Source

XEmacs / man / xemacs / building.texi

Full commit
  1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613

@node Running, Packages, Programs, Top
@chapter Compiling and Testing Programs

  The previous chapter discusses the Emacs commands that are useful for
making changes in programs.  This chapter deals with commands that assist
in the larger process of developing and maintaining programs.

@menu
* Compilation::        Compiling programs in languages other than Lisp
                        (C, Pascal, etc.)
* Modes: Lisp Modes.   Various modes for editing Lisp programs, with
                       different facilities for running the Lisp programs.
* Libraries: Lisp Libraries.      Creating Lisp programs to run in Emacs.
* Eval: Lisp Eval.     Executing a single Lisp expression in Emacs.
* Debug: Lisp Debug.   Debugging Lisp programs running in Emacs.
* Interaction: Lisp Interaction.  Executing Lisp in an Emacs buffer.
* External Lisp::      Communicating through Emacs with a separate Lisp.
@end menu

@node Compilation, Lisp Modes, Running, Running
@section Running ``make'', or Compilers Generally
@cindex inferior process
@cindex make
@cindex compilation errors
@cindex error log

  Emacs can run compilers for non-interactive languages like C and
Fortran as inferior processes, feeding the error log into an Emacs buffer.
It can also parse the error messages and visit the files in which errors
are found, moving point to the line where the error occurred.

@table @kbd
@item M-x compile
Run a compiler asynchronously under Emacs, with error messages to
@samp{*compilation*} buffer.
@item M-x grep
Run @code{grep} asynchronously under Emacs, with matching lines
listed in the buffer named @samp{*compilation*}.
@item M-x kill-compilation
Kill the process made by the @code{M-x compile} command.
@item M-x kill-grep
Kill the running compilation or @code{grep} subprocess.
@item C-x `
Visit the next compiler error message or @code{grep} match.
@end table

@findex compile
  To run @code{make} or another compiler, type @kbd{M-x compile}.  This
command reads a shell command line using the minibuffer, then executes
the specified command line in an inferior shell with output going to the
buffer named @samp{*compilation*}.  By default, the current buffer's
default directory is used as the working directory for the execution of
the command; therefore, the makefile comes from this directory.

@vindex compile-command
  When the shell command line is read, the minibuffer appears containing a
default command line (the command you used the last time you typed
@kbd{M-x compile}).  If you type just @key{RET}, the same command line is used
again.  The first @kbd{M-x compile} provides @code{make -k} as the default.
The default is taken from the variable @code{compile-command}; if the
appropriate compilation command for a file is something other than
@code{make -k}, it can be useful to have the file specify a local value for
@code{compile-command} (@pxref{File Variables}).

@cindex compiling files
  When you start a compilation, the buffer @samp{*compilation*} is
displayed in another window but not selected.  Its mode line displays
the word @samp{run} or @samp{exit} in the parentheses to tell you whether
compilation is finished.  You do not have to keep this buffer visible;
compilation continues in any case.

@findex kill-compilation
  To kill the compilation process, type @kbd{M-x-kill-compilation}.  The mode
line of the @samp{*compilation*} buffer changes to say @samp{signal}
instead of @samp{run}.  Starting a new compilation also kills any
running compilation, as only one can occur at any time.  Starting a new
compilation prompts for confirmation before actually killing a
compilation that is running.@refill

@kindex C-x `
@findex next-error
  To parse the compiler error messages, type @kbd{C-x `}
(@code{next-error}).  The character following @kbd{C-x} is the grave
accent, not the single quote.  The command displays the buffer
@samp{*compilation*} in one window and the buffer in which the next
error occurred in another window.  Point in that buffer is moved to the
line where the error was found.  The corresponding error message is
scrolled to the top of the window in which @samp{*compilation*} is
displayed.

  The first time you use @kbd{C-x `} after the start of a compilation, it
parses all the error messages, visits all the files that have error
messages, and creates markers pointing at the lines the error messages
refer to.  It then moves to the first error message location.  Subsequent
uses of @kbd{C-x `} advance down the data set up by the first use.  When
the preparsed error messages are exhausted, the next @kbd{C-x `} checks for
any more error messages that have come in; this is useful if you start
editing compiler errors while compilation is still going on.  If no
additional error messages have come in, @kbd{C-x `} reports an error.

