mingw/info/gdb/Automatic-Overlay-Debugging.html

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  12 Node:<a name="Automatic%20Overlay%20Debugging">Automatic Overlay Debugging</a>,
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  19 <h3 class="section">Automatic Overlay Debugging</h3>
  20
  21    GDB can automatically track which overlays are mapped and which
  22 are not, given some simple co-operation from the overlay manager in the
  23 inferior.  If you enable automatic overlay debugging with the
  24 <code>overlay auto</code> command (see <a href="Overlay-Commands.html#Overlay%20Commands">Overlay Commands</a>), GDB
  25 looks in the inferior's memory for certain variables describing the
  26 current state of the overlays.
  27
  28    <p>Here are the variables your overlay manager must define to support
  29 GDB's automatic overlay debugging:
  30
  31      <dl>
  32
  33      <br><dt><code>_ovly_table</code>:
  34      <dd>This variable must be an array of the following structures:
  35
  36      <pre class="example">          struct
  37           {
  38             /* The overlay's mapped address.  */
  39             unsigned long vma;
  40
  41             /* The size of the overlay, in bytes.  */
  42             unsigned long size;
  43
  44             /* The overlay's load address.  */
  45             unsigned long lma;
  46
  47             /* Non-zero if the overlay is currently mapped;
  48                zero otherwise.  */
  49             unsigned long mapped;
  50           }
  51           </pre>
  52
  53      <br><dt><code>_novlys</code>:
  54      <dd>This variable must be a four-byte signed integer, holding the total
  55 number of elements in <code>_ovly_table</code>.
  56
  57    </dl>
  58
  59    <p>To decide whether a particular overlay is mapped or not, GDB
  60 looks for an entry in <code>_ovly_table</code> whose <code>vma</code> and
  61 <code>lma</code> members equal the VMA and LMA of the overlay's section in the
  62 executable file.  When GDB finds a matching entry, it consults
  63 the entry's <code>mapped</code> member to determine whether the overlay is
  64 currently mapped.
  65
  66    <p>In addition, your overlay manager may define a function called
  67 <code>_ovly_debug_event</code>.  If this function is defined, GDB
  68 will silently set a breakpoint there.  If the overlay manager then
  69 calls this function whenever it has changed the overlay table, this
  70 will enable GDB to accurately keep track of which overlays
  71 are in program memory, and update any breakpoints that may be set
  72 in overlays.  This will allow breakpoints to work even if the
  73 overlays are kept in ROM or other non-writable memory while they
  74 are not being executed.
  75
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