Cannot set watchpoint in GDB

Question

I am doing debugging and wanted to check the place where the value of the variable changes .For this I tried setting a watch point by saying something like watch 'structure->somefunc.getvalue()' which is not a simple variable (probably some portion of a bigger structure invoking a function ) .When I do this gdb says No symbol 'structure->somefunc..' in current context .When I do a grep in the same directory I get 3-4 instances of the same expression.Am I missing out something?

Answer 1

Am I missing out something?

Yes, you appear to be missing at least a couple of things:

• The expression structure->somefunc.getvalue() doesn't make any sense. You probably meant some_variable->some_field.getvalue()

• For that expression to be valid, you must be in a context where some_variable exists. The fact that some_variable shows up in grep output doesn't mean GDB can currently evaluate it. It may be able to evaluate it when you stop the program in correct context.

• It makes no sense (and is impossible) to set a watchpoint on return value of getvalue(). Watchpoints only make sense if you can specify memory location that you want to watch. If (as is likely) getvalue() returns something like this->m_value, then what you really want is to set a watchpoint on *(&some_variable->some_field.m_value).