The most important time to enter the debugger is when a Lisp error happens. This allows you to investigate the immediate causes of the error. However, entry to the debugger is not a normal consequence of an error. Many commands signal Lisp errors when invoked inappropriately, and during ordinary editing it would be very inconvenient to enter the debugger each time this happens. So if you want errors to enter the debugger, set the variable debug-on-error to non-nil. (The command toggle-debug-on-error provides an easy way to do this.)

debug-on-error

This variable determines whether the debugger is called when an error is signaled and not handled. If debug-on-error is t, all kinds of errors call the debugger, except those listed in debug-ignored-errors (see below). If it is nil, none call the debugger. The value can also be a list of error conditions (Signaling Errors). Then the debugger is called only for error conditions in this list (except those also listed in debug-ignored-errors). For example, if you set debug-on-error to the list (void-variable), the debugger is only called for errors about a variable that has no value. Note that eval-expression-debug-on-error overrides this variable in some cases; see below. When this variable is non-nil, Emacs does not create an error handler around process filter functions and sentinels. Therefore, errors in these functions also invoke the debugger. Processes.

debug-ignored-errors

This variable specifies errors which should not enter the debugger, regardless of the value of debug-on-error. Its value is a list of error condition symbols and/or regular expressions. If the error has any of those condition symbols, or if the error message matches any of the regular expressions, then that error does not enter the debugger. The normal value of this variable includes user-error, as well as several errors that happen often during editing but rarely result from bugs in Lisp programs. However, "rarely" is not "never"; if your program fails with an error that matches this list, you may try changing this list to debug the error. The easiest way is usually to set debug-ignored-errors to nil.

eval-expression-debug-on-error

If this variable has a non-nil value (the default), running the command eval-expression causes debug-on-error to be temporarily bound to t. Evaluating Emacs Lisp Expressions. If eval-expression-debug-on-error is nil, then the value of debug-on-error is not changed during eval-expression.

debug-on-signal

Normally, errors caught by condition-case never invoke the debugger. The condition-case gets a chance to handle the error before the debugger gets a chance. If you change debug-on-signal to a non-nil value, the debugger gets the first chance at every error, regardless of the presence of condition-case. (To invoke the debugger, the error must still fulfill the criteria specified by debug-on-error and debug-ignored-errors.) For example, setting this variable is useful to get a backtrace from code evaluated by emacsclient's --eval option. If Lisp code evaluated by emacsclient signals an error while this variable is non-nil, the backtrace will popup in the running Emacs. Warning: Setting this variable to non-nil may have annoying effects. Various parts of Emacs catch errors in the normal course of affairs, and you may not even realize that errors happen there. If you need to debug code wrapped in condition-case, consider using condition-case-unless-debug (Handling Errors).

debug-on-event

If you set debug-on-event to a special event (Special Events), Emacs will try to enter the debugger as soon as it receives this event, bypassing special-event-map. At present, the only supported values correspond to the signals SIGUSR1 and SIGUSR2 (this is the default). This can be helpful when inhibit-quit is set and Emacs is not otherwise responding.

debug-on-message

If you set debug-on-message to a regular expression, Emacs will enter the debugger if it displays a matching message in the echo area. For example, this can be useful when trying to find the cause of a particular message.

To debug an error that happens during loading of the init file, use the option --debug-init. This binds debug-on-error to t while loading the init file, and bypasses the condition-case which normally catches errors in the init file.

When a program loops infinitely and fails to return, your first problem is to stop the loop. On most operating systems, you can do this with C-g, which causes a quit. Quitting. Ordinary quitting gives no information about why the program was looping. To get more information, you can set the variable debug-on-quit to non-nil. Once you have the debugger running in the middle of the infinite loop, you can proceed from the debugger using the stepping commands. If you step through the entire loop, you may get enough information to solve the problem. Quitting with C-g is not considered an error, and debug-on-error has no effect on the handling of C-g. Likewise, debug-on-quit has no effect on errors.

debug-on-quit

This variable determines whether the debugger is called when quit is signaled and not handled. If debug-on-quit is non-nil, then the debugger is called whenever you quit (that is, type C-g). If debug-on-quit is nil (the default), then the debugger is not called when you quit.

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

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

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.

Sometimes a problem with a function is due to a wrong setting of a variable. Setting up the debugger to trigger whenever the variable is changed is a quick way to find the origin of the setting.

Command debug-on-variable-change

This function arranges for the debugger to be called whenever variable is modified. It is implemented using the watchpoint mechanism, so it inherits the same characteristics and limitations: all aliases of variable will be watched together, only dynamic variables can be watched, and changes to the objects referenced by variables are not detected. For details, see Watching Variables.

Command cancel-debug-on-variable-change

This function undoes the effect of debug-on-variable-change on variable. When called interactively, it prompts for variable in the minibuffer. If variable is omitted or nil, it cancels break-on-change for all variables. Calling cancel-debug-on-variable-change does nothing to a variable which is not currently set up to break on change.

You can cause the debugger to be called at a certain point in your program by writing the expression (debug) at that point. To do this, visit the source file, insert the text (debug) at the proper place, and type C-M-x (eval-defun, a Lisp mode key binding). Warning: if you do this for temporary debugging purposes, be sure to undo this insertion before you save the file! The place where you insert (debug) must be a place where an additional form can be evaluated and its value ignored. (If the value of (debug) isn't ignored, it will alter the execution of the program!) The most common suitable places are inside a progn or an implicit progn (Sequencing). If you don't know exactly where in the source code you want to put the debug statement, but you want to display a backtrace when a certain message is displayed, you can set debug-on-message to a regular expression matching the desired message.

