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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
fd897522 GM |
3 | @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1998, 1999 |
4 | @c Free Software Foundation, Inc. | |
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5 | @c See the file elisp.texi for copying conditions. |
6 | @setfilename ../info/debugging | |
a9f0a989 | 7 | @node Debugging, Read and Print, Advising Functions, Top |
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8 | @chapter Debugging Lisp Programs |
9 | ||
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. | |
12 | ||
13 | @itemize @bullet | |
14 | @item | |
15 | If the problem occurs when you run the program, you can use a Lisp | |
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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. | |
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19 | |
20 | @item | |
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. | |
23 | ||
24 | @item | |
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. | |
27 | @end itemize | |
28 | ||
29 | @menu | |
30 | * Debugger:: How the Emacs Lisp debugger is implemented. | |
a9f0a989 | 31 | * Edebug:: A source-level Emacs Lisp debugger. |
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32 | * Syntax Errors:: How to find syntax errors. |
33 | * Compilation Errors:: How to find errors that show up in byte compilation. | |
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34 | @end menu |
35 | ||
36 | Another useful debugging tool is the dribble file. When a dribble | |
37 | file is open, Emacs copies all keyboard input characters to that file. | |
38 | Afterward, you can examine the file to find out what input was used. | |
39 | @xref{Terminal Input}. | |
40 | ||
41 | For debugging problems in terminal descriptions, the | |
42 | @code{open-termscript} function can be useful. @xref{Terminal Output}. | |
43 | ||
44 | @node Debugger | |
45 | @section The Lisp Debugger | |
46 | @cindex debugger | |
47 | @cindex Lisp debugger | |
48 | @cindex break | |
49 | ||
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50 | The ordinary @dfn{Lisp debugger} provides the ability to suspend |
51 | evaluation of a form. While evaluation is suspended (a state that is | |
52 | commonly known as a @dfn{break}), you may examine the run time stack, | |
53 | examine the values of local or global variables, or change those values. | |
54 | Since a break is a recursive edit, all the usual editing facilities of | |
55 | Emacs are available; you can even run programs that will enter the | |
56 | debugger recursively. @xref{Recursive Editing}. | |
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57 | |
58 | @menu | |
59 | * Error Debugging:: Entering the debugger when an error happens. | |
60 | * Infinite Loops:: Stopping and debugging a program that doesn't exit. | |
61 | * Function Debugging:: Entering it when a certain function is called. | |
62 | * Explicit Debug:: Entering it at a certain point in the program. | |
63 | * Using Debugger:: What the debugger does; what you see while in it. | |
64 | * Debugger Commands:: Commands used while in the debugger. | |
65 | * Invoking the Debugger:: How to call the function @code{debug}. | |
66 | * Internals of Debugger:: Subroutines of the debugger, and global variables. | |
67 | @end menu | |
68 | ||
69 | @node Error Debugging | |
70 | @subsection Entering the Debugger on an Error | |
71 | @cindex error debugging | |
72 | @cindex debugging errors | |
73 | ||
74 | The most important time to enter the debugger is when a Lisp error | |
75 | happens. This allows you to investigate the immediate causes of the | |
76 | error. | |
77 | ||
78 | However, entry to the debugger is not a normal consequence of an | |
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79 | error. Many commands frequently cause Lisp errors when invoked |
80 | inappropriately (such as @kbd{C-f} at the end of the buffer), and during | |
81 | ordinary editing it would be very inconvenient to enter the debugger | |
82 | each time this happens. So if you want errors to enter the debugger, set | |
83 | the variable @code{debug-on-error} to non-@code{nil}. (The command | |
84 | @code{toggle-debug-on-error} provides an easy way to do this.) | |
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85 | |
86 | @defopt debug-on-error | |
ae4a3857 | 87 | This variable determines whether the debugger is called when an error is |
73804d4b | 88 | signaled and not handled. If @code{debug-on-error} is @code{t}, all |
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89 | kinds of errors call the debugger (except those listed in |
90 | @code{debug-ignored-errors}). If it is @code{nil}, none call the | |
91 | debugger. | |
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92 | |
93 | The value can also be a list of error conditions that should call the | |
94 | debugger. For example, if you set it to the list | |
95 | @code{(void-variable)}, then only errors about a variable that has no | |
96 | value invoke the debugger. | |
22697dac | 97 | |
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98 | When this variable is non-@code{nil}, Emacs does not create an error |
