may exhaust
some resource in the course of <a class="reference internal" href="#mps_fmt_create_A" title="mps_fmt_create_A"><tt class="xref c c-func docutils literal"><span class="pre">mps_fmt_create_A()</span></tt></a> and will
return an appropriate <a class="reference internal" href="../glossary/r.html#term-result-code"><em class="xref std std-term">result code</em></a> if so.</p>
<p>After this function returns, the object format description pointed
to be <tt class="docutils literal"><span class="pre">fmt_A</span></tt> is no longer needed and may be discarded. The object
format pointed to by <tt class="docutils literal"><span class="pre">fmt_o</span></tt> persists until it is destroyed by
calling <a class="reference internal" href="#mps_fmt_destroy" title="mps_fmt_destroy"><tt class="xref c c-func docutils literal"><span class="pre">mps_fmt_destroy()</span></tt></a>.</p>
</dd></dl>
<dl class="type">
<dt id="mps_fmt_B_s">
<tt class="descname">mps_fmt_B_s</tt><a class="headerlink" href="#mps_fmt_B_s" title="Permalink to this definition"></a></dt>
<dd><p>The type of the structure used to create an <a class="reference internal" href="../glossary/o.html#term-object-format"><em class="xref std std-term">object format</em></a>
of variant B.</p>
<div class="highlight-c"><div class="highlight"><pre><span class="k">typedef</span> <span class="k">struct</span> <span class="n">mps_fmt_B_s</span> <span class="p">{</span>
<span class="n">mps_align_t</span> <span class="n">align</span><span class="p">;</span>
<span class="n">mps_fmt_scan_t</span> <span class="n">scan</span><span class="p">;</span>
<span class="n">mps_fmt_skip_t</span> <span class="n">skip</span><span class="p">;</span>
<span class="n">mps_fmt_copy_t</span> <span class="n">copy</span><span class="p">;</span>
<span class="n">mps_fmt_fwd_t</span> <span class="n">fwd</span><span class="p">;</span>
<span class="n">mps_fmt_isfwd_t</span> <span class="n">isfwd</span><span class="p">;</span>
<span class="n">mps_fmt_pad_t</span> <span class="n">pad</span><span class="p">;</span>
<span class="n">mps_fmt_class_t</span> <span class="n">mps_class</span><span class="p">;</span>
<span class="p">}</span> <span class="n">mps_fmt_B_s</span><span class="p">;</span>
</pre></div>
</div>
<p>Variant B is the same as variant A except for the addition of the
<tt class="docutils literal"><span class="pre">mps_class</span></tt> method. See <a class="reference internal" href="#mps_fmt_A_s" title="mps_fmt_A_s"><tt class="xref c c-type docutils literal"><span class="pre">mps_fmt_A_s</span></tt></a>.</p>
</dd></dl>
<dl class="function">
<dt id="mps_fmt_create_B">
<a class="reference internal" href="error.html#mps_res_t" title="mps_res_t">mps_res_t</a> <tt class="descname">mps_fmt_create_B</tt><big>(</big><a class="reference internal" href="#mps_fmt_t" title="mps_fmt_t">mps_fmt_t</a><em>&nbsp;*fmt_o</em>, <a class="reference internal" href="arena.html#mps_arena_t" title="mps_arena_t">mps_arena_t</a><em>&nbsp;arena</em>, <a class="reference internal" href="#mps_fmt_B_s" title="mps_fmt_B_s">mps_fmt_B_s</a><em>&nbsp;*fmt_B</em><big>)</big><a class="headerlink" href="#mps_fmt_create_B" title="Permalink to this definition"></a></dt>
<dd><p>Create an <a class="reference internal" href="../glossary/o.html#term-object-format"><em class="xref std std-term">object format</em></a> of variant B.</p>
<p><tt class="docutils literal"><span class="pre">fmt_o</span></tt> points to a location that will hold the address of the new
object format.</p>
<p><tt class="docutils literal"><span class="pre">arena</span></tt> is the arena in which to create the format.</p>
<p><tt class="docutils literal"><span class="pre">fmt_B</span></tt> points to a description of an object format of variant B.</p>
<p>Returns <a class="reference internal" href="error.html#MPS_RES_OK" title="MPS_RES_OK"><tt class="xref c c-macro docutils literal"><span class="pre">MPS_RES_OK</span></tt></a> if successful. The MPS may exhaust
some resource in the course of <a class="reference internal" href="#mps_fmt_create_B" title="mps_fmt_create_B"><tt class="xref c c-func docutils literal"><span class="pre">mps_fmt_create_B()</span></tt></a> and will
return an appropriate <a class="reference internal" href="../glossary/r.html#term-result-code"><em class="xref std std-term">result code</em></a> if so.</p>
</dd></dl>
<dl class="type">
<dt id="mps_fmt_auto_header_s">
<tt class="descname">mps_fmt_auto_header_s</tt><a class="headerlink" href="#mps_fmt_auto_header_s" title="Permalink to this definition"></a></dt>
<dd><p>The type of the structure used to create an <a class="reference internal" href="../glossary/o.html#term-object-format"><em class="xref std std-term">object format</em></a>
of variant auto-header.</p>
<div class="highlight-c"><div class="highlight"><pre><span class="k">typedef</span> <span class="k">struct</span> <span class="n">mps_fmt_auto_header_s</span> <span class="p">{</span>
<span class="n">mps_align_t</span> <span class="n">align</span><span class="p">;</span>