When the debugger is entered, it displays the previously selected buffer in one window and a buffer named *Backtrace* in another window. The backtrace buffer contains one line for each level of Lisp function execution currently going on. At the beginning of this buffer is a message describing the reason that the debugger was invoked (such as the error message and associated data, if it was invoked due to an error). The backtrace buffer is read-only and uses a special major mode, Debugger mode, in which letters are defined as debugger commands. The usual Emacs editing commands are available; thus, you can switch windows to examine the buffer that was being edited at the time of the error, switch buffers, visit files, or do any other sort of editing. However, the debugger is a recursive editing level (Recursive Editing) and it is wise to go back to the backtrace buffer and exit the debugger (with the q command) when you are finished with it. Exiting the debugger gets out of the recursive edit and buries the backtrace buffer. (You can customize what the q command does with the backtrace buffer by setting the variable debugger-bury-or-kill. For example, set it to kill if you prefer to kill the buffer rather than bury it. Consult the variable's documentation for more possibilities.) When the debugger has been entered, the debug-on-error variable is temporarily set according to eval-expression-debug-on-error. If the latter variable is non-nil, debug-on-error will temporarily be set to t. This means that any further errors that occur while doing a debugging session will (by default) trigger another backtrace. If this is not what you want, you can either set eval-expression-debug-on-error to nil, or set debug-on-error to nil in debugger-mode-hook. The debugger itself must be run byte-compiled, since it makes assumptions about the state of the Lisp interpreter. These assumptions are false if the debugger is running interpreted.

Debugger mode is derived from Backtrace mode, which is also used to show backtraces by Edebug and ERT. (Edebug, and the ERT manual.) The backtrace buffer shows you the functions that are executing and their argument values. When a backtrace buffer is created, it shows each stack frame on one, possibly very long, line. (A stack frame is the place where the Lisp interpreter records information about a particular invocation of a function.) The most recently called function will be at the top. In a backtrace you can specify a stack frame by moving point to a line describing that frame. The frame whose line point is on is considered the current frame. If a function name is underlined, that means Emacs knows where its source code is located. You can click with the mouse on that name, or move to it and type RET, to visit the source code. You can also type RET while point is on any name of a function or variable which is not underlined, to see help information for that symbol in a help buffer, if any exists. The xref-find-definitions command, bound to M-., can also be used on any identifier in a backtrace (Looking Up Identifiers). In backtraces, the tails of long lists and the ends of long strings, vectors or structures, as well as objects which are deeply nested, will be printed as underlined "…". You can click with the mouse on a "…", or type RET while point is on it, to show the part of the object that was hidden. To control how much abbreviation is done, customize backtrace-line-length. Here is a list of commands for navigating and viewing backtraces:

v

Toggle the display of local variables of the current stack frame.

p

Move to the beginning of the frame, or to the beginning of the previous frame.

n

Move to the beginning of the next frame.

+

Add line breaks and indentation to the top-level Lisp form at point to make it more readable.

-

Collapse the top-level Lisp form at point back to a single line.

#

Toggle print-circle for the frame at point.

:

Toggle print-gensym for the frame at point.

.

Expand all the forms abbreviated with "…" in the frame at point.

The debugger buffer (in Debugger mode) provides special commands in addition to the usual Emacs commands and to the Backtrace mode commands described in the previous section. The most important use of debugger commands is for stepping through code, so that you can see how control flows. The debugger can step through the control structures of an interpreted function, but cannot do so in a byte-compiled function. If you would like to step through a byte-compiled function, replace it with an interpreted definition of the same function. (To do this, visit the source for the function and type C-M-x on its definition.) You cannot use the Lisp debugger to step through a primitive function. Some of the debugger commands operate on the current frame. If a frame starts with a star, that means that exiting that frame will call the debugger again. This is useful for examining the return value of a function. Here is a list of Debugger mode commands:

c

Exit the debugger and continue execution. This resumes execution of the program as if the debugger had never been entered (aside from any side-effects that you caused by changing variable values or data structures while inside the debugger).

d

Continue execution, but enter the debugger the next time any Lisp function is called. This allows you to step through the subexpressions of an expression, seeing what values the subexpressions compute, and what else they do. The stack frame made for the function call which enters the debugger in this way will be flagged automatically so that the debugger will be called again when the frame is exited. You can use the u command to cancel this flag.

b

Flag the current frame so that the debugger will be entered when the frame is exited. Frames flagged in this way are marked with stars in the backtrace buffer.

u

Don't enter the debugger when the current frame is exited. This cancels a b command on that frame. The visible effect is to remove the star from the line in the backtrace buffer.

j

Flag the current frame like b. Then continue execution like c, but temporarily disable break-on-entry for all functions that are set up to do so by debug-on-entry.

e

Read a Lisp expression in the minibuffer, evaluate it (with the relevant lexical environment, if applicable), and print the value in the echo area. The debugger alters certain important variables, and the current buffer, as part of its operation; e temporarily restores their values from outside the debugger, so you can examine and change them. This makes the debugger more transparent. By contrast, M-: does nothing special in the debugger; it shows you the variable values within the debugger.

R

Like e, but also save the result of evaluation in the buffer *Debugger-record*.

q

Terminate the program being debugged; return to top-level Emacs command execution. If the debugger was entered due to a C-g but you really want to quit, and not debug, use the q command.

r

Return a value from the debugger. The value is computed by reading an expression with the minibuffer and evaluating it. The r command is useful when the debugger was invoked due to exit from a Lisp call frame (as requested with b or by entering the frame with d); then the value specified in the r command is used as the value of that frame. It is also useful if you call debug and use its return value. Otherwise, r has the same effect as c, and the specified return value does not matter. You can't use r when the debugger was entered due to an error.

l

Display a list of functions that will invoke the debugger when called. This is a list of functions that are set to break on entry by means of debug-on-entry.

