2 @c This is part of the GNU Emacs Lisp Reference Manual.
3 @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1998, 1999, 2005
4 @c Free Software Foundation, Inc.
5 @c See the file elisp.texi for copying conditions.
6 @setfilename ../info/debugging
7 @node Debugging, Read and Print, Advising Functions, Top
8 @chapter Debugging Lisp Programs
10 There are three ways to investigate a problem in an Emacs Lisp program,
11 depending on what you are doing with the program when the problem appears.
15 If the problem occurs when you run the program, you can use a Lisp
16 debugger to investigate what is happening during execution. In addition
17 to the ordinary debugger, Emacs comes with a source-level debugger,
18 Edebug. This chapter describes both of them.
21 If the problem is syntactic, so that Lisp cannot even read the program,
22 you can use the Emacs facilities for editing Lisp to localize it.
25 If the problem occurs when trying to compile the program with the byte
26 compiler, you need to know how to examine the compiler's input buffer.
30 * Debugger:: How the Emacs Lisp debugger is implemented.
31 * Edebug:: A source-level Emacs Lisp debugger.
32 * Syntax Errors:: How to find syntax errors.
33 * Test Coverage:: Ensuring you have tested all branches in your code.
34 * Compilation Errors:: How to find errors that show up in byte compilation.
37 Another useful debugging tool is the dribble file. When a dribble
38 file is open, Emacs copies all keyboard input characters to that file.
39 Afterward, you can examine the file to find out what input was used.
40 @xref{Terminal Input}.
42 For debugging problems in terminal descriptions, the
43 @code{open-termscript} function can be useful. @xref{Terminal Output}.
46 @section The Lisp Debugger
51 The ordinary @dfn{Lisp debugger} provides the ability to suspend
52 evaluation of a form. While evaluation is suspended (a state that is
53 commonly known as a @dfn{break}), you may examine the run time stack,
54 examine the values of local or global variables, or change those values.
55 Since a break is a recursive edit, all the usual editing facilities of
56 Emacs are available; you can even run programs that will enter the
57 debugger recursively. @xref{Recursive Editing}.
60 * Error Debugging:: Entering the debugger when an error happens.
61 * Infinite Loops:: Stopping and debugging a program that doesn't exit.
62 * Function Debugging:: Entering it when a certain function is called.
63 * Explicit Debug:: Entering it at a certain point in the program.
64 * Using Debugger:: What the debugger does; what you see while in it.
65 * Debugger Commands:: Commands used while in the debugger.
66 * Invoking the Debugger:: How to call the function @code{debug}.
67 * Internals of Debugger:: Subroutines of the debugger, and global variables.
71 @subsection Entering the Debugger on an Error
72 @cindex error debugging
73 @cindex debugging errors
75 The most important time to enter the debugger is when a Lisp error
76 happens. This allows you to investigate the immediate causes of the
79 However, entry to the debugger is not a normal consequence of an
80 error. Many commands frequently cause Lisp errors when invoked
81 inappropriately (such as @kbd{C-f} at the end of the buffer), and during
82 ordinary editing it would be very inconvenient to enter the debugger
83 each time this happens. So if you want errors to enter the debugger, set
84 the variable @code{debug-on-error} to non-@code{nil}. (The command
85 @code{toggle-debug-on-error} provides an easy way to do this.)
87 @defopt debug-on-error
88 This variable determines whether the debugger is called when an error is
89 signaled and not handled. If @code{debug-on-error} is @code{t}, all
90 kinds of errors call the debugger (except those listed in
91 @code{debug-ignored-errors}). If it is @code{nil}, none call the
94 The value can also be a list of error conditions that should call the
95 debugger. For example, if you set it to the list
96 @code{(void-variable)}, then only errors about a variable that has no
97 value invoke the debugger.
99 When this variable is non-@code{nil}, Emacs does not create an error
100 handler around process filter functions and sentinels. Therefore,
101 errors in these functions also invoke the debugger. @xref{Processes}.