  @kbd{C-u C-x `} discards the preparsed error message data and parses the
@samp{*compilation*} buffer again, then displays the first error.
This way, you can process the same set of errors again.

  Instead of running a compiler, you can run @code{grep} and see the
lines on which matches were found.  To do this, type @kbd{M-x grep} with
an argument line that contains the same arguments you would give to
@code{grep}: a @code{grep}-style regexp (usually in single quotes to
quote the shell's special characters) followed by filenames, which may
use wildcard characters.  The output from @code{grep} goes in the
@samp{*compilation*} buffer.  You can use @kbd{C-x `} to find the lines that
match as if they were compilation errors.

  Note: a shell is used to run the compile command, but the shell is not
run in interactive mode.  In particular, this means that the shell starts
up with no prompt.  If you find your usual shell prompt making an
unsightly appearance in the @samp{*compilation*} buffer, it means you
have made a mistake in your shell's initialization file (@file{.cshrc}
or @file{.shrc} or @dots{}) by setting the prompt unconditionally.  The
shell initialization file should set the prompt only if there already is
a prompt.  Here's how to do it in @code{csh}:

@example
if ($?prompt) set prompt = ...
@end example

@node Lisp Modes, Lisp Libraries, Compilation, Running
@section Major Modes for Lisp

  Emacs has four different major modes for Lisp.  They are the same in
terms of editing commands, but differ in the commands for executing Lisp
expressions.

@table @asis
@item Emacs-Lisp mode
The mode for editing source files of programs to run in Emacs Lisp.
This mode defines @kbd{C-M-x} to evaluate the current defun.
@xref{Lisp Libraries}.
@item Lisp Interaction mode
The mode for an interactive session with Emacs Lisp.  It defines
@key{LFD} to evaluate the sexp before point and insert its value in the
buffer.  @xref{Lisp Interaction}.
@item Lisp mode
The mode for editing source files of programs that run in other dialects
of Lisp than Emacs Lisp.  This mode defines @kbd{C-M-x} to send the
current defun to an inferior Lisp process.  @xref{External Lisp}.
@item Inferior Lisp mode
The mode for an interactive session with an inferior Lisp process.
This mode combines the special features of Lisp mode and Shell mode
(@pxref{Shell Mode}).
@item Scheme mode
Like Lisp mode but for Scheme programs.
@item Inferior Scheme mode
The mode for an interactive session with an inferior Scheme process.
@end table

@node Lisp Libraries, Lisp Eval, Lisp Modes, Running
@section Libraries of Lisp Code for Emacs
@cindex libraries
@cindex loading Lisp code

  Lisp code for Emacs editing commands is stored in files whose names
conventionally end in @file{.el}.  This ending tells Emacs to edit them in
Emacs-Lisp mode (@pxref{Lisp Modes}).

@menu
* Loading::		Loading libraries of Lisp code into Emacs for use.
* Compiling Libraries:: Compiling a library makes it load and run faster.
* Mocklisp::		Converting Mocklisp to Lisp so XEmacs can run it.
@end menu

@node Loading, Compiling Libraries, Lisp Libraries, Lisp Libraries
@subsection Loading Libraries

@table @kbd
@item M-x load-file @var{file}
Load the file @var{file} of Lisp code.
@item M-x load-library @var{library}
Load the library named @var{library}.
@item M-x locate-library @var{library} &optional @var{nosuffix}
Show the full path name of Emacs library @var{library}.
@end table

@findex load-file
  To execute a file of Emacs Lisp, use @kbd{M-x load-file}.  This
command reads the file name you provide in the minibuffer, then executes
the contents of that file as Lisp code.  It is not necessary to visit
the file first; in fact, this command reads the file as found on
disk, not the text in an Emacs buffer.

@findex load
@findex load-library
  Once a file of Lisp code is installed in the Emacs Lisp library
directories, users can load it using @kbd{M-x load-library}.  Programs can
load it by calling @code{load-library}, or with @code{load}, a more primitive
function that is similar but accepts some additional arguments.

  @kbd{M-x load-library} differs from @kbd{M-x load-file} in that it
searches a sequence of directories and tries three file names in each
directory.  The three names are: first, the specified name with @file{.elc}
appended; second, the name with @file{.el} appended; third, the specified
name alone.  A @file{.elc} file would be the result of compiling the Lisp
file into byte code;  if possible, it is loaded in preference to the Lisp
file itself because the compiled file loads and runs faster.

