debug-on-entry

A second way to start debug on a function is to enter the debugger when you call the function. You can do this by calling debug-on-entry.

Type:

M-x debug-on-entry RET triangle-bugged RET

Now, evaluate the following:

(triangle-bugged 5)

Emacs will create a `*Backtrace*' buffer and tell you that it is beginning to evaluate the triangle-bugged function:

---------- Buffer: *Backtrace* ----------
Entering:
* triangle-bugged(5)
  eval((triangle-bugged 5))
  eval-last-sexp(nil)
* call-interactively(eval-last-sexp)
---------- Buffer: *Backtrace* ----------

In the `*Backtrace*' buffer, type d. Emacs will evaluate the first expression in triangle-bugged; the buffer will look like this:

---------- Buffer: *Backtrace* ----------
Beginning evaluation of function call form:
* (let ((total 0)) (while (> number 0) (setq total ...)
        (setq number ...)) total))
  triangle-bugged(5)
* eval((triangle-bugged 5))
  eval-last-sexp(nil)
* call-interactively(eval-last-sexp)
---------- Buffer: *Backtrace* ----------

Now, type d again, eight times, slowly. Each time you type d, Emacs will evaluate another expression in the function definition. Eventually, the buffer will look like this:

---------- Buffer: *Backtrace* ----------
Beginning evaluation of function call form:
* (setq number (1= number)))
* (while (> number 0) (setq total (+ total number)) 
        (setq number (1= number))))
* (let ((total 0)) (while (> number 0) 
        (setq total ...) (setq number ...)) total))
  triangle-bugged(5)
* eval((triangle-bugged 5))
  eval-last-sexp(nil)
* call-interactively(eval-last-sexp)
---------- Buffer: *Backtrace* ----------

Finally, after you type d two more times, Emacs will reach the error, and the top two lines of the `*Backtrace*' buffer will look like this:

---------- Buffer: *Backtrace* ----------
Signalling: (void-function 1=)
* (1= number))
...
---------- Buffer: *Backtrace* ----------

By typing d, you were able to step through the function.

You can quit a `*Backtrace*' buffer by typing q; this quits the trace, but does not cancel debug-on-entry.

To cancel the effect of debug-on-entry, call cancel-debug-on-entry and the name of the function, like this:

M-x cancel-debug-on-entry RET triangle-debugged RET

(If you are reading this in Info, cancel debug-on-entry now.)

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