<span class="n">mps_fmt_scan_t</span> <span class="n">scan</span><span class="p">;</span>
<span class="n">mps_fmt_skip_t</span> <span class="n">skip</span><span class="p">;</span>
<span class="n">mps_fmt_fwd_t</span> <span class="n">fwd</span><span class="p">;</span>
<span class="n">mps_fmt_isfwd_t</span> <span class="n">isfwd</span><span class="p">;</span>
<span class="n">mps_fmt_pad_t</span> <span class="n">pad</span><span class="p">;</span>
<span class="kt">size_t</span> <span class="n">mps_headerSize</span><span class="p">;</span>
<span class="p">}</span> <span class="n">mps_fmt_auto_header_s</span><span class="p">;</span>
</pre></div>
</div>
<p>Variant auto-header is the same as variant A except for the
removal of the unused <tt class="docutils literal"><span class="pre">copy</span></tt> method, and the addition of the
<tt class="docutils literal"><span class="pre">mps_headerSize</span></tt> field. See <a class="reference internal" href="#mps_fmt_A_s" title="mps_fmt_A_s"><tt class="xref c c-type docutils literal"><span class="pre">mps_fmt_A_s</span></tt></a>.</p>
<p>Broadly speaking, the object formats of this variant are suitable
for use in <a class="reference internal" href="../glossary/a.html#term-automatic-memory-management"><em class="xref std std-term">automatic memory management</em></a> for objects with
<a class="reference internal" href="../glossary/i.html#term-in-band-header"><em class="xref std std-term">headers</em></a> (hence the name). More precisely,
this variant is intended for formats where the <a class="reference internal" href="../glossary/c.html#term-client-program"><em class="xref std std-term">client
program&#8217;s</em></a> pointers point some distance into the
memory <a class="reference internal" href="../glossary/b.html#term-block"><em class="xref std std-term">block</em></a> containing the object. This typically happens
when the objects have a common header used for memory management
or class system purposes, but this situation also arises when the
low bits of a pointer are used for a tag. The MPS does not care
what the reason is, only about the offset of the pointer in
relation to the memory block.</p>
<p><tt class="docutils literal"><span class="pre">mps_headerSize</span></tt> is the size of the header, that is, the offset of
a client pointer from the base of the memory block.</p>
<div class="admonition-note admonition">
<p class="first admonition-title">Note</p>
<p>Format methods (other than the <a class="reference internal" href="../glossary/p.html#term-padding-method"><em class="xref std std-term">padding method</em></a>) for
formats of this variant will receive <em>client pointers</em> (that
is, pointers past the header) but all other MPS functions
expect to receive and return <em>base pointers</em> (that is,
pointers to the base of the block where the header is stored).</p>
<p class="last">In particular, <a class="reference internal" href="allocation.html#mps_reserve" title="mps_reserve"><tt class="xref c c-func docutils literal"><span class="pre">mps_reserve()</span></tt></a> and <a class="reference internal" href="allocation.html#mps_alloc" title="mps_alloc"><tt class="xref c c-func docutils literal"><span class="pre">mps_alloc()</span></tt></a>
always hand out base pointers, and <a class="reference internal" href="allocation.html#mps_free" title="mps_free"><tt class="xref c c-func docutils literal"><span class="pre">mps_free()</span></tt></a> expects
to receive one.</p>
</div>
<div class="admonition-note admonition">
<p class="first admonition-title">Note</p>
<p class="last">For technical reasons, formatted objects must be longer than
the header. In other words, objects consisting of only a
header are not supported.</p>
</div>
<div class="admonition-note admonition">
<p class="first admonition-title">Note</p>
<p class="last">Even if the header size is larger than or equal to
<a class="reference internal" href="../glossary/a.html#term-alignment"><em class="xref std std-term">alignment</em></a>, the <a class="reference internal" href="../glossary/p.html#term-padding-method"><em class="xref std std-term">padding method</em></a> must still be
able to create <a class="reference internal" href="../glossary/p.html#term-padding-object"><em class="xref std std-term">padding objects</em></a> down
to the alignment size.</p>
</div>
</dd></dl>
<dl class="function">
<dt id="mps_fmt_create_auto_header">
<a class="reference internal" href="error.html#mps_res_t" title="mps_res_t">mps_res_t</a> <tt class="descname">mps_fmt_create_auto_header</tt><big>(</big><a class="reference internal" href="#mps_fmt_t" title="mps_fmt_t">mps_fmt_t</a><em>&nbsp;*fmt_o</em>, <a class="reference internal" href="arena.html#mps_arena_t" title="mps_arena_t">mps_arena_t</a><em>&nbsp;arena</em>, <a class="reference internal" href="#mps_fmt_auto_header_s" title="mps_fmt_auto_header_s">mps_fmt_auto_header_s</a><em>&nbsp;*fmt_ah</em><big>)</big><a class="headerlink" href="#mps_fmt_create_auto_header" title="Permalink to this definition"></a></dt>
