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.
This is the function used by
debug to fill up the
*Backtrace* buffer. It is written in C, since it must have access
to the stack to determine which function calls are active. The return
value is always
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 values of the function’s arguments if they are all known; if they are still being computed, the line says so. The arguments of special forms are 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 ...computing arguments...)
(progn ...) eval((progn (1+ var) (list (quote 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
debugger sets debug-on-next-call to
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.
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 frame-number is out of range,