Here we describe in full detail the function debug that is used to invoke the debugger.

Command debug

This function enters the debugger. It switches buffers to a buffer named *Backtrace* (or *Backtrace*<2> if it is the second recursive entry to the debugger, etc.), and fills it with information about the stack of Lisp function calls. It then enters a recursive edit, showing the backtrace buffer in Debugger mode. The Debugger mode c, d, j, and r commands exit the recursive edit; then debug switches back to the previous buffer and returns to whatever called debug. This is the only way the function debug can return to its caller. The use of the debugger-args is that debug displays the rest of its arguments at the top of the *Backtrace* buffer, so that the user can see them. Except as described below, this is the only way these arguments are used. However, certain values for first argument to debug have a special significance. (Normally, these values are used only by the internals of Emacs, and not by programmers calling debug.) Here is a table of these special values:

lambda

A first argument of lambda means debug was called because of entry to a function when debug-on-next-call was non-nil. The debugger displays Debugger entered--entering a function: as a line of text at the top of the buffer.

debug

debug as first argument means debug was called because of entry to a function that was set to debug on entry. The debugger displays the string Debugger entered--entering a function:, just as in the lambda case. It also marks the stack frame for that function so that it will invoke the debugger when exited.

t

When the first argument is t, this indicates a call to debug due to evaluation of a function call form when debug-on-next-call is non-nil. The debugger displays Debugger entered--beginning evaluation of function call form: as the top line in the buffer.

exit

When the first argument is exit, it indicates the exit of a stack frame previously marked to invoke the debugger on exit. The second argument given to debug in this case is the value being returned from the frame. The debugger displays Debugger entered--returning value: in the top line of the buffer, followed by the value being returned.

error

When the first argument is error, the debugger indicates that it is being entered because an error or quit was signaled and not handled, by displaying Debugger entered--Lisp error: followed by the error signaled and any arguments to signal. For example, (let ((debug-on-error t)) (/ 1 0)) —— Buffer: Backtrace —— Debugger entered–Lisp error: (arith-error) /(1 0) … —— Buffer: Backtrace —— If an error was signaled, presumably the variable debug-on-error is non-nil. If quit was signaled, then presumably the variable debug-on-quit is non-nil.

nil

Use nil as the first of the debugger-args when you want to enter the debugger explicitly. The rest of the debugger-args are printed on the top line of the buffer. You can use this feature to display messages—for example, to remind yourself of the conditions under which debug is called.

This section describes functions and variables used internally by the debugger.

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 debug. 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 (Invoking the Debugger).

backtrace

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 backtrace explicitly. 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 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 ------------

debugger-stack-frame-as-list

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. With debugger-stack-frame-as-list non-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 ------------

debug-on-next-call

If this variable is non-nil, it says to call the debugger before the next eval, apply or funcall. Entering the debugger sets debug-on-next-call to nil. The d command in the debugger works by setting this variable.

backtrace-debug

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.

command-debug-status

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-frame

The function 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 (t FUNCTION ARG-VALUES...). 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, backtrace-frame returns nil.

mapbacktrace

The function 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 nil). 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 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.

To debug a Lisp program with Edebug, you must first instrument the Lisp code that you want to debug. A simple way to do this is to first move point into the definition of a function or macro and then do C-u C-M-x (eval-defun with a prefix argument). See Instrumenting, for alternative ways to instrument code. Once a function is instrumented, any call to the function activates Edebug. Depending on which Edebug execution mode you have selected, activating Edebug may stop execution and let you step through the function, or it may update the display and continue execution while checking for debugging commands. The default execution mode is step, which stops execution. Edebug Execution Modes. Within Edebug, you normally view an Emacs buffer showing the source of the Lisp code you are debugging. This is referred to as the source code buffer, and it is temporarily read-only. An arrow in the left fringe indicates the line where the function is executing. Point initially shows where within the line the function is executing, but this ceases to be true if you move point yourself. If you instrument the definition of fac (shown below) and then execute (fac 3), here is what you would normally see. Point is at the open-parenthesis before if.

(defun fac (n)
=>⋆(if (< 0 n)
      (* n (fac (1- n)))
    1))

The places within a function where Edebug can stop execution are called stop points. These occur both before and after each subexpression that is a list, and also after each variable reference. Here we use periods to show the stop points in the function fac:

(defun fac (n)
  .(if .(< 0 n.).
      .(* n. .(fac .(1- n.).).).
    1).)

The special commands of Edebug are available in the source code buffer in addition to the commands of Emacs Lisp mode. For example, you can type the Edebug command SPC to execute until the next stop point. If you type SPC once after entry to fac, here is the display you will see:

(defun fac (n)
=>(if ⋆(< 0 n)
      (* n (fac (1- n)))
    1))

When Edebug stops execution after an expression, it displays the expression's value in the echo area. Other frequently used commands are b to set a breakpoint at a stop point, g to execute until a breakpoint is reached, and q to exit Edebug and return to the top-level command loop. Type ? to display a list of all Edebug commands.

