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Debugging Lisp Programs

There are three ways to investigate a problem in an Emacs Lisp program, depending on what you are doing with the program when the problem appears.

Another useful debugging tool is a dribble file. When a dribble file is open, Emacs copies all keyboard input characters to that file. Afterward, you can examine the file to find out what input was used. See section Terminal Input.

For debugging problems in terminal descriptions, the open-termscript function can be useful. See section Terminal Output.

The Lisp Debugger

The Lisp debugger provides you with 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. See section Recursive Editing.

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 frequently get Lisp errors when invoked in inappropriate contexts (such as C-f at the end of the buffer) and during ordinary editing it would be very unpleasant to enter the debugger each time this happens. If you want errors to enter the debugger, set the variable debug-on-error to non-nil.

User Option: debug-on-error

This variable determines whether the debugger is called when a error is signaled and not handled. If debug-on-error is t, all errors call the debugger. If it is nil, none call the debugger.

The value can also be a list of error conditions that should call the debugger. For example, if you set it to the list (void-variable), then only errors about a variable that has no value invoke the debugger.

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 quit.

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. Quitting with C-g is not considered an error, and debug-on-error has no effect on the handling of C-g. Contrariwise, debug-on-quit has no effect on errors.

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 will probably get enough information to solve the problem.

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, then the debugger is not called when you quit. See section Quitting.

Entering the Debugger on a Function Call

To investigate a problem that happens in the middle of a program, one useful technique is to cause the debugger to be entered when 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. It works by inserting the form (debug 'debug) into the function definition as the first form.

Any function defined as Lisp code may be set to break on entry, regardless of whether it is interpreted code or compiled code. Even functions that are commands may be debugged--they will enter the debugger when called inside a function, or when called interactively (after the reading of the arguments). Primitive functions (i.e., those written in C) may not be debugged.

When debug-on-entry is called interactively, it prompts for function-name in the minibuffer.

Caveat: if debug-on-entry is called more than once on the same function, the second call does nothing. If you redefine a function after using debug-on-entry on it, the code to enter the debugger is lost.

debug-on-entry returns function-name.

(defun fact (n)
  (if (zerop n) 1
      (* n (fact (1- n)))))
     => fact
(debug-on-entry 'fact)
     => fact
(fact 3)
     => 6

------ Buffer: *Backtrace* ------
Entering:
* fact(3)
  eval-region(4870 4878 t)
  byte-code("...")
  eval-last-sexp(nil)
  (let ...)
  eval-insert-last-sexp(nil)
* call-interactively(eval-insert-last-sexp)
------ Buffer: *Backtrace* ------

(symbol-function 'fact)
     => (lambda (n)
          (debug (quote debug))
          (if (zerop n) 1 (* n (fact (1- n)))))

Command: cancel-debug-on-entry 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 cancel-debug-on-entry is called more than once on the same function, the second call does nothing. cancel-debug-on-entry returns function-name.

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. 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 isn't ignored, it will alter the execution of the program!) Usually this means inside a progn or an implicit progn (see section Sequencing).

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 (see section 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 kills the backtrace buffer.

The contents of the backtrace buffer show you the functions that are executing and the arguments that were given to them. 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.

The debugger itself should always 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.

Debugger Commands

Inside the debugger (in Debugger mode), these special commands are available in addition to the usual cursor motion commands. (Keep in mind that all the usual facilities of Emacs, such as switching windows or buffers, are still available.)

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 file for the function and type C-M-x on its definition.)

c
Exit the debugger and continue execution. When continuing is possible, it resumes execution of the program as if the debugger had never been entered (aside from the effect of any variables or data structures you may have changed while inside the debugger).

Continuing is possible after entry to the debugger due to function entry or exit, explicit invocation, quitting or certain errors. Most errors cannot be continued; trying to continue an unsuitable error causes the same error to occur again.

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.

e
Read a Lisp expression in the minibuffer, evaluate it, and print the value in the echo area. This is the same as the command M-ESC, except that e is not normally disabled like M-ESC.

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 makes a difference when the debugger was invoked due to exit from a Lisp call frame (as requested with b); then the value specified in the r command is used as the value of that frame.

You can't use r when the debugger was entered due to an error.

Invoking the Debugger

Here we describe fully the function used to invoke the debugger.

Function: 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, leaving that buffer in Debugger mode and displayed in the selected window.