@cindex loading libraries
  Because the argument to @code{load-library} is usually not in itself
a valid file name, file name completion is not available.  In fact, when
using this command, you usually do not know exactly what file name
will be used.

@vindex load-path
  The sequence of directories searched by @kbd{M-x load-library} is
specified by the variable @code{load-path}, a list of strings that are
directory names.  The elements of this list may not begin with "@samp{~}",
so you must call @code{expand-file-name} on them before adding them to
the list.  The default value of the list contains the directory where
the Lisp code for Emacs itself is stored.  If you have libraries of your
own, put them in a single directory and add that directory to
@code{load-path}.  @code{nil} in this list stands for the current
default directory, but it is probably not a good idea to put @code{nil}
in the list.  If you start wishing that @code{nil} were in the list, you
should probably use @kbd{M-x load-file} for this case.

The variable is initialized by the @b{EMACSLOADPATH} environment
variable. If no value is specified, the variable takes the default value
specified in the file @file{paths.h} when Emacs was built. If a path
isn't specified in @file{paths.h}, a default value is obtained from the
file system, near the directory in which the Emacs executable resides.

@findex locate-library
 Like @kbd{M-x load-library}, @kbd{M-x locate-library} searches the 
directories in @code{load-path} to find the file that @kbd{M-x load-library}
would load.  If the optional second argument @var{nosuffix} is
non-@code{nil}, the suffixes @file{.elc} or @file{.el} are not added to
the specified name @var{library} (like calling @code{load} instead of
@code{load-library}).

@cindex autoload
   You often do not have to give any command to load a library, because the
commands defined in the library are set up to @dfn{autoload} that library.
Running any of those commands causes @code{load} to be called to load the
library; this replaces the autoload definitions with the real ones from the
library.

  If autoloading a file does not finish, either because of an error or
because of a @kbd{C-g} quit, all function definitions made by the file
are undone automatically.  So are any calls to @code{provide}.  As a
consequence, the entire file is loaded a second time if you use one of
the autoloadable commands again.  This prevents problems when the
command is no longer autoloading but is working incorrectly because the file
was only partially loaded.  Function definitions are undone only for
autoloading; explicit calls to @code{load} do not undo anything if
loading is not completed.

@vindex after-load-alist
The variable @code{after-load-alist} takes an alist of expressions to be
evaluated when particular files are loaded.  Each element has the form
@code{(@var{filename} forms...)}.  When @code{load} is run and the filename
argument is @var{filename}, the forms in the corresponding element are
executed at the end of loading.

@var{filename} must match exactly.  Normally @var{filename} is the
name of a library, with no directory specified, since that is how load
is normally called.  An error in @code{forms} does not undo the load, but
it does prevent execution of the rest of the @code{forms}.

@node Compiling Libraries, Mocklisp, Loading, Lisp Libraries
@subsection Compiling Libraries

@cindex byte code
  Emacs Lisp code can be compiled into byte-code which loads faster,
takes up less space when loaded, and executes faster.

@table @kbd
@item M-x batch-byte-compile
Run byte-compile-file on the files remaining on the command line.
@item M-x byte-compile-buffer &optional @var{buffer}
Byte-compile and evaluate contents of @var{buffer} (default is current 
buffer).
@item M-x byte-compile-file
Compile a file of Lisp code named @var{filename} into a file of byte code.
@item M-x byte-compile-and-load-file @var{filename}
Compile a file of Lisp code named @var{filename} into a file of byte
code and load it.
@item M-x byte-recompile-directory @var{directory}
Recompile every @file{.el} file in @var{directory} that needs recompilation.
@item M-x disassemble
Print disassembled code for @var{object} on (optional) @var{stream}.
@findex make-obsolete
@item M-x make-obsolete @var{function new} 
Make the byte-compiler warn that @var{function} is obsolete and @var{new} 
should be used instead.
@end table

@findex byte-compile-file
@findex byte-compile-and-load-file
@findex byte-compile-buffer
 @kbd{byte-compile-file} creates a byte-code compiled file from an
Emacs-Lisp source file.  The default argument for this function is the
file visited in the current buffer.  The function reads the specified
file, compiles it into byte code, and writes an output file whose name
is made by appending @file{c} to the input file name.  Thus, the file
@file{rmail.el} would be compiled into @file{rmail.elc}. To compile a
file of Lisp code named @var{filename} into a file of byte code and
then load it, use @code{byte-compile-and-load-file}. To compile and
evaluate Lisp code in a given buffer, use @code{byte-compile-buffer}.

