To investigate a problem that happens in the middle of a program, one useful technique is to enter the debugger whenever a certain function is called. You can do this to the function in which the problem occurs, and then step through the function, or you can do this to a function called shortly before the problem, step quickly over the call to that function, and then step through its caller.
This function requests function-name to invoke the debugger each time it is called. It works by inserting the form
(implement-debug-on-entry)into the function definition as the first form.
Any function or macro defined as Lisp code may be set to break on entry, regardless of whether it is interpreted code or compiled code. If the function is a command, it will enter the debugger when called from Lisp and when called interactively (after the reading of the arguments). You can also set debug-on-entry for primitive functions (i.e., those written in C) this way, but it only takes effect when the primitive is called from Lisp code. Debug-on-entry is not allowed for special forms.
debug-on-entryis called interactively, it prompts for function-name in the minibuffer. If the function is already set up to invoke the debugger on entry,
debug-on-entryalways returns function-name.
Warning: if you redefine a function after using
debug-on-entryon it, the code to enter the debugger is discarded by the redefinition. In effect, redefining the function cancels the break-on-entry feature for that function.
Here's an example to illustrate use of this function:(defun fact (n) (if (zerop n) 1 (* n (fact (1- n))))) ⇒ fact (debug-on-entry 'fact) ⇒ fact (fact 3) ------ Buffer: *Backtrace* ------ Debugger entered--entering a function: * fact(3) eval((fact 3)) eval-last-sexp-1(nil) eval-last-sexp(nil) call-interactively(eval-last-sexp) ------ Buffer: *Backtrace* ------ (symbol-function 'fact) ⇒ (lambda (n) (debug (quote debug)) (if (zerop n) 1 (* n (fact (1- n)))))
This function undoes the effect of
debug-on-entryon function-name. When called interactively, it prompts for function-name in the minibuffer. If function-name is omitted or
nil, it cancels break-on-entry for all functions. Calling
cancel-debug-on-entrydoes nothing to a function which is not currently set up to break on entry.