Debugger mode provides a c command which operates by exiting the recursive edit, switching back to the previous buffer, and returning to whatever called debug. The r command also returns from debug. These are the only ways the function debug can return to its caller.

If the first of the debugger-args passed to debug is nil (or if it is not one of the following special values), then the rest of the arguments to debug are printed at the top of the `*Backtrace*' buffer. This mechanism is used to display a message to the user.

However, if the first argument passed to debug is one of the following special values, then it has special significance. Normally, these values are passed to debug only by the internals of Emacs and the debugger, and not by programmers calling debug.

The special values are:

lambda
When the first argument is lambda, the debugger displays `Entering:' as a line of text at the top of the buffer. This means that a function is being entered when debug-on-next-call is non-nil.

debug
When the first argument is debug, the debugger displays `Entering:' just as in the lambda case. However, debug as the argument indicates that the reason for entering the debugger is that a function set to debug on entry is being entered.

In addition, debug as the first argument directs the debugger to mark the function that called debug so that it will invoke the debugger when exited. (When lambda is the first argument, the debugger does not do this, because it has already been done by the interpreter.)

t
When the first argument is t, the debugger displays the following as the top line in the buffer:

Beginning evaluation of function call form:

This indicates that it was entered due to the evaluation of a list form at a time when debug-on-next-call is non-nil.

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 `Return value:' on 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 `Signaling:' followed by the error signaled and any arguments to signal. For example,

(let ((debug-on-error t))
     (/ 1 0))

------ Buffer: *Backtrace* ------
Signaling: (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.

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). Presumably this function will enter 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.

Command: 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, backtrace is called explicitly in a Lisp expression. When the expression is evaluated, the backtrace is printed to the stream standard-output: in this case, to the buffer `backtrace-output'. Each line of the backtrace represents one function call. If the arguments of the function call are all known, they are displayed; if they are being computed, that fact is stated. 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))))))))

     => 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-region(1973 2142 #<buffer *scratch*>)
  byte-code("...  for eval-print-last-sexp ...")
  eval-print-last-sexp(nil)
* call-interactively(eval-print-last-sexp)
----------- Buffer: backtrace-output ------------

The character `*' indicates a frame whose debug-on-exit flag is set.

Variable: debug-on-next-call

This variable determines whether the debugger is called before the next eval, apply or funcall. It is automatically reset to nil when the debugger is entered.

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 eval frame level levels down to flag. If flag is non-nil, this will cause the debugger to be entered when that frame exits. Even a nonlocal exit through that frame will enter the debugger.

The debug-on-exit flag is an entry in the stack frame of a function call. This flag is examined on every exit from a function.

Normally, this function is only called by the debugger.

Variable: command-debug-status

This variable records the debugging status of 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.

The advantage of using this variable rather that defining another global variable is that the data will never carry over to a later other command invocation.

Function: backtrace-frame frame-number

The function backtrace-frame is intended for use in Lisp debuggers. It returns information about what computation is happening in the eval frame level 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 value is (t function arg-values...).

In the return value, function is whatever was supplied as CAR of 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 the argument is out of range, backtrace-frame returns nil.

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 not 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.

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 insert a close parenthesis at the end of the file and type C-M-b (backward-sexp). This will move you to the beginning of the defun that is unbalanced. (Then type C-SPC C-_ C-u C-SPC to set the mark there, undo the insertion of the close parenthesis, and finally return to the mark.)

The next step is to determine precisely what is wrong. There is no way to be sure of this except to study 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.

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.

Then 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, since the old indentation is probably appropriate to the intended parentheses.

After you think you have fixed the problem, use C-M-q again. It should not change anything, if the problem is really fixed.

Excess Close Parentheses

To deal with an excess close parenthesis, first insert an open parenthesis at the beginning of the file and type C-M-f to find the end of the unbalanced defun. (Then type C-SPC C-_ C-u C-SPC to set the mark there, undo the insertion of the open parenthesis, and finally return to the mark.)

Then find the actual matching close parenthesis by typing C-M-f at the beginning of the 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, since the old indentation is probably appropriate to the intended parentheses.

Debugging Problems in Compilation

When an error happens during byte compilation, it is normally due to invalid syntax in the program you are compiling. The compiler prints a suitable error message in the `*Compile-Log*' buffer, and then stops. The message may state a function name in which the error was found, or it may not. Regardless, here is how to find out where in the file the error occurred.

