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1 Debugging Lisp Programs

There are several ways to find and investigate problems in an Emacs Lisp program.

Other useful tools for debugging input and output problems are the dribble file (@pxref{Terminal Input}) and the open-termscript function (@pxref{Terminal Output}).


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1.1 The Lisp Debugger

The ordinary Lisp debugger provides the ability to suspend evaluation of a form. While evaluation is suspended (a state that is commonly known as a break), you may examine the run time stack, examine the values of local or global variables, or change those values. Since a break is a recursive edit, all the usual editing facilities of Emacs are available; you can even run programs that will enter the debugger recursively. @xref{Recursive Editing}.


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1.1.1 Entering the Debugger on an Error

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.)

User Option: 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 (@pxref{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. @xref{Processes}.

User Option: 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.

User Option: 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. See Evaluating Emacs-Lisp Expressions in The GNU Emacs Manual.

If eval-expression-debug-on-error is nil, then the value of debug-on-error is not changed during eval-expression.

User Option: 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 (@pxref{Handling Errors}).

User Option: debug-on-event

If you set debug-on-event to a special event (@pxref{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.

Variable: 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.


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1.1.2 Debugging Infinite Loops

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. @xref{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.

User Option: 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.


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1.1.3 Entering the Debugger on a Function Call

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 function-name

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 &optional function-name

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.


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1.1.4 Entering the debugger when a variable is modified

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 variable

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 @ref{Watching Variables}.

Command: cancel-debug-on-variable-change &optional variable

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.


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1.1.5 Explicit Entry to the Debugger

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 (@pxref{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.


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1.1.6 Using the Debugger

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 (@pxref{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 backtrace buffer shows you the functions that are executing and their argument values. It also allows you to specify a stack frame by moving point to the line describing that frame. (A stack frame is the place where the Lisp interpreter records information about a particular invocation of a function.) The frame whose line point is on is considered the current frame. Some of the debugger commands operate on the current frame. If a line 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.

If a function name is underlined, that means the debugger 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.

The debugger itself must be run byte-compiled, since it makes assumptions about how many stack frames are used for the debugger itself. These assumptions are false if the debugger is running interpreted.


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1.1.7 Debugger Commands

The debugger buffer (in Debugger mode) provides special commands in addition to the usual Emacs commands. 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.

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.

v

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


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1.1.8 Invoking the Debugger

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

Command: debug &rest debugger-args

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.


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1.1.9 Internals of the Debugger

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

Variable: 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 (see section Invoking the Debugger).

Function: backtrace

This function prints a trace of Lisp function calls currently active. This is the function used by debug to fill up the ‘*Backtrace*’ buffer. It is written in C, since it must have access to the stack to determine which function calls are active. The return value is always 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 values of the function’s arguments if they are all known; if they are still being computed, the line says so. The arguments of special forms are elided.

(with-output-to-temp-buffer "backtrace-output"
  (let ((var 1))
    (save-excursion
      (setq var (eval '(progn
                         (1+ var)
                         (list 'testing (backtrace))))))))

     ⇒ (testing nil)
----------- Buffer: backtrace-output ------------
  backtrace()
  (list ...computing arguments...)
  (progn ...)
  eval((progn (1+ var) (list (quote testing) (backtrace))))
  (setq ...)
  (save-excursion ...)
  (let ...)
  (with-output-to-temp-buffer ...)
  eval((with-output-to-temp-buffer ...))
  eval-last-sexp-1(nil)
  eval-last-sexp(nil)
  call-interactively(eval-last-sexp)
----------- Buffer: backtrace-output ------------
User Option: 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 ...computing arguments...)
  (progn ...)
  (eval (progn (1+ var) (list (quote testing) (backtrace))))
  (setq ...)
  (save-excursion ...)
  (let ...)
  (with-output-to-temp-buffer ...)
  (eval (with-output-to-temp-buffer ...))
  (eval-last-sexp-1 nil)
  (eval-last-sexp nil)
  (call-interactively eval-last-sexp)
----------- Buffer: backtrace-output ------------
Variable: 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.

Function: backtrace-debug level flag

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.

Variable: 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.

Function: backtrace-frame frame-number &optional base

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.

Function: mapbacktrace function &optional base

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.


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1.2 Edebug

Edebug is a source-level debugger for Emacs Lisp programs, with which you can:

The first three sections below should tell you enough about Edebug to start using it.