<dd><p>Create an <a class="reference internal" href="../glossary/o.html#term-object-format"><em class="xref std std-term">object format</em></a> of variant auto-header.</p>
<p><tt class="docutils literal"><span class="pre">fmt_o</span></tt> points to a location that will hold the address of the new
object format.</p>
<p><tt class="docutils literal"><span class="pre">arena</span></tt> is the arena in which to create the format.</p>
<p><tt class="docutils literal"><span class="pre">fmt_ah</span></tt> points to a description of an object format of variant
auto-header.</p>
<p>Returns <a class="reference internal" href="error.html#MPS_RES_OK" title="MPS_RES_OK"><tt class="xref c c-macro docutils literal"><span class="pre">MPS_RES_OK</span></tt></a> if successful. The MPS may exhaust
some resource in the course of
<a class="reference internal" href="#mps_fmt_create_auto_header" title="mps_fmt_create_auto_header"><tt class="xref c c-func docutils literal"><span class="pre">mps_fmt_create_auto_header()</span></tt></a> and will return an
appropriate <a class="reference internal" href="../glossary/r.html#term-result-code"><em class="xref std std-term">result code</em></a> if so.</p>
</dd></dl>
<dl class="function">
<dt id="mps_fmt_destroy">
void <tt class="descname">mps_fmt_destroy</tt><big>(</big><a class="reference internal" href="#mps_fmt_t" title="mps_fmt_t">mps_fmt_t</a><em>&nbsp;fmt</em><big>)</big><a class="headerlink" href="#mps_fmt_destroy" title="Permalink to this definition"></a></dt>
<dd><p>Destroy an <a class="reference internal" href="../glossary/o.html#term-object-format"><em class="xref std std-term">object format</em></a>.</p>
<p><tt class="docutils literal"><span class="pre">fmt</span></tt> is the object format to destroy.</p>
<p>It is an error to destroy an object format if there exists a
<a class="reference internal" href="../glossary/p.html#term-pool"><em class="xref std std-term">pool</em></a> using the format. The pool must be destroyed first.</p>
</dd></dl>
</div>
<div class="section" id="cautions">
<span id="topic-format-cautions"></span><span id="index-2"></span><h2>7.2. Cautions<a class="headerlink" href="#cautions" title="Permalink to this headline"></a></h2>
<ol class="arabic">
<li><p class="first">The MPS guarantees that format methods have exclusive access to the
object for the duration of the call. This guarantee may entail
suspending arbitrary threads. The methods that manipulate the
object must not perform any sort of inter-thread locking or
communication.</p>
</li>
<li><p class="first">The MPS may call format methods in the context of an exception
handler or a signal handler. For example, the following sequence of
events is common:</p>
<ol class="loweralpha simple">
<li>the MPS places a <a class="reference internal" href="../glossary/r.html#term-read-barrier"><em class="xref std std-term">read barrier</em></a> on a block of memory;</li>
<li>the client program attempts to read from this block;</li>
<li>the hardware raises a <a class="reference internal" href="../glossary/p.html#term-protection-fault"><em class="xref std std-term">protection fault</em></a>;</li>
<li>the MPS signal handler is called;</li>
<li>the MPS ensures that the contents of the block are correct and
consistent: this may involve inspection of formatted objects in
the block (or indeed, elsewhere), and so</li>
<li>the MPS calls format methods.</li>
</ol>
<p>Therefore, the format methods must be able to be run at any time,
including asynchronously or in parallel with the rest of the
program.</p>
</li>
<li><p class="first">Format methods must be re-entrant.</p>
</li>
<li><p class="first">Format methods must not:</p>
<ol class="loweralpha simple">
<li>call library code;</li>
<li>perform a non-local exit (for example, by calling <tt class="docutils literal"><span class="pre">longjmp</span></tt>);</li>
<li>call any functions in the MPS other than the fix functions
(<a class="reference internal" href="scanning.html#mps_fix" title="mps_fix"><tt class="xref c c-func docutils literal"><span class="pre">mps_fix()</span></tt></a>, <a class="reference internal" href="scanning.html#MPS_FIX1" title="MPS_FIX1"><tt class="xref c c-func docutils literal"><span class="pre">MPS_FIX1()</span></tt></a>, <a class="reference internal" href="scanning.html#MPS_FIX12" title="MPS_FIX12"><tt class="xref c c-func docutils literal"><span class="pre">MPS_FIX12()</span></tt></a>, and
<a class="reference internal" href="scanning.html#MPS_FIX2" title="MPS_FIX2"><tt class="xref c c-func docutils literal"><span class="pre">MPS_FIX2()</span></tt></a>).</li>
</ol>
<p>It&#8217;s permissible to call other functions in the client program, but
see <a class="reference internal" href="scanning.html#MPS_FIX_CALL" title="MPS_FIX_CALL"><tt class="xref c c-func docutils literal"><span class="pre">MPS_FIX_CALL()</span></tt></a> for a restriction on passing the
<a class="reference internal" href="../glossary/s.html#term-scan-state"><em class="xref std std-term">scan state</em></a>.</p>
</li>
<li><p class="first">Subject to the above constraints, format methods can freely access:</p>