104 @defopt debug-ignored-errors
105 This variable specifies certain kinds of errors that should not enter
106 the debugger. Its value is a list of error condition symbols and/or
107 regular expressions. If the error has any of those condition symbols,
108 or if the error message matches any of the regular expressions, then
109 that error does not enter the debugger, regardless of the value of
110 @code{debug-on-error}.
112 The normal value of this variable lists several errors that happen often
113 during editing but rarely result from bugs in Lisp programs. However,
114 ``rarely'' is not ``never''; if your program fails with an error that
115 matches this list, you will need to change this list in order to debug
116 the error. The easiest way is usually to set
117 @code{debug-ignored-errors} to @code{nil}.
120 @defopt eval-expression-debug-on-error
121 If you set this variable to a non-@code{nil} value, then
122 @code{debug-on-error} will be set to @code{t} when evaluating with the
123 command @code{eval-expression}. If
124 @code{eval-expression-debug-on-error} is @code{nil}, then the value of
125 @code{debug-on-error} is not changed. @xref{Lisp Eval,, Evaluating
126 Emacs-Lisp Expressions, emacs, The GNU Emacs Manual}.
129 @defopt debug-on-signal
130 Normally, errors that are caught by @code{condition-case} never run the
131 debugger, even if @code{debug-on-error} is non-@code{nil}. In other
132 words, @code{condition-case} gets a chance to handle the error before
133 the debugger gets a chance.
135 If you set @code{debug-on-signal} to a non-@code{nil} value, then the
136 debugger gets the first chance at every error; an error will invoke the
137 debugger regardless of any @code{condition-case}, if it fits the
138 criteria specified by the values of @code{debug-on-error} and
139 @code{debug-ignored-errors}.
141 @strong{Warning:} This variable is strong medicine! Various parts of
142 Emacs handle errors in the normal course of affairs, and you may not
143 even realize that errors happen there. If you set
144 @code{debug-on-signal} to a non-@code{nil} value, those errors will
147 @strong{Warning:} @code{debug-on-signal} has no effect when
148 @code{debug-on-error} is @code{nil}.
151 To debug an error that happens during loading of the init
152 file, use the option @samp{--debug-init}. This binds
153 @code{debug-on-error} to @code{t} while loading the init file, and
154 bypasses the @code{condition-case} which normally catches errors in the
157 If your init file sets @code{debug-on-error}, the effect may
158 not last past the end of loading the init file. (This is an undesirable
159 byproduct of the code that implements the @samp{--debug-init} command
160 line option.) The best way to make the init file set
161 @code{debug-on-error} permanently is with @code{after-init-hook}, like
165 (add-hook 'after-init-hook
166 (lambda () (setq debug-on-error t)))
169 When the debugger is entered, it shows a backtrace (@pxref{Using
170 Debugger}). If you like to see the backtrace when an error happens,
171 but you do not want to enter the debugger, you can set the variable
172 @code{stack-trace-on-error} to non-@code{nil}.
174 @defopt stack-trace-on-error
175 This variable determines whether a backtrace buffer is shown when an
176 error is signalled and not handled. If @code{stack-trace-on-error} is
177 @code{t}, all kinds of errors display a backtrace; if it is
178 @code{nil}, none do. If the value is a list, an error only means to
179 display a backtrace if one of its condition symbols appears in the
184 @subsection Debugging Infinite Loops
185 @cindex infinite loops
186 @cindex loops, infinite
187 @cindex quitting from infinite loop
188 @cindex stopping an infinite loop
190 When a program loops infinitely and fails to return, your first
191 problem is to stop the loop. On most operating systems, you can do this
192 with @kbd{C-g}, which causes a @dfn{quit}.