What you should do is switch to the buffer ` *Compiler Input*'. (Note that the buffer name starts with a space, so it does not show up in M-x list-buffers.) This buffer contains the program being compiled, and point shows how far the byte compiler was able to read.

If the error was due to invalid Lisp syntax, point shows exactly where the invalid syntax was detected. The cause of the error is not necessarily near by! Use the techniques in the previous section to find the error.

If the error was detected while compiling a form that had been read successfully, then point is located at the end of the form. In this case, it can't localize the error precisely, but can still show you which function to check.

Edebug

Edebug is a source-level debugger for Emacs Lisp programs that provides the following features:

The first three sections of this chapter should tell you enough about Edebug to enable you to use it.

Using Edebug

To debug a Lisp program with Edebug, you must first prepare the Lisp functions that you want to debug. See section Preparing Functions for Edebug.

Once a function is prepared, any call to the function activates Edebug. This involves entering a recursive edit which is a level of Edebug activation.

Activating Edebug may stop execution and let you step through the function, or it may continue execution while checking for debugging commands, depending on the selected Edebug execution mode. See section Edebug Modes.

Within Edebug, you normally view an Emacs buffer showing the source of the Lisp function you are debugging. We call this the Edebug buffer---but note that it is not always the same buffer, and it is not reserved for Edebug use.

An arrow at the left margin 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 prepare the definition of fac (shown below) for Edebug and then execute (fac 3), here is what you 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. Stop points before variables are optional, under the control of the value of edebug-stop-before-symbols. Here we show with periods the stop points normally found in the function fac:

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

While a buffer is the Edebug buffer, the special commands of Edebug are available in it, instead of many usual editing commands. Type ? to display a list of Edebug commands. In particular, you can exit the innermost Edebug activation level with C-], and you can return all the way to top level with q.

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 state that you get:

(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. Use the r command to display the value again later.

While Edebug is active, it catches all errors (if debug-on-error is non-nil) and quits (if debug-on-quit is non-nil) instead of the standard debugger. When this happens, Edebug displays the last stop point that it knows about. This may be the location of a call to a function which was not prepared for Edebug debugging, within which the error actually occurred.

Preparing Functions for Edebug

In order to use Edebug to debug a function, you must first prepare the function. Preparing a function inserts additional code into it which invokes Edebug at the proper places.

Any call to an Edebug-prepared function activates Edebug. This may or may not stop execution, depending on the Edebug execution mode in use. Some Edebug modes only update the display to indicate the progress of the evaluation without stopping execution. The default initial Edebug mode is step which does stop execution. See section Edebug Modes.

Once you have loaded Edebug, the command C-M-x is redefined so that when used on a function or macro definition, it prepares the function or macro if given a prefix argument. If the variable edebug-all-defuns is non-nil, that inverts the meaning of the prefix argument: then C-M-x prepares the function or macro unless it has a prefix argument. The default value of edebug-all-defuns is nil. The command M-x edebug-all-defuns toggles the value of the variable edebug-all-defuns.

If edebug-all-defuns is non-nil, then the commands eval-region and eval-current-buffer also prepare any functions and macros whose definitions they evaluate.

Loading a file does not prepare functions and macros for Edebug.

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

Edebug Modes

Edebug supports several execution modes for running the program you are debugging. We call these alternatives Edebug modes; do not confuse them with major modes or minor modes. The current Edebug mode determines how Edebug displays the progress of the evaluation, whether it stops at each stop point, or continues to the next breakpoint, for example.

Normally, you specify the Edebug mode for execution 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 for now, just wait for more Edebug commands.

SPC
Step: stop at the next stop point encountered.

t
Trace: pause one second at each Edebug stop point.

T
Rapid trace: mention each stop point, but don't actually pause.

g
Go: run until the next breakpoint. See section Breakpoints.

c
Continue: pause for one second at each breakpoint, but don't stop.

C
Continue: mention each breakpoint, but don't actually pause.

G
Non-stop: ignore breakpoints. You can still stop the program by typing S.

In general, the execution modes earlier in the above list run the program more slowly or stop sooner.

When you enter a new Edebug level, the mode comes from the value of the variable edebug-initial-mode. By default, this specifies step mode. If the mode thus specified is not stop mode, then the Edebug level executes the program (or part of it).