@findex byte-recompile-directory
  To recompile all changed Lisp files in a directory, use @kbd{M-x
byte-recompile-directory}.  Specify just the directory name as an argument.
Each @file{.el} file that has been byte-compiled before is byte-compiled
again if it has changed since the previous compilation.  A numeric argument
to this command tells it to offer to compile each @file{.el} file that has
not been compiled yet.  You must answer @kbd{y} or @kbd{n} to each
offer.

@findex batch-byte-compile
  You can use the function @code{batch-byte-compile} to invoke Emacs
non-interactively from the shell to do byte compilation.  When you use
this function, the files to be compiled are specified with command-line
arguments.  Use a shell command of the form:

@example
emacs -batch -f batch-byte-compile @var{files}...
@end example

  Directory names may also be given as arguments; in that case,
@code{byte-recompile-directory} is invoked on each such directory.
@code{batch-byte-compile} uses all remaining command-line arguments as
file or directory names, then kills the Emacs process.

@findex disassemble
  @kbd{M-x disassemble} explains the result of byte compilation.  Its
argument is a function name.  It displays the byte-compiled code in a help
window in symbolic form, one instruction per line.  If the instruction
refers to a variable or constant, that is shown, too.

@node Mocklisp,,Compiling Libraries,Lisp Libraries
@subsection Converting Mocklisp to Lisp

@cindex mocklisp
@findex convert-mocklisp-buffer
  XEmacs can run Mocklisp files by converting them to Emacs Lisp first.
To convert a Mocklisp file, visit it and then type @kbd{M-x
convert-mocklisp-buffer}.  Then save the resulting buffer of Lisp file in a
file whose name ends in @file{.el} and use the new file as a Lisp library.

  You cannot currently byte-compile converted Mocklisp code.
The reason is that converted Mocklisp code uses some special Lisp features
to deal with Mocklisp's incompatible ideas of how arguments are evaluated
and which values signify ``true'' or ``false''.

@node Lisp Eval, Lisp Debug, Lisp Libraries, Running
@section Evaluating Emacs-Lisp Expressions
@cindex Emacs-Lisp mode

@findex emacs-lisp-mode
  Lisp programs intended to be run in Emacs should be edited in
Emacs-Lisp mode; this will happen automatically for file names ending in
@file{.el}.  By contrast, Lisp mode itself should be used for editing
Lisp programs intended for other Lisp systems.  Emacs-Lisp mode can be
selected with the command @kbd{M-x emacs-lisp-mode}.

  For testing of Lisp programs to run in Emacs, it is useful to be able
to evaluate part of the program as it is found in the Emacs buffer.  For
example, if you change the text of a Lisp function definition and then
evaluate the definition, Emacs installs the change for future calls to the
function.  Evaluation of Lisp expressions is also useful in any kind of
editing task for invoking non-interactive functions (functions that are
not commands).

@table @kbd
@item M-@key{ESC}
Read a Lisp expression in the minibuffer, evaluate it, and print the
value in the minibuffer (@code{eval-expression}).
@item C-x C-e
Evaluate the Lisp expression before point, and print the value in the
minibuffer (@code{eval-last-sexp}).
@item C-M-x
Evaluate the defun containing point or after point, and print the value in
the minibuffer (@code{eval-defun}).
@item M-x eval-region
Evaluate all the Lisp expressions in the region.
@item M-x eval-current-buffer
Evaluate all the Lisp expressions in the buffer.
@end table

@kindex M-ESC
@findex eval-expression
  @kbd{M-@key{ESC}} (@code{eval-expression}) is the most basic command
for evaluating a Lisp expression interactively.  It reads the expression
using the minibuffer, so you can execute any expression on a buffer
regardless of what the buffer contains.  When evaluation is complete,
the current buffer is once again the buffer that was current when
@kbd{M-@key{ESC}} was typed.