194 Ordinary quitting gives no information about why the program was
195 looping. To get more information, you can set the variable
196 @code{debug-on-quit} to non-@code{nil}. Quitting with @kbd{C-g} is not
197 considered an error, and @code{debug-on-error} has no effect on the
198 handling of @kbd{C-g}. Likewise, @code{debug-on-quit} has no effect on
201 Once you have the debugger running in the middle of the infinite loop,
202 you can proceed from the debugger using the stepping commands. If you
203 step through the entire loop, you will probably get enough information
204 to solve the problem.
206 @defopt debug-on-quit
207 This variable determines whether the debugger is called when @code{quit}
208 is signaled and not handled. If @code{debug-on-quit} is non-@code{nil},
209 then the debugger is called whenever you quit (that is, type @kbd{C-g}).
210 If @code{debug-on-quit} is @code{nil}, then the debugger is not called
211 when you quit. @xref{Quitting}.
214 @node Function Debugging
215 @subsection Entering the Debugger on a Function Call
216 @cindex function call debugging
217 @cindex debugging specific functions
219 To investigate a problem that happens in the middle of a program, one
220 useful technique is to enter the debugger whenever a certain function is
221 called. You can do this to the function in which the problem occurs,
222 and then step through the function, or you can do this to a function
223 called shortly before the problem, step quickly over the call to that
224 function, and then step through its caller.
226 @deffn Command debug-on-entry function-name
227 This function requests @var{function-name} to invoke the debugger each time
228 it is called. It works by inserting the form @code{(debug 'debug)} into
229 the function definition as the first form.
231 Any function defined as Lisp code may be set to break on entry,
232 regardless of whether it is interpreted code or compiled code. If the
233 function is a command, it will enter the debugger when called from Lisp
234 and when called interactively (after the reading of the arguments). You
235 can't debug primitive functions (i.e., those written in C) this way.
237 When @code{debug-on-entry} is called interactively, it prompts for
238 @var{function-name} in the minibuffer. If the function is already set
239 up to invoke the debugger on entry, @code{debug-on-entry} does nothing.
240 @code{debug-on-entry} always returns @var{function-name}.
242 @strong{Warning:} if you redefine a function after using
243 @code{debug-on-entry} on it, the code to enter the debugger is
244 discarded by the redefinition. In effect, redefining the function
245 cancels the break-on-entry feature for that function.
247 Here's an example to illustrate use of this function:
253 (* n (fact (1- n)))))
257 (debug-on-entry 'fact)
265 ------ Buffer: *Backtrace* ------
266 Debugger entered--entering a function:
269 eval-last-sexp-1(nil)
271 call-interactively(eval-last-sexp)
272 ------ Buffer: *Backtrace* ------
276 (symbol-function 'fact)
277 @result{} (lambda (n)
278 (debug (quote debug))
279 (if (zerop n) 1 (* n (fact (1- n)))))
284 @deffn Command cancel-debug-on-entry function-name
285 This function undoes the effect of @code{debug-on-entry} on
286 @var{function-name}. When called interactively, it prompts for
287 @var{function-name} in the minibuffer. If @var{function-name} is
288 @code{nil} or the empty string, it cancels break-on-entry for all
291 Calling @code{cancel-debug-on-entry} does nothing to a function which is
292 not currently set up to break on entry. It always returns
297 @subsection Explicit Entry to the Debugger
299 You can cause the debugger to be called at a certain point in your
300 program by writing the expression @code{(debug)} at that point. To do
301 this, visit the source file, insert the text @samp{(debug)} at the
302 proper place, and type @kbd{C-M-x} (@code{eval-defun}, a Lisp mode key
303 binding). @strong{Warning:} if you do this for temporary debugging
304 purposes, be sure to undo this insertion before you save the file!
306 The place where you insert @samp{(debug)} must be a place where an
307 additional form can be evaluated and its value ignored. (If the value
308 of @code{(debug)} isn't ignored, it will alter the execution of the
309 program!) The most common suitable places are inside a @code{progn} or
310 an implicit @code{progn} (@pxref{Sequencing}).
