Re: Reverse debugging

[Michael Snyder <msnyder at vmware dot com>]

Nick Roberts wrote:
> On i386, with current GDB, if I do:
>
>   (gdb) reverse-next
>
> I get:
>
>   Target child does not support this command.
>
> However if I do:
>
>   (gdb) target record
>
> first, and proceed, then "reverse-next" does indeed work as I would expect.

That's because target exec does not support reverse,
but target record does.  Reverse is also potentially
supported under target remote, depending on what remote
target you're connected to.

> In the "Reverse Execution" node of the GDB manual it talks about
> "a target environment that supports reverse execution" but doesn't clarify
> what those targets are.  The node "Process Record and Replay" comes next
> in the manual but these two seem to be inimately related.

Process record (target record) is one of a small number of
targets that currently support reverse debugging.  The others,
so far, are all remote targets (eg. SID from Red Hat, and
Simics from Virtutech).

> Is it possible to reverse debug without doing "target record" first? 

Yes -- but not under target exec (normal native debugging).

> If not
> then I think "Reverse Execution" should be a sub-node of "Process Record and
> Replay".  In any case it would be helpful to mention "target record" in the
> "Reverse Execution" node if most platforms don't support reverse execution
> outside record and replay.

No, reverse execution is not tied to process record -- it just
isn't available under target exec.

> Is there a GDB command that shows if process recording is started/stopped?
> "target record" doesn't appear to influence the output of 'info target".

Good question.  We've been discussing the same question under
the thread "Testing of reverse debug commands", and I think
we're leaning toward the conclusion "we need such a command".