0.9.13.35:

[sbcl.git] / src / code / debug.lisp

;;;; the debugger

;;;; This software is part of the SBCL system. See the README file for
;;;; more information.
;;;;
;;;; This software is derived from the CMU CL system, which was
;;;; written at Carnegie Mellon University and released into the
;;;; public domain. The software is in the public domain and is
;;;; provided with absolutely no warranty. See the COPYING and CREDITS
;;;; files for more information.

(in-package "SB!DEBUG")
\f
;;;; variables and constants

;;; things to consider when tweaking these values:
;;;   * We're afraid to just default them to NIL and NIL, in case the
;;;     user inadvertently causes a hairy data structure to be printed
;;;     when he inadvertently enters the debugger.
;;;   * We don't want to truncate output too much. These days anyone
;;;     can easily run their Lisp in a windowing system or under Emacs,
;;;     so it's not the end of the world even if the worst case is a
;;;     few thousand lines of output.
;;;   * As condition :REPORT methods are converted to use the pretty
;;;     printer, they acquire *PRINT-LEVEL* constraints, so e.g. under
;;;     sbcl-0.7.1.28's old value of *DEBUG-PRINT-LEVEL*=3, an
;;;     ARG-COUNT-ERROR printed as
;;;       error while parsing arguments to DESTRUCTURING-BIND:
;;;         invalid number of elements in
;;;           #
;;;         to satisfy lambda list
;;;           #:
;;;         exactly 2 expected, but 5 found
(defvar *debug-print-variable-alist* nil
  #!+sb-doc
  "an association list describing new bindings for special variables
to be used within the debugger. Eg.

 ((*PRINT-LENGTH* . 10) (*PRINT-LEVEL* . 6) (*PRINT-PRETTY* . NIL))

The variables in the CAR positions are bound to the values in the CDR
during the execution of some debug commands. When evaluating arbitrary
expressions in the debugger, the normal values of the printer control
variables are in effect.

Initially empty, *DEBUG-PRINT-VARIABLE-ALIST* is typically used to
provide bindings for printer control variables.")

(defvar *debug-readtable*
  ;; KLUDGE: This can't be initialized in a cold toplevel form,
  ;; because the *STANDARD-READTABLE* isn't initialized until after
  ;; cold toplevel forms have run. So instead we initialize it
  ;; immediately after *STANDARD-READTABLE*. -- WHN 20000205
  nil
  #!+sb-doc
  "*READTABLE* for the debugger")

(defvar *in-the-debugger* nil
  #!+sb-doc
  "This is T while in the debugger.")

;;; nestedness inside debugger command loops
(defvar *debug-command-level* 0)

;;; If this is bound before the debugger is invoked, it is used as the
;;; stack top by the debugger.
(defvar *stack-top-hint* nil)

(defvar *stack-top* nil)
(defvar *real-stack-top* nil)

(defvar *current-frame* nil)

;;; Beginner-oriented help messages are important because you end up
;;; in the debugger whenever something bad happens, or if you try to
;;; get out of the system with Ctrl-C or (EXIT) or EXIT or whatever.
;;; But after memorizing them the wasted screen space gets annoying..
(defvar *debug-beginner-help-p* t
  "Should the debugger display beginner-oriented help messages?")

(defun debug-prompt (stream)
  (sb!thread::get-foreground)
  (format stream
          "~%~W~:[~;[~W~]] "
          (sb!di:frame-number *current-frame*)
          (> *debug-command-level* 1)
          *debug-command-level*))

(defparameter *debug-help-string*
"The debug prompt is square brackets, with number(s) indicating the current
  control stack level and, if you've entered the debugger recursively, how
  deeply recursed you are.
Any command -- including the name of a restart -- may be uniquely abbreviated.
The debugger rebinds various special variables for controlling i/o, sometimes
  to defaults (much like WITH-STANDARD-IO-SYNTAX does) and sometimes to
  its own special values, based on SB-EXT:*DEBUG-PRINT-VARIABLE-ALIST*.
Debug commands do not affect *, //, and similar variables, but evaluation in
  the debug loop does affect these variables.
SB-DEBUG:*FLUSH-DEBUG-ERRORS* controls whether errors at the debug prompt
  drop you deeper into the debugger. The default NIL allows recursive entry
  to debugger.

Getting in and out of the debugger:
  TOPLEVEL, TOP  exits debugger and returns to top level REPL
  RESTART        invokes restart numbered as shown (prompt if not given).
  ERROR          prints the error condition and restart cases.

  The number of any restart, or its name, or a unique abbreviation for its
   name, is a valid command, and is the same as using RESTART to invoke
   that restart.

Changing frames:
  UP     up frame         DOWN     down frame
  BOTTOM bottom frame     FRAME n  frame n (n=0 for top frame)

Inspecting frames:
  BACKTRACE [n]  shows n frames going down the stack.
  LIST-LOCALS, L lists locals in current frame.
  PRINT, P       displays function call for current frame.
  SOURCE [n]     displays frame's source form with n levels of enclosing forms.

Stepping:
  STEP  Selects the CONTINUE restart if one exists and starts
        single-stepping. Single stepping affects only code compiled with
        under high DEBUG optimization quality. See User Manual for details.

Function and macro commands:
 (SB-DEBUG:ARG n)
    Return the n'th argument in the current frame.
 (SB-DEBUG:VAR string-or-symbol [id])
    Returns the value of the specified variable in the current frame.

Other commands:
  RETURN expr
    Return the values resulting from evaluation of expr from the
    current frame, if this frame was compiled with a sufficiently high
    DEBUG optimization quality.

  SLURP
    Discard all pending input on *STANDARD-INPUT*. (This can be
    useful when the debugger was invoked to handle an error in
    deeply nested input syntax, and now the reader is confused.)")
\f

;;; If LOC is an unknown location, then try to find the block start
;;; location. Used by source printing to some information instead of
;;; none for the user.
(defun maybe-block-start-location (loc)
  (if (sb!di:code-location-unknown-p loc)
      (let* ((block (sb!di:code-location-debug-block loc))
             (start (sb!di:do-debug-block-locations (loc block)
                      (return loc))))
        (cond ((and (not (sb!di:debug-block-elsewhere-p block))
                    start)
               (format *debug-io* "~%unknown location: using block start~%")
               start)
              (t
               loc)))
      loc))
\f
;;;; BACKTRACE

(defun backtrace (&optional (count most-positive-fixnum) (stream *debug-io*))
  #!+sb-doc
  "Show a listing of the call stack going down from the current frame.
In the debugger, the current frame is indicated by the prompt. COUNT
is how many frames to show."
  (fresh-line stream)
  (do ((frame (if *in-the-debugger* *current-frame* (sb!di:top-frame))
              (sb!di:frame-down frame))
       (count count (1- count)))
      ((or (null frame) (zerop count)))
    (print-frame-call frame stream :number t))
  (fresh-line stream)
  (values))