313 @subsection Using the Debugger
315 When the debugger is entered, it displays the previously selected
316 buffer in one window and a buffer named @samp{*Backtrace*} in another
317 window. The backtrace buffer contains one line for each level of Lisp
318 function execution currently going on. At the beginning of this buffer
319 is a message describing the reason that the debugger was invoked (such
320 as the error message and associated data, if it was invoked due to an
323 The backtrace buffer is read-only and uses a special major mode,
324 Debugger mode, in which letters are defined as debugger commands. The
325 usual Emacs editing commands are available; thus, you can switch windows
326 to examine the buffer that was being edited at the time of the error,
327 switch buffers, visit files, or do any other sort of editing. However,
328 the debugger is a recursive editing level (@pxref{Recursive Editing})
329 and it is wise to go back to the backtrace buffer and exit the debugger
330 (with the @kbd{q} command) when you are finished with it. Exiting
331 the debugger gets out of the recursive edit and kills the backtrace
334 @cindex current stack frame
335 The backtrace buffer shows you the functions that are executing and
336 their argument values. It also allows you to specify a stack frame by
337 moving point to the line describing that frame. (A stack frame is the
338 place where the Lisp interpreter records information about a particular
339 invocation of a function.) The frame whose line point is on is
340 considered the @dfn{current frame}. Some of the debugger commands
341 operate on the current frame. If a line starts with a star, that means
342 that exiting that frame will call the debugger again. This is useful
343 for examining the return value of a function.
345 If a function name is underlined, that means the debugger knows
346 where its source code is located. You can click @kbd{Mouse-2} on that
347 name, or move to it and type @key{RET}, to visit the source code.
349 The debugger itself must be run byte-compiled, since it makes
350 assumptions about how many stack frames are used for the debugger
351 itself. These assumptions are false if the debugger is running
354 @node Debugger Commands
355 @subsection Debugger Commands
356 @cindex debugger command list
358 The debugger buffer (in Debugger mode) provides special commands in
359 addition to the usual Emacs commands. The most important use of
360 debugger commands is for stepping through code, so that you can see
361 how control flows. The debugger can step through the control
362 structures of an interpreted function, but cannot do so in a
363 byte-compiled function. If you would like to step through a
364 byte-compiled function, replace it with an interpreted definition of
365 the same function. (To do this, visit the source for the function and
366 type @kbd{C-M-x} on its definition.)
368 Here is a list of Debugger mode commands:
372 Exit the debugger and continue execution. When continuing is possible,
373 it resumes execution of the program as if the debugger had never been
374 entered (aside from any side-effects that you caused by changing
375 variable values or data structures while inside the debugger).
377 Continuing is possible after entry to the debugger due to function entry
378 or exit, explicit invocation, or quitting. You cannot continue if the
379 debugger was entered because of an error.
382 Continue execution, but enter the debugger the next time any Lisp
383 function is called. This allows you to step through the
384 subexpressions of an expression, seeing what values the subexpressions
385 compute, and what else they do.
387 The stack frame made for the function call which enters the debugger in
388 this way will be flagged automatically so that the debugger will be
389 called again when the frame is exited. You can use the @kbd{u} command
393 Flag the current frame so that the debugger will be entered when the
394 frame is exited. Frames flagged in this way are marked with stars
395 in the backtrace buffer.
398 Don't enter the debugger when the current frame is exited. This
399 cancels a @kbd{b} command on that frame. The visible effect is to
400 remove the star from the line in the backtrace buffer.
403 Flag the current frame like @kbd{b}. Then continue execution like
404 @kbd{c}, but temporarily disable break-on-entry for all functions that
405 are set up to do so by @code{debug-on-entry}.