In order to use Edebug to debug Lisp code, you must first instrument the code. Instrumenting code inserts additional code into it, to invoke Edebug at the proper places. When you invoke command C-M-x (eval-defun) with a prefix argument on a function definition, it instruments the definition before evaluating it. (This does not modify the source code itself.) If the variable edebug-all-defs is non-nil, that inverts the meaning of the prefix argument: in this case, C-M-x instruments the definition unless it has a prefix argument. The default value of edebug-all-defs is nil. The command M-x edebug-all-defs toggles the value of the variable edebug-all-defs. If edebug-all-defs is non-nil, then the commands eval-region, eval-current-buffer, and eval-buffer also instrument any definitions they evaluate. Similarly, edebug-all-forms controls whether eval-region should instrument any form, even non-defining forms. This doesn't apply to loading or evaluations in the minibuffer. The command M-x edebug-all-forms toggles this option. Another command, M-x edebug-eval-top-level-form, is available to instrument any top-level form regardless of the values of edebug-all-defs and edebug-all-forms. edebug-defun is an alias for edebug-eval-top-level-form. While Edebug is active, the command I (edebug-instrument-callee) instruments the definition of the function or macro called by the list form after point, if it is not already instrumented. This is possible only if Edebug knows where to find the source for that function; for this reason, after loading Edebug, eval-region records the position of every definition it evaluates, even if not instrumenting it. See also the i command (Jumping), which steps into the call after instrumenting the function. Edebug knows how to instrument all the standard special forms, interactive forms with an expression argument, anonymous lambda expressions, and other defining forms. However, Edebug cannot determine on its own what a user-defined macro will do with the arguments of a macro call, so you must provide that information using Edebug specifications; for details, Edebug and Macros. When Edebug is about to instrument code for the first time in a session, it runs the hook edebug-setup-hook, then sets it to nil. You can use this to load Edebug specifications associated with a package you are using, but only when you use Edebug. If Edebug detects a syntax error while instrumenting, it leaves point at the erroneous code and signals an invalid-read-syntax error. Example:

error--> Invalid read syntax: "Expected lambda expression"

One potential reason for such a failure to instrument is that some macro definitions are not yet known to Emacs. To work around this, load the file which defines the function you are about to instrument. To remove instrumentation from a definition, simply re-evaluate its definition in a way that does not instrument. There are two ways of evaluating forms that never instrument them: from a file with load, and from the minibuffer with eval-expression (M-:). A different way to remove the instrumentation from a definition is to use the edebug-remove-instrumentation command. It also allows removing the instrumentation from everything that has been instrumented. Edebug Eval, for other evaluation functions available inside of Edebug.

Edebug supports several execution modes for running the program you are debugging. We call these alternatives Edebug execution modes; do not confuse them with major or minor modes. The current Edebug execution mode determines how far Edebug continues execution before stopping—whether it stops at each stop point, or continues to the next breakpoint, for example—and how much Edebug displays the progress of the evaluation before it stops. Normally, you specify the Edebug execution mode by typing a command to continue the program in a certain mode. Here is a table of these commands; all except for S resume execution of the program, at least for a certain distance.

S

Stop: don't execute any more of the program, but wait for more Edebug commands (edebug-stop).

SPC

Step: stop at the next stop point encountered (edebug-step-mode).

n

Next: stop at the next stop point encountered after an expression (edebug-next-mode). Also see edebug-forward-sexp in Jumping.

t

Trace: pause (normally one second) at each Edebug stop point (edebug-trace-mode).

T

Rapid trace: update the display at each stop point, but don't actually pause (edebug-Trace-fast-mode).

g

Go: run until the next breakpoint (edebug-go-mode). Breakpoints.

c

Continue: pause one second at each breakpoint, and then continue (edebug-continue-mode).

C

Rapid continue: move point to each breakpoint, but don't pause (edebug-Continue-fast-mode).

G

Go non-stop: ignore breakpoints (edebug-Go-nonstop-mode). You can still stop the program by typing S, or any editing command.

In general, the execution modes earlier in the above list run the program more slowly or stop sooner than the modes later in the list. When you enter a new Edebug level, Edebug will normally stop at the first instrumented function it encounters. If you prefer to stop only at a break point, or not at all (for example, when gathering coverage data), change the value of edebug-initial-mode from its default step to go, or Go-nonstop, or one of its other values (Edebug Options). You can do this readily with C-x C-a C-m (edebug-set-initial-mode):

Command edebug-set-initial-mode

This command, bound to C-x C-a C-m, sets edebug-initial-mode. It prompts you for a key to indicate the mode. You should enter one of the eight keys listed above, which sets the corresponding mode.

Note that you may reenter the same Edebug level several times if, for example, an instrumented function is called several times from one command. While executing or tracing, you can interrupt the execution by typing any Edebug command. Edebug stops the program at the next stop point and then executes the command you typed. For example, typing t during execution switches to trace mode at the next stop point. You can use S to stop execution without doing anything else. If your function happens to read input, a character you type intending to interrupt execution may be read by the function instead. You can avoid such unintended results by paying attention to when your program wants input. Keyboard macros containing the commands in this section do not completely work: exiting from Edebug, to resume the program, loses track of the keyboard macro. This is not easy to fix. Also, defining or executing a keyboard macro outside of Edebug does not affect commands inside Edebug. This is usually an advantage. See also the edebug-continue-kbd-macro option in Edebug Options.

edebug-sit-for-seconds

This option specifies how many seconds to wait between execution steps in trace mode or continue mode. The default is 1 second.

The commands described in this section execute until they reach a specified location. All except i make a temporary breakpoint to establish the place to stop, then switch to go mode. Any other breakpoint reached before the intended stop point will also stop execution. Breakpoints, for the details on breakpoints. These commands may fail to work as expected in case of nonlocal exit, as that can bypass the temporary breakpoint where you expected the program to stop.

h

Proceed to the stop point near where point is (edebug-goto-here).

f

Run the program for one expression (edebug-forward-sexp).

o

Run the program until the end of the containing sexp (edebug-step-out).

i

Step into the function or macro called by the form after point (edebug-step-in).