(defun backtrace-as-list (&optional (count most-positive-fixnum))
  #!+sb-doc "Return a list representing the current BACKTRACE."
  (do ((reversed-result nil)
       (frame (if *in-the-debugger* *current-frame* (sb!di:top-frame))
              (sb!di:frame-down frame))
       (count count (1- count)))
      ((or (null frame) (zerop count))
       (nreverse reversed-result))
    (push (frame-call-as-list frame) reversed-result)))

(defun frame-call-as-list (frame)
  (multiple-value-bind (name args) (frame-call frame)
    (cons name args)))
\f
;;;; frame printing

(eval-when (:compile-toplevel :execute)

;;; This is a convenient way to express what to do for each type of
;;; lambda-list element.
(sb!xc:defmacro lambda-list-element-dispatch (element
                                              &key
                                              required
                                              optional
                                              rest
                                              keyword
                                              deleted)
  `(etypecase ,element
     (sb!di:debug-var
      ,@required)
     (cons
      (ecase (car ,element)
        (:optional ,@optional)
        (:rest ,@rest)
        (:keyword ,@keyword)))
     (symbol
      (aver (eq ,element :deleted))
      ,@deleted)))

(sb!xc:defmacro lambda-var-dispatch (variable location deleted valid other)
  (let ((var (gensym)))
    `(let ((,var ,variable))
       (cond ((eq ,var :deleted) ,deleted)
             ((eq (sb!di:debug-var-validity ,var ,location) :valid)
              ,valid)
             (t ,other)))))

) ; EVAL-WHEN

;;; This is used in constructing arg lists for debugger printing when
;;; the arg list is unavailable, some arg is unavailable or unused, etc.
(defstruct (unprintable-object
            (:constructor make-unprintable-object (string))
            (:print-object (lambda (x s)
                             (print-unreadable-object (x s)
                               (write-string (unprintable-object-string x)
                                             s))))
            (:copier nil))
  string)

;;; Extract the function argument values for a debug frame.
(defun frame-args-as-list (frame)
  (let ((debug-fun (sb!di:frame-debug-fun frame))
        (loc (sb!di:frame-code-location frame))
        (reversed-result nil))
    (handler-case
        (progn
          (dolist (ele (sb!di:debug-fun-lambda-list debug-fun))
            (lambda-list-element-dispatch ele
             :required ((push (frame-call-arg ele loc frame) reversed-result))
             :optional ((push (frame-call-arg (second ele) loc frame)
                              reversed-result))
             :keyword ((push (second ele) reversed-result)
                       (push (frame-call-arg (third ele) loc frame)
                             reversed-result))
             :deleted ((push (frame-call-arg ele loc frame) reversed-result))
             :rest ((lambda-var-dispatch (second ele) loc
                     nil
                     (progn
                       (setf reversed-result
                             (append (reverse (sb!di:debug-var-value
                                               (second ele) frame))
                                     reversed-result))
                       (return))
                     (push (make-unprintable-object
                            "unavailable &REST argument")
                     reversed-result)))))
          ;; As long as we do an ordinary return (as opposed to SIGNALing
          ;; a CONDITION) from the DOLIST above:
          (nreverse reversed-result))
      (sb!di:lambda-list-unavailable
       ()
       (make-unprintable-object "unavailable lambda list")))))
(legal-fun-name-p '(lambda ()))
(defvar *show-entry-point-details* nil)

(defun clean-xep (name args)
  (values (second name)
          (if (consp args)
              (let ((count (first args))
                    (real-args (rest args)))
                (if (fixnump count)
                    (subseq real-args 0
                            (min count (length real-args)))
                    real-args))
              args)))

(defun clean-&more-processor (name args)
  (values (second name)
          (if (consp args)
              (let* ((more (last args 2))
                     (context (first more))
                     (count (second more)))
                (append
                 (butlast args 2)
                 (if (fixnump count)
                     (multiple-value-list
                      (sb!c:%more-arg-values context 0 count))
                     (list
                      (make-unprintable-object "more unavailable arguments")))))
              args)))

(defun frame-call (frame)
  (labels ((clean-name-and-args (name args)
             (if (and (consp name) (not *show-entry-point-details*))
                 ;; FIXME: do we need to deal with
                 ;; HAIRY-FUNCTION-ENTRY here? I can't make it or
                 ;; &AUX-BINDINGS appear in backtraces, so they are
                 ;; left alone for now. --NS 2005-02-28
                 (case (first name)
                   ((sb!c::xep sb!c::tl-xep)
                    (clean-xep name args))
                   ((sb!c::&more-processor)
                    (clean-&more-processor name args))
                   ((sb!c::hairy-arg-processor
                     sb!c::varargs-entry sb!c::&optional-processor)
                    (clean-name-and-args (second name) args))
                   (t
                    (values name args)))
                 (values name args))))
    (let ((debug-fun (sb!di:frame-debug-fun frame)))
      (multiple-value-bind (name args)
          (clean-name-and-args (sb!di:debug-fun-name debug-fun)
                                (frame-args-as-list frame))
        (values name args
                (when *show-entry-point-details*
                  (sb!di:debug-fun-kind debug-fun)))))))

(defun ensure-printable-object (object)
  (handler-case
      (with-open-stream (out (make-broadcast-stream))
        (prin1 object out)
        object)
    (error (cond)
      (declare (ignore cond))
      (make-unprintable-object "error printing object"))))

(defun frame-call-arg (var location frame)
  (lambda-var-dispatch var location
    (make-unprintable-object "unused argument")
    (sb!di:debug-var-value var frame)
    (make-unprintable-object "unavailable argument")))

;;; Prints a representation of the function call causing FRAME to
;;; exist. VERBOSITY indicates the level of information to output;
;;; zero indicates just printing the DEBUG-FUN's name, and one
;;; indicates displaying call-like, one-liner format with argument
;;; values.
(defun print-frame-call (frame stream &key (verbosity 1) (number nil))
  (when number
    (format stream "~&~S: " (sb!di:frame-number frame)))
  (if (zerop verbosity)
      (let ((*print-readably* nil))
        (prin1 frame stream))
      (multiple-value-bind (name args kind) (frame-call frame)
        (pprint-logical-block (stream nil :prefix "(" :suffix ")")
          ;; Since we go to some trouble to make nice informative function
          ;; names like (PRINT-OBJECT :AROUND (CLOWN T)), let's make sure
          ;; that they aren't truncated by *PRINT-LENGTH* and *PRINT-LEVEL*.
          ;; For the function arguments, we can just print normally.
          (let ((*print-length* nil)
                (*print-level* nil))
            (prin1 (ensure-printable-object name) stream))
          ;; If we hit a &REST arg, then print as many of the values as
          ;; possible, punting the loop over lambda-list variables since any
          ;; other arguments will be in the &REST arg's list of values.
          (let ((args (ensure-printable-object args)))
            (if (listp args)
                (format stream "~{ ~_~S~}" args)
                (format stream " ~S" args))))
        (when kind
          (format stream "[~S]" kind))))
  (when (>= verbosity 2)
    (let ((loc (sb!di:frame-code-location frame)))
      (handler-case
          (progn
            ;; FIXME: Is this call really necessary here? If it is,
            ;; then the reason for it should be unobscured.
            (sb!di:code-location-debug-block loc)
            (format stream "~%source: ")
            (prin1 (code-location-source-form loc 0) stream))
        (sb!di:debug-condition (ignore)
          ignore)
        (error (c)
          (format stream "~&error finding source: ~A" c))))))
\f
;;;; INVOKE-DEBUGGER

(defvar *debugger-hook* nil
  #!+sb-doc
  "This is either NIL or a function of two arguments, a condition and the value
   of *DEBUGGER-HOOK*. This function can either handle the condition or return
   which causes the standard debugger to execute. The system passes the value
   of this variable to the function because it binds *DEBUGGER-HOOK* to NIL
   around the invocation.")