408 Read a Lisp expression in the minibuffer, evaluate it, and print the
409 value in the echo area. The debugger alters certain important
410 variables, and the current buffer, as part of its operation; @kbd{e}
411 temporarily restores their values from outside the debugger, so you can
412 examine and change them. This makes the debugger more transparent. By
413 contrast, @kbd{M-:} does nothing special in the debugger; it shows you
414 the variable values within the debugger.
417 Like @kbd{e}, but also save the result of evaluation in the
418 buffer @samp{*Debugger-record*}.
421 Terminate the program being debugged; return to top-level Emacs
424 If the debugger was entered due to a @kbd{C-g} but you really want
425 to quit, and not debug, use the @kbd{q} command.
428 Return a value from the debugger. The value is computed by reading an
429 expression with the minibuffer and evaluating it.
431 The @kbd{r} command is useful when the debugger was invoked due to exit
432 from a Lisp call frame (as requested with @kbd{b} or by entering the
433 frame with @kbd{d}); then the value specified in the @kbd{r} command is
434 used as the value of that frame. It is also useful if you call
435 @code{debug} and use its return value. Otherwise, @kbd{r} has the same
436 effect as @kbd{c}, and the specified return value does not matter.
438 You can't use @kbd{r} when the debugger was entered due to an error.
441 Display a list of functions that will invoke the debugger when called.
442 This is a list of functions that are set to break on entry by means of
443 @code{debug-on-entry}. @strong{Warning:} if you redefine such a
444 function and thus cancel the effect of @code{debug-on-entry}, it may
445 erroneously show up in this list.
448 @node Invoking the Debugger
449 @subsection Invoking the Debugger
451 Here we describe in full detail the function @code{debug} that is used
452 to invoke the debugger.
454 @defun debug &rest debugger-args
455 This function enters the debugger. It switches buffers to a buffer
456 named @samp{*Backtrace*} (or @samp{*Backtrace*<2>} if it is the second
457 recursive entry to the debugger, etc.), and fills it with information
458 about the stack of Lisp function calls. It then enters a recursive
459 edit, showing the backtrace buffer in Debugger mode.
461 The Debugger mode @kbd{c}, @kbd{d}, @kbd{j}, and @kbd{r} commands exit
462 the recursive edit; then @code{debug} switches back to the previous
463 buffer and returns to whatever called @code{debug}. This is the only
464 way the function @code{debug} can return to its caller.
466 The use of the @var{debugger-args} is that @code{debug} displays the
467 rest of its arguments at the top of the @samp{*Backtrace*} buffer, so
468 that the user can see them. Except as described below, this is the
469 @emph{only} way these arguments are used.
471 However, certain values for first argument to @code{debug} have a
472 special significance. (Normally, these values are used only by the
473 internals of Emacs, and not by programmers calling @code{debug}.) Here
474 is a table of these special values:
478 @cindex @code{lambda} in debug
479 A first argument of @code{lambda} means @code{debug} was called
480 because of entry to a function when @code{debug-on-next-call} was
481 non-@code{nil}. The debugger displays @samp{Debugger
482 entered--entering a function:} as a line of text at the top of the
486 @code{debug} as first argument indicates a call to @code{debug}
487 because of entry to a function that was set to debug on entry. The
488 debugger displays @samp{Debugger entered--entering a function:}, just
489 as in the @code{lambda} case. It also marks the stack frame for that
490 function so that it will invoke the debugger when exited.
493 When the first argument is @code{t}, this indicates a call to
494 @code{debug} due to evaluation of a list form when
495 @code{debug-on-next-call} is non-@code{nil}. The debugger displays
496 @samp{Debugger entered--beginning evaluation of function call form:}
497 as the top line in the buffer.
500 When the first argument is @code{exit}, it indicates the exit of a
501 stack frame previously marked to invoke the debugger on exit. The
502 second argument given to @code{debug} in this case is the value being
503 returned from the frame. The debugger displays @samp{Debugger
504 entered--returning value:} in the top line of the buffer, followed by
505 the value being returned.
