Re: execute_control_command may not remove its cleanups

Dave Allan
2004-02-19T19:14:53Z

Ok. I see your point. How about setting old_chain to cleanup_chain
unconditionally at the beginning of the function and doing the cleanups
unconditionally at the end? That way, we're safe against both
scenarios: against doing cleanups prematurely, but also safe against
getting into the function with cleanup_chain null and then freeing
something random at a later point.

Dave

On Thu, 2004-02-19 at 13:47, Daniel Jacobowitz wrote:
> On Thu, Feb 19, 2004 at 01:21:52PM -0500, Dave Allan wrote:
> > > > However, it seems from code inspection and the gdb internals
> > > > documentation that the call to do_cleanups ought to be unconditional.
> > > > Does that seem right?
> > >
> > > No, instead, the cleanup chain should always have an item on it. If
> > > make_cleanup is not called then old_chain will remain NULL, and
> > > do_cleanups (NULL) means "do all cleanups", not "do nothing". It looks
> > > to me like command_handler is responsible for there always being a
> > > cleanup on the chain:
> > > old_chain = make_cleanup (null_cleanup, 0);
> > > but maybe I'm mistaken about that; it's a bit far down the tree.
> >
> > I definitely understand that do_cleanups(NULL) will do all cleanups
> > which is not what's wanted here. The call is do_cleanups(old_chain),
> > though, so if there are cleanups on the chain already, they are
> > preserved. The problem isn't the do_cleanups call, it's the fact that
> > the do_cleanups call is conditional. The solution is to remove the if
> > (old_chain) statement and always do the cleanup.
> >
> > Given what's stated in the docs, that a function must always remove the
> > cleanups it creates, it would seem to me that regardless of the state of
> > cleanup_chain at the beginning of execute_control_command, whether it's
> > NULL or contains cleanups, we want to get back to that state before we
> > return.
> >
> > Looking at what cleanups execute_control_command puts on cleanup_chain,
> > that is correct. Either one or two cleanups are put on the chain where
> > arg is an automatic variable and function is free_current_contents. If
> > these cleanups aren't done before the stack frame is destroyed,
> > something undefined will later be freed when the cleanups are done.
>
> Think about this again. Both of those cleanups are conditionally
> created. If neither of them is created, old_chain will still be NULL.
> This will lead to running cleanups prematurely. If the cleanup chain
> is non-empty, things work OK.
>
> The alternative is null_cleanup.