(defvar *invoke-debugger-hook* nil
  #!+sb-doc
  "This is either NIL or a designator for a function of two arguments,
   to be run when the debugger is about to be entered.  The function is
   run with *INVOKE-DEBUGGER-HOOK* bound to NIL to minimize recursive
   errors, and receives as arguments the condition that triggered
   debugger entry and the previous value of *INVOKE-DEBUGGER-HOOK*

   This mechanism is an SBCL extension similar to the standard *DEBUGGER-HOOK*.
   In contrast to *DEBUGGER-HOOK*, it is observed by INVOKE-DEBUGGER even when
   called by BREAK.")

;;; These are bound on each invocation of INVOKE-DEBUGGER.
(defvar *debug-restarts*)
(defvar *debug-condition*)
(defvar *nested-debug-condition*)

;;; Oh, what a tangled web we weave when we preserve backwards
;;; compatibility with 1968-style use of global variables to control
;;; per-stream i/o properties; there's really no way to get this
;;; quite right, but we do what we can.
(defun funcall-with-debug-io-syntax (fun &rest rest)
  (declare (type function fun))
  ;; Try to force the other special variables into a useful state.
  (let (;; Protect from WITH-STANDARD-IO-SYNTAX some variables where
        ;; any default we might use is less useful than just reusing
        ;; the global values.
        (original-package *package*)
        (original-print-pretty *print-pretty*))
    (with-standard-io-syntax
      (with-sane-io-syntax
          (let (;; We want the printer and reader to be in a useful
                ;; state, regardless of where the debugger was invoked
                ;; in the program. WITH-STANDARD-IO-SYNTAX and
                ;; WITH-SANE-IO-SYNTAX do much of what we want, but
                ;;   * It doesn't affect our internal special variables
                ;;     like *CURRENT-LEVEL-IN-PRINT*.
                ;;   * It isn't customizable.
                ;;   * It sets *PACKAGE* to COMMON-LISP-USER, which is not
                ;;     helpful behavior for a debugger.
                ;;   * There's no particularly good debugger default for
                ;;     *PRINT-PRETTY*, since T is usually what you want
                ;;     -- except absolutely not what you want when you're
                ;;     debugging failures in PRINT-OBJECT logic.
                ;; We try to address all these issues with explicit
                ;; rebindings here.
                (sb!kernel:*current-level-in-print* 0)
                (*package* original-package)
                (*print-pretty* original-print-pretty)
                ;; Clear the circularity machinery to try to to reduce the
                ;; pain from sharing the circularity table across all
                ;; streams; if these are not rebound here, then setting
                ;; *PRINT-CIRCLE* within the debugger when debugging in a
                ;; state where something circular was being printed (e.g.,
                ;; because the debugger was entered on an error in a
                ;; PRINT-OBJECT method) makes a hopeless mess. Binding them
                ;; here does seem somewhat ugly because it makes it more
                ;; difficult to debug the printing-of-circularities code
                ;; itself; however, as far as I (WHN, 2004-05-29) can see,
                ;; that's almost entirely academic as long as there's one
                ;; shared *C-H-T* for all streams (i.e., it's already
                ;; unreasonably difficult to debug print-circle machinery
                ;; given the buggy crosstalk between the debugger streams
                ;; and the stream you're trying to watch), and any fix for
                ;; that buggy arrangement will likely let this hack go away
                ;; naturally.
                (sb!impl::*circularity-hash-table* . nil)
                (sb!impl::*circularity-counter* . nil)
                (*readtable* *debug-readtable*))
            (progv
                ;; (Why NREVERSE? PROGV makes the later entries have
                ;; precedence over the earlier entries.
                ;; *DEBUG-PRINT-VARIABLE-ALIST* is called an alist, so it's
                ;; expected that its earlier entries have precedence. And
                ;; the earlier-has-precedence behavior is mostly more
                ;; convenient, so that programmers can use PUSH or LIST* to
                ;; customize *DEBUG-PRINT-VARIABLE-ALIST*.)
                (nreverse (mapcar #'car *debug-print-variable-alist*))
                (nreverse (mapcar #'cdr *debug-print-variable-alist*))
              (apply fun rest)))))))

(defun invoke-debugger (condition)
  #!+sb-doc
  "Enter the debugger."

  ;; call *INVOKE-DEBUGGER-HOOK* first, so that *DEBUGGER-HOOK* is not
  ;; called when the debugger is disabled
  (let ((old-hook *invoke-debugger-hook*))
    (when old-hook
      (let ((*invoke-debugger-hook* nil))
        (funcall old-hook condition old-hook))))
  (let ((old-hook *debugger-hook*))
    (when old-hook
      (let ((*debugger-hook* nil))
        (funcall old-hook condition old-hook))))

  ;; We definitely want *PACKAGE* to be of valid type.
  ;;
  ;; Elsewhere in the system, we use the SANE-PACKAGE function for
  ;; this, but here causing an exception just as we're trying to handle
  ;; an exception would be confusing, so instead we use a special hack.
  (unless (and (packagep *package*)
               (package-name *package*))
    (setf *package* (find-package :cl-user))
    (format *error-output*
            "The value of ~S was not an undeleted PACKAGE. It has been
reset to ~S."
            '*package* *package*))

  ;; Before we start our own output, finish any pending output.
  ;; Otherwise, if the user tried to track the progress of his program
  ;; using PRINT statements, he'd tend to lose the last line of output
  ;; or so, which'd be confusing.
  (flush-standard-output-streams)

  (funcall-with-debug-io-syntax #'%invoke-debugger condition))

(defun %print-debugger-invocation-reason (condition stream)
  (format stream "~2&")
  ;; Note: Ordinarily it's only a matter of taste whether to use
  ;; FORMAT "~<...~:>" or to use PPRINT-LOGICAL-BLOCK directly, but
  ;; until bug 403 is fixed, PPRINT-LOGICAL-BLOCK (STREAM NIL) is
  ;; definitely preferred, because the FORMAT alternative was acting odd.
  (pprint-logical-block (stream nil)
    (format stream
            "debugger invoked on a ~S~@[ in thread ~A~]: ~2I~_~A"
            (type-of condition)
            #!+sb-thread sb!thread:*current-thread*
            #!-sb-thread nil
            condition))
  (terpri stream))

