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<title>Debugging with GDB: Automatic Overlay Debugging</title>
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<a name="Automatic-Overlay-Debugging"></a>
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<div class="header">
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<p>
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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="previous">Overlay Commands</a>, Up: <a href="Overlays.html#Overlays" accesskey="u" rel="up">Overlays</a> [<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>
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<h3 class="section">14.3 Automatic Overlay Debugging</h3>
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<a name="index-automatic-overlay-debugging"></a>
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<p><small>GDB</small> can automatically track which overlays are mapped and which
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are not, given some simple co-operation from the overlay manager in the
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inferior. If you enable automatic overlay debugging with the
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<code>overlay auto</code> command (see <a href="Overlay-Commands.html#Overlay-Commands">Overlay Commands</a>), <small>GDB</small>
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looks in the inferior’s memory for certain variables describing the
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current state of the overlays.
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</p>
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<p>Here are the variables your overlay manager must define to support
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<small>GDB</small>’s automatic overlay debugging:
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</p>
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<dl compact="compact">
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<dt><code>_ovly_table</code>:</dt>
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<dd><p>This variable must be an array of the following structures:
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</p>
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<div class="smallexample">
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<pre class="smallexample">struct
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{
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/* The overlay's mapped address. */
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unsigned long vma;
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/* The size of the overlay, in bytes. */
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unsigned long size;
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/* The overlay's load address. */
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unsigned long lma;
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/* Non-zero if the overlay is currently mapped;
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zero otherwise. */
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unsigned long mapped;
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}
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</pre></div>
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</dd>
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<dt><code>_novlys</code>:</dt>
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<dd><p>This variable must be a four-byte signed integer, holding the total
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number of elements in <code>_ovly_table</code>.
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</p>
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</dd>
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</dl>
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<p>To decide whether a particular overlay is mapped or not, <small>GDB</small>
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looks for an entry in <code><span class="nolinebreak">_ovly_table</span></code><!-- /@w --> whose <code>vma</code> and
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<code>lma</code> members equal the VMA and LMA of the overlay’s section in the
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executable file. When <small>GDB</small> finds a matching entry, it consults
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the entry’s <code>mapped</code> member to determine whether the overlay is
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currently mapped.
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</p>
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<p>In addition, your overlay manager may define a function called
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<code>_ovly_debug_event</code>. If this function is defined, <small>GDB</small>
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will silently set a breakpoint there. If the overlay manager then
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calls this function whenever it has changed the overlay table, this
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will enable <small>GDB</small> to accurately keep track of which overlays
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are in program memory, and update any breakpoints that may be set
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in overlays. This will allow breakpoints to work even if the
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overlays are kept in ROM or other non-writable memory while they
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are not being executed.
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</p>
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<hr>
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<p>
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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="previous">Overlay Commands</a>, Up: <a href="Overlays.html#Overlays" accesskey="u" rel="up">Overlays</a> [<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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