Red Hat ENTERPRISE LINUX 4 - DEVELOPER TOOLS GUIDE, Manual

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106 Chapter 13. Debugging Programs That Use Overlays
(gdb) overlay list
Section .ov.foo.text, loaded at 0x100000 - 0x100034,
mapped at 0x1016 - 0x104a
(gdb) print foo
$6 = {int (int)} 0x1016
.
foo
/
When overlay debugging is enabled, gdb can find the correct address for functions and variables in
an overlay, whether or not the overlay is mapped. This allows most gdb commands, like
break
and
disassemble
, to work normally, even on unmapped code. However, gdb’s breakpoint support has
some limitations:
•
You can set breakpoints in functions in unmapped overlays, as long as gdb can write to the overlay
at its load address.
•
gdb can not set hardware or simulator-based breakpoints in unmapped overlays. However, if you
set a breakpoint at the end of your overlay manager (and tell gdb which overlays are now mapped,
if you are using manual overlay management), gdb will re-set its breakpoints properly.
13.3. Automatic Overlay Debugging
gdb 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
overlay auto
command (refer to Section 13.2 Overlay Commands ), gdb looks in the inferior’s
memory for certain variables describing the current state of the overlays.
Here are the variables your overlay manager must define to support gdb’s automatic overlay debug-
ging:
_ovly_table
:
This variable must be an array of the following structures:
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;
}
_novlys
:
This variable must be a four-byte signed integer, holding the total number of elements in
_ovly_table
.
To decide whether a particular overlay is mapped or not, gdb looks for an entry in
_ovly_table
whose
vma
and
lma
members equal the VMA and LMA of the overlay’s section in the executable file.
When gdb finds a matching entry, it consults the entry’s
mapped
member to determine whether the
overlay is currently mapped.