(defun %invoke-debugger (condition)

  (let ((*debug-condition* condition)
        (*debug-restarts* (compute-restarts condition))
        (*nested-debug-condition* nil))
    (handler-case
        ;; (The initial output here goes to *ERROR-OUTPUT*, because the
        ;; initial output is not interactive, just an error message, and
        ;; when people redirect *ERROR-OUTPUT*, they could reasonably
        ;; expect to see error messages logged there, regardless of what
        ;; the debugger does afterwards.)
        (%print-debugger-invocation-reason condition *error-output*)
      (error (condition)
        (setf *nested-debug-condition* condition)
        (let ((ndc-type (type-of *nested-debug-condition*)))
          (format *error-output*
                  "~&~@<(A ~S was caught when trying to print ~S when ~
                      entering the debugger. Printing was aborted and the ~
                      ~S was stored in ~S.)~@:>~%"
                  ndc-type
                  '*debug-condition*
                  ndc-type
                  '*nested-debug-condition*))
        (when (typep *nested-debug-condition* 'cell-error)
          ;; what we really want to know when it's e.g. an UNBOUND-VARIABLE:
          (format *error-output*
                  "~&(CELL-ERROR-NAME ~S) = ~S~%"
                  '*nested-debug-condition*
                  (cell-error-name *nested-debug-condition*)))))

    (let ((background-p (sb!thread::debugger-wait-until-foreground-thread
                         *debug-io*)))

      ;; After the initial error/condition/whatever announcement to
      ;; *ERROR-OUTPUT*, we become interactive, and should talk on
      ;; *DEBUG-IO* from now on. (KLUDGE: This is a normative
      ;; statement, not a description of reality.:-| There's a lot of
      ;; older debugger code which was written to do i/o on whatever
      ;; stream was in fashion at the time, and not all of it has
      ;; been converted to behave this way. -- WHN 2000-11-16)

      (unwind-protect
           (let (;; We used to bind *STANDARD-OUTPUT* to *DEBUG-IO*
                 ;; here as well, but that is probably bogus since it
                 ;; removes the users ability to do output to a redirected
                 ;; *S-O*. Now we just rebind it so that users can temporarily
                 ;; frob it. FIXME: This and other "what gets bound when"
                 ;; behaviour should be documented in the manual.
                 (*standard-output* *standard-output*)
                 ;; This seems reasonable: e.g. if the user has redirected
                 ;; *ERROR-OUTPUT* to some log file, it's probably wrong
                 ;; to send errors which occur in interactive debugging to
                 ;; that file, and right to send them to *DEBUG-IO*.
                 (*error-output* *debug-io*))
             (unless (typep condition 'step-condition)
               (when *debug-beginner-help-p*
                 (format *debug-io*
                         "~%~@<Type HELP for debugger help, or ~
                               (SB-EXT:QUIT) to exit from SBCL.~:@>~2%"))
               (show-restarts *debug-restarts* *debug-io*))
             (internal-debug))
        (when background-p
          (sb!thread::release-foreground))))))

;;; this function is for use in *INVOKE-DEBUGGER-HOOK* when ordinary
;;; ANSI behavior has been suppressed by the "--disable-debugger"
;;; command-line option
(defun debugger-disabled-hook (condition me)
  (declare (ignore me))
  ;; There is no one there to interact with, so report the
  ;; condition and terminate the program.
  (flet ((failure-quit (&key recklessly-p)
           (/show0 "in FAILURE-QUIT (in --disable-debugger debugger hook)")
           (quit :unix-status 1 :recklessly-p recklessly-p)))
    ;; This HANDLER-CASE is here mostly to stop output immediately
    ;; (and fall through to QUIT) when there's an I/O error. Thus,
    ;; when we're run under a shell script or something, we can die
    ;; cleanly when the script dies (and our pipes are cut), instead
    ;; of falling into ldb or something messy like that. Similarly, we
    ;; can terminate cleanly even if BACKTRACE dies because of bugs in
    ;; user PRINT-OBJECT methods.
    (handler-case
        (progn
          (format *error-output*
                  "~&~@<unhandled ~S~@[ in thread ~S~]: ~2I~_~A~:>~2%"
                  (type-of condition)
                  #!+sb-thread sb!thread:*current-thread*
                  #!-sb-thread nil
                  condition)
          ;; Flush *ERROR-OUTPUT* even before the BACKTRACE, so that
          ;; even if we hit an error within BACKTRACE (e.g. a bug in
          ;; the debugger's own frame-walking code, or a bug in a user
          ;; PRINT-OBJECT method) we'll at least have the CONDITION
          ;; printed out before we die.
          (finish-output *error-output*)
          ;; (Where to truncate the BACKTRACE is of course arbitrary, but
          ;; it seems as though we should at least truncate it somewhere.)
          (sb!debug:backtrace 128 *error-output*)
          (format
           *error-output*
           "~%unhandled condition in --disable-debugger mode, quitting~%")
          (finish-output *error-output*)
          (failure-quit))
      (condition ()
        ;; We IGNORE-ERRORS here because even %PRIMITIVE PRINT can
        ;; fail when our output streams are blown away, as e.g. when
        ;; we're running under a Unix shell script and it dies somehow
        ;; (e.g. because of a SIGINT). In that case, we might as well
        ;; just give it up for a bad job, and stop trying to notify
        ;; the user of anything.
        ;;
        ;; Actually, the only way I've run across to exercise the
        ;; problem is to have more than one layer of shell script.
        ;; I have a shell script which does
        ;;   time nice -10 sh make.sh "$1" 2>&1 | tee make.tmp
        ;; and the problem occurs when I interrupt this with Ctrl-C
        ;; under Linux 2.2.14-5.0 and GNU bash, version 1.14.7(1).
        ;; I haven't figured out whether it's bash, time, tee, Linux, or
        ;; what that is responsible, but that it's possible at all
        ;; means that we should IGNORE-ERRORS here. -- WHN 2001-04-24
        (ignore-errors
         (%primitive print
                     "Argh! error within --disable-debugger error handling"))
        (failure-quit :recklessly-p t)))))

;;; halt-on-failures and prompt-on-failures modes, suitable for
;;; noninteractive and interactive use respectively
(defun disable-debugger ()
  ;; Why conditionally? Why not disable it even if user has frobbed
  ;; this hook? We could just save the old value in case of a later
  ;; ENABLE-DEBUGGER.
  (when (eql *invoke-debugger-hook* nil)
    ;; *DEBUG-IO* used to be set here to *ERROR-OUTPUT* which is sort
    ;; of unexpected but mostly harmless, but then ENABLE-DEBUGGER had
    ;; to set it to a suitable value again and be very careful,
    ;; especially if the user has also set it. -- MG 2005-07-15
    (setf *invoke-debugger-hook* 'debugger-disabled-hook)
    (sb!alien:alien-funcall (sb!alien:extern-alien "disable_lossage_handler" (function sb!alien:void)))))

