[RFC] Interaction between HW watchpoints and 'set var'.

Paul Pluzhnikov
2009-11-25T17:52:10Z

Greetings,

Consider the following test case:

--- cut ---
int x;
int main()
{
int i, j;

for (i = 0; i < 500; ++i) {
j = 0; // break here
x = 42;
j = i; // expect HW watchpoint stop
}
return 0;
}
--- cut ---

gdb64-cvs -q ./a.out

Reading symbols from /tmp/a.out...done.
(gdb) b 7
Breakpoint 1 at 0x400455: file foo.c, line 7.
(gdb) r

Breakpoint 1, main () at foo.c:7
7 j = 0; // break here
(gdb) watch x
Hardware watchpoint 2: x
(gdb) c
Hardware watchpoint 2: x

Old value = 0
New value = 42
main () at foo.c:9
9 j = i; // expect HW watchpoint stop
(gdb) c

Breakpoint 1, main () at foo.c:7
7 j = 0; // break here

So far, everything is working just as one would expect.

(gdb) p x
$1 = 42
(gdb) set var x = 1
(gdb) print x
$2 = 1
(gdb) c

Breakpoint 1, main () at foo.c:7
7 j = 0; // break here
(gdb) print x
$3 = 42

Why didn't HW watchpoint fire? Clearly 'x' changed from 1 to 42.

Setting 'debug infrun' shows that the watchpoint does indeed fire,
but is ignored by watchpoint_check() because GDB thinks the old
value was 42 as well. And GDB believes that because value_assign()
does not update breakpoints.

Now, I can fix that by installing deprecated_memory_changed_hook,
but the deprecated part gives me pause.

Should I create a new memory_changed observer and remove
deprecated_memory_changed_hook instead?

Thanks,
--
Paul Pluzhnikov