99 | handler around process filter functions and sentinels. Therefore, | |
100 | errors in these functions also invoke the debugger. @xref{Processes}. | |
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101 | @end defopt |
102 | ||
103 | @defopt debug-ignored-errors | |
104 | This variable specifies certain kinds of errors that should not enter | |
105 | the debugger. Its value is a list of error condition symbols and/or | |
106 | regular expressions. If the error has any of those condition symbols, | |
107 | or if the error message matches any of the regular expressions, then | |
108 | that error does not enter the debugger, regardless of the value of | |
109 | @code{debug-on-error}. | |
110 | ||
111 | The normal value of this variable lists several errors that happen often | |
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112 | during editing but rarely result from bugs in Lisp programs. However, |
113 | ``rarely'' is not ``never''; if your program fails with an error that | |
114 | matches this list, you will need to change this list in order to debug | |
115 | the error. The easiest way is usually to set | |
116 | @code{debug-ignored-errors} to @code{nil}. | |
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117 | @end defopt |
118 | ||
119 | @defopt debug-on-signal | |
120 | Normally, errors that are caught by @code{condition-case} never run the | |
121 | debugger, even if @code{debug-on-error} is non-@code{nil}. In other | |
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122 | words, @code{condition-case} gets a chance to handle the error before |
123 | the debugger gets a chance. | |
f9f59935 | 124 | |
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125 | If you set @code{debug-on-signal} to a non-@code{nil} value, then the |
126 | debugger gets the first chance at every error; an error will invoke the | |
127 | debugger regardless of any @code{condition-case}, if it fits the | |
1911e6e5 | 128 | criteria specified by the values of @code{debug-on-error} and |
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129 | @code{debug-ignored-errors}. |
130 | ||
969fe9b5 | 131 | @strong{Warning:} This variable is strong medicine! Various parts of |
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132 | Emacs handle errors in the normal course of affairs, and you may not |
133 | even realize that errors happen there. If you set | |
134 | @code{debug-on-signal} to a non-@code{nil} value, those errors will | |
135 | enter the debugger. | |
136 | ||
137 | @strong{Warning:} @code{debug-on-signal} has no effect when | |
138 | @code{debug-on-error} is @code{nil}. | |
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139 | @end defopt |
140 | ||
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141 | To debug an error that happens during loading of the init |
142 | file, use the option @samp{--debug-init}. This binds | |
05aea714 | 143 | @code{debug-on-error} to @code{t} while loading the init file, and |
a9f0a989 | 144 | bypasses the @code{condition-case} which normally catches errors in the |
1911e6e5 | 145 | init file. |
73804d4b | 146 | |
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147 | If your init file sets @code{debug-on-error}, the effect may |
148 | not last past the end of loading the init file. (This is an undesirable | |
a9f0a989 | 149 | byproduct of the code that implements the @samp{--debug-init} command |
a40d4712 | 150 | line option.) The best way to make the init file set |
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151 | @code{debug-on-error} permanently is with @code{after-init-hook}, like |
152 | this: | |
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153 | |
154 | @example | |
155 | (add-hook 'after-init-hook | |
00c804d5 | 156 | (lambda () (setq debug-on-error t))) |
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157 | @end example |
158 | ||
159 | @node Infinite Loops | |
160 | @subsection Debugging Infinite Loops | |
161 | @cindex infinite loops | |
162 | @cindex loops, infinite | |
163 | @cindex quitting from infinite loop | |
164 | @cindex stopping an infinite loop | |
165 | ||
166 | When a program loops infinitely and fails to return, your first | |
167 | problem is to stop the loop. On most operating systems, you can do this | |
b6ae404e | 168 | with @kbd{C-g}, which causes a @dfn{quit}. |
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169 | |
170 | Ordinary quitting gives no information about why the program was | |
171 | looping. To get more information, you can set the variable | |
172 | @code{debug-on-quit} to non-@code{nil}. Quitting with @kbd{C-g} is not | |
173 | considered an error, and @code{debug-on-error} has no effect on the | |
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174 | handling of @kbd{C-g}. Likewise, @code{debug-on-quit} has no effect on |
175 | errors. | |
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176 | |
177 | Once you have the debugger running in the middle of the infinite loop, | |
178 | you can proceed from the debugger using the stepping commands. If you | |
179 | step through the entire loop, you will probably get enough information | |