508 @cindex @code{error} in debug
509 When the first argument is @code{error}, the debugger indicates that
510 it is being entered because an error or @code{quit} was signaled and
511 not handled, by displaying @samp{Debugger entered--Lisp error:}
512 followed by the error signaled and any arguments to @code{signal}.
517 (let ((debug-on-error t))
522 ------ Buffer: *Backtrace* ------
523 Debugger entered--Lisp error: (arith-error)
526 ------ Buffer: *Backtrace* ------
530 If an error was signaled, presumably the variable
531 @code{debug-on-error} is non-@code{nil}. If @code{quit} was signaled,
532 then presumably the variable @code{debug-on-quit} is non-@code{nil}.
535 Use @code{nil} as the first of the @var{debugger-args} when you want
536 to enter the debugger explicitly. The rest of the @var{debugger-args}
537 are printed on the top line of the buffer. You can use this feature to
538 display messages---for example, to remind yourself of the conditions
539 under which @code{debug} is called.
543 @node Internals of Debugger
544 @subsection Internals of the Debugger
546 This section describes functions and variables used internally by the
550 The value of this variable is the function to call to invoke the
551 debugger. Its value must be a function of any number of arguments, or,
552 more typically, the name of a function. This function should invoke
553 some kind of debugger. The default value of the variable is
556 The first argument that Lisp hands to the function indicates why it
557 was called. The convention for arguments is detailed in the description
558 of @code{debug} (@pxref{Invoking the Debugger}).
561 @deffn Command backtrace
562 @cindex run time stack
564 This function prints a trace of Lisp function calls currently active.
565 This is the function used by @code{debug} to fill up the
566 @samp{*Backtrace*} buffer. It is written in C, since it must have access
567 to the stack to determine which function calls are active. The return
568 value is always @code{nil}.
570 In the following example, a Lisp expression calls @code{backtrace}
571 explicitly. This prints the backtrace to the stream
572 @code{standard-output}, which, in this case, is the buffer
573 @samp{backtrace-output}.
575 Each line of the backtrace represents one function call. The line shows
576 the values of the function's arguments if they are all known; if they
577 are still being computed, the line says so. The arguments of special
582 (with-output-to-temp-buffer "backtrace-output"
585 (setq var (eval '(progn
587 (list 'testing (backtrace))))))))
589 @result{} (testing nil)
593 ----------- Buffer: backtrace-output ------------
595 (list ...computing arguments...)
598 eval((progn (1+ var) (list (quote testing) (backtrace))))
602 (with-output-to-temp-buffer ...)
603 eval((with-output-to-temp-buffer ...))
604 eval-last-sexp-1(nil)
607 call-interactively(eval-last-sexp)
608 ----------- Buffer: backtrace-output ------------
613 @ignore @c Not worth mentioning
614 @defopt stack-trace-on-error
616 This variable controls whether Lisp automatically displays a
617 backtrace buffer after every error that is not handled. A quit signal
618 counts as an error for this variable. If it is non-@code{nil} then a
619 backtrace is shown in a pop-up buffer named @samp{*Backtrace*} on every
620 error. If it is @code{nil}, then a backtrace is not shown.
622 When a backtrace is shown, that buffer is not selected. If either
623 @code{debug-on-quit} or @code{debug-on-error} is also non-@code{nil}, then
624 a backtrace is shown in one buffer, and the debugger is popped up in
625 another buffer with its own backtrace.
627 We consider this feature to be obsolete and superseded by the debugger
632 @defvar debug-on-next-call
633 @cindex @code{eval}, and debugging
634 @cindex @code{apply}, and debugging
635 @cindex @code{funcall}, and debugging
636 If this variable is non-@code{nil}, it says to call the debugger before
637 the next @code{eval}, @code{apply} or @code{funcall}. Entering the
638 debugger sets @code{debug-on-next-call} to @code{nil}.