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 that you typed. For example, typing t during execution switches to trace mode at the next stop point.

You can use the S command to stop execution without doing anything else.

If your function happens to read input, a character you hit 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.

Stepping

f
Run the program forward over one expression. More precisely, set a temporary breakpoint at the position that C-M-f would reach, then execute in go mode so that the program will stop at breakpoints. See section Breakpoints for the details on 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.

Be careful that the position C-M-f finds is a place that the program will really get to; this may not be true in a condition-case, for example.

This command does forward-sexp starting at point rather than the stop point, thus providing more flexibility. If you want to execute one expression from the current stop point, type w first, to move point there.

o
Run the program until the end of the containing sexp. If the containing sexp is the top level defun, run until just before the function returns. If that is where you are now, return from the function and then stop.

This command does not exit the currently executing function unless you are positioned after the last sexp of the function.

If the program does a non-local exit, it may fail to reach the temporary breakpoint that this command sets.

i
Step into the function about to be called. Use this command before any of the arguments of the function call are evaluated, since otherwise it is too late.

One undesirable side effect of using edebug-step-in is that the next time the stepped-into function is called, Edebug will be called there as well.

h
Proceed to the stop point near where point is. This uses a temporary breakpoint.

The f command runs the program forward over one expression. More precisely, set a temporary breakpoint at the position that C-M-f would reach, then execute in go mode so that the program will stop at breakpoints. See section Breakpoints for the details on 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.

Be careful that the position C-M-f finds is a place that the program will really get to; this may not be true in a condition-case, for example.

The f command uses the existing value of point as the basis for setting the breakpoint, because that is more flexible. To execute one expression from the current stop point, type w and then f.

The o command continues "out of" an expression. It places a temporary breakpoints at the end of the containing sexp. If the containing sexp is the top level defun, it continues until just before the function returns. If that is where you are now, it returns from the function and then stops.

This command does not exit the currently executing function unless you are positioned after the last sexp of the function.

The i command steps into the function about to be called. Use this command before any of the arguments of the function call are evaluated, since otherwise it is too late.

One undesirable side effect of using i is that the next time the stepped-into function is called, Edebug will be called there as well.

The h command proceeds to the stop point near where point is, using a temporary breakpoint.

All the commands in this section may fail to work as expected in case of nonlocal exit, because a nonlocal exit can bypass the temporary breakpoint where you expected the program to stop.

Miscellaneous

Some miscellaneous commands are described here.

C-]
Abort one level of Edebug activity.

q
Return to the top level editor command loop. This exits all recursive editing levels, including all levels of Edebug activity.

r
Redisplay the result of the previous expression in the echo area.

d
Display a backtrace, excluding Edebug's own functions for clarity.

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.

Breakpoints

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

b
Set a breakpoint at the stop point at or after point. 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 the current point.

x cond RET
Set a conditional breakpoint which stops the program only if cond evaluates to a non-nil value. If you use a prefix argument, the breakpoint is temporary (it turns off the first time it stops the program).

B
Move point to the next breakpoint in the current function definition.

While in Edebug, you can set a breakpoint with b (edebug-set-breakpoint) and unset one with u (edebug-unset-breakpoint). First you must move point to a position at or before the desired Edebug stop point, then hit the key to change the breakpoint. Unsetting a breakpoint that has not been set does nothing.

Reevaluating the function with edebug-defun clears all breakpoints in the function.

A conditional breakpoint tests a condition each time the program gets there, to decide whether to stop. To set a conditional breakpoint, use x, and specify the condition expression in the minibuffer.

You can make both conditional and unconditional breakpoints temporary by using a prefix arg to the command to set the breakpoint. After breaking at a temporary breakpoint, it is automatically cleared.

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 (edebug-next-breakpoint) command, which moves point to the next breakpoint in the function following point, or to the first breakpoint if there are no following breakpoints. This command does not continue execution--it just moves point in the buffer.

Views

These Edebug commands let you view aspects of the buffer and window status that obtained before entry to Edebug.

v
View the outside window configuration.

p
Temporarily display the outside current buffer with point at its outside position.

w
Switch back to the buffer showing the currently executing function, and move point back to the current stop point.

W
Forget the saved outside window configuration--so that the current window configuration will remain unchanged when you next exit Edebug (by continuing the program). Also toggle the edebug-save-windows variable.