180 | to solve the problem. | |
181 | ||
182 | @defopt debug-on-quit | |
183 | This variable determines whether the debugger is called when @code{quit} | |
184 | is signaled and not handled. If @code{debug-on-quit} is non-@code{nil}, | |
185 | then the debugger is called whenever you quit (that is, type @kbd{C-g}). | |
186 | If @code{debug-on-quit} is @code{nil}, then the debugger is not called | |
187 | when you quit. @xref{Quitting}. | |
188 | @end defopt | |
189 | ||
190 | @node Function Debugging | |
191 | @subsection Entering the Debugger on a Function Call | |
192 | @cindex function call debugging | |
193 | @cindex debugging specific functions | |
194 | ||
195 | To investigate a problem that happens in the middle of a program, one | |
196 | useful technique is to enter the debugger whenever a certain function is | |
197 | called. You can do this to the function in which the problem occurs, | |
198 | and then step through the function, or you can do this to a function | |
199 | called shortly before the problem, step quickly over the call to that | |
200 | function, and then step through its caller. | |
201 | ||
202 | @deffn Command debug-on-entry function-name | |
969fe9b5 | 203 | This function requests @var{function-name} to invoke the debugger each time |
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204 | it is called. It works by inserting the form @code{(debug 'debug)} into |
205 | the function definition as the first form. | |
206 | ||
969fe9b5 | 207 | Any function defined as Lisp code may be set to break on entry, |
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208 | regardless of whether it is interpreted code or compiled code. If the |
209 | function is a command, it will enter the debugger when called from Lisp | |
210 | and when called interactively (after the reading of the arguments). You | |
211 | can't debug primitive functions (i.e., those written in C) this way. | |
212 | ||
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213 | When @code{debug-on-entry} is called interactively, it prompts for |
214 | @var{function-name} in the minibuffer. If the function is already set | |
215 | up to invoke the debugger on entry, @code{debug-on-entry} does nothing. | |
216 | @code{debug-on-entry} always returns @var{function-name}. | |
73804d4b | 217 | |
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218 | @strong{Note:} if you redefine a function after using |
219 | @code{debug-on-entry} on it, the code to enter the debugger is discarded | |
220 | by the redefinition. In effect, redefining the function cancels | |
221 | the break-on-entry feature for that function. | |
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222 | |
223 | @example | |
224 | @group | |
225 | (defun fact (n) | |
226 | (if (zerop n) 1 | |
227 | (* n (fact (1- n))))) | |
228 | @result{} fact | |
229 | @end group | |
230 | @group | |
231 | (debug-on-entry 'fact) | |
232 | @result{} fact | |
233 | @end group | |
234 | @group | |
235 | (fact 3) | |
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236 | @end group |
237 | ||
238 | @group | |
239 | ------ Buffer: *Backtrace* ------ | |
240 | Entering: | |
241 | * fact(3) | |
242 | eval-region(4870 4878 t) | |
243 | byte-code("...") | |
244 | eval-last-sexp(nil) | |
245 | (let ...) | |
246 | eval-insert-last-sexp(nil) | |
247 | * call-interactively(eval-insert-last-sexp) | |
248 | ------ Buffer: *Backtrace* ------ | |
249 | @end group | |
250 | ||
251 | @group | |
252 | (symbol-function 'fact) | |
253 | @result{} (lambda (n) | |
254 | (debug (quote debug)) | |
255 | (if (zerop n) 1 (* n (fact (1- n))))) | |
256 | @end group | |
257 | @end example | |
258 | @end deffn | |
259 | ||
260 | @deffn Command cancel-debug-on-entry function-name | |
261 | This function undoes the effect of @code{debug-on-entry} on | |
262 | @var{function-name}. When called interactively, it prompts for | |
bfe721d1 | 263 | @var{function-name} in the minibuffer. If @var{function-name} is |
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264 | @code{nil} or the empty string, it cancels break-on-entry for all |
265 | functions. | |
73804d4b | 266 | |
969fe9b5 RS |
267 | Calling @code{cancel-debug-on-entry} does nothing to a function which is |
268 | not currently set up to break on entry. It always returns | |
269 | @var{function-name}. | |
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270 | @end deffn |
271 | ||
272 | @node Explicit Debug | |
273 | @subsection Explicit Entry to the Debugger | |
274 | ||
275 | You can cause the debugger to be called at a certain point in your | |
276 | program by writing the expression @code{(debug)} at that point. To do | |
277 | this, visit the source file, insert the text @samp{(debug)} at the | |
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278 | proper place, and type @kbd{C-M-x}. @strong{Warning:} if you do this |
279 | for temporary debugging purposes, be sure to undo this insertion before | |
280 | you save the file! | |
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281 | |
282 | The place where you insert @samp{(debug)} must be a place where an | |