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1.2.1 Using Edebug

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 Edebug, 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. See section 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.


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1.2.2 Instrumenting for Edebug

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 (see section 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, see section 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-:).

See section Evaluation, for other evaluation functions available inside of Edebug.


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1.2.3 Edebug Execution Modes

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). See section Edebug 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 (see section 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.

User Option: 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.


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1.2.4 Jumping

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. See section Edebug 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. See section 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.


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1.2.5 Miscellaneous Edebug Commands

Some miscellaneous Edebug commands are described here.

?

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

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-backtrace).

You cannot use debugger commands in the backtrace buffer in Edebug as you would in the standard debugger.

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.


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1.2.6 Breaks

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.


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1.2.6.1 Edebug Breakpoints

While using Edebug, you can specify breakpoints in the program you are testing: these are places where execution should stop. You can set a breakpoint at any stop point, as defined in Using Edebug. For setting and unsetting breakpoints, the stop point that is affected is the first one at or after point in the source code buffer. Here are the Edebug commands for breakpoints:

b

Set a breakpoint at the stop point at or after point (edebug-set-breakpoint). If you use a prefix argument, the breakpoint is temporary—it turns off the first time it stops the program.

u

Unset the breakpoint (if any) at the stop point at or after point (edebug-unset-breakpoint).

x condition <RET>

Set a conditional breakpoint which stops the program only if evaluating condition produces a non-nil value (edebug-set-conditional-breakpoint). With a prefix argument, the breakpoint is temporary.

B

Move point to the next breakpoint in the current definition (edebug-next-breakpoint).

While in Edebug, you can set a breakpoint with b and unset one with u. First move point to the Edebug stop point of your choice, then type b or u to set or unset a breakpoint there. Unsetting a breakpoint where none has been set has no effect.

Re-evaluating or reinstrumenting a definition removes all of its previous breakpoints.

A conditional breakpoint tests a condition each time the program gets there. Any errors that occur as a result of evaluating the condition are ignored, as if the result were nil. To set a conditional breakpoint, use x, and specify the condition expression in the minibuffer. Setting a conditional breakpoint at a stop point that has a previously established conditional breakpoint puts the previous condition expression in the minibuffer so you can edit it.

You can make a conditional or unconditional breakpoint temporary by using a prefix argument with the command to set the breakpoint. When a temporary breakpoint stops the program, it is automatically unset.

Edebug always stops or pauses at a breakpoint, except when the Edebug mode is Go-nonstop. In that mode, it ignores breakpoints entirely.

To find out where your breakpoints are, use the B command, which moves point to the next breakpoint following point, within the same function, or to the first breakpoint if there are no following breakpoints. This command does not continue execution—it just moves point in the buffer.


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1.2.6.2 Global Break Condition

A global break condition stops execution when a specified condition is satisfied, no matter where that may occur. Edebug evaluates the global break condition at every stop point; if it evaluates to a non-nil value, then execution stops or pauses depending on the execution mode, as if a breakpoint had been hit. If evaluating the condition gets an error, execution does not stop.

The condition expression is stored in edebug-global-break-condition. You can specify a new expression using the X command from the source code buffer while Edebug is active, or using C-x X X from any buffer at any time, as long as Edebug is loaded (edebug-set-global-break-condition).

The global break condition is the simplest way to find where in your code some event occurs, but it makes code run much more slowly. So you should reset the condition to nil when not using it.


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1.2.6.3 Source Breakpoints

All breakpoints in a definition are forgotten each time you reinstrument it. If you wish to make a breakpoint that won’t be forgotten, you can write a source breakpoint, which is simply a call to the function edebug in your source code. You can, of course, make such a call conditional. For example, in the fac function, you can insert the first line as shown below, to stop when the argument reaches zero:

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

When the fac definition is instrumented and the function is called, the call to edebug acts as a breakpoint. Depending on the execution mode, Edebug stops or pauses there.

If no instrumented code is being executed when edebug is called, that function calls debug.


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1.2.7 Trapping Errors

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 (see section Miscellaneous Edebug Commands).

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.


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1.2.8 Edebug Views

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.

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.


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1.2.9 Evaluation

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. See section 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).

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.


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1.2.10 Evaluation List Buffer

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 (see Lisp Interaction in The GNU Emacs Manual) 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).

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.