The h command proceeds to the stop point at or after the current location of point, using a temporary breakpoint. The f command runs the program forward over one expression. More precisely, it sets a temporary breakpoint at the position that forward-sexp would reach, then executes in go mode so that the program will stop at breakpoints. With a prefix argument n, the temporary breakpoint is placed n sexps beyond point. If the containing list ends before n more elements, then the place to stop is after the containing expression. You must check that the position forward-sexp finds is a place that the program will really get to. In cond, for example, this may not be true. For flexibility, the f command does forward-sexp starting at point, rather than at the stop point. If you want to execute one expression from the current stop point, first type w (edebug-where) to move point there, and then type f. The o command continues out of an expression. It places a temporary breakpoint at the end of the sexp containing point. If the containing sexp is a function definition itself, o continues until just before the last sexp in the definition. If that is where you are now, it returns from the function and then stops. In other words, this command does not exit the currently executing function unless you are positioned after the last sexp. Normally, the h, f, and o commands display "Break" and pause for edebug-sit-for-seconds before showing the result of the form just evaluated. You can avoid this pause by setting edebug-sit-on-break to nil. Edebug Options. The i command steps into the function or macro called by the list form after point, and stops at its first stop point. Note that the form need not be the one about to be evaluated. But if the form is a function call about to be evaluated, remember to use this command before any of the arguments are evaluated, since otherwise it will be too late. The i command instruments the function or macro it's supposed to step into, if it isn't instrumented already. This is convenient, but keep in mind that the function or macro remains instrumented unless you explicitly arrange to deinstrument it.

Some miscellaneous Edebug commands are described here.

?

Display the help message for Edebug (edebug-help).

a, C-]

Abort one level back to the previous command level (abort-recursive-edit).

q

Return to the top level editor command loop (top-level). This exits all recursive editing levels, including all levels of Edebug activity. However, instrumented code protected with unwind-protect or condition-case forms may resume debugging.

Q

Like q, but don't stop even for protected code (edebug-top-level-nonstop).

r

Redisplay the most recently known expression result in the echo area (edebug-previous-result).

d

Display a backtrace, excluding Edebug's own functions for clarity (edebug-pop-to-backtrace). Backtraces, for a description of backtraces and the commands which work on them. If you would like to see Edebug's functions in the backtrace, use M-x edebug-backtrace-show-instrumentation. To hide them again use M-x edebug-backtrace-hide-instrumentation. If a backtrace frame starts with > that means that Edebug knows where the source code for the frame is located. Use s to jump to the source code for the current frame. The backtrace buffer is killed automatically when you continue execution.

You can invoke commands from Edebug that activate Edebug again recursively. Whenever Edebug is active, you can quit to the top level with q or abort one recursive edit level with C-]. You can display a backtrace of all the pending evaluations with d.

Edebug's step mode stops execution when the next stop point is reached. There are three other ways to stop Edebug execution once it has started: breakpoints, the global break condition, and source breakpoints.

(defun fac (n)
  (if (= n 0) (edebug))
  (if (< 0 n)
      (* n (fac (1- n)))
    1))

Emacs normally displays an error message when an error is signaled and not handled with condition-case. While Edebug is active and executing instrumented code, it normally responds to all unhandled errors. You can customize this with the options edebug-on-error and edebug-on-quit; see Edebug Options. When Edebug responds to an error, it shows the last stop point encountered before the error. This may be the location of a call to a function which was not instrumented, and within which the error actually occurred. For an unbound variable error, the last known stop point might be quite distant from the offending variable reference. In that case, you might want to display a full backtrace (Edebug Misc). If you change debug-on-error or debug-on-quit while Edebug is active, these changes will be forgotten when Edebug becomes inactive. Furthermore, during Edebug's recursive edit, these variables are bound to the values they had outside of Edebug.

These Edebug commands let you view aspects of the buffer and window status as they were before entry to Edebug. The outside window configuration is the collection of windows and contents that were in effect outside of Edebug.

P, v

Switch to viewing the outside window configuration (edebug-view-outside). Type C-x X w to return to Edebug.

p

Temporarily display the outside current buffer with point at its outside position (edebug-bounce-point), pausing for one second before returning to Edebug. With a prefix argument n, pause for n seconds instead.

w

Move point back to the current stop point in the source code buffer (edebug-where). If you use this command in a different window displaying the same buffer, that window will be used instead to display the current definition in the future.

W

Toggle whether Edebug saves and restores the outside window configuration (edebug-toggle-save-windows). With a prefix argument, W only toggles saving and restoring of the selected window. To specify a window that is not displaying the source code buffer, you must use C-x X W from the global keymap.

You can view the outside window configuration with v or just bounce to the point in the current buffer with p, even if it is not normally displayed. After moving point, you may wish to jump back to the stop point. You can do that with w from a source code buffer. You can jump back to the stop point in the source code buffer from any buffer using C-x X w. Each time you use W to turn saving off, Edebug forgets the saved outside window configuration—so that even if you turn saving back on, the current window configuration remains unchanged when you next exit Edebug (by continuing the program). However, the automatic redisplay of *edebug* and *edebug-trace* may conflict with the buffers you wish to see unless you have enough windows open.

While within Edebug, you can evaluate expressions as if Edebug were not running. Edebug tries to be invisible to the expression's evaluation and printing. Evaluation of expressions that cause side effects will work as expected, except for changes to data that Edebug explicitly saves and restores. The Outside Context, for details on this process.

e EXP RET

Evaluate expression exp in the context outside of Edebug (edebug-eval-expression). That is, Edebug tries to minimize its interference with the evaluation.