283 | additional form can be evaluated and its value ignored. (If the value | |
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284 | of @code{(debug)} isn't ignored, it will alter the execution of the |
285 | program!) The most common suitable places are inside a @code{progn} or | |
286 | an implicit @code{progn} (@pxref{Sequencing}). | |
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287 | |
288 | @node Using Debugger | |
289 | @subsection Using the Debugger | |
290 | ||
291 | When the debugger is entered, it displays the previously selected | |
292 | buffer in one window and a buffer named @samp{*Backtrace*} in another | |
293 | window. The backtrace buffer contains one line for each level of Lisp | |
294 | function execution currently going on. At the beginning of this buffer | |
295 | is a message describing the reason that the debugger was invoked (such | |
296 | as the error message and associated data, if it was invoked due to an | |
297 | error). | |
298 | ||
299 | The backtrace buffer is read-only and uses a special major mode, | |
300 | Debugger mode, in which letters are defined as debugger commands. The | |
301 | usual Emacs editing commands are available; thus, you can switch windows | |
302 | to examine the buffer that was being edited at the time of the error, | |
303 | switch buffers, visit files, or do any other sort of editing. However, | |
304 | the debugger is a recursive editing level (@pxref{Recursive Editing}) | |
305 | and it is wise to go back to the backtrace buffer and exit the debugger | |
306 | (with the @kbd{q} command) when you are finished with it. Exiting | |
307 | the debugger gets out of the recursive edit and kills the backtrace | |
308 | buffer. | |
309 | ||
310 | @cindex current stack frame | |
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311 | The backtrace buffer shows you the functions that are executing and |
312 | their argument values. It also allows you to specify a stack frame by | |
313 | moving point to the line describing that frame. (A stack frame is the | |
314 | place where the Lisp interpreter records information about a particular | |
315 | invocation of a function.) The frame whose line point is on is | |
316 | considered the @dfn{current frame}. Some of the debugger commands | |
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317 | operate on the current frame. |
318 | ||
319 | The debugger itself must be run byte-compiled, since it makes | |
320 | assumptions about how many stack frames are used for the debugger | |
321 | itself. These assumptions are false if the debugger is running | |
322 | interpreted. | |
323 | ||
324 | @need 3000 | |
325 | ||
326 | @node Debugger Commands | |
327 | @subsection Debugger Commands | |
328 | @cindex debugger command list | |
329 | ||
330 | Inside the debugger (in Debugger mode), these special commands are | |
331 | available in addition to the usual cursor motion commands. (Keep in | |
332 | mind that all the usual facilities of Emacs, such as switching windows | |
333 | or buffers, are still available.) | |
334 | ||
335 | The most important use of debugger commands is for stepping through | |
336 | code, so that you can see how control flows. The debugger can step | |
337 | through the control structures of an interpreted function, but cannot do | |
338 | so in a byte-compiled function. If you would like to step through a | |
339 | byte-compiled function, replace it with an interpreted definition of the | |
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340 | same function. (To do this, visit the source for the function and type |
341 | @kbd{C-M-x} on its definition.) | |
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342 | |
343 | Here is a list of Debugger mode commands: | |
344 | ||
345 | @table @kbd | |
346 | @item c | |
347 | Exit the debugger and continue execution. When continuing is possible, | |
348 | it resumes execution of the program as if the debugger had never been | |
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349 | entered (aside from any side-effects that you caused by changing |
350 | variable values or data structures while inside the debugger). | |
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351 | |
352 | Continuing is possible after entry to the debugger due to function entry | |
353 | or exit, explicit invocation, or quitting. You cannot continue if the | |
354 | debugger was entered because of an error. | |
355 | ||
356 | @item d | |
357 | Continue execution, but enter the debugger the next time any Lisp | |
358 | function is called. This allows you to step through the | |
359 | subexpressions of an expression, seeing what values the subexpressions | |
360 | compute, and what else they do. | |
361 | ||
362 | The stack frame made for the function call which enters the debugger in | |
363 | this way will be flagged automatically so that the debugger will be | |
364 | called again when the frame is exited. You can use the @kbd{u} command | |
365 | to cancel this flag. | |
366 | ||
367 | @item b | |
368 | Flag the current frame so that the debugger will be entered when the | |
369 | frame is exited. Frames flagged in this way are marked with stars | |