640 The @kbd{d} command in the debugger works by setting this variable.
643 @defun backtrace-debug level flag
644 This function sets the debug-on-exit flag of the stack frame @var{level}
645 levels down the stack, giving it the value @var{flag}. If @var{flag} is
646 non-@code{nil}, this will cause the debugger to be entered when that
647 frame later exits. Even a nonlocal exit through that frame will enter
650 This function is used only by the debugger.
653 @defvar command-debug-status
654 This variable records the debugging status of the current interactive
655 command. Each time a command is called interactively, this variable is
656 bound to @code{nil}. The debugger can set this variable to leave
657 information for future debugger invocations during the same command
660 The advantage of using this variable rather than an ordinary global
661 variable is that the data will never carry over to a subsequent command
665 @defun backtrace-frame frame-number
666 The function @code{backtrace-frame} is intended for use in Lisp
667 debuggers. It returns information about what computation is happening
668 in the stack frame @var{frame-number} levels down.
670 If that frame has not evaluated the arguments yet, or is a special
671 form, the value is @code{(nil @var{function} @var{arg-forms}@dots{})}.
673 If that frame has evaluated its arguments and called its function
674 already, the return value is @code{(t @var{function}
675 @var{arg-values}@dots{})}.
677 In the return value, @var{function} is whatever was supplied as the
678 @sc{car} of the evaluated list, or a @code{lambda} expression in the
679 case of a macro call. If the function has a @code{&rest} argument, that
680 is represented as the tail of the list @var{arg-values}.
682 If @var{frame-number} is out of range, @code{backtrace-frame} returns
689 @section Debugging Invalid Lisp Syntax
691 The Lisp reader reports invalid syntax, but cannot say where the real
692 problem is. For example, the error ``End of file during parsing'' in
693 evaluating an expression indicates an excess of open parentheses (or
694 square brackets). The reader detects this imbalance at the end of the
695 file, but it cannot figure out where the close parenthesis should have
696 been. Likewise, ``Invalid read syntax: ")"'' indicates an excess close
697 parenthesis or missing open parenthesis, but does not say where the
698 missing parenthesis belongs. How, then, to find what to change?
700 If the problem is not simply an imbalance of parentheses, a useful
701 technique is to try @kbd{C-M-e} at the beginning of each defun, and see
702 if it goes to the place where that defun appears to end. If it does
703 not, there is a problem in that defun.
705 However, unmatched parentheses are the most common syntax errors in
706 Lisp, and we can give further advice for those cases. (In addition,
707 just moving point through the code with Show Paren mode enabled might
711 * Excess Open:: How to find a spurious open paren or missing close.
712 * Excess Close:: How to find a spurious close paren or missing open.
716 @subsection Excess Open Parentheses
718 The first step is to find the defun that is unbalanced. If there is
719 an excess open parenthesis, the way to do this is to go to the end of
720 the file and type @kbd{C-u C-M-u}. This will move you to the
721 beginning of the first defun that is unbalanced.
723 The next step is to determine precisely what is wrong. There is no
724 way to be sure of this except by studying the program, but often the
725 existing indentation is a clue to where the parentheses should have
726 been. The easiest way to use this clue is to reindent with @kbd{C-M-q}
727 and see what moves. @strong{But don't do this yet!} Keep reading,
730 Before you do this, make sure the defun has enough close parentheses.
731 Otherwise, @kbd{C-M-q} will get an error, or will reindent all the rest
732 of the file until the end. So move to the end of the defun and insert a
733 close parenthesis there. Don't use @kbd{C-M-e} to move there, since
734 that too will fail to work until the defun is balanced.
736 Now you can go to the beginning of the defun and type @kbd{C-M-q}.
737 Usually all the lines from a certain point to the end of the function
738 will shift to the right. There is probably a missing close parenthesis,
739 or a superfluous open parenthesis, near that point. (However, don't
740 assume this is true; study the code to make sure.) Once you have found
741 the discrepancy, undo the @kbd{C-M-q} with @kbd{C-_}, since the old
742 indentation is probably appropriate to the intended parentheses.