M-: EXP RET

Evaluate expression exp in the context of Edebug itself (eval-expression).

C-x C-e

Evaluate the expression before point, in the context outside of Edebug (edebug-eval-last-sexp). With the prefix argument of zero (C-u 0 C-x C-e), don't shorten long items (like strings and lists).

Edebug supports evaluation of expressions containing references to lexically bound symbols created by the following constructs in cl.el: lexical-let, macrolet, and symbol-macrolet.

You can use the evaluation list buffer, called *edebug*, to evaluate expressions interactively. You can also set up the evaluation list of expressions to be evaluated automatically each time Edebug updates the display.

E

Switch to the evaluation list buffer *edebug* (edebug-visit-eval-list).

In the *edebug* buffer you can use the commands of Lisp Interaction mode (Lisp Interaction) as well as these special commands:

C-j

Evaluate the expression before point, in the outside context, and insert the value in the buffer (edebug-eval-print-last-sexp). With prefix argument of zero (C-u 0 C-j), don't shorten long items (like strings and lists).

C-x C-e

Evaluate the expression before point, in the context outside of Edebug (edebug-eval-last-sexp).

C-c C-u

Build a new evaluation list from the contents of the buffer (edebug-update-eval-list).

C-c C-d

Delete the evaluation list group that point is in (edebug-delete-eval-item).

C-c C-w

Switch back to the source code buffer at the current stop point (edebug-where).

You can evaluate expressions in the evaluation list window with C-j or C-x C-e, just as you would in *scratch*; but they are evaluated in the context outside of Edebug. The expressions you enter interactively (and their results) are lost when you continue execution; but you can set up an evaluation list consisting of expressions to be evaluated each time execution stops. To do this, write one or more evaluation list groups in the evaluation list buffer. An evaluation list group consists of one or more Lisp expressions. Groups are separated by comment lines. The command C-c C-u (edebug-update-eval-list) rebuilds the evaluation list, scanning the buffer and using the first expression of each group. (The idea is that the second expression of the group is the value previously computed and displayed.) Each entry to Edebug redisplays the evaluation list by inserting each expression in the buffer, followed by its current value. It also inserts comment lines so that each expression becomes its own group. Thus, if you type C-c C-u again without changing the buffer text, the evaluation list is effectively unchanged. If an error occurs during an evaluation from the evaluation list, the error message is displayed in a string as if it were the result. Therefore, expressions using variables that are not currently valid do not interrupt your debugging. Here is an example of what the evaluation list window looks like after several expressions have been added to it:

(current-buffer)
#<buffer *scratch*>
;---------------------------------------------------------------
(selected-window)
#<window 16 on *scratch*>
;---------------------------------------------------------------
(point)
196
;---------------------------------------------------------------
bad-var
"Symbol's value as variable is void: bad-var"
;---------------------------------------------------------------
(recursion-depth)
0
;---------------------------------------------------------------
this-command
eval-last-sexp
;---------------------------------------------------------------

To delete a group, move point into it and type C-c C-d, or simply delete the text for the group and update the evaluation list with C-c C-u. To add a new expression to the evaluation list, insert the expression at a suitable place, insert a new comment line, then type C-c C-u. You need not insert dashes in the comment line—its contents don't matter. After selecting *edebug*, you can return to the source code buffer with C-c C-w. The *edebug* buffer is killed when you continue execution, and recreated next time it is needed.

If an expression in your program produces a value containing circular list structure, you may get an error when Edebug attempts to print it. One way to cope with circular structure is to set print-length or print-level to truncate the printing. Edebug does this for you; it binds print-length and print-level to the values of the variables edebug-print-length and edebug-print-level (so long as they have non-nil values). Output Variables.

edebug-print-length

If non-nil, Edebug binds print-length to this value while printing results. The default value is 50.

edebug-print-level

If non-nil, Edebug binds print-level to this value while printing results. The default value is 50.

You can also print circular structures and structures that share elements more informatively by binding print-circle to a non-nil value. Here is an example of code that creates a circular structure:

(setq a (list 'x 'y))
(setcar a a)

If print-circle is non-nil, printing functions (e.g., prin1) will print a as #1=(#1# y). The #1= notation labels the structure that follows it with the label 1, and the #1# notation references the previously labeled structure. This notation is used for any shared elements of lists or vectors.

edebug-print-circle

If non-nil, Edebug binds print-circle to this value while printing results. The default value is t.

For further details about how printing can be customized, see Output Functions.

Edebug can record an execution trace, storing it in a buffer named *edebug-trace*. This is a log of function calls and returns, showing the function names and their arguments and values. To enable trace recording, set edebug-trace to a non-nil value. Making a trace buffer is not the same thing as using trace execution mode (Edebug Execution Modes). When trace recording is enabled, each function entry and exit adds lines to the trace buffer. A function entry record consists of ::::{, followed by the function name and argument values. A function exit record consists of ::::}, followed by the function name and result of the function. The number of :=s in an entry shows its recursion depth. You can use the braces in the trace buffer to find the matching beginning or end of function calls. You can customize trace recording for function entry and exit by redefining the functions =edebug-print-trace-before and edebug-print-trace-after.

edebug-tracing

This macro requests additional trace information around the execution of the body forms. The argument string specifies text to put in the trace buffer, after the { or }. All the arguments are evaluated, and edebug-tracing returns the value of the last form in body.

edebug-trace

This function inserts text in the trace buffer. It computes the text with (apply 'format FORMAT-STRING FORMAT-ARGS). It also appends a newline to separate entries.

edebug-tracing and edebug-trace insert lines in the trace buffer whenever they are called, even if Edebug is not active. Adding text to the trace buffer also scrolls its window to show the last lines inserted.