370 | in the backtrace buffer. | |
371 | ||
372 | @item u | |
373 | Don't enter the debugger when the current frame is exited. This | |
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374 | cancels a @kbd{b} command on that frame. The visible effect is to |
375 | remove the star from the line in the backtrace buffer. | |
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376 | |
377 | @item e | |
378 | Read a Lisp expression in the minibuffer, evaluate it, and print the | |
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379 | value in the echo area. The debugger alters certain important |
380 | variables, and the current buffer, as part of its operation; @kbd{e} | |
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381 | temporarily restores their values from outside the debugger, so you can |
382 | examine and change them. This makes the debugger more transparent. By | |
383 | contrast, @kbd{M-:} does nothing special in the debugger; it shows you | |
384 | the variable values within the debugger. | |
73804d4b | 385 | |
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386 | @item R |
387 | Like @kbd{e}, but also save the result of evaluation in the | |
388 | buffer @samp{*Debugger-record*}. | |
389 | ||
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390 | @item q |
391 | Terminate the program being debugged; return to top-level Emacs | |
392 | command execution. | |
393 | ||
394 | If the debugger was entered due to a @kbd{C-g} but you really want | |
395 | to quit, and not debug, use the @kbd{q} command. | |
396 | ||
397 | @item r | |
398 | Return a value from the debugger. The value is computed by reading an | |
399 | expression with the minibuffer and evaluating it. | |
400 | ||
ae4a3857 | 401 | The @kbd{r} command is useful when the debugger was invoked due to exit |
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402 | from a Lisp call frame (as requested with @kbd{b} or by entering the |
403 | frame with @kbd{d}); then the value specified in the @kbd{r} command is | |
404 | used as the value of that frame. It is also useful if you call | |
405 | @code{debug} and use its return value. Otherwise, @kbd{r} has the same | |
406 | effect as @kbd{c}, and the specified return value does not matter. | |
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407 | |
408 | You can't use @kbd{r} when the debugger was entered due to an error. | |
409 | @end table | |
410 | ||
411 | @node Invoking the Debugger | |
412 | @subsection Invoking the Debugger | |
413 | ||
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414 | Here we describe in full detail the function @code{debug} that is used |
415 | to invoke the debugger. | |
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416 | |
417 | @defun debug &rest debugger-args | |
418 | This function enters the debugger. It switches buffers to a buffer | |
419 | named @samp{*Backtrace*} (or @samp{*Backtrace*<2>} if it is the second | |
420 | recursive entry to the debugger, etc.), and fills it with information | |
421 | about the stack of Lisp function calls. It then enters a recursive | |
422 | edit, showing the backtrace buffer in Debugger mode. | |
423 | ||
424 | The Debugger mode @kbd{c} and @kbd{r} commands exit the recursive edit; | |
425 | then @code{debug} switches back to the previous buffer and returns to | |
426 | whatever called @code{debug}. This is the only way the function | |
427 | @code{debug} can return to its caller. | |
428 | ||
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429 | The use of the @var{debugger-args} is that @code{debug} displays the |
430 | rest of its arguments at the top of the @samp{*Backtrace*} buffer, so | |
431 | that the user can see them. Except as described below, this is the | |
432 | @emph{only} way these arguments are used. | |
73804d4b | 433 | |
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434 | However, certain values for first argument to @code{debug} have a |
435 | special significance. (Normally, these values are used only by the | |
436 | internals of Emacs, and not by programmers calling @code{debug}.) Here | |
437 | is a table of these special values: | |
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438 | |
439 | @table @code | |
440 | @item lambda | |
441 | @cindex @code{lambda} in debug | |
442 | A first argument of @code{lambda} means @code{debug} was called because | |
443 | of entry to a function when @code{debug-on-next-call} was | |
444 | non-@code{nil}. The debugger displays @samp{Entering:} as a line of | |
445 | text at the top of the buffer. | |
446 | ||
447 | @item debug | |
448 | @code{debug} as first argument indicates a call to @code{debug} because | |
449 | of entry to a function that was set to debug on entry. The debugger | |
450 | displays @samp{Entering:}, just as in the @code{lambda} case. It also | |
451 | marks the stack frame for that function so that it will invoke the | |
452 | debugger when exited. | |
453 | ||
454 | @item t | |
455 | When the first argument is @code{t}, this indicates a call to | |
456 | @code{debug} due to evaluation of a list form when | |
457 | @code{debug-on-next-call} is non-@code{nil}. The debugger displays the | |