(defun enable-debugger ()
  (when (eql *invoke-debugger-hook* 'debugger-disabled-hook)
    (setf *invoke-debugger-hook* nil)
    (sb!alien:alien-funcall (sb!alien:extern-alien "enable_lossage_handler" (function sb!alien:void)))))

(defun show-restarts (restarts s)
  (cond ((null restarts)
         (format s
                 "~&(no restarts: If you didn't do this on purpose, ~
                  please report it as a bug.)~%"))
        (t
         (format s "~&restarts (invokable by number or by ~
                    possibly-abbreviated name):~%")
         (let ((count 0)
               (names-used '(nil))
               (max-name-len 0))
           (dolist (restart restarts)
             (let ((name (restart-name restart)))
               (when name
                 (let ((len (length (princ-to-string name))))
                   (when (> len max-name-len)
                     (setf max-name-len len))))))
           (unless (zerop max-name-len)
             (incf max-name-len 3))
           (dolist (restart restarts)
             (let ((name (restart-name restart)))
               ;; FIXME: maybe it would be better to display later names
               ;; in parens instead of brakets, not just omit them fully.
               ;; Call BREAK, call BREAK in the debugger, and tell me
               ;; it's not confusing looking. --NS 20050310
               (cond ((member name names-used)
                      (format s "~& ~2D: ~V@T~A~%" count max-name-len restart))
                     (t
                      (format s "~& ~2D: [~VA] ~A~%"
                              count (- max-name-len 3) name restart)
                      (push name names-used))))
             (incf count))))))

(defvar *debug-loop-fun* #'debug-loop-fun
  "a function taking no parameters that starts the low-level debug loop")

;;; This calls DEBUG-LOOP, performing some simple initializations
;;; before doing so. INVOKE-DEBUGGER calls this to actually get into
;;; the debugger. SB!KERNEL::ERROR-ERROR calls this in emergencies
;;; to get into a debug prompt as quickly as possible with as little
;;; risk as possible for stepping on whatever is causing recursive
;;; errors.
(defun internal-debug ()
  (let ((*in-the-debugger* t)
        (*read-suppress* nil))
    (unless (typep *debug-condition* 'step-condition)
      (clear-input *debug-io*))
    (funcall *debug-loop-fun*)))
\f
;;;; DEBUG-LOOP

;;; Note: This defaulted to T in CMU CL. The changed default in SBCL
;;; was motivated by desire to play nicely with ILISP.
(defvar *flush-debug-errors* nil
  #!+sb-doc
  "When set, avoid calling INVOKE-DEBUGGER recursively when errors occur while
   executing in the debugger.")

(defun debug-read (stream)
  (declare (type stream stream))
  (let* ((eof-marker (cons nil nil))
         (form (read stream nil eof-marker)))
    (if (eq form eof-marker)
        (abort)
        form)))

(defun debug-loop-fun ()
  (let* ((*debug-command-level* (1+ *debug-command-level*))
         (*real-stack-top* (sb!di:top-frame))
         (*stack-top* (or *stack-top-hint* *real-stack-top*))
         (*stack-top-hint* nil)
         (*current-frame* *stack-top*))
    (handler-bind ((sb!di:debug-condition
                    (lambda (condition)
                      (princ condition *debug-io*)
                      (/show0 "handling d-c by THROWing DEBUG-LOOP-CATCHER")
                      (throw 'debug-loop-catcher nil))))
      (terpri *debug-io*)
      (print-frame-call *current-frame* *debug-io* :verbosity 2)
      (loop
       (catch 'debug-loop-catcher
         (handler-bind ((error (lambda (condition)
                                 (when *flush-debug-errors*
                                   (clear-input *debug-io*)
                                   (princ condition *debug-io*)
                                   (format *debug-io*
                                           "~&error flushed (because ~
                                             ~S is set)"
                                           '*flush-debug-errors*)
                                   (/show0 "throwing DEBUG-LOOP-CATCHER")
                                   (throw 'debug-loop-catcher nil)))))
           ;; We have to bind LEVEL for the restart function created by
           ;; WITH-SIMPLE-RESTART.
           (let ((level *debug-command-level*)
                 (restart-commands (make-restart-commands)))
             (flush-standard-output-streams)
             (debug-prompt *debug-io*)
             (force-output *debug-io*)
             (let* ((exp (debug-read *debug-io*))
                    (cmd-fun (debug-command-p exp restart-commands)))
               (with-simple-restart (abort
                                     "~@<Reduce debugger level (to debug level ~W).~@:>"
                                     level)
                 (cond ((not cmd-fun)
                        (debug-eval-print exp))
                       ((consp cmd-fun)
                        (format *debug-io*
                                "~&Your command, ~S, is ambiguous:~%"
                                exp)
                        (dolist (ele cmd-fun)
                          (format *debug-io* "   ~A~%" ele)))
                       (t
                        (funcall cmd-fun))))))))))))

(defun debug-eval-print (expr)
  (/noshow "entering DEBUG-EVAL-PRINT" expr)
  (let ((values (multiple-value-list (interactive-eval expr))))
    (/noshow "done with EVAL in DEBUG-EVAL-PRINT")
    (dolist (value values)
      (fresh-line *debug-io*)
      (prin1 value *debug-io*)))
  (force-output *debug-io*))
\f
;;;; debug loop functions

;;; These commands are functions, not really commands, so that users
;;; can get their hands on the values returned.

(eval-when (:execute :compile-toplevel)