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1.2.11 Printing in Edebug

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). @xref{Output Variables}.

User Option: edebug-print-length

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

User Option: 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 '(x y))
(setcar a a)

Custom printing prints this as ‘Result: #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.

User Option: edebug-print-circle

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

Other programs can also use custom printing; see ‘cust-print.el’ for details.


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1.2.12 Trace Buffer

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 (see section 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.

Macro: edebug-tracing string body…

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.

Function: edebug-trace format-string &rest format-args

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.


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1.2.13 Coverage Testing

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.


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1.2.14 The Outside Context

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.


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1.2.14.1 Checking Whether to Stop

Whenever Edebug is entered, it needs to save and restore certain data before even deciding whether to make trace information or stop the program.


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1.2.14.2 Edebug Display Update

When Edebug needs to display something (e.g., in trace mode), it saves the current window configuration from outside Edebug (@pxref{Window Configurations}). When you exit Edebug, it restores the previous window configuration.

Emacs redisplays only when it pauses. Usually, when you continue execution, the program re-enters Edebug at a breakpoint or after stepping, without pausing or reading input in between. In such cases, Emacs never gets a chance to redisplay the outside configuration. Consequently, what you see is the same window configuration as the last time Edebug was active, with no interruption.

Entry to Edebug for displaying something also saves and restores the following data (though some of them are deliberately not restored if an error or quit signal occurs).


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1.2.14.3 Edebug Recursive Edit

When Edebug is entered and actually reads commands from the user, it saves (and later restores) these additional data:


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1.2.15 Edebug and Macros

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


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1.2.15.1 Instrumenting Macro Calls

When Edebug instruments an expression that calls a Lisp macro, it needs additional information about the macro to do the job properly. This is because there is no a-priori way to tell which subexpressions of the macro call are forms to be evaluated. (Evaluation may occur explicitly in the macro body, or when the resulting expansion is evaluated, or any time later.)

Therefore, you must define an Edebug specification for each macro that Edebug will encounter, to explain the format of calls to that macro. To do this, add a debug declaration to the macro definition. Here is a simple example that shows the specification for the for example macro (@pxref{Argument Evaluation}).

(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)))
  ...)

The Edebug specification says which parts of a call to the macro are forms to be evaluated. For simple macros, the specification often looks very similar to the formal argument list of the macro definition, but specifications are much more general than macro arguments. @xref{Defining Macros}, for more explanation of the declare form.

Take care to ensure that the specifications are known to Edebug when you instrument code. If you are instrumenting a function which uses a macro defined in another file, you may first need to either evaluate the require forms in the file containing your function, or explicitly load the file containing the macro. If the definition of a macro is wrapped by eval-when-compile, you may need to evaluate it.

You can also define an edebug specification for a macro separately from the macro definition with def-edebug-spec. Adding debug declarations is preferred, and more convenient, for macro definitions in Lisp, but def-edebug-spec makes it possible to define Edebug specifications for special forms implemented in C.

Macro: def-edebug-spec macro specification

Specify which expressions of a call to macro macro are forms to be evaluated. specification should be the edebug specification. Neither argument is evaluated.

The macro argument can actually be any symbol, not just a macro name.

Here is a table of the possibilities for specification and how each directs processing of arguments.

t

All arguments are instrumented for evaluation.

0

None of the arguments is instrumented.

a symbol

The symbol must have an Edebug specification, which is used instead. This indirection is repeated until another kind of specification is found. This allows you to inherit the specification from another macro.

a list

The elements of the list describe the types of the arguments of a calling form. The possible elements of a specification list are described in the following sections.

If a macro has no Edebug specification, neither through a debug declaration nor through a def-edebug-spec call, the variable edebug-eval-macro-args comes into play.

User Option: edebug-eval-macro-args

This controls the way Edebug treats macro arguments with no explicit Edebug specification. If it is nil (the default), none of the arguments is instrumented for evaluation. Otherwise, all arguments are instrumented.


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1.2.15.2 Specification List

A specification list is required for an Edebug specification if some arguments of a macro call are evaluated while others are not. Some elements in a specification list match one or more arguments, but others modify the processing of all following elements. The latter, called specification keywords, are symbols beginning with ‘&’ (such as &optional).

A specification list may contain sublists, which match arguments that are themselves lists, or it may contain vectors used for grouping. Sublists and groups thus subdivide the specification list into a hierarchy of levels. Specification keywords apply only to the remainder of the sublist or group they are contained in.

