gdb watchpoint except when triggered in specific locations? - gdb

Question

I have a variable getting corrupted. I want to see where this is happening, so a watchpoint is the obvious choice, except this variable frequently gets set (in this case, processing network packets and it gets set once for each packet). I would like to exempt the watchpoint from the location where the variable is supposed to get written. Basically, what I would like is "watch variable if line_number != x"; the gdb docs seem to be a bit thin on the conditional expressions, so I'm not sure this is possible...

Answer 1

There are a couple of ways to do this, in the following source file.
struct stuff {
int watched;
};
void dont_hit(struct stuff *foo) {
begin:
for (int i = 0; i < 100; i++)
foo->watched = i;
end:
return;
}
void hit(struct stuff *foo) {
foo->watched = 1024;
}
int main()
{
struct stuff foo;
setup:
dont_hit(&foo);
hit(&foo);
dont_hit(&foo);
}
We can then use the watchpoint condition: if $pc != ...
as shown below:
(gdb) break main
(gdb) r
(gdb) watch foo.watched
(gdb) c
Hardware watchpoint 2: foo.watched
Old value = -8320
New value = 0
dont_hit (foo=0x7fffffffde90) at main.c:7
7 for (int i = 0; i < 100; i++)
(gdb) p $pc
$1 = (void (*)()) 0x4004c0 <dont_hit+26>
(gdb) watch -location foo->watched if $pc != 0x4004c0
ardware watchpoint 3: -location foo->watched
(gdb) delete 2
(gdb) c
Continuing.
Hardware watchpoint 3: -location foo->watched
Old value = 99
New value = 1024
hit (foo=0x7fffffffde90) at main.c:15
Another way is to use breakpoints to enable/disable the watchpoint in places we don't want it to trigger, I'm using the label's here setup/begin/end for clarity, but you can replace main:label with filename:line_number
break main:setup
run
watch -location foo.watched
break dont_hit:begin
commands
silent
disable 2
cont
end
break dont_hit:end
commands
silent
enable 2
cont
end
A third way, is going to use Python, and FinishBreakpoints to do the enabling/disabling.