(sb!xc:defmacro define-var-operation (ref-or-set &optional value-var)
  `(let* ((temp (etypecase name
                  (symbol (sb!di:debug-fun-symbol-vars
                           (sb!di:frame-debug-fun *current-frame*)
                           name))
                  (simple-string (sb!di:ambiguous-debug-vars
                                  (sb!di:frame-debug-fun *current-frame*)
                                  name))))
          (location (sb!di:frame-code-location *current-frame*))
          ;; Let's only deal with valid variables.
          (vars (remove-if-not (lambda (v)
                                 (eq (sb!di:debug-var-validity v location)
                                     :valid))
                               temp)))
     (declare (list vars))
     (cond ((null vars)
            (error "No known valid variables match ~S." name))
           ((= (length vars) 1)
            ,(ecase ref-or-set
               (:ref
                '(sb!di:debug-var-value (car vars) *current-frame*))
               (:set
                `(setf (sb!di:debug-var-value (car vars) *current-frame*)
                       ,value-var))))
           (t
            ;; Since we have more than one, first see whether we have
            ;; any variables that exactly match the specification.
            (let* ((name (etypecase name
                           (symbol (symbol-name name))
                           (simple-string name)))
                   ;; FIXME: REMOVE-IF-NOT is deprecated, use STRING/=
                   ;; instead.
                   (exact (remove-if-not (lambda (v)
                                           (string= (sb!di:debug-var-symbol-name v)
                                                    name))
                                         vars))
                   (vars (or exact vars)))
              (declare (simple-string name)
                       (list exact vars))
              (cond
               ;; Check now for only having one variable.
               ((= (length vars) 1)
                ,(ecase ref-or-set
                   (:ref
                    '(sb!di:debug-var-value (car vars) *current-frame*))
                   (:set
                    `(setf (sb!di:debug-var-value (car vars) *current-frame*)
                           ,value-var))))
               ;; If there weren't any exact matches, flame about
               ;; ambiguity unless all the variables have the same
               ;; name.
               ((and (not exact)
                     (find-if-not
                      (lambda (v)
                        (string= (sb!di:debug-var-symbol-name v)
                                 (sb!di:debug-var-symbol-name (car vars))))
                      (cdr vars)))
                (error "specification ambiguous:~%~{   ~A~%~}"
                       (mapcar #'sb!di:debug-var-symbol-name
                               (delete-duplicates
                                vars :test #'string=
                                :key #'sb!di:debug-var-symbol-name))))
               ;; All names are the same, so see whether the user
               ;; ID'ed one of them.
               (id-supplied
                (let ((v (find id vars :key #'sb!di:debug-var-id)))
                  (unless v
                    (error
                     "invalid variable ID, ~W: should have been one of ~S"
                     id
                     (mapcar #'sb!di:debug-var-id vars)))
                  ,(ecase ref-or-set
                     (:ref
                      '(sb!di:debug-var-value v *current-frame*))
                     (:set
                      `(setf (sb!di:debug-var-value v *current-frame*)
                             ,value-var)))))
               (t
                (error "Specify variable ID to disambiguate ~S. Use one of ~S."
                       name
                       (mapcar #'sb!di:debug-var-id vars)))))))))

) ; EVAL-WHEN

;;; FIXME: This doesn't work. It would be real nice we could make it
;;; work! Alas, it doesn't seem to work in CMU CL X86 either..
(defun var (name &optional (id 0 id-supplied))
  #!+sb-doc
  "Return a variable's value if possible. NAME is a simple-string or symbol.
   If it is a simple-string, it is an initial substring of the variable's name.
   If name is a symbol, it has the same name and package as the variable whose
   value this function returns. If the symbol is uninterned, then the variable
   has the same name as the symbol, but it has no package.

   If name is the initial substring of variables with different names, then
   this return no values after displaying the ambiguous names. If name
   determines multiple variables with the same name, then you must use the
   optional id argument to specify which one you want. If you left id
   unspecified, then this returns no values after displaying the distinguishing
   id values.

   The result of this function is limited to the availability of variable
   information. This is SETF'able."
  (define-var-operation :ref))
(defun (setf var) (value name &optional (id 0 id-supplied))
  (define-var-operation :set value))

;;; This returns the COUNT'th arg as the user sees it from args, the
;;; result of SB!DI:DEBUG-FUN-LAMBDA-LIST. If this returns a
;;; potential DEBUG-VAR from the lambda-list, then the second value is
;;; T. If this returns a keyword symbol or a value from a rest arg,
;;; then the second value is NIL.
;;;
;;; FIXME: There's probably some way to merge the code here with
;;; FRAME-ARGS-AS-LIST. (A fair amount of logic is already shared
;;; through LAMBDA-LIST-ELEMENT-DISPATCH, but I suspect more could be.)
(declaim (ftype (function (index list)) nth-arg))
(defun nth-arg (count args)
  (let ((n count))
    (dolist (ele args (error "The argument specification ~S is out of range."
                             n))
      (lambda-list-element-dispatch ele
        :required ((if (zerop n) (return (values ele t))))
        :optional ((if (zerop n) (return (values (second ele) t))))
        :keyword ((cond ((zerop n)
                         (return (values (second ele) nil)))
                        ((zerop (decf n))
                         (return (values (third ele) t)))))
        :deleted ((if (zerop n) (return (values ele t))))
        :rest ((let ((var (second ele)))
                 (lambda-var-dispatch var (sb!di:frame-code-location
                                           *current-frame*)
                   (error "unused &REST argument before n'th argument")
                   (dolist (value
                            (sb!di:debug-var-value var *current-frame*)
                            (error
                             "The argument specification ~S is out of range."
                             n))
                     (if (zerop n)
                         (return-from nth-arg (values value nil))
                         (decf n)))
                   (error "invalid &REST argument before n'th argument")))))
      (decf n))))

(defun arg (n)
  #!+sb-doc
  "Return the N'th argument's value if possible. Argument zero is the first
   argument in a frame's default printed representation. Count keyword/value
   pairs as separate arguments."
  (multiple-value-bind (var lambda-var-p)
      (nth-arg n (handler-case (sb!di:debug-fun-lambda-list
                                (sb!di:frame-debug-fun *current-frame*))
                   (sb!di:lambda-list-unavailable ()
                     (error "No argument values are available."))))
    (if lambda-var-p
        (lambda-var-dispatch var (sb!di:frame-code-location *current-frame*)
          (error "Unused arguments have no values.")
          (sb!di:debug-var-value var *current-frame*)
          (error "invalid argument value"))
        var)))
\f
;;;; machinery for definition of debug loop commands

(defvar *debug-commands* nil)

;;; Interface to *DEBUG-COMMANDS*. No required arguments in args are
;;; permitted.
(defmacro !def-debug-command (name args &rest body)
  (let ((fun-name (symbolicate name "-DEBUG-COMMAND")))
    `(progn
       (setf *debug-commands*
             (remove ,name *debug-commands* :key #'car :test #'string=))
       (defun ,fun-name ,args
         (unless *in-the-debugger*
           (error "invoking debugger command while outside the debugger"))
         ,@body)
       (push (cons ,name #',fun-name) *debug-commands*)
       ',fun-name)))

(defun !def-debug-command-alias (new-name existing-name)
  (let ((pair (assoc existing-name *debug-commands* :test #'string=)))
    (unless pair (error "unknown debug command name: ~S" existing-name))
    (push (cons new-name (cdr pair)) *debug-commands*))
  new-name)

;;; This takes a symbol and uses its name to find a debugger command,
;;; using initial substring matching. It returns the command function
;;; if form identifies only one command, but if form is ambiguous,
;;; this returns a list of the command names. If there are no matches,
;;; this returns nil. Whenever the loop that looks for a set of
;;; possibilities encounters an exact name match, we return that
;;; command function immediately.
(defun debug-command-p (form &optional other-commands)
  (if (or (symbolp form) (integerp form))
      (let* ((name
              (if (symbolp form)
                  (symbol-name form)
                  (format nil "~W" form)))
             (len (length name))
             (res nil))
        (declare (simple-string name)
                 (fixnum len)
                 (list res))