Edebug provides rudimentary coverage testing and display of execution frequency. Coverage testing works by comparing the result of each expression with the previous result; each form in the program is considered covered if it has returned two different values since you began testing coverage in the current Emacs session. Thus, to do coverage testing on your program, execute it under various conditions and note whether it behaves correctly; Edebug will tell you when you have tried enough different conditions that each form has returned two different values. Coverage testing makes execution slower, so it is only done if edebug-test-coverage is non-nil. Frequency counting is performed for all executions of an instrumented function, even if the execution mode is Go-nonstop, and regardless of whether coverage testing is enabled. Use C-x X = (edebug-display-freq-count) to display both the coverage information and the frequency counts for a definition. Just = (edebug-temp-display-freq-count) displays the same information temporarily, only until you type another key.

Command edebug-display-freq-count

This command displays the frequency count data for each line of the current definition. It inserts frequency counts as comment lines after each line of code. You can undo all insertions with one undo command. The counts appear under the ( before an expression or the ) after an expression, or on the last character of a variable. To simplify the display, a count is not shown if it is equal to the count of an earlier expression on the same line. The character = following the count for an expression says that the expression has returned the same value each time it was evaluated. In other words, it is not yet covered for coverage testing purposes. To clear the frequency count and coverage data for a definition, simply reinstrument it with eval-defun.

For example, after evaluating (fac 5) with a source breakpoint, and setting edebug-test-coverage to t, when the breakpoint is reached, the frequency data looks like this:

(defun fac (n)
  (if (= n 0) (edebug))
;#6           1      = =5
  (if (< 0 n)
;#5         =
      (* n (fac (1- n)))
;#    5               0
    1))
;#   0

The comment lines show that fac was called 6 times. The first if statement returned 5 times with the same result each time; the same is true of the condition on the second if. The recursive call of fac did not return at all.

Edebug tries to be transparent to the program you are debugging, but it does not succeed completely. Edebug also tries to be transparent when you evaluate expressions with e or with the evaluation list buffer, by temporarily restoring the outside context. This section explains precisely what context Edebug restores, and how Edebug fails to be completely transparent.

To make Edebug properly instrument expressions that call macros, some extra care is needed. This subsection explains the details.

(defmacro for (var from init to final do &rest body)
  "Execute a simple \"for\" loop.
For example, (for i from 1 to 10 do (print i))."
  (declare (debug (symbolp "from" form "to" form "do" &rest form)))
  ...)

(def-edebug-spec my-test-generator (&rest sexp))

(def-edebug-spec let
  ((&rest
    &or symbolp (gate symbolp &optional form))
   body))

(def-edebug-spec defun
  (&define name lambda-list
           [&optional stringp]   ; Match the doc string
           [&optional ("interactive" interactive)]
           def-body))

(def-edebug-elem-spec 'lambda-list
  '(([&rest arg]
     [&optional ["&optional" arg &rest arg]]
     &optional ["&rest" arg]
     )))

(def-edebug-elem-spec 'interactive
  '(&optional &or stringp def-form))    ; Notice: def-form

(def-edebug-spec \` (backquote-form))   ; Alias just for clarity.

(def-edebug-elem-spec 'backquote-form
  '(&or ([&or "," ",@"] &or ("quote" backquote-form) form)
        (backquote-form . [&or nil backquote-form])
        (vector &rest backquote-form)
        sexp))

These options affect the behavior of Edebug:

edebug-setup-hook

Functions to call before Edebug is used. Each time it is set to a new value, Edebug will call those functions once and then reset edebug-setup-hook to nil. You could use this to load up Edebug specifications associated with a package you are using, but only when you also use Edebug. Instrumenting.

edebug-all-defs

If this is non-nil, normal evaluation of defining forms such as defun and defmacro instruments them for Edebug. This applies to eval-defun, eval-region, eval-buffer, and eval-current-buffer. Use the command M-x edebug-all-defs to toggle the value of this option. Instrumenting.

edebug-all-forms

If this is non-nil, the commands eval-defun, eval-region, eval-buffer, and eval-current-buffer instrument all forms, even those that don't define anything. This doesn't apply to loading or evaluations in the minibuffer. Use the command M-x edebug-all-forms to toggle the value of this option. Instrumenting.

edebug-eval-macro-args

When this is non-nil, all macro arguments will be instrumented in the generated code. For any macro, the debug declaration overrides this option. So to specify exceptions for macros that have some arguments evaluated and some not, use the debug declaration specify an Edebug form specification.

edebug-save-windows

If this is non-nil, Edebug saves and restores the window configuration. That takes some time, so if your program does not care what happens to the window configurations, it is better to set this variable to nil. If the value is a list, only the listed windows are saved and restored. You can use the W command in Edebug to change this variable interactively. Edebug Display Update.

edebug-save-displayed-buffer-points

If this is non-nil, Edebug saves and restores point in all displayed buffers. Saving and restoring point in other buffers is necessary if you are debugging code that changes the point of a buffer that is displayed in a non-selected window. If Edebug or the user then selects the window, point in that buffer will move to the window's value of point. Saving and restoring point in all buffers is expensive, since it requires selecting each window twice, so enable this only if you need it. Edebug Display Update.

edebug-initial-mode

If this variable is non-nil, it specifies the initial execution mode for Edebug when it is first activated. Possible values are step, next, go, Go-nonstop, trace, Trace-fast, continue, and Continue-fast. The default value is step. This variable can be set interactively with C-x C-a C-m (edebug-set-initial-mode). Edebug Execution Modes.