458 | following as the top line in the buffer: | |
459 | ||
460 | @smallexample | |
461 | Beginning evaluation of function call form: | |
462 | @end smallexample | |
463 | ||
464 | @item exit | |
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465 | When the first argument is @code{exit}, it indicates the exit of a stack |
466 | frame previously marked to invoke the debugger on exit. The second | |
467 | argument given to @code{debug} in this case is the value being returned | |
468 | from the frame. The debugger displays @samp{Return value:} in the top | |
469 | line of the buffer, followed by the value being returned. | |
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470 | |
471 | @item error | |
472 | @cindex @code{error} in debug | |
473 | When the first argument is @code{error}, the debugger indicates that | |
474 | it is being entered because an error or @code{quit} was signaled and not | |
475 | handled, by displaying @samp{Signaling:} followed by the error signaled | |
476 | and any arguments to @code{signal}. For example, | |
477 | ||
478 | @example | |
479 | @group | |
480 | (let ((debug-on-error t)) | |
481 | (/ 1 0)) | |
482 | @end group | |
483 | ||
484 | @group | |
485 | ------ Buffer: *Backtrace* ------ | |
486 | Signaling: (arith-error) | |
487 | /(1 0) | |
488 | ... | |
489 | ------ Buffer: *Backtrace* ------ | |
490 | @end group | |
491 | @end example | |
492 | ||
493 | If an error was signaled, presumably the variable | |
494 | @code{debug-on-error} is non-@code{nil}. If @code{quit} was signaled, | |
495 | then presumably the variable @code{debug-on-quit} is non-@code{nil}. | |
496 | ||
497 | @item nil | |
498 | Use @code{nil} as the first of the @var{debugger-args} when you want | |
499 | to enter the debugger explicitly. The rest of the @var{debugger-args} | |
500 | are printed on the top line of the buffer. You can use this feature to | |
501 | display messages---for example, to remind yourself of the conditions | |
502 | under which @code{debug} is called. | |
503 | @end table | |
504 | @end defun | |
505 | ||
73804d4b RS |
506 | @node Internals of Debugger |
507 | @subsection Internals of the Debugger | |
508 | ||
509 | This section describes functions and variables used internally by the | |
510 | debugger. | |
511 | ||
512 | @defvar debugger | |
513 | The value of this variable is the function to call to invoke the | |
8241495d RS |
514 | debugger. Its value must be a function of any number of arguments, or, |
515 | more typically, the name of a function. This function should invoke | |
516 | some kind of debugger. The default value of the variable is | |
73804d4b RS |
517 | @code{debug}. |
518 | ||
519 | The first argument that Lisp hands to the function indicates why it | |
520 | was called. The convention for arguments is detailed in the description | |
521 | of @code{debug}. | |
522 | @end defvar | |
523 | ||
524 | @deffn Command backtrace | |
525 | @cindex run time stack | |
526 | @cindex call stack | |
527 | This function prints a trace of Lisp function calls currently active. | |
528 | This is the function used by @code{debug} to fill up the | |
529 | @samp{*Backtrace*} buffer. It is written in C, since it must have access | |
530 | to the stack to determine which function calls are active. The return | |
531 | value is always @code{nil}. | |
532 | ||
533 | In the following example, a Lisp expression calls @code{backtrace} | |
534 | explicitly. This prints the backtrace to the stream | |
8241495d RS |
535 | @code{standard-output}, which, in this case, is the buffer |
536 | @samp{backtrace-output}. | |
537 | ||
538 | Each line of the backtrace represents one function call. The line shows | |
539 | the values of the function's arguments if they are all known; if they | |
540 | are still being computed, the line says so. The arguments of special | |
541 | forms are elided. | |
73804d4b RS |
542 | |
543 | @smallexample | |
544 | @group | |
545 | (with-output-to-temp-buffer "backtrace-output" | |
546 | (let ((var 1)) | |
547 | (save-excursion | |
548 | (setq var (eval '(progn | |
549 | (1+ var) | |
550 | (list 'testing (backtrace)))))))) | |
551 | ||
552 | @result{} nil | |
553 | @end group | |
554 | ||
555 | @group | |
556 | ----------- Buffer: backtrace-output ------------ | |
557 | backtrace() | |
558 | (list ...computing arguments...) | |
7dd3d99f | 559 | @end group |
73804d4b RS |
560 | (progn ...) |
561 | eval((progn (1+ var) (list (quote testing) (backtrace)))) | |
562 | (setq ...) | |
563 | (save-excursion ...) | |
564 | (let ...) | |
565 | (with-output-to-temp-buffer ...) | |
566 | eval-region(1973 2142 #<buffer *scratch*>) | |
567 | byte-code("... for eval-print-last-sexp ...") | |
7dd3d99f | 568 | @group |
73804d4b RS |
569 | eval-print-last-sexp(nil) |
570 | * call-interactively(eval-print-last-sexp) | |
571 | ----------- Buffer: backtrace-output ------------ | |
572 | @end group | |
573 | @end smallexample | |
574 | ||
575 | The character @samp{*} indicates a frame whose debug-on-exit flag is | |
576 | set. | |
577 | @end deffn | |
578 | ||
579 | @ignore @c Not worth mentioning | |
580 | @defopt stack-trace-on-error | |
581 | @cindex stack trace | |