        ;; Find matching commands, punting if exact match.
        (flet ((match-command (ele)
                 (let* ((str (car ele))
                        (str-len (length str)))
                   (declare (simple-string str)
                            (fixnum str-len))
                   (cond ((< str-len len))
                         ((= str-len len)
                          (when (string= name str :end1 len :end2 len)
                            (return-from debug-command-p (cdr ele))))
                         ((string= name str :end1 len :end2 len)
                          (push ele res))))))
          (mapc #'match-command *debug-commands*)
          (mapc #'match-command other-commands))

        ;; Return the right value.
        (cond ((not res) nil)
              ((= (length res) 1)
               (cdar res))
              (t ; Just return the names.
               (do ((cmds res (cdr cmds)))
                   ((not cmds) res)
                 (setf (car cmds) (caar cmds))))))))

;;; Return a list of debug commands (in the same format as
;;; *DEBUG-COMMANDS*) that invoke each active restart.
;;;
;;; Two commands are made for each restart: one for the number, and
;;; one for the restart name (unless it's been shadowed by an earlier
;;; restart of the same name, or it is NIL).
(defun make-restart-commands (&optional (restarts *debug-restarts*))
  (let ((commands)
        (num 0))                        ; better be the same as show-restarts!
    (dolist (restart restarts)
      (let ((name (string (restart-name restart))))
        (let ((restart-fun
                (lambda ()
                  (/show0 "in restart-command closure, about to i-r-i")
                  (invoke-restart-interactively restart))))
          (push (cons (prin1-to-string num) restart-fun) commands)
          (unless (or (null (restart-name restart))
                      (find name commands :key #'car :test #'string=))
            (push (cons name restart-fun) commands))))
    (incf num))
  commands))
\f
;;;; frame-changing commands

(!def-debug-command "UP" ()
  (let ((next (sb!di:frame-up *current-frame*)))
    (cond (next
           (setf *current-frame* next)
           (print-frame-call next *debug-io*))
          (t
           (format *debug-io* "~&Top of stack.")))))

(!def-debug-command "DOWN" ()
  (let ((next (sb!di:frame-down *current-frame*)))
    (cond (next
           (setf *current-frame* next)
           (print-frame-call next *debug-io*))
          (t
           (format *debug-io* "~&Bottom of stack.")))))

(!def-debug-command-alias "D" "DOWN")

(!def-debug-command "BOTTOM" ()
  (do ((prev *current-frame* lead)
       (lead (sb!di:frame-down *current-frame*) (sb!di:frame-down lead)))
      ((null lead)
       (setf *current-frame* prev)
       (print-frame-call prev *debug-io*))))

(!def-debug-command-alias "B" "BOTTOM")

(!def-debug-command "FRAME" (&optional
                             (n (read-prompting-maybe "frame number: ")))
  (setf *current-frame*
        (multiple-value-bind (next-frame-fun limit-string)
            (if (< n (sb!di:frame-number *current-frame*))
                (values #'sb!di:frame-up "top")
              (values #'sb!di:frame-down "bottom"))
          (do ((frame *current-frame*))
              ((= n (sb!di:frame-number frame))
               frame)
            (let ((next-frame (funcall next-frame-fun frame)))
              (cond (next-frame
                     (setf frame next-frame))
                    (t
                     (format *debug-io*
                             "The ~A of the stack was encountered.~%"
                             limit-string)
                     (return frame)))))))
  (print-frame-call *current-frame* *debug-io*))

(!def-debug-command-alias "F" "FRAME")
\f
;;;; commands for entering and leaving the debugger

(!def-debug-command "TOPLEVEL" ()
  (throw 'toplevel-catcher nil))

;;; make T safe
(!def-debug-command-alias "TOP" "TOPLEVEL")

(!def-debug-command "RESTART" ()
  (/show0 "doing RESTART debug-command")
  (let ((num (read-if-available :prompt)))
    (when (eq num :prompt)
      (show-restarts *debug-restarts* *debug-io*)
      (write-string "restart: " *debug-io*)
      (force-output *debug-io*)
      (setf num (read *debug-io*)))
    (let ((restart (typecase num
                     (unsigned-byte
                      (nth num *debug-restarts*))
                     (symbol
                      (find num *debug-restarts* :key #'restart-name
                            :test (lambda (sym1 sym2)
                                    (string= (symbol-name sym1)
                                             (symbol-name sym2)))))
                     (t
                      (format *debug-io* "~S is invalid as a restart name.~%"
                              num)
                      (return-from restart-debug-command nil)))))
      (/show0 "got RESTART")
      (if restart
          (invoke-restart-interactively restart)
          (princ "There is no such restart." *debug-io*)))))
\f
;;;; information commands

(!def-debug-command "HELP" ()
  ;; CMU CL had a little toy pager here, but "if you aren't running
  ;; ILISP (or a smart windowing system, or something) you deserve to
  ;; lose", so we've dropped it in SBCL. However, in case some
  ;; desperate holdout is running this on a dumb terminal somewhere,
  ;; we tell him where to find the message stored as a string.
  (format *debug-io*
          "~&~A~2%(The HELP string is stored in ~S.)~%"
          *debug-help-string*
          '*debug-help-string*))

(!def-debug-command-alias "?" "HELP")

(!def-debug-command "ERROR" ()
  (format *debug-io* "~A~%" *debug-condition*)
  (show-restarts *debug-restarts* *debug-io*))

(!def-debug-command "BACKTRACE" ()
  (backtrace (read-if-available most-positive-fixnum)))

(!def-debug-command "PRINT" ()
  (print-frame-call *current-frame* *debug-io*))

(!def-debug-command-alias "P" "PRINT")

(!def-debug-command "LIST-LOCALS" ()
  (let ((d-fun (sb!di:frame-debug-fun *current-frame*)))
    (if (sb!di:debug-var-info-available d-fun)
        (let ((*standard-output* *debug-io*)
              (location (sb!di:frame-code-location *current-frame*))
              (prefix (read-if-available nil))
              (any-p nil)
              (any-valid-p nil))
          (dolist (v (sb!di:ambiguous-debug-vars
                        d-fun
                        (if prefix (string prefix) "")))
            (setf any-p t)
            (when (eq (sb!di:debug-var-validity v location) :valid)
              (setf any-valid-p t)
              (format *debug-io* "~S~:[#~W~;~*~]  =  ~S~%"
                      (sb!di:debug-var-symbol v)
                      (zerop (sb!di:debug-var-id v))
                      (sb!di:debug-var-id v)
                      (sb!di:debug-var-value v *current-frame*))))

          (cond
           ((not any-p)
            (format *debug-io*
                    "There are no local variables ~@[starting with ~A ~]~
                    in the function."
                    prefix))
           ((not any-valid-p)
            (format *debug-io*
                    "All variables ~@[starting with ~A ~]currently ~
                    have invalid values."
                    prefix))))
        (write-line "There is no variable information available."
                    *debug-io*))))

(!def-debug-command-alias "L" "LIST-LOCALS")

(!def-debug-command "SOURCE" ()
  (print (code-location-source-form (sb!di:frame-code-location *current-frame*)
                                    (read-if-available 0))
         *debug-io*))
\f
;;;; source location printing