Evaluation

While within Edebug, you can evaluate expressions "as if" Edebug were not running. Edebug tries to be invisible to the expression's evaluation.

e exp RET
Evaluate expression exp in the context outside of Edebug. That is, Edebug tries to avoid altering the effect of exp.

M-ESC exp RET
Evaluate expression exp in the context of Edebug itself.

C-x C-e
Evaluate the expression in the buffer before point, in the context outside of Edebug.

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 is reentered.

E
Switch to the evaluation list buffer `*edebug*'.

In the `*edebug*' buffer you can use the commands of Lisp Interaction as well as these special commands:

LFD
Evaluate the expression before point, in the context outside of Edebug, and insert the value in the buffer.

C-x C-e
Evaluate the expression before point, in the context outside of Edebug.

C-c C-u
Build a new evaluation list from the first expression of each group, reevaluate and redisplay. Groups are separated by a line starting with a comment.

C-c C-d
Delete the evaluation list group that point is in.

C-c C-w
Switch back to the Edebug buffer at the current stop point.

You can evaluate expressions in the evaluation list window with LFD 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 of your function unless you add them to the evaluation list with C-c C-u (edebug-update-eval-list). This command builds a new list from the first expression of each evaluation list group. Groups are separated by a line starting with a comment.

When the evaluation list is redisplayed, each expression is displayed followed by the result of evaluating it, and a comment line. If an error occurs during an evaluation, the error message is displayed in a string as if it were the result. Therefore expressions that use variables 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*>
;---------------------------------------------------------------
(point-min)
1
;---------------------------------------------------------------
(point-max)
2
;---------------------------------------------------------------
edebug-outside-point-max
"Symbol's value as variable is void: edebug-outside-point-max"
;---------------------------------------------------------------
(recursion-depth)
0
;---------------------------------------------------------------
this-command
eval-last-sexp
;---------------------------------------------------------------

To delete a group, move point into it and type C-c C-d (edebug-delete-eval-item), or simply delete the text for it and update the evaluation list with C-c C-u. When you add a new group, be sure to add a comment at the beginning.

After selecting `*edebug*', you can return to the source code buffer (the Edebug buffer) with C-c C-w. The *edebug* buffer is killed when you continue execution of your function, and recreated next time it is needed.

Printing

If the results of your expressions contain circular references to other parts of the same structure, you can print them more usefully with the `custom-print'.

To load the package and activate custom printing only for Edebug, simply use the command edebug-install-custom-print-funcs. Then set the variable print-circle to enable special handling of circular structure. To restore the standard print functions, use edebug-reset-print-funcs.

The Outside Context

Edebug tries to be transparent to the program you are debugging, but it does not succeed completely. In addition, most evaluations you do within Edebug (see section Evaluation) occur in the same outside context which is temporarily restored for the evaluation. This section explains precisely how use Edebug fails to be completely transparent.

Just Checking

Whenever Edebug is entered just to think about whether to take some action, it needs to save and restore certain data.

Outside Window Configuration

When Edebug needs to display something (e.g., in trace mode), it saves the current window configuration from "outside" Edebug (see section Window Configurations). When you exit Edebug (by continuing the program), it restores the previous window configuration.

Emacs redisplays only when it pauses. Usually, when you continue Edebug, the program comes back into 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. What you see is the window configuration for within Edebug, with no interruption.

The window configuration proper does not include which buffer is current or where point and mark are in the current buffer, but Edebug saves and restores these also.

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

Recursive Edit

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

Side Effects

Edebug operation unavoidably alters some data in Emacs, and this can interfere with debugging certain programs.

Macro Calls

When Edebug prepares for stepping through an expression that uses a Lisp macro, it needs additional advice to do the job properly. This is because there is no way to tell which parts of the macro call are forms to be evaluated. You must explain the format of calls to each macro to enable Edebug to handle it. To do this, use def-edebug-form-spec to define the format of calls to a given macro.

Macro: def-edebug-form-spec macro argpattern

Specify which parts of a call to macro macro are subexpressions to be evaluated. The second argument, argpattern, details what the argument list looks like.

Here is a table of the possibilities for argpattern and its subexpressions:

t
A list of any number of evaluated arguments.

