Watchpoint issues

Vladimir Prus ghost at
Thu Apr 20 10:32:16 UTC 2006

I have some issues with current watchpoint logic in KDevelop, but before 
changing it completely wanted to discuss this on list.

Problem 1. If I set a watchpoint on anything but a global var, and then 
restart KDevelop, I immediately get error that expression is not valid, when 
KDevelop tries to insert that watchpoint.

Problem 2. Suppose I have a function like this:

  void do_that(My_class* ptr)
		ptr->i = .....;

I notice that ptr->i holds a valid value at the moment, and want to set 
watchpoint on 'i', to detect where that value is damaged. Unfortunately, as 
soon as gdb leaves 'do_that', gdb reports:

  Watchpoint N deleted because the program has left the block in
  which its expression is valid.

This is technically correct, but makes watchpoints unusable.

In fact, when debugging with console gdb, I've learned to always set 
breakpoints on address.

How about making KDevelop behave like this:

1. For each watchpoint, KDevelop evaluates the address and sets gdb watchpoint 
on the address. The original expression is remembered by KDevelop.

2. If the address is in the current stack frame, then leaving the stack frame 
deletes watchpoints.

3. When debugger has stopped, all watchpoints end up in disabled state. Later, 
user can re-enable any watchpoint. In this case, the stored expression value 
will be reevaluated again.

4. If gdb can't set hardware watchpoint, we should produce a big error 
message, saying "While gdb has software watchpoints, they are too slow to be 

Any problems with the above?

- Volodya

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