  @kbd{M-@key{ESC}} can easily confuse users, especially on keyboards
with autorepeat, where it can result from holding down the @key{ESC} key
for too long.  Therefore, @code{eval-expression} is normally a disabled
command.  Attempting to use this command asks for confirmation and gives
you the option of enabling it; once you enable the command, you are no
longer required to confirm.  @xref{Disabling}.@refill

@kindex C-M-x
@findex eval-defun
  In Emacs-Lisp mode, the key @kbd{C-M-x} is bound to the function
@code{eval-defun}, which parses the defun containing point or following point
as a Lisp expression and evaluates it.  The value is printed in the echo
area.  This command is convenient for installing in the Lisp environment
changes that you have just made in the text of a function definition.

@kindex C-x C-e
@findex eval-last-sexp
  The command @kbd{C-x C-e} (@code{eval-last-sexp}) performs a similar job
but is available in all major modes, not just Emacs-Lisp mode.  It finds
the sexp before point, reads it as a Lisp expression, evaluates it, and
prints the value in the echo area.  It is sometimes useful to type in an
expression and then, with point still after it, type @kbd{C-x C-e}.

  If @kbd{C-M-x} or @kbd{C-x C-e} are given a numeric argument, they
print the value by inserting it into the current buffer at point, rather
than in the echo area.  The argument value does not matter.

@findex eval-region
@findex eval-current-buffer
  The most general command for evaluating Lisp expressions from a buffer
is @code{eval-region}.  @kbd{M-x eval-region} parses the text of the
region as one or more Lisp expressions, evaluating them one by one.
@kbd{M-x eval-current-buffer} is similar, but it evaluates the entire
buffer.  This is a reasonable way to install the contents of a file of
Lisp code that you are just ready to test.  After finding and fixing a
bug, use @kbd{C-M-x} on each function that you change, to keep the Lisp
world in step with the source file.

@node Lisp Debug, Lisp Interaction, Lisp Eval, Running
@section The Emacs-Lisp Debugger
@cindex debugger

@vindex debug-on-error
@vindex debug-on-quit
  XEmacs contains a debugger for Lisp programs executing inside it.
This debugger is normally not used; many commands frequently get Lisp
errors when invoked in inappropriate contexts (such as @kbd{C-f} at the
end of the buffer) and it would be unpleasant to enter a special
debugging mode in this case.  When you want to make Lisp errors invoke
the debugger, you must set the variable @code{debug-on-error} to
non-@code{nil}.  Quitting with @kbd{C-g} is not considered an error, and
@code{debug-on-error} has no effect on the handling of @kbd{C-g}.
However, if you set @code{debug-on-quit} to be non-@code{nil}, @kbd{C-g} will
invoke the debugger.  This can be useful for debugging an infinite loop;
type @kbd{C-g} once the loop has had time to reach its steady state.
@code{debug-on-quit} has no effect on errors.@refill

@findex debug-on-entry
@findex cancel-debug-on-entry
@findex debug
  You can make Emacs enter the debugger when a specified function
is called or at a particular place in Lisp code.  Use @kbd{M-x
debug-on-entry} with argument @var{fun-name} to have Emacs enter the
debugger as soon as @var{fun-name} is called. Use
@kbd{M-x cancel-debug-on-entry} to make the function stop entering the
debugger when called.  (Redefining the function also does this.)  To enter
the debugger from some other place in Lisp code, you must insert the
expression @code{(debug)} there and install the changed code with
@kbd{C-M-x}.  @xref{Lisp Eval}.@refill

  When the debugger is entered, it displays the previously selected buffer
in one window and a buffer named @samp{*Backtrace*} in another window.  The
backtrace buffer contains one line for each level of Lisp function
execution currently going on.  At the beginning of the buffer is a message
describing the reason that the debugger was invoked, for example, an
error message if it was invoked due to an error.

  The backtrace buffer is read-only and is in Backtrace mode, a special
major mode in which letters are defined as debugger commands.  The
usual Emacs editing commands are available; you can switch windows to
examine the buffer that was being edited at the time of the error, and
you can switch buffers, visit files, and perform any other editing
operations.  However, the debugger is a recursive editing level
(@pxref{Recursive Edit}); it is a good idea to return to the backtrace
buffer and explictly exit the debugger when you don't want to use it any
more.  Exiting the debugger kills the backtrace buffer.

