This section describes functions and variables used internally by the debugger.
The value of this variable is the function to call to invoke the
debugger. Its value must be a function of any number of arguments, or,
more typically, the name of a function. This function should invoke
some kind of debugger. The default value of the variable is
The first argument that Lisp hands to the function indicates why it
was called. The convention for arguments is detailed in the description
debug (see Invoking the Debugger).
This function prints a trace of Lisp function calls currently active.
The trace is identical to the one that
debug would show in the
*Backtrace* buffer. The return value is always nil.
In the following example, a Lisp expression calls
explicitly. This prints the backtrace to the stream
standard-output, which, in this case, is the buffer
Each line of the backtrace represents one function call. The line shows the function followed by a list of the values of the function’s arguments if they are all known; if they are still being computed, the line consists of a list containing the function and its unevaluated arguments. Long lists or deeply nested structures may be elided.
(with-output-to-temp-buffer "backtrace-output" (let ((var 1)) (save-excursion (setq var (eval '(progn (1+ var) (list 'testing (backtrace)))))))) ⇒ (testing nil)
----------- Buffer: backtrace-output ------------ backtrace() (list 'testing (backtrace))
(progn ...) eval((progn (1+ var) (list 'testing (backtrace)))) (setq ...) (save-excursion ...) (let ...) (with-output-to-temp-buffer ...) eval((with-output-to-temp-buffer ...)) eval-last-sexp-1(nil)
eval-last-sexp(nil) call-interactively(eval-last-sexp) ----------- Buffer: backtrace-output ------------
If this variable is non-
nil, every stack frame of the backtrace
is displayed as a list. This aims at improving the backtrace
readability at the cost of special forms no longer being visually
different from regular function calls.
nil, the above
example would look as follows:
----------- Buffer: backtrace-output ------------ (backtrace) (list 'testing (backtrace))
(progn ...) (eval (progn (1+ var) (list 'testing (backtrace)))) (setq ...) (save-excursion ...) (let ...) (with-output-to-temp-buffer ...) (eval (with-output-to-temp-buffer ...)) (eval-last-sexp-1 nil)
(eval-last-sexp nil) (call-interactively eval-last-sexp) ----------- Buffer: backtrace-output ------------
If this variable is non-
nil, it says to call the debugger before
funcall. Entering the
The d command in the debugger works by setting this variable.
This function sets the debug-on-exit flag of the stack frame level
levels down the stack, giving it the value flag. If flag is
nil, this will cause the debugger to be entered when that
frame later exits. Even a nonlocal exit through that frame will enter
This function is used only by the debugger.
This variable records the debugging status of the current interactive
command. Each time a command is called interactively, this variable is
nil. The debugger can set this variable to leave
information for future debugger invocations during the same command
The advantage of using this variable rather than an ordinary global variable is that the data will never carry over to a subsequent command invocation.
This variable is obsolete and will be removed in future versions.
backtrace-frame is intended for use in Lisp
debuggers. It returns information about what computation is happening
in the stack frame frame-number levels down.
If that frame has not evaluated the arguments yet, or is a special
form, the value is
(nil function arg-forms…).
If that frame has evaluated its arguments and called its function
already, the return value is
In the return value, function is whatever was supplied as the
CAR of the evaluated list, or a
lambda expression in the
case of a macro call. If the function has a
&rest argument, that
is represented as the tail of the list arg-values.
If base is specified, frame-number counts relative to the topmost frame whose function is base.
If frame-number is out of range,
mapbacktrace calls function once for each
frame in the backtrace, starting at the first frame whose function is
base (or from the top if base is omitted or
function is called with four arguments: evald, func, args, and flags.
If a frame has not evaluated its arguments yet or is a special form,
nil and args is a list of forms.
If a frame has evaluated its arguments and called its function
already, evald is
t and args is a list of values.
flags is a plist of properties of the current frame: currently,
the only supported property is
:debug-on-exit, which is
t if the stack frame’s
debug-on-exit flag is set.