When a specification list involves alternatives or repetition, matching it against an actual macro call may require backtracking. For more details, see section Backtracking in Specifications.

Edebug specifications provide the power of regular expression matching, plus some context-free grammar constructs: the matching of sublists with balanced parentheses, recursive processing of forms, and recursion via indirect specifications.

Here’s a table of the possible elements of a specification list, with their meanings (see Specification Examples, for the referenced examples):

sexp

A single unevaluated Lisp object, which is not instrumented.

form

A single evaluated expression, which is instrumented. If your macro wraps the expression with lambda before it is evaluated, use def-form instead. See def-form below.

place

A generalized variable. @xref{Generalized Variables}.

body

Short for &rest form. See &rest below. If your macro wraps its body of code with lambda before it is evaluated, use def-body instead. See def-body below.

function-form

A function form: either a quoted function symbol, a quoted lambda expression, or a form (that should evaluate to a function symbol or lambda expression). This is useful when an argument that’s a lambda expression might be quoted with quote rather than function, since it instruments the body of the lambda expression either way.

lambda-expr

A lambda expression with no quoting.

&optional

All following elements in the specification list are optional; as soon as one does not match, Edebug stops matching at this level.

To make just a few elements optional, followed by non-optional elements, use [&optional specs…]. To specify that several elements must all match or none, use &optional [specs…]. See the defun example.

&rest

All following elements in the specification list are repeated zero or more times. In the last repetition, however, it is not a problem if the expression runs out before matching all of the elements of the specification list.

To repeat only a few elements, use [&rest specs…]. To specify several elements that must all match on every repetition, use &rest [specs…].

&or

Each of the following elements in the specification list is an alternative. One of the alternatives must match, or the &or specification fails.

Each list element following &or is a single alternative. To group two or more list elements as a single alternative, enclose them in […].

&not

Each of the following elements is matched as alternatives as if by using &or, but if any of them match, the specification fails. If none of them match, nothing is matched, but the &not specification succeeds.

&define

Indicates that the specification is for a defining form. Edebug’s definition of a defining form is a form containing one or more code forms which are saved and executed later, after the execution of the defining form.

The defining form itself is not instrumented (that is, Edebug does not stop before and after the defining form), but forms inside it typically will be instrumented. The &define keyword should be the first element in a list specification.

nil

This is successful when there are no more arguments to match at the current argument list level; otherwise it fails. See sublist specifications and the backquote example.

gate

No argument is matched but backtracking through the gate is disabled while matching the remainder of the specifications at this level. This is primarily used to generate more specific syntax error messages. See Backtracking in Specifications, for more details. Also see the let example.

other-symbol

Any other symbol in a specification list may be a predicate or an indirect specification.

If the symbol has an Edebug specification, this indirect specification should be either a list specification that is used in place of the symbol, or a function that is called to process the arguments. The specification may be defined with def-edebug-spec just as for macros. See the defun example.

Otherwise, the symbol should be a predicate. The predicate is called with the argument, and if the predicate returns nil, the specification fails and the argument is not instrumented.

Some suitable predicates include symbolp, integerp, stringp, vectorp, and atom.

[elements…]

A vector of elements groups the elements into a single group specification. Its meaning has nothing to do with vectors.

"string"

The argument should be a symbol named string. This specification is equivalent to the quoted symbol, 'symbol, where the name of symbol is the string, but the string form is preferred.

(vector elements…)

The argument should be a vector whose elements must match the elements in the specification. See the backquote example.

(elements…)

Any other list is a sublist specification and the argument must be a list whose elements match the specification elements.

A sublist specification may be a dotted list and the corresponding list argument may then be a dotted list. Alternatively, the last CDR of a dotted list specification may be another sublist specification (via a grouping or an indirect specification, e.g., (spec . [(more specs…)])) whose elements match the non-dotted list arguments. This is useful in recursive specifications such as in the backquote example. Also see the description of a nil specification above for terminating such recursion.

Note that a sublist specification written as (specs . nil) is equivalent to (specs), and (specs . (sublist-elements…)) is equivalent to (specs sublist-elements…).

Here is a list of additional specifications that may appear only after &define. See the defun example.

name

The argument, a symbol, is the name of the defining form.

A defining form is not required to have a name field; and it may have multiple name fields.