0
A list of unevaluated arguments.

sexp
A single unevaluated object.

form
A single evaluated expression.

symbolp
An unevaluated symbol.

integerp
An unevaluated number.

stringp
An unevaluated string.

vectorp
An unevaluated vector.

atom
An unevaluated object that is not a cons cell.

function
A function argument: a quoted symbol, a quoted lambda expression, or a form (that should evaluate to a function or lambda expression). Edebug treats the body of a lambda expression treated as evaluated.

function
A function serves as a predicate--it designates the set of possible arguments for which it would return non-nil.

'object
The precise object object, treated as unevaluated.

(patterns)
A list whose elements are described by patterns. A sublist of the same format as the top level, processed recursively.

[patterns]
A sequence of arguments that are described by patterns.

&optional
This symbol serves as a flag saying that all following elements in the specification list at this level are optional. They may or may not match arguments; as soon as one does not match, processing of the specification list at this level terminates. To make just one item optional, use [&optional pattern].

&rest
This symbol serves as a flag saying that the following elements in the specification list at this level may be repeated, in order, zero or more times. Only one &rest may appear at the same level of a specification list, and &rest must not be followed by &optional.

To specify repetition of certain types of arguments, followed by dissimilar arguments, use [&rest patterns...].

&or
This symbol serves as an operator saying that the following elements in the specification list at this level are alternatives. To group two or more list elements as one alternative, bracket them in [...]. Only one &or may appear in a list, and it may not be followed by &optional or &rest. One of the alternatives must match, unless the &or is preceded by &optional or &rest.

If the actual arguments of a macro call fail to match the specification, taking account of alternatives, optional arguments and repeated arguments, Edebug reports a syntax error in use of the macro.

The combination of backtracking, &optional, &rest, &or, and [...] for grouping provides the equivalent of regular expressions. The (...) lists require balanced parentheses, which is the only context free (finite state with stack) construct supported.

Here are some examples of using def-edebug-form-spec. First, for the let special form:

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

Here's the spec for the for loop macro (see section Common Problems Using Macros) and for the case and do macros in `cl.el':

(def-edebug-form-spec for
  '(symbolp 'from form 'to form 'do &rest form))

(def-edebug-form-spec case
  '(form &rest (sexp form)))

(def-edebug-form-spec do
  '((&rest &or symbolp (symbolp &optional form form))
    (form &rest form)
    &rest body))

Finally, the functions mapcar, mapconcat, mapatoms, apply, and funcall all take function arguments, and Edebug defines specifications for them. Here's one example:

(def-edebug-form-spec apply '(function &rest form))

The backquote (`) macro results in an expression that is not necessarily evaluated. Edebug cannot step through code generated by use of backquote.

Edebug Options

These options affect the behavior of Edebug:

User Option: edebug-all-defuns

If non-nil, normal evaluation of defun and defmacro forms prepares the functions and macros for stepping with Edebug. This applies to eval-defun, eval-region and eval-current-buffer.

The default value is nil.

User Option: edebug-stop-before-symbols

If non-nil, Edebug places stop points before symbols as well as after.

This option takes effect for a function when you prepare it for stepping with Edebug. Changing the option's value during execution of Edebug has no effect on the functions already set up for Edebug execution.

User Option: edebug-save-windows

If non-nil, save and restore window configuration on Edebug calls. It takes some time to save and restore, so if your program does not care what happens to the window configurations, it is better to set this variable to nil.

The default value is t.

User Option: edebug-save-point

If non-nil, Edebug saves and restores point and the mark in source code buffers. The default value is t.

User Option: edebug-save-displayed-buffer-points

If non-nil, save and restore point in all buffers when entering Edebug mode.

Saving and restoring point in other buffers is necessary if you are debugging code that changes the point of a buffer which is displayed in a non-selected window. If Edebug or the user then selects the window, the buffer's point will be changed to the window's point.

Saving and restoring is an expensive operation since it visits each window and each displayed buffer twice for each Edebug call, so it is best to avoid it if you can.

The default value is nil.

User Option: edebug-initial-mode

If this variable is non-nil, it specifies an Edebug mode to start in each time the program enters a new Edebug recursive-edit level. Possible values are step, go, Go-nonstop, trace, Trace-fast, continue, and Continue-fast.

The default value is step.

User Option: edebug-trace

Non-nil means display a trace of 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. You can customize this display by replacing the functions edebug-print-trace-entry and edebug-print-trace-exit.

The default value is nil.

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