<ol class="loweralpha simple">
<li>memory inside the object or block that they have been asked to
look at;</li>
<li>memory managed by the MPS that is in pools that do not protect
their contents;</li>
<li>memory not managed by the MPS;</li>
</ol>
<p>They must not access other memory managed by the MPS.</p>
</li>
</ol>
</div>
<div class="section" id="format-methods">
<span id="index-3"></span><h2>7.3. Format methods<a class="headerlink" href="#format-methods" title="Permalink to this headline"></a></h2>
<dl class="type">
<dt id="mps_fmt_class_t">
<a class="reference internal" href="interface.html#mps_addr_t" title="mps_addr_t">mps_addr_t</a> <tt class="descname">(*mps_fmt_class_t)</tt><big>(</big><a class="reference internal" href="interface.html#mps_addr_t" title="mps_addr_t">mps_addr_t</a><em>&nbsp;addr</em><big>)</big><a class="headerlink" href="#mps_fmt_class_t" title="Permalink to this definition"></a></dt>
<dd><p>The type of the class method of an <a class="reference internal" href="../glossary/o.html#term-object-format"><em class="xref std std-term">object format</em></a>.</p>
<p><tt class="docutils literal"><span class="pre">addr</span></tt> is the address of the object whose class is of interest.</p>
<p>Returns an address that is related to the class or type of the
object, or a null pointer if this is not possible.</p>
<p>It is recommended that a null pointer be returned for
<a class="reference internal" href="../glossary/p.html#term-padding-object"><em class="xref std std-term">padding objects</em></a> and <a class="reference internal" href="../glossary/f.html#term-forwarding-object"><em class="xref std std-term">forwarding objects</em></a>.</p>
</dd></dl>
<dl class="type">
<dt id="mps_fmt_fwd_t">
void <tt class="descname">(*mps_fmt_fwd_t)</tt><big>(</big><a class="reference internal" href="interface.html#mps_addr_t" title="mps_addr_t">mps_addr_t</a><em>&nbsp;old</em>, <a class="reference internal" href="interface.html#mps_addr_t" title="mps_addr_t">mps_addr_t</a><em>&nbsp;new</em><big>)</big><a class="headerlink" href="#mps_fmt_fwd_t" title="Permalink to this definition"></a></dt>
<dd><p>The type of the <a class="reference internal" href="../glossary/f.html#term-forward-method"><em class="xref std std-term">forward method</em></a> of an <a class="reference internal" href="../glossary/o.html#term-object-format"><em class="xref std std-term">object format</em></a>.</p>
<p><tt class="docutils literal"><span class="pre">old</span></tt> is the address of an object.</p>
<p><tt class="docutils literal"><span class="pre">new</span></tt> is the address to where the object has been moved.</p>
<p>The MPS calls the forward method for an object format when it has
relocated an object belonging to that format. The forward method
must replace the object at <tt class="docutils literal"><span class="pre">old</span></tt> with a <a class="reference internal" href="../glossary/f.html#term-forwarding-marker"><em class="xref std std-term">forwarding marker</em></a>
that points to the address &#8216;new&#8217;. The forwarding marker must meet
the following requirements:</p>
<ol class="arabic simple">
<li>It must be possible for the MPS to call other methods in the
object format (the <a class="reference internal" href="../glossary/s.html#term-scan-method"><em class="xref std std-term">scan method</em></a>, the <a class="reference internal" href="../glossary/s.html#term-skip-method"><em class="xref std std-term">skip method</em></a>
and so on) with the address of a forwarding marker as the
argument.</li>
<li>The forwarding marker must not be bigger than the original
object.</li>
<li>It must be possible for the <a class="reference internal" href="../glossary/i.html#term-is-forwarded-method"><em class="xref std std-term">is-forwarded method</em></a> of the
object format to distinguish the forwarding marker from
ordinary objects, and the is-forwarded method method must
return the address <tt class="docutils literal"><span class="pre">new</span></tt>. See <a class="reference internal" href="#mps_fmt_isfwd_t" title="mps_fmt_isfwd_t"><tt class="xref c c-type docutils literal"><span class="pre">mps_fmt_isfwd_t</span></tt></a>.</li>
</ol>
<div class="admonition-note admonition">
<p class="first admonition-title">Note</p>
<p class="last">This method is never invoked by the <a class="reference internal" href="../glossary/g.html#term-garbage-collector"><em class="xref std std-term">garbage collector</em></a>
on an object in a <a class="reference internal" href="../glossary/n.html#term-non-moving-garbage-collector"><em class="xref std std-term">non-moving</em></a> <a class="reference internal" href="../glossary/p.html#term-pool"><em class="xref std std-term">pool</em></a>.</p>
</div>
</dd></dl>
<dl class="type">
<dt id="mps_fmt_isfwd_t">
<a class="reference internal" href="interface.html#mps_addr_t" title="mps_addr_t">mps_addr_t</a> <tt class="descname">(*mps_fmt_isfwd_t)</tt><big>(</big><a class="reference internal" href="interface.html#mps_addr_t" title="mps_addr_t">mps_addr_t</a><em>&nbsp;addr</em><big>)</big><a class="headerlink" href="#mps_fmt_isfwd_t" title="Permalink to this definition"></a></dt>