:name

This construct does not actually match an argument. The element following :name should be a symbol; it is used as an additional name component for the definition. You can use this to add a unique, static component to the name of the definition. It may be used more than once.

arg

The argument, a symbol, is the name of an argument of the defining form. However, lambda-list keywords (symbols starting with ‘&’) are not allowed.

lambda-list

This matches a lambda list—the argument list of a lambda expression.

def-body

The argument is the body of code in a definition. This is like body, described above, but a definition body must be instrumented with a different Edebug call that looks up information associated with the definition. Use def-body for the highest level list of forms within the definition.

def-form

The argument is a single, highest-level form in a definition. This is like def-body, except it is used to match a single form rather than a list of forms. As a special case, def-form also means that tracing information is not output when the form is executed. See the interactive example.


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1.2.15.3 Backtracking in Specifications

If a specification fails to match at some point, this does not necessarily mean a syntax error will be signaled; instead, backtracking will take place until all alternatives have been exhausted. Eventually every element of the argument list must be matched by some element in the specification, and every required element in the specification must match some argument.

When a syntax error is detected, it might not be reported until much later, after higher-level alternatives have been exhausted, and with the point positioned further from the real error. But if backtracking is disabled when an error occurs, it can be reported immediately. Note that backtracking is also reenabled automatically in several situations; when a new alternative is established by &optional, &rest, or &or, or at the start of processing a sublist, group, or indirect specification. The effect of enabling or disabling backtracking is limited to the remainder of the level currently being processed and lower levels.

Backtracking is disabled while matching any of the form specifications (that is, form, body, def-form, and def-body). These specifications will match any form so any error must be in the form itself rather than at a higher level.

Backtracking is also disabled after successfully matching a quoted symbol or string specification, since this usually indicates a recognized construct. But if you have a set of alternative constructs that all begin with the same symbol, you can usually work around this constraint by factoring the symbol out of the alternatives, e.g., ["foo" &or [first case] [second case] ...].

Most needs are satisfied by these two ways that backtracking is automatically disabled, but occasionally it is useful to explicitly disable backtracking by using the gate specification. This is useful when you know that no higher alternatives could apply. See the example of the let specification.


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1.2.15.4 Specification Examples

It may be easier to understand Edebug specifications by studying the examples provided here.

A let special form has a sequence of bindings and a body. Each of the bindings is either a symbol or a sublist with a symbol and optional expression. In the specification below, notice the gate inside of the sublist to prevent backtracking once a sublist is found.

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

Edebug uses the following specifications for defun and the associated argument list and interactive specifications. It is necessary to handle interactive forms specially since an expression argument is actually evaluated outside of the function body. (The specification for defmacro is very similar to that for defun, but allows for the declare statement.)

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

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

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

The specification for backquote below illustrates how to match dotted lists and use nil to terminate recursion. It also illustrates how components of a vector may be matched. (The actual specification defined by Edebug is a little different, and does not support dotted lists because doing so causes very deep recursion that could fail.)

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

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

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1.2.16 Edebug Options

These options affect the behavior of Edebug:

User Option: 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. See section Instrumenting for Edebug.

User Option: 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. See section Instrumenting for Edebug.

User Option: 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. See section Instrumenting for Edebug.

User Option: edebug-eval-macro-args

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

User Option: 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. See section Edebug Display Update.

User Option: 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. See section Edebug Display Update.

User Option: 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). See section Edebug Execution Modes.

User Option: 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.

User Option: edebug-test-coverage

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

User Option: 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. See section Edebug Execution Modes.

User Option: edebug-print-length

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

User Option: edebug-print-level

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

User Option: edebug-print-circle

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

User Option: 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.

User Option: edebug-on-error

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

User Option: edebug-on-quit

Edebug binds debug-on-quit to this value, if debug-on-quit was previously nil. See section 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.

User Option: 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. See section Global Break Condition.

User Option: edebug-sit-for-seconds

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

User Option: 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.


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1.3 Debugging Invalid Lisp Syntax

The Lisp reader reports invalid syntax, but cannot say where the real problem is. For example, the error ‘End of file during parsing’ in evaluating an expression indicates an excess of open parentheses (or square brackets). The reader detects this imbalance at the end of the file, but it cannot figure out where the close parenthesis should have been. Likewise, ‘Invalid read syntax: ")"’ indicates an excess close parenthesis or missing open parenthesis, but does not say where the missing parenthesis belongs. How, then, to find what to change?