582 | This variable controls whether Lisp automatically displays a | |
583 | backtrace buffer after every error that is not handled. A quit signal | |
584 | counts as an error for this variable. If it is non-@code{nil} then a | |
585 | backtrace is shown in a pop-up buffer named @samp{*Backtrace*} on every | |
586 | error. If it is @code{nil}, then a backtrace is not shown. | |
587 | ||
588 | When a backtrace is shown, that buffer is not selected. If either | |
589 | @code{debug-on-quit} or @code{debug-on-error} is also non-@code{nil}, then | |
590 | a backtrace is shown in one buffer, and the debugger is popped up in | |
591 | another buffer with its own backtrace. | |
592 | ||
593 | We consider this feature to be obsolete and superseded by the debugger | |
594 | itself. | |
595 | @end defopt | |
596 | @end ignore | |
597 | ||
598 | @defvar debug-on-next-call | |
599 | @cindex @code{eval}, and debugging | |
600 | @cindex @code{apply}, and debugging | |
601 | @cindex @code{funcall}, and debugging | |
602 | If this variable is non-@code{nil}, it says to call the debugger before | |
603 | the next @code{eval}, @code{apply} or @code{funcall}. Entering the | |
604 | debugger sets @code{debug-on-next-call} to @code{nil}. | |
605 | ||
606 | The @kbd{d} command in the debugger works by setting this variable. | |
607 | @end defvar | |
608 | ||
609 | @defun backtrace-debug level flag | |
610 | This function sets the debug-on-exit flag of the stack frame @var{level} | |
ae4a3857 | 611 | levels down the stack, giving it the value @var{flag}. If @var{flag} is |
73804d4b RS |
612 | non-@code{nil}, this will cause the debugger to be entered when that |
613 | frame later exits. Even a nonlocal exit through that frame will enter | |
614 | the debugger. | |
615 | ||
ae4a3857 | 616 | This function is used only by the debugger. |
73804d4b RS |
617 | @end defun |
618 | ||
619 | @defvar command-debug-status | |
bfe721d1 | 620 | This variable records the debugging status of the current interactive |
73804d4b RS |
621 | command. Each time a command is called interactively, this variable is |
622 | bound to @code{nil}. The debugger can set this variable to leave | |
f9f59935 RS |
623 | information for future debugger invocations during the same command |
624 | invocation. | |
73804d4b | 625 | |
8241495d RS |
626 | The advantage of using this variable rather than an ordinary global |
627 | variable is that the data will never carry over to a subsequent command | |
628 | invocation. | |
73804d4b RS |
629 | @end defvar |
630 | ||
631 | @defun backtrace-frame frame-number | |
632 | The function @code{backtrace-frame} is intended for use in Lisp | |
633 | debuggers. It returns information about what computation is happening | |
634 | in the stack frame @var{frame-number} levels down. | |
635 | ||
8241495d RS |
636 | If that frame has not evaluated the arguments yet, or is a special |
637 | form, the value is @code{(nil @var{function} @var{arg-forms}@dots{})}. | |
73804d4b RS |
638 | |
639 | If that frame has evaluated its arguments and called its function | |
8241495d | 640 | already, the return value is @code{(t @var{function} |
73804d4b RS |
641 | @var{arg-values}@dots{})}. |
642 | ||
ae4a3857 RS |
643 | In the return value, @var{function} is whatever was supplied as the |
644 | @sc{car} of the evaluated list, or a @code{lambda} expression in the | |
645 | case of a macro call. If the function has a @code{&rest} argument, that | |
646 | is represented as the tail of the list @var{arg-values}. | |
73804d4b | 647 | |
ae4a3857 | 648 | If @var{frame-number} is out of range, @code{backtrace-frame} returns |
73804d4b RS |
649 | @code{nil}. |
650 | @end defun | |
651 | ||
a9f0a989 RS |
652 | @include edebug.texi |
653 | ||
73804d4b RS |
654 | @node Syntax Errors |
655 | @section Debugging Invalid Lisp Syntax | |
656 | ||
657 | The Lisp reader reports invalid syntax, but cannot say where the real | |
658 | problem is. For example, the error ``End of file during parsing'' in | |
659 | evaluating an expression indicates an excess of open parentheses (or | |
660 | square brackets). The reader detects this imbalance at the end of the | |
661 | file, but it cannot figure out where the close parenthesis should have | |
662 | been. Likewise, ``Invalid read syntax: ")"'' indicates an excess close | |
663 | parenthesis or missing open parenthesis, but does not say where the | |
664 | missing parenthesis belongs. How, then, to find what to change? | |
665 | ||
666 | If the problem is not simply an imbalance of parentheses, a useful | |
667 | technique is to try @kbd{C-M-e} at the beginning of each defun, and see | |
668 | if it goes to the place where that defun appears to end. If it does | |
669 | not, there is a problem in that defun. | |
670 | ||
671 | However, unmatched parentheses are the most common syntax errors in | |
a9f0a989 RS |
672 | Lisp, and we can give further advice for those cases. (In addition, |
673 | just moving point through the code with Show Paren mode enabled might | |
674 | find the mismatch.) | |
73804d4b RS |
675 | |
676 | @menu | |
677 | * Excess Open:: How to find a spurious open paren or missing close. | |