<dd><p>The type of the <a class="reference internal" href="../glossary/i.html#term-is-forwarded-method"><em class="xref std std-term">is-forwarded method</em></a> of an <a class="reference internal" href="../glossary/o.html#term-object-format"><em class="xref std std-term">object
format</em></a>.</p>
<p><tt class="docutils literal"><span class="pre">addr</span></tt> is the address of a candidate object.</p>
<p>If the <tt class="docutils literal"><span class="pre">addr</span></tt> is the address of a <a class="reference internal" href="../glossary/f.html#term-forwarding-object"><em class="xref std std-term">forwarding object</em></a>, return
the address where the object was moved to. This must be the value
of the <tt class="docutils literal"><span class="pre">new</span></tt> argument supplied to the <a class="reference internal" href="../glossary/f.html#term-forward-method"><em class="xref std std-term">forward method</em></a> when
the object was moved. If not, return a null pointer.</p>
<div class="admonition-note admonition">
<p class="first admonition-title">Note</p>
<p class="last">This method is never invoked by the <a class="reference internal" href="../glossary/g.html#term-garbage-collector"><em class="xref std std-term">garbage collector</em></a>
on an object in a <a class="reference internal" href="../glossary/n.html#term-non-moving-garbage-collector"><em class="xref std std-term">non-moving</em></a> <a class="reference internal" href="../glossary/p.html#term-pool"><em class="xref std std-term">pool</em></a>.</p>
</div>
</dd></dl>
<dl class="type">
<dt id="mps_fmt_pad_t">
void <tt class="descname">(*mps_fmt_pad_t)</tt><big>(</big><a class="reference internal" href="interface.html#mps_addr_t" title="mps_addr_t">mps_addr_t</a><em>&nbsp;addr</em>, size_t<em>&nbsp;size</em><big>)</big><a class="headerlink" href="#mps_fmt_pad_t" title="Permalink to this definition"></a></dt>
<dd><p>The type of the <a class="reference internal" href="../glossary/p.html#term-padding-method"><em class="xref std std-term">padding method</em></a> of an <a class="reference internal" href="../glossary/o.html#term-object-format"><em class="xref std std-term">object
format</em></a>.</p>
<p><tt class="docutils literal"><span class="pre">addr</span></tt> is the address at which to create a <a class="reference internal" href="../glossary/p.html#term-padding-object"><em class="xref std std-term">padding object</em></a>.</p>
<p><tt class="docutils literal"><span class="pre">size</span></tt> is the <a class="reference internal" href="../glossary/s.html#term-size"><em class="xref std std-term">size</em></a> of the padding object to be created.</p>
<p>The MPS calls a padding method when it wants to create a padding
object. Typically the MPS creates padding objects to fill in
otherwise unused gaps in memory; they allow the MPS to pack
objects into fixed-size units (such as operating system
<a class="reference internal" href="../glossary/p.html#term-page"><em class="xref std std-term">pages</em></a>).</p>
<p>The padding method must create a padding object of the specified
size at the specified address. The size can be any aligned (to the
format alignment) size. A padding object must be acceptable to
other methods in the format (the <a class="reference internal" href="../glossary/s.html#term-scan-method"><em class="xref std std-term">scan method</em></a>, the
<a class="reference internal" href="../glossary/s.html#term-skip-method"><em class="xref std std-term">skip method</em></a>, and so on).</p>
<div class="admonition-note admonition">
<p class="first admonition-title">Note</p>
<p class="last">The padding method always receives a base pointer, even if the
object format belongs to variant auto-header.</p>
</div>
</dd></dl>
<dl class="type">
<dt id="mps_fmt_scan_t">
<a class="reference internal" href="error.html#mps_res_t" title="mps_res_t">mps_res_t</a> <tt class="descname">(*mps_fmt_scan_t)</tt><big>(</big><a class="reference internal" href="scanning.html#mps_ss_t" title="mps_ss_t">mps_ss_t</a><em>&nbsp;ss</em>, <a class="reference internal" href="interface.html#mps_addr_t" title="mps_addr_t">mps_addr_t</a><em>&nbsp;base</em>, <a class="reference internal" href="interface.html#mps_addr_t" title="mps_addr_t">mps_addr_t</a><em>&nbsp;limit</em><big>)</big><a class="headerlink" href="#mps_fmt_scan_t" title="Permalink to this definition"></a></dt>
<dd><p>The type of the <a class="reference internal" href="../glossary/s.html#term-scan-method"><em class="xref std std-term">scan method</em></a> of an <a class="reference internal" href="../glossary/o.html#term-object-format"><em class="xref std std-term">object format</em></a>.</p>
<p><tt class="docutils literal"><span class="pre">ss</span></tt> is the <a class="reference internal" href="../glossary/s.html#term-scan-state"><em class="xref std std-term">scan state</em></a>. It must be passed to