edebug-trace

If this is non-nil, trace each function entry and exit. Tracing output is displayed in a buffer named *edebug-trace*, one function entry or exit per line, indented by the recursion level. Also see edebug-tracing, in Trace Buffer.

edebug-test-coverage

If non-nil, Edebug tests coverage of all expressions debugged. Coverage Testing.

edebug-continue-kbd-macro

If non-nil, continue defining or executing any keyboard macro that is executing outside of Edebug. Use this with caution since it is not debugged. Edebug Execution Modes.

edebug-print-length

If non-nil, the default value of print-length for printing results in Edebug. Output Variables.

edebug-print-level

If non-nil, the default value of print-level for printing results in Edebug. Output Variables.

edebug-print-circle

If non-nil, the default value of print-circle for printing results in Edebug. Output Variables.

edebug-unwrap-results

If non-nil, Edebug tries to remove any of its own instrumentation when showing the results of expressions. This is relevant when debugging macros where the results of expressions are themselves instrumented expressions. As a very artificial example, suppose that the example function fac has been instrumented, and consider a macro of the form:

(defmacro test () "Edebug example."
  (if (symbol-function 'fac)
      ...))

If you instrument the test macro and step through it, then by default the result of the symbol-function call has numerous edebug-after and edebug-before forms, which can make it difficult to see the actual result. If edebug-unwrap-results is non-nil, Edebug tries to remove these forms from the result.

edebug-on-error

Edebug binds debug-on-error to this value, if debug-on-error was previously nil. Trapping Errors.

edebug-on-quit

Edebug binds debug-on-quit to this value, if debug-on-quit was previously nil. Trapping Errors.

If you change the values of edebug-on-error or edebug-on-quit while Edebug is active, their values won't be used until the next time Edebug is invoked via a new command.

edebug-global-break-condition

If non-nil, an expression to test for at every stop point. If the result is non-nil, then break. Errors are ignored. Global Break Condition.

edebug-sit-for-seconds

Number of seconds to pause when a breakpoint is reached and the execution mode is trace or continue. Edebug Execution Modes.

edebug-sit-on-break

Whether or not to pause for edebug-sit-for-seconds on reaching a breakpoint. Set to nil to prevent the pause, non-nil to allow it.

edebug-behavior-alist

By default, this alist contains one entry with the key edebug and a list of three functions, which are the default implementations of the functions inserted in instrumented code: edebug-enter, edebug-before and edebug-after. To change Edebug's behavior globally, modify the default entry. Edebug's behavior may also be changed on a per-definition basis by adding an entry to this alist, with a key of your choice and three functions. Then set the edebug-behavior symbol property of an instrumented definition to the key of the new entry, and Edebug will call the new functions in place of its own for that definition.

edebug-new-definition-function

A function run by Edebug after it wraps the body of a definition or closure. After Edebug has initialized its own data, this function is called with one argument, the symbol associated with the definition, which may be the actual symbol defined or one generated by Edebug. This function may be used to set the edebug-behavior symbol property of each definition instrumented by Edebug.

edebug-after-instrumentation-function

To inspect or modify Edebug's instrumentation before it is used, set this variable to a function which takes one argument, an instrumented top-level form, and returns either the same or a replacement form, which Edebug will then use as the final result of instrumentation.

The first step is to find the defun that is unbalanced. If there is an excess open parenthesis, the way to do this is to go to the end of the file and type C-u C-M-u (backward-up-list, Moving by Parens). This will move you to the beginning of the first defun that is unbalanced. The next step is to determine precisely what is wrong. There is no way to be sure of this except by studying the program, but often the existing indentation is a clue to where the parentheses should have been. The easiest way to use this clue is to reindent with C-M-q (indent-pp-sexp, Multi-line Indent) and see what moves. But don't do this yet! Keep reading, first. Before you do this, make sure the defun has enough close parentheses. Otherwise, C-M-q will get an error, or will reindent all the rest of the file until the end. So move to the end of the defun and insert a close parenthesis there. Don't use C-M-e (end-of-defun) to move there, since that too will fail to work until the defun is balanced. Now you can go to the beginning of the defun and type C-M-q. Usually all the lines from a certain point to the end of the function will shift to the right. There is probably a missing close parenthesis, or a superfluous open parenthesis, near that point. (However, don't assume this is true; study the code to make sure.) Once you have found the discrepancy, undo the C-M-q with C-_ (undo), since the old indentation is probably appropriate to the intended parentheses. After you think you have fixed the problem, use C-M-q again. If the old indentation actually fit the intended nesting of parentheses, and you have put back those parentheses, C-M-q should not change anything.

To deal with an excess close parenthesis, first go to the beginning of the file, then type C-u -1 C-M-u (backward-up-list with an argument of −1) to find the end of the first unbalanced defun. Then find the actual matching close parenthesis by typing C-M-f (forward-sexp, Expressions) at the beginning of that defun. This will leave you somewhere short of the place where the defun ought to end. It is possible that you will find a spurious close parenthesis in that vicinity. If you don't see a problem at that point, the next thing to do is to type C-M-q (indent-pp-sexp) at the beginning of the defun. A range of lines will probably shift left; if so, the missing open parenthesis or spurious close parenthesis is probably near the first of those lines. (However, don't assume this is true; study the code to make sure.) Once you have found the discrepancy, undo the C-M-q with C-_ (undo), since the old indentation is probably appropriate to the intended parentheses. After you think you have fixed the problem, use C-M-q again. If the old indentation actually fits the intended nesting of parentheses, and you have put back those parentheses, C-M-q should not change anything.