How do I set persistent and conditional watchpoints on locally scoped variables?

Question

1. If I set a watchpoint for a variable local to the current scope, it will be auto deleted when going out of the scope. Is there any way to set it once and keep it auto alive whenever entering the same scope?

2. Is there anyway to set conditional watchpoint, like watch var1 if var1==0? In my case, the condition does't work. gdb stops whenever var1's value is changed, instead of untill var1 == 0 is true. My gdb is GNU gdb 6.8-debian.

Answer 1

I agree with Dave that a conditional breakpoint is the way to go.

However, to do what you asked, you can use GDB's commands command to set a list of GDB commands to execute whenever a breakpoint is hit. I find this incredibly useful.

I suggest writing your GDB commands into a file so that they are easy to edit and easy to reload with the source command. Or you can specify command files to load on the GDB command line or use .gdbinit to make them load automatically.

An example of a good use of commands:
Suppose that I have a function format that is called by a lot of other functions. I want to break on it, but only after function do_step_3 has been called.

break do_step_3 commands   break format   continue end 

You could use this for your problem with something like:

break func commands   watch var   continue end 

Answer 2

You can set conditions on watchpoints in the same way that you do with breakpoints. This is in the documentation but admittedly it hardly calls attention to itself.

So watch my_var if my_var > 3 works just fine, as does the condition command.

To recreate the watchpoint if the variable it is watching goes out of scope, have gdb do this automatically using a breakpoint at the start of the function as Zan has described.