| Red Hat Enterprise Linux 4: Debugging with gdb | ||
|---|---|---|
| Prev | Chapter 13. Debugging Programs That Use Overlays | Next |
To use gdb's overlay support, each overlay in your program must correspond to a separate section of the executable file. The section's virtual memory address and load memory address must be the overlay's mapped and load addresses. Identifying overlays with sections allows gdb to determine the appropriate address of a function or variable, depending on whether the overlay is mapped or not.
gdb's overlay commands all start with the word overlay; you can abbreviate this as ov or ovly. The commands are:
Disable gdb's overlay support. When overlay support is disabled, gdb assumes that all functions and variables are always present at their mapped addresses. By default, gdb's overlay support is disabled.
Enable manual overlay debugging. In this mode, gdb relies on you to tell it which overlays are mapped, and which are not, using the overlay map-overlay and overlay unmap-overlay commands described below.
Tell gdb that overlay is now mapped; overlay must be the name of the object file section containing the overlay. When an overlay is mapped, gdb assumes it can find the overlay's functions and variables at their mapped addresses. gdb assumes that any other overlays whose mapped ranges overlap that of overlay are now unmapped.
Tell gdb that overlay is no longer mapped; overlay must be the name of the object file section containing the overlay. When an overlay is unmapped, gdb assumes it can find the overlay's functions and variables at their load addresses.
Enable automatic overlay debugging. In this mode, gdb consults a data structure the overlay manager maintains in the inferior to see which overlays are mapped. For details, (refer to Section 13.3 Automatic Overlay Debugging.
Re-read the overlay table from the inferior. Normally, gdb re-reads the table gdb automatically each time the inferior stops, so this command should only be necessary if you have changed the overlay mapping yourself using gdb. This command is only useful when using automatic overlay debugging.
Display a list of the overlays currently mapped, along with their mapped addresses, load addresses, and sizes.
Normally, when gdb prints a code address, it includes the name of the function the address falls in:
(gdb) print main
$3 = {int ()} 0x11a0 <main> |
When overlay debugging is enabled, gdb recognizes code in unmapped overlays, and prints the names of unmapped functions with asterisks around them. For example, if foo is a function in an unmapped overlay, gdb prints it this way:
(gdb) overlay list
No sections are mapped.
(gdb) print foo
$5 = {int (int)} 0x100000 <*foo*> |
When foo's overlay is mapped, gdb prints the function's name normally:
(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.