<a class="reference internal" href="scanning.html#MPS_SCAN_BEGIN" title="MPS_SCAN_BEGIN"><tt class="xref c c-func docutils literal"><span class="pre">MPS_SCAN_BEGIN()</span></tt></a> and <a class="reference internal" href="scanning.html#MPS_SCAN_END" title="MPS_SCAN_END"><tt class="xref c c-func docutils literal"><span class="pre">MPS_SCAN_END()</span></tt></a> to delimit a
sequence of fix operations, and to the functions
<a class="reference internal" href="scanning.html#MPS_FIX1" title="MPS_FIX1"><tt class="xref c c-func docutils literal"><span class="pre">MPS_FIX1()</span></tt></a> and <a class="reference internal" href="scanning.html#MPS_FIX2" title="MPS_FIX2"><tt class="xref c c-func docutils literal"><span class="pre">MPS_FIX2()</span></tt></a> when fixing a
<a class="reference internal" href="../glossary/r.html#term-reference"><em class="xref std std-term">reference</em></a>.</p>
<p><tt class="docutils literal"><span class="pre">base</span></tt> points to the first <a class="reference internal" href="../glossary/f.html#term-formatted-object"><em class="xref std std-term">formatted object</em></a> in the block
of memory to be scanned.</p>
<p><tt class="docutils literal"><span class="pre">limit</span></tt> points to the location just beyond the end of the block to
be scanned. Note that there might not be any object at this
location.</p>
<p>Returns a <a class="reference internal" href="../glossary/r.html#term-result-code"><em class="xref std std-term">result code</em></a>. If a fix function returns a value
other than <a class="reference internal" href="error.html#MPS_RES_OK" title="MPS_RES_OK"><tt class="xref c c-macro docutils literal"><span class="pre">MPS_RES_OK</span></tt></a>, the scan method must return that
value, and may return without fixing any further references.
Generally, it is better if it returns as soon as possible. If the
scanning is completed successfully, the function should return
<a class="reference internal" href="error.html#MPS_RES_OK" title="MPS_RES_OK"><tt class="xref c c-macro docutils literal"><span class="pre">MPS_RES_OK</span></tt></a>.</p>
<p>The scan method for an object format is called when the MPS needs
to scan objects in a block of memory containing objects belonging
to that format. The scan method is called with a scan state and
the base and limit of the block of objects to scan. It must then
indicate references within the objects by calling
<a class="reference internal" href="scanning.html#MPS_FIX1" title="MPS_FIX1"><tt class="xref c c-func docutils literal"><span class="pre">MPS_FIX1()</span></tt></a> and <a class="reference internal" href="scanning.html#MPS_FIX2" title="MPS_FIX2"><tt class="xref c c-func docutils literal"><span class="pre">MPS_FIX2()</span></tt></a>.</p>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference internal" href="scanning.html#topic-scanning"><em>Scanning</em></a>.</p>
</div>
</dd></dl>
<dl class="type">
<dt id="mps_fmt_skip_t">
<a class="reference internal" href="interface.html#mps_addr_t" title="mps_addr_t">mps_addr_t</a> <tt class="descname">(*mps_fmt_skip_t)</tt><big>(</big><a class="reference internal" href="interface.html#mps_addr_t" title="mps_addr_t">mps_addr_t</a><em>&nbsp;addr</em><big>)</big><a class="headerlink" href="#mps_fmt_skip_t" title="Permalink to this definition"></a></dt>
<dd><p>The type of the <a class="reference internal" href="../glossary/s.html#term-skip-method"><em class="xref std std-term">skip method</em></a> of an <a class="reference internal" href="../glossary/o.html#term-object-format"><em class="xref std std-term">object format</em></a>.</p>
<p><tt class="docutils literal"><span class="pre">addr</span></tt> is the address of the object to be skipped.</p>
<p>Returns the address of the &#8220;next object&#8221;. In an object format
without headers (for example, a format of variant A), this is the
address just past the end of this object. In an object format with
headers (for example, a format of variant auto-header), it&#8217;s the
address just past where the header of next object would be, if
there were one.</p>
<div class="admonition-note admonition">
<p class="first admonition-title">Note</p>
<p class="last">In either case, the result is the sum of <tt class="docutils literal"><span class="pre">addr</span></tt> and the size
of the block containing the object.</p>
</div>
<p>A skip method is not allowed to fail.</p>
<div class="admonition-note admonition">
<p class="first admonition-title">Note</p>
<p class="last">The MPS uses this method to determine the size of objects (by
subtracting <tt class="docutils literal"><span class="pre">addr</span></tt> from the result) as well as skipping over
them.</p>
</div>
</dd></dl>
</div>
<div class="section" id="object-format-introspection">
<span id="index-4"></span><h2>7.4. Object format introspection<a class="headerlink" href="#object-format-introspection" title="Permalink to this headline"></a></h2>
<dl class="function">
<dt id="mps_addr_fmt">