;;; We cache a stream to the last valid file debug source so that we
;;; won't have to repeatedly open the file.
;;;
;;; KLUDGE: This sounds like a bug, not a feature. Opening files is fast
;;; in the 1990s, so the benefit is negligible, less important than the
;;; potential of extra confusion if someone changes the source during
;;; a debug session and the change doesn't show up. And removing this
;;; would simplify the system, which I like. -- WHN 19990903
(defvar *cached-debug-source* nil)
(declaim (type (or sb!di:debug-source null) *cached-debug-source*))
(defvar *cached-source-stream* nil)
(declaim (type (or stream null) *cached-source-stream*))

;;; To suppress the read-time evaluation #. macro during source read,
;;; *READTABLE* is modified. *READTABLE* is cached to avoid
;;; copying it each time, and invalidated when the
;;; *CACHED-DEBUG-SOURCE* has changed.
(defvar *cached-readtable* nil)
(declaim (type (or readtable null) *cached-readtable*))

;;; Stuff to clean up before saving a core
(defun debug-deinit ()
  (setf *cached-debug-source* nil
        *cached-source-stream* nil
        *cached-readtable* nil))

;;; We also cache the last toplevel form that we printed a source for
;;; so that we don't have to do repeated reads and calls to
;;; FORM-NUMBER-TRANSLATIONS.
(defvar *cached-toplevel-form-offset* nil)
(declaim (type (or index null) *cached-toplevel-form-offset*))
(defvar *cached-toplevel-form*)
(defvar *cached-form-number-translations*)

;;; Given a code location, return the associated form-number
;;; translations and the actual top level form. We check our cache ---
;;; if there is a miss, we dispatch on the kind of the debug source.
(defun get-toplevel-form (location)
  (let ((d-source (sb!di:code-location-debug-source location)))
    (if (and (eq d-source *cached-debug-source*)
             (eql (sb!di:code-location-toplevel-form-offset location)
                  *cached-toplevel-form-offset*))
        (values *cached-form-number-translations* *cached-toplevel-form*)
        (let* ((offset (sb!di:code-location-toplevel-form-offset location))
               (res
                (ecase (sb!di:debug-source-from d-source)
                  (:file (get-file-toplevel-form location))
                  (:lisp (svref (sb!di:debug-source-name d-source) offset)))))
          (setq *cached-toplevel-form-offset* offset)
          (values (setq *cached-form-number-translations*
                        (sb!di:form-number-translations res offset))
                  (setq *cached-toplevel-form* res))))))

;;; Locate the source file (if it still exists) and grab the top level
;;; form. If the file is modified, we use the top level form offset
;;; instead of the recorded character offset.
(defun get-file-toplevel-form (location)
  (let* ((d-source (sb!di:code-location-debug-source location))
         (tlf-offset (sb!di:code-location-toplevel-form-offset location))
         (local-tlf-offset (- tlf-offset
                              (sb!di:debug-source-root-number d-source)))
         (char-offset
          (aref (or (sb!di:debug-source-start-positions d-source)
                    (error "no start positions map"))
                local-tlf-offset))
         (name (sb!di:debug-source-name d-source)))
    (unless (eq d-source *cached-debug-source*)
      (unless (and *cached-source-stream*
                   (equal (pathname *cached-source-stream*)
                          (pathname name)))
        (setq *cached-readtable* nil)
        (when *cached-source-stream* (close *cached-source-stream*))
        (setq *cached-source-stream* (open name :if-does-not-exist nil))
        (unless *cached-source-stream*
          (error "The source file no longer exists:~%  ~A" (namestring name)))
        (format *debug-io* "~%; file: ~A~%" (namestring name)))

        (setq *cached-debug-source*
              (if (= (sb!di:debug-source-created d-source)
                     (file-write-date name))
                  d-source nil)))

    (cond
     ((eq *cached-debug-source* d-source)
      (file-position *cached-source-stream* char-offset))
     (t
      (format *debug-io*
              "~%; File has been modified since compilation:~%;   ~A~@
                 ; Using form offset instead of character position.~%"
              (namestring name))
      (file-position *cached-source-stream* 0)
      (let ((*read-suppress* t))
        (dotimes (i local-tlf-offset)
          (read *cached-source-stream*)))))
    (unless *cached-readtable*
      (setq *cached-readtable* (copy-readtable))
      (set-dispatch-macro-character
       #\# #\.
       (lambda (stream sub-char &rest rest)
         (declare (ignore rest sub-char))
         (let ((token (read stream t nil t)))
           (format nil "#.~S" token)))
       *cached-readtable*))
    (let ((*readtable* *cached-readtable*))
      (read *cached-source-stream*))))

(defun code-location-source-form (location context)
  (let* ((location (maybe-block-start-location location))
         (form-num (sb!di:code-location-form-number location)))
    (multiple-value-bind (translations form) (get-toplevel-form location)
      (unless (< form-num (length translations))
        (error "The source path no longer exists."))
      (sb!di:source-path-context form
                                 (svref translations form-num)
                                 context))))
\f
;;; step to the next steppable form
(!def-debug-command "STEP" ()
  (let ((restart (find-restart 'continue *debug-condition*)))
    (cond (restart
           (setf *stepping* t
                 *step* t)
           (invoke-restart restart))
          (t
           (format *debug-io* "~&Non-continuable error, cannot step.~%")))))

;;; miscellaneous commands

(!def-debug-command "DESCRIBE" ()
  (let* ((curloc (sb!di:frame-code-location *current-frame*))
         (debug-fun (sb!di:code-location-debug-fun curloc))
         (function (sb!di:debug-fun-fun debug-fun)))
    (if function
        (describe function)
        (format *debug-io* "can't figure out the function for this frame"))))

(!def-debug-command "SLURP" ()
  (loop while (read-char-no-hang *standard-input*)))

(!def-debug-command "RETURN" (&optional
                              (return (read-prompting-maybe
                                       "return: ")))
  (let ((tag (find-if (lambda (x)
                        (and (typep (car x) 'symbol)
                             (not (symbol-package (car x)))
                             (string= (car x) "SB-DEBUG-CATCH-TAG")))
                      (sb!di::frame-catches *current-frame*))))
    (if tag
        (throw (car tag)
          (funcall (sb!di:preprocess-for-eval
                    return
                    (sb!di:frame-code-location *current-frame*))
                   *current-frame*))
        (format *debug-io*
                "~@<can't find a tag for this frame ~
                 ~2I~_(hint: try increasing the DEBUG optimization quality ~
                 and recompiling)~:@>"))))
\f
;;;; debug loop command utilities

(defun read-prompting-maybe (prompt)
  (unless (sb!int:listen-skip-whitespace *debug-io*)
    (princ prompt *debug-io*)
    (force-output *debug-io*))
  (read *debug-io*))

(defun read-if-available (default)
  (if (sb!int:listen-skip-whitespace *debug-io*)
      (read *debug-io*)
      default))