SBCL Internals The pages on this CLiki-driven site can be edited by anybody at any time. No warranty of any kind can therefore be made; any implied warranties of merchantability or fitness for a particular purpose are expressly disclaimed

The stepper works by having the compiler insert STEP-CONDITION signalling forms into code compiled with high DEBUG optimization policy, and providing a handler for these.

The built-in interface can be replaced: SB-EXT:*STEPPER-HOOK* is called with each step-condition when single-stepping.

Different subtypes of STEP-CONDITION are signalled under different circumstances:

• STEP-FORM-CONDITION is signalled prior to function call (including evaluation of function call arguments.

• STEP-VALUES-CONDITION is signalled after function call. (Only for fully steppable code.)

• STEP-VARIABLE-CONDITION is signalled for variable references as function call arguments. (Only for fully steppable code.)

Around the signalling of STEP-FORM-CONDITION a number of restarts are established: SB-EXT:STEP-NEXT, SB-EXT:STEP-INTO, SB-EXT:STEP-CONTINUE. The stepper hook can control stepping by invoking any of these. For more details, see the source & documentation strings.

TODO aka shortcomings

Using whatever hardware/kernel support is available for debugging wouldn't hurt as far as speed goes.
• It would be nice to have sensible stepping for some special forms as well.

• The heuristic for deciding which forms to instrument is a bit nasty.

• EVAL doesn't currently signal STEP-CONDITIONS unless it indirects through the compiler.

• The debugger integration is pretty weak: examining local variables and backtraces from within the stepper would be good. Forms introduced by the stepper may also make for more confusing backtraces.

• The stepper interface should be a REPL -- currently no evaluation there is possible.

• The output format of the stepper is not quite ideal. It should be possible to indent the printed values according to nesting depth, making it easier to see what's going on.

• no tests/step.impure.lisp yet.

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