Watchpoints in GDB
Used to stop execution whenever the value of an expression changes, without having to predict a particular place where this might happen. The expression may be as simple as the value of a single variable or complex as a combination.
- A reference to the value of a single variable.
- An address which is typecasted into a type (int)0x123456.
- A complex expression, like
a+b*5.
Watchpoint works by watching the expression every time an instruction is stepped through, which is called as a software watchpoint. This can be a lot slower. However, some x86 and PPC support hardware watchpoints, which do not slow down the machine.
watch expr [thread thread-id]
-
> watch var, watch a single exp. (var) for change in it’s value, when detected, it breaks there. -
> rwatch var, watch a single exp. (var) for being read from “var”, when detected, it breaks there. -
> awatch varis a combination for both read and write. -
thread thread-id can be mentioned when it should be stopped only in that particular thread. This only works on a hardware breakpoint.
-
> watch *(type*)0x1234, the above formats apply for the content at the address. -
> info watchpointsprints all the watchpoints set.
GDB always prefers to use hardware watchpoints before falling back to software watchpoints. It can be however forced to use software breakpoints using set can-use-hw-watchpoints 0. The vice-versa can also done with the command by replacing 0 with 1. To see the status of the watchpoint, use show can-use-hw-watchpoints.
GDB Breakpoints:
GDB Commands:
> info program: displays information about the status of your program whether it is running or not and why it is stopped.