If the problem is not simply an imbalance of parentheses, a useful technique is to try C-M-e at the beginning of each defun, and see if it goes to the place where that defun appears to end. If it does not, there is a problem in that defun.

However, unmatched parentheses are the most common syntax errors in Lisp, and we can give further advice for those cases. (In addition, just moving point through the code with Show Paren mode enabled might find the mismatch.)


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1.3.1 Excess Open Parentheses

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. 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 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 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-_, 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.


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1.3.2 Excess Close Parentheses

To deal with an excess close parenthesis, first go to the beginning of the file, then type C-u -1 C-M-u to find the end of the first unbalanced defun.

Then find the actual matching close parenthesis by typing C-M-f 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 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-_, 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.


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1.4 Test Coverage

You can do coverage testing for a file of Lisp code by loading the testcover library and using the command M-x testcover-start <RET> file <RET> to instrument the code. Then test your code by calling it one or more times. Then use the command M-x testcover-mark-all to display colored highlights on the code to show where coverage is insufficient. The command M-x testcover-next-mark will move point forward to the next highlighted spot.

Normally, a red highlight indicates the form was never completely evaluated; a brown highlight means it always evaluated to the same value (meaning there has been little testing of what is done with the result). However, the red highlight is skipped for forms that can’t possibly complete their evaluation, such as error. The brown highlight is skipped for forms that are expected to always evaluate to the same value, such as (setq x 14).

For difficult cases, you can add do-nothing macros to your code to give advice to the test coverage tool.

Macro: 1value form

Evaluate form and return its value, but inform coverage testing that form’s value should always be the same.

Macro: noreturn form

Evaluate form, informing coverage testing that form should never return. If it ever does return, you get a run-time error.

Edebug also has a coverage testing feature (see section Coverage Testing). These features partly duplicate each other, and it would be cleaner to combine them.


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1.5 Profiling

If your program is working correctly, but not fast enough, and you want to make it run more quickly or efficiently, the first thing to do is profile your code so that you know where it spends most of the execution time. If you find that one particular function is responsible for a significant portion of the execution time, you can start looking for ways to optimize that piece.

Emacs has built-in support for this. To begin profiling, type M-x profiler-start. You can choose to profile by processor usage, memory usage, or both. Then run the code you’d like to speed up. After that, type M-x profiler-report to display a summary buffer for each resource (cpu and memory) that you chose to profile. The names of the report buffers include the times at which the reports were generated, so you can generate another report later on without erasing previous results. When you have finished profiling, type M-x profiler-stop (there is a small overhead associated with profiling, so we don’t recommend leaving it active except when you are actually running the code you want to examine).

The profiler report buffer shows, on each line, a function that was called, followed by how much resources (cpu or memory) it used in absolute and percentage terms since profiling started. If a given line has a ‘+’ symbol at the left-hand side, you can expand that line by typing <RET>, in order to see the function(s) called by the higher-level function. Use a prefix argument (C-u <RET>) to see the whole call tree below a function. Pressing <RET> again will collapse back to the original state.

Press j or mouse-2 to jump to the definition of a function at point. Press d to view a function’s documentation. You can save a profile to a file using C-x C-w. You can compare two profiles using =.

The ‘elp’ library offers an alternative approach, which is useful when you know in advance which Lisp function(s) you want to profile. Using that library, you begin by setting elp-function-list to the list of function symbols—those are the functions you want to profile. Then type M-x elp-instrument-list <RET> nil <RET> to arrange for profiling those functions. After running the code you want to profile, invoke M-x elp-results to display the current results. See the file ‘elp.el’ for more detailed instructions. This approach is limited to profiling functions written in Lisp, it cannot profile Emacs primitives.

You can measure the time it takes to evaluate individual Emacs Lisp forms using the ‘benchmark’ library. See the macros benchmark-run and benchmark-run-compiled in ‘benchmark.el’. You can also use the benchmark command for timing forms interactively.

To profile Emacs at the level of its C code, you can build it using the ‘--enable-profiling’ option of configure. When Emacs exits, it generates a file ‘gmon.out’ that you can examine using the gprof utility. This feature is mainly useful for debugging Emacs. It actually stops the Lisp-level M-x profiler-… commands described above from working.


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