678 | * Excess Close:: How to find a spurious close paren or missing open. | |
679 | @end menu | |
680 | ||
681 | @node Excess Open | |
682 | @subsection Excess Open Parentheses | |
683 | ||
684 | The first step is to find the defun that is unbalanced. If there is | |
8241495d RS |
685 | an excess open parenthesis, the way to do this is to go to the end of |
686 | the file and type @kbd{C-u C-M-u}. This will move you to the beginning | |
687 | of the defun that is unbalanced. | |
73804d4b RS |
688 | |
689 | The next step is to determine precisely what is wrong. There is no | |
f9f59935 | 690 | way to be sure of this except by studying the program, but often the |
73804d4b RS |
691 | existing indentation is a clue to where the parentheses should have |
692 | been. The easiest way to use this clue is to reindent with @kbd{C-M-q} | |
969fe9b5 RS |
693 | and see what moves. @strong{But don't do this yet!} Keep reading, |
694 | first. | |
73804d4b RS |
695 | |
696 | Before you do this, make sure the defun has enough close parentheses. | |
697 | Otherwise, @kbd{C-M-q} will get an error, or will reindent all the rest | |
698 | of the file until the end. So move to the end of the defun and insert a | |
699 | close parenthesis there. Don't use @kbd{C-M-e} to move there, since | |
700 | that too will fail to work until the defun is balanced. | |
701 | ||
702 | Now you can go to the beginning of the defun and type @kbd{C-M-q}. | |
703 | Usually all the lines from a certain point to the end of the function | |
704 | will shift to the right. There is probably a missing close parenthesis, | |
705 | or a superfluous open parenthesis, near that point. (However, don't | |
706 | assume this is true; study the code to make sure.) Once you have found | |
ae4a3857 RS |
707 | the discrepancy, undo the @kbd{C-M-q} with @kbd{C-_}, since the old |
708 | indentation is probably appropriate to the intended parentheses. | |
73804d4b RS |
709 | |
710 | After you think you have fixed the problem, use @kbd{C-M-q} again. If | |
711 | the old indentation actually fit the intended nesting of parentheses, | |
712 | and you have put back those parentheses, @kbd{C-M-q} should not change | |
713 | anything. | |
714 | ||
715 | @node Excess Close | |
716 | @subsection Excess Close Parentheses | |
717 | ||
8241495d RS |
718 | To deal with an excess close parenthesis, first go to the beginning of |
719 | the file, then type @kbd{C-u -1 C-M-u} to find the end of the unbalanced | |
720 | defun. | |
73804d4b RS |
721 | |
722 | Then find the actual matching close parenthesis by typing @kbd{C-M-f} | |
969fe9b5 | 723 | at the beginning of that defun. This will leave you somewhere short of |
73804d4b RS |
724 | the place where the defun ought to end. It is possible that you will |
725 | find a spurious close parenthesis in that vicinity. | |
726 | ||
727 | If you don't see a problem at that point, the next thing to do is to | |
728 | type @kbd{C-M-q} at the beginning of the defun. A range of lines will | |
729 | probably shift left; if so, the missing open parenthesis or spurious | |
730 | close parenthesis is probably near the first of those lines. (However, | |
731 | don't assume this is true; study the code to make sure.) Once you have | |
ae4a3857 RS |
732 | found the discrepancy, undo the @kbd{C-M-q} with @kbd{C-_}, since the |
733 | old indentation is probably appropriate to the intended parentheses. | |
734 | ||
735 | After you think you have fixed the problem, use @kbd{C-M-q} again. If | |
2bdedac1 | 736 | the old indentation actually fits the intended nesting of parentheses, |
ae4a3857 RS |
737 | and you have put back those parentheses, @kbd{C-M-q} should not change |
738 | anything. | |
73804d4b | 739 | |
1911e6e5 | 740 | @node Compilation Errors |
73804d4b RS |
741 | @section Debugging Problems in Compilation |
742 | ||
743 | When an error happens during byte compilation, it is normally due to | |
744 | invalid syntax in the program you are compiling. The compiler prints a | |
745 | suitable error message in the @samp{*Compile-Log*} buffer, and then | |
746 | stops. The message may state a function name in which the error was | |
747 | found, or it may not. Either way, here is how to find out where in the | |
748 | file the error occurred. | |
749 | ||
750 | What you should do is switch to the buffer @w{@samp{ *Compiler Input*}}. | |
751 | (Note that the buffer name starts with a space, so it does not show | |
752 | up in @kbd{M-x list-buffers}.) This buffer contains the program being | |
753 | compiled, and point shows how far the byte compiler was able to read. | |
754 | ||
755 | If the error was due to invalid Lisp syntax, point shows exactly where | |
756 | the invalid syntax was @emph{detected}. The cause of the error is not | |
757 | necessarily near by! Use the techniques in the previous section to find | |
758 | the error. | |
759 | ||
760 | If the error was detected while compiling a form that had been read | |
761 | successfully, then point is located at the end of the form. In this | |
ae4a3857 RS |
762 | case, this technique can't localize the error precisely, but can still |
763 | show you which function to check. |