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 of
debug(see Invoking the Debugger).
This function prints a trace of Lisp function calls currently active. This is the function used by
debugto 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
backtraceexplicitly. This prints the backtrace to the stream
standard-output, which, in this case, is the buffer ‘backtrace-output’.
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, 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 ...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 ------------
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 non-
nil, this will cause the debugger to be entered when that frame later exits. Even a nonlocal exit through that frame will enter the debugger.
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 bound to
nil. The debugger can set this variable to leave information for future debugger invocations during the same command invocation.
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-frameis 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
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
lambdaexpression in the case of a macro call. If the function has a
&restargument, 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,
mapbacktracecalls 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, evald is
niland args is a list of forms.
If a frame has evaluated its arguments and called its function already, evald is
tand 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
tif the stack frame's
debug-on-exitflag is set.