<a class="reference internal" href="interface.html#mps_bool_t" title="mps_bool_t">mps_bool_t</a> <tt class="descname">mps_addr_fmt</tt><big>(</big><a class="reference internal" href="#mps_fmt_t" title="mps_fmt_t">mps_fmt_t</a><em>&nbsp;*fmt_o</em>, <a class="reference internal" href="arena.html#mps_arena_t" title="mps_arena_t">mps_arena_t</a><em>&nbsp;arena</em>, <a class="reference internal" href="interface.html#mps_addr_t" title="mps_addr_t">mps_addr_t</a><em>&nbsp;addr</em><big>)</big><a class="headerlink" href="#mps_addr_fmt" title="Permalink to this definition"></a></dt>
<dd><p>Determine the <a class="reference internal" href="../glossary/o.html#term-object-format"><em class="xref std std-term">object format</em></a> to which an address belongs.</p>
<p><tt class="docutils literal"><span class="pre">fmt_o</span></tt> points to a location that will hold the address of the
object format, if one is found.</p>
<p><tt class="docutils literal"><span class="pre">arena</span></tt> is the arena whose object formats will be considered.</p>
<p><tt class="docutils literal"><span class="pre">addr</span></tt> is the address.</p>
<p>If <tt class="docutils literal"><span class="pre">addr</span></tt> is the address of a location inside a block allocated
from a pool in <tt class="docutils literal"><span class="pre">arena</span></tt>, and that pool has an object format, then
update the location pointed to by <tt class="docutils literal"><span class="pre">fmt_o</span></tt> with the address of
the object format, and return true.</p>
<p>If <tt class="docutils literal"><span class="pre">addr</span></tt> is the address of a location inside a block allocated
from a pool in <tt class="docutils literal"><span class="pre">arena</span></tt>, but that pool has no object format,
return false.</p>
<p>If <tt class="docutils literal"><span class="pre">addr</span></tt> points to a location that is not managed by <tt class="docutils literal"><span class="pre">arena</span></tt>,
return false.</p>
<p>If none of the above conditions is satisfied,
<a class="reference internal" href="#mps_addr_fmt" title="mps_addr_fmt"><tt class="xref c c-func docutils literal"><span class="pre">mps_addr_fmt()</span></tt></a> may return either true or false.</p>
<div class="admonition-note admonition">
<p class="first admonition-title">Note</p>
<p class="last">This function might return a false positive by returning true
if you ask about an address that happens to be inside memory
managed by a pool with an object format, but which is not
inside a block allocated by that pool. It never returns a
false negative.</p>
</div>
</dd></dl>
<dl class="function">
<dt id="mps_arena_formatted_objects_walk">
void <tt class="descname">mps_arena_formatted_objects_walk</tt><big>(</big><a class="reference internal" href="arena.html#mps_arena_t" title="mps_arena_t">mps_arena_t</a><em>&nbsp;arena</em>, <a class="reference internal" href="#mps_formatted_objects_stepper_t" title="mps_formatted_objects_stepper_t">mps_formatted_objects_stepper_t</a><em>&nbsp;f</em>, void<em>&nbsp;*p</em>, size_t<em>&nbsp;s</em><big>)</big><a class="headerlink" href="#mps_arena_formatted_objects_walk" title="Permalink to this definition"></a></dt>
<dd><p>Visit all <a class="reference internal" href="../glossary/f.html#term-formatted-object"><em class="xref std std-term">formatted objects</em></a> in an
<a class="reference internal" href="../glossary/a.html#term-arena"><em class="xref std std-term">arena</em></a>.</p>
<p><tt class="docutils literal"><span class="pre">arena</span></tt> is the arena whose formatted objects you want to visit.</p>
<p><tt class="docutils literal"><span class="pre">f</span></tt> is a formatted objects stepper function. It will be called for
each formatted object in the arena. See
<a class="reference internal" href="#mps_formatted_objects_stepper_t" title="mps_formatted_objects_stepper_t"><tt class="xref c c-type docutils literal"><span class="pre">mps_formatted_objects_stepper_t</span></tt></a>.</p>
<p><tt class="docutils literal"><span class="pre">p</span></tt> and <tt class="docutils literal"><span class="pre">s</span></tt> are arguments that will be passed to <tt class="docutils literal"><span class="pre">f</span></tt> each time it
is called. This is intended to make it easy to pass, for example,
an array and its size as parameters.</p>
<p>Each <a class="reference internal" href="../glossary/p.html#term-pool-class"><em class="xref std std-term">pool class</em></a> determines for which objects the stepper
function is called. Typically, all validly formatted objects are
visited. During a <a class="reference internal" href="../glossary/t.html#term-trace"><em class="xref std std-term">trace</em></a> this will in general be only the
<a class="reference internal" href="../glossary/b.html#term-black"><em class="xref std std-term">black</em></a> objects, though the <a class="reference internal" href="../pool/lo.html#pool-lo"><em>LO (Leaf Object)</em></a> pool, for
example, will walk all objects since they are validly formatted
whether they are black or <a class="reference internal" href="../glossary/w.html#term-white"><em class="xref std std-term">white</em></a>. <a class="reference internal" href="../glossary/p.html#term-padding-object"><em class="xref std std-term">Padding objects</em></a>