744 After you think you have fixed the problem, use @kbd{C-M-q} again. If
745 the old indentation actually fit the intended nesting of parentheses,
746 and you have put back those parentheses, @kbd{C-M-q} should not change
750 @subsection Excess Close Parentheses
752 To deal with an excess close parenthesis, first go to the beginning
753 of the file, then type @kbd{C-u -1 C-M-u} to find the end of the first
756 Then find the actual matching close parenthesis by typing @kbd{C-M-f}
757 at the beginning of that defun. This will leave you somewhere short of
758 the place where the defun ought to end. It is possible that you will
759 find a spurious close parenthesis in that vicinity.
761 If you don't see a problem at that point, the next thing to do is to
762 type @kbd{C-M-q} at the beginning of the defun. A range of lines will
763 probably shift left; if so, the missing open parenthesis or spurious
764 close parenthesis is probably near the first of those lines. (However,
765 don't assume this is true; study the code to make sure.) Once you have
766 found the discrepancy, undo the @kbd{C-M-q} with @kbd{C-_}, since the
767 old indentation is probably appropriate to the intended parentheses.
769 After you think you have fixed the problem, use @kbd{C-M-q} again. If
770 the old indentation actually fits the intended nesting of parentheses,
771 and you have put back those parentheses, @kbd{C-M-q} should not change
775 @section Test Coverage
776 @cindex coverage testing
778 @findex testcover-start
779 @findex testcover-mark-all
780 @findex testcover-next-mark
781 You can do coverage testing for a file of Lisp code by loading the
782 @code{testcover} library and using the command @kbd{M-x
783 testcover-start @key{RET} @var{file} @key{RET}} to instrument the
784 code. Then test your code by calling it one or more times. Then use
785 the command @kbd{M-x testcover-mark-all} to display colored highlights
786 on the code to show where coverage is insufficient. The command
787 @kbd{M-x testcover-next-mark} will move point forward to the next
790 Normally, a red highlight indicates the form was never completely
791 evaluated; a brown highlight means it always evaluated to the same
792 value (meaning there has been little testing of what is done with the
793 result). However, the red highlight is skipped for forms that can't
794 possibly complete their evaluation, such as @code{error}. The brown
795 highlight is skipped for forms that are expected to always evaluate to
796 the same value, such as @code{(setq x 14)}.
798 For difficult cases, you can add do-nothing macros to your code to
799 give advice to the test coverage tool.
802 Evaluate @var{form} and return its value, but inform coverage testing
803 that @var{form}'s value should always be the same.
806 @defmac noreturn form
807 Evaluate @var{form}, informing coverage testing that @var{form} should
808 never return. If it ever does return, you get a run-time error.
811 @node Compilation Errors
812 @section Debugging Problems in Compilation
814 When an error happens during byte compilation, it is normally due to
815 invalid syntax in the program you are compiling. The compiler prints a
816 suitable error message in the @samp{*Compile-Log*} buffer, and then
817 stops. The message may state a function name in which the error was
818 found, or it may not. Either way, here is how to find out where in the
819 file the error occurred.
821 What you should do is switch to the buffer @w{@samp{ *Compiler Input*}}.
822 (Note that the buffer name starts with a space, so it does not show
823 up in @kbd{M-x list-buffers}.) This buffer contains the program being
824 compiled, and point shows how far the byte compiler was able to read.
826 If the error was due to invalid Lisp syntax, point shows exactly where
827 the invalid syntax was @emph{detected}. The cause of the error is not
828 necessarily near by! Use the techniques in the previous section to find
831 If the error was detected while compiling a form that had been read
832 successfully, then point is located at the end of the form. In this
833 case, this technique can't localize the error precisely, but can still
834 show you which function to check.
837 arch-tag: ddc57378-b0e6-4195-b7b6-43f8777395a7