Info Catalog elisp: Infinite Loops elisp: Debugger elisp: Explicit Debug

elisp: Function Debugging

 
 17.1.3 Entering the Debugger on a Function Call
 -----------------------------------------------
 
 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.
 
  -- Command: debug-on-entry function-name
      This function requests FUNCTION-NAME to invoke the debugger each
      time it is called.
 
      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.
 
      When ‘debug-on-entry’ is 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-entry’ does nothing.
      ‘debug-on-entry’ always returns FUNCTION-NAME.
 
      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* ------
 
 
  -- Command: cancel-debug-on-entry &optional function-name
      This function undoes the effect of ‘debug-on-entry’ on
      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-entry’ does nothing to a function which is not
      currently set up to break on entry.
 

Info Catalog elisp: Infinite Loops elisp: Debugger elisp: Explicit Debug