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<a name="Automatic-Overlay-Debugging"></a>
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Next: <a href="Overlay-Sample-Program.html#Overlay-Sample-Program" accesskey="n" rel="next">Overlay Sample Program</a>, Previous: <a href="Overlay-Commands.html#Overlay-Commands" accesskey="p" rel="prev">Overlay Commands</a>, Up: <a href="Overlays.html#Overlays" accesskey="u" rel="up">Overlays</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Automatic-Overlay-Debugging-1"></a>
<h3 class="section">14.3 Automatic Overlay Debugging</h3>
<a name="index-automatic-overlay-debugging"></a>

<p><small>GDB</small> can automatically track which overlays are mapped and which
are not, given some simple co-operation from the overlay manager in the
inferior.  If you enable automatic overlay debugging with the
<code>overlay auto</code> command (see <a href="Overlay-Commands.html#Overlay-Commands">Overlay Commands</a>), <small>GDB</small>
looks in the inferior&rsquo;s memory for certain variables describing the
current state of the overlays.
</p>
<p>Here are the variables your overlay manager must define to support
<small>GDB</small>&rsquo;s automatic overlay debugging:
</p>
<dl compact="compact">
<dt><code>_ovly_table</code>:</dt>
<dd><p>This variable must be an array of the following structures:
</p>
<div class="smallexample">
<pre class="smallexample">struct
{
  /* The overlay's mapped address.  */
  unsigned long vma;

  /* The size of the overlay, in bytes.  */
  unsigned long size;

  /* The overlay's load address.  */
  unsigned long lma;

  /* Non-zero if the overlay is currently mapped;
     zero otherwise.  */
  unsigned long mapped;
}
</pre></div>

</dd>
<dt><code>_novlys</code>:</dt>
<dd><p>This variable must be a four-byte signed integer, holding the total
number of elements in <code>_ovly_table</code>.
</p>
</dd>
</dl>

<p>To decide whether a particular overlay is mapped or not, <small>GDB</small>
looks for an entry in <code><span class="nolinebreak">_ovly_table</span></code><!-- /@w --> whose <code>vma</code> and
<code>lma</code> members equal the VMA and LMA of the overlay&rsquo;s section in the
executable file.  When <small>GDB</small> finds a matching entry, it consults
the entry&rsquo;s <code>mapped</code> member to determine whether the overlay is
currently mapped.
</p>
<p>In addition, your overlay manager may define a function called
<code>_ovly_debug_event</code>.  If this function is defined, <small>GDB</small>
will silently set a breakpoint there.  If the overlay manager then
calls this function whenever it has changed the overlay table, this
will enable <small>GDB</small> to accurately keep track of which overlays
are in program memory, and update any breakpoints that may be set
in overlays.  This will allow breakpoints to work even if the
overlays are kept in ROM or other non-writable memory while they
are not being executed.
</p>
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