[slime-devel] how do I invoke the debugger and step through a function?

[Helmut Eller]

ramb at sonic.net writes:

> I've created a function (defun tester () ...) that does almost
> what I want - but its not totally correct. I would like to use
> the slime debugger to start this function and single step through
> its various parts to see where the mistake is. How do I do that with
> slime?

The support for stepping is, unfortunately, not very good.  I usually
insert a call to BREAK at the beginning of the function like:

  (defun tester () (break) ...)

recompile the function, and then run the function.  The debugger will
pop up and you can use `s' for stepping.

`s' inserts a breakpoint at the next "interesting" code location(s) and
continues.  The debugger will usually pop up again at the next
breakpoint.

Interesting code locations are: beginning of blocks, call-sites, and
return points. I think "block" means a sequence of instructions
without branches in CMUCL terminology.

There are also sldb-break and sldb-break-on-return commands, but
inserting an explicit BREAK works better in my experience.


Here are some limitations of the current support:

- Sometimes two code locations are at the same source location, i.e.,
  the same position in the Emacs buffer.  This can be a bit confusing,
  but hasn't annoyed me enough yet to fix it.  The problem is more
  severe if your code contains a lot of macros.

- There's currently no support for a gdb-like "step into" command,
  i.e., the ability to continue stepping in a called function.

- I think there's also a problem/bug when stepping through
  unwind-protect code which can cause segfaults.  (Segfaults on
  CMUCL/x86 are currently serious show stoppers because the signal
  handler trashes the control stack.)

- Another (minor) problem are stale breakpoints.  Breakpoints are
  sometimes not properly deleted and are still there if you execute
  the same function later.

Helmut.