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.

Answer 3

You can set a watchpoint that does not go out of scope by setting it to the memory address.

(gdb) p &var1
$1 = (int *) 0x41523c0
(gdb) watch *(int *)0x41523c0
Hardware watchpoint 1: *(int *)0x41523c0

This also works for other data types and pointers.

Answer 4

1. I'm not sure which language us are using, so the exact answer will vary, but could you change the variable to either be static, global, or dynamically allocated (and don't free it when the function returns?). This way it's raw address won't change, and gdb will be able breakpoint on it.

2. Instead of watching the value whe it equals a specific value; you should set a conditional break point on the line where you want to check the value of var1. This should effectively have the same effect

e.g.

(gdb) break main.c:123 if (var1 == 0)