may be visited at the pool class&#8217;s discretion: the <a class="reference internal" href="../glossary/c.html#term-client-program"><em class="xref std std-term">client
program</em></a> should handle this case.</p>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference internal" href="arena.html#topic-arena"><em>Arenas</em></a>.</p>
</div>
<div class="admonition-note admonition">
<p class="first admonition-title">Note</p>
<p class="last">This function is intended for heap analysis, tuning, and
debugging, not for frequent use in production.</p>
</div>
</dd></dl>
<dl class="type">
<dt id="mps_formatted_objects_stepper_t">
void <tt class="descname">(*mps_formatted_objects_stepper_t)</tt><big>(</big><a class="reference internal" href="interface.html#mps_addr_t" title="mps_addr_t">mps_addr_t</a><em>&nbsp;addr</em>, <a class="reference internal" href="#mps_fmt_t" title="mps_fmt_t">mps_fmt_t</a><em>&nbsp;fmt</em>, <a class="reference internal" href="pool.html#mps_pool_t" title="mps_pool_t">mps_pool_t</a><em>&nbsp;pool</em>, void<em>&nbsp;*p</em>, size_t<em>&nbsp;s</em><big>)</big><a class="headerlink" href="#mps_formatted_objects_stepper_t" title="Permalink to this definition"></a></dt>
<dd><p>The type of a <a class="reference internal" href="../glossary/f.html#term-formatted-object"><em class="xref std std-term">formatted objects</em></a>
<a class="reference internal" href="../glossary/s.html#term-stepper-function"><em class="xref std std-term">stepper function</em></a>.</p>
<p>A function of this type can be passed to
<a class="reference internal" href="#mps_arena_formatted_objects_walk" title="mps_arena_formatted_objects_walk"><tt class="xref c c-func docutils literal"><span class="pre">mps_arena_formatted_objects_walk()</span></tt></a>, in which case it will
be called for each formatted object in an <a class="reference internal" href="../glossary/a.html#term-arena"><em class="xref std std-term">arena</em></a>. It
receives five arguments:</p>
<p><tt class="docutils literal"><span class="pre">addr</span></tt> is the address of the object.</p>
<p><tt class="docutils literal"><span class="pre">fmt</span></tt> is the <a class="reference internal" href="../glossary/o.html#term-object-format"><em class="xref std std-term">object format</em></a> for that object.</p>
<p><tt class="docutils literal"><span class="pre">pool</span></tt> is the <a class="reference internal" href="../glossary/p.html#term-pool"><em class="xref std std-term">pool</em></a> to which the object belongs.</p>
<p><tt class="docutils literal"><span class="pre">p</span></tt> and <tt class="docutils literal"><span class="pre">s</span></tt> are the corresponding values that were passed to
<a class="reference internal" href="#mps_arena_formatted_objects_walk" title="mps_arena_formatted_objects_walk"><tt class="xref c c-func docutils literal"><span class="pre">mps_arena_formatted_objects_walk()</span></tt></a>.</p>
<p>The function may not call any function in the MPS. It may access:</p>
<ol class="loweralpha simple">
<li>memory inside the object or block pointed to by <tt class="docutils literal"><span class="pre">addr</span></tt>;</li>
<li>memory managed by the MPS that is in pools that do not protect
their contents;</li>
<li>memory not managed by the MPS;</li>
</ol>
<p>It must not access other memory managed by the MPS.</p>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference internal" href="arena.html#topic-arena"><em>Arenas</em></a>.</p>
</div>
</dd></dl>
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<h3><a href="../index.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">7. Object formats</a><ul>
<li><a class="reference internal" href="#creating-an-object-format">7.1. Creating an object format</a></li>
<li><a class="reference internal" href="#cautions">7.2. Cautions</a></li>
<li><a class="reference internal" href="#format-methods">7.3. Format methods</a></li>
<li><a class="reference internal" href="#object-format-introspection">7.4. Object format introspection</a></li>
</ul>
</li>
</ul>
<h4>Previous topic</h4>
<p class="topless"><a href="allocation.html"
title="previous chapter">6. Allocation</a></p>
<h4>Next topic</h4>
<p class="topless"><a href="scanning.html"
title="next chapter">8. Scanning</a></p><h4>Downloads</h4>
<p class="topless">
<a href="http://www.ravenbrook.com/project/mps/release/1.111.0/">MPS Kit release 1.111.0</a><br>
<a href="http://www.ravenbrook.com/project/mps/release/">All MPS Kit releases</a>
</p>
<h4>Issues</h4>
<p class="topless">
<a href="http://www.ravenbrook.com/project/mps/issue/?action=list&amp;view=status%3dopen&amp;display=Job:Priority:Title&amp;sort=Priority">Known issues</a><br>
<a href="http://www.ravenbrook.com/project/mps/issue/?action=fixed&release_fixed=1.111.0">Issues fixed in release 1.111.0</a>
</p><h4>Contact us</h4>
<p class="topless"><a href="mailto:mps-questions@ravenbrook.com">mps-questions@ravenbrook.com</a></p>
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