@cindex current stack frame
  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.  The frame whose line point is on is considered the @dfn{current
frame}.  Some of the debugger commands operate on the current frame.
Debugger commands are mainly used for stepping through code one
expression at a time.  Here is a list of them:

@table @kbd
@item c
Exit the debugger and continue execution.  In most cases, execution of
the program continues 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).  This includes entry to the debugger due to
function entry or exit, explicit invocation, and quitting or certain
errors.  Most errors cannot be continued; trying to continue an error usually
causes the same error to occur again.
@item d
Continue execution, but enter the debugger the next time a Lisp
function is called.  This allows you to step through the
subexpressions of an expression, and see what the subexpressions do and
what values they compute.

When you enter the debugger this way, Emacs flags the stack frame for the
function call from which you entered.  The same function is then called
when you exit the frame.  To cancel this flag, use @kbd{u}.
@item b
Set up to enter the debugger when the current frame is exited.  Frames
that invoke the debugger on exit are flagged with stars.
@item u
Don't enter the debugger when the current frame is exited.  This
cancels a @kbd{b} command on a frame.
@item e
Read a Lisp expression in the minibuffer, evaluate it, and print the
value in the echo area.  This is equivalent to the command @kbd{M-@key{ESC}},
except that @kbd{e} is not normally disabled like @kbd{M-@key{ESC}}.
@item q
Terminate the program being debugged; return to top-level Emacs
command execution.

If the debugger was entered due to a @kbd{C-g} but you really want
to quit, not to debug, use the @kbd{q} command.
@item r
Return a value from the debugger.  The value is computed by reading an
expression with the minibuffer and evaluating it.

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

The debugger's return value also matters with many errors.  For example,
@code{wrong-type-argument} errors will use the debugger's return value
instead of the invalid argument; @code{no-catch} errors will use the
debugger value as a throw tag instead of the tag that was not found.
If an error was signaled by calling the Lisp function @code{signal},
the debugger's return value is returned as the value of @code{signal}.
@end table

@node Lisp Interaction, External Lisp, Lisp Debug, Running
@section Lisp Interaction Buffers

  The buffer @samp{*scratch*}, which is selected when Emacs starts up, is
provided for evaluating Lisp expressions interactively inside Emacs.  Both
the expressions you evaluate and their output goes in the buffer.

  The @samp{*scratch*} buffer's major mode is Lisp Interaction mode, which
is the same as Emacs-Lisp mode except for one command, @key{LFD}.  In
Emacs-Lisp mode, @key{LFD} is an indentation command.  In Lisp
Interaction mode, @key{LFD} is bound to @code{eval-print-last-sexp}.  This
function reads the Lisp expression before point, evaluates it, and inserts
the value in printed representation before point.

 The way to use the @samp{*scratch*} buffer is to insert Lisp
expressions at the end, ending each one with @key{LFD} so that it will
be evaluated.  The result is a complete typescript of the expressions
you have evaluated and their values.

@findex lisp-interaction-mode
  The rationale for this feature is that Emacs must have a buffer when it
starts up, but that buffer is not useful for editing files since a new
buffer is made for every file that you visit.  The Lisp interpreter
typescript is the most useful thing I can think of for the initial buffer
to do.  @kbd{M-x lisp-interaction-mode} will put any buffer in Lisp
Interaction mode.

@node External Lisp,, Lisp Interaction, Running
@section Running an External Lisp

  Emacs has facilities for running programs in other Lisp systems.  You can
run a Lisp process as an inferior of Emacs, and pass expressions to it to
be evaluated.  You can also pass changed function definitions directly from
the Emacs buffers in which you edit the Lisp programs to the inferior Lisp
process.

@findex run-lisp
  To run an inferior Lisp process, type @kbd{M-x run-lisp}.  This runs the
program named @code{lisp}, the same program you would run by typing
@code{lisp} as a shell command, with both input and output going through an
Emacs buffer named @samp{*lisp*}.  In other words, any ``terminal output''
from Lisp will go into the buffer, advancing point, and any ``terminal
input'' for Lisp comes from text in the buffer.  To give input to Lisp, go
to the end of the buffer and type the input, terminated by @key{RET}.  The
@samp{*lisp*} buffer is in Inferior Lisp mode, which has all the
special characteristics of Lisp mode and Shell mode (@pxref{Shell Mode}).

@findex lisp-mode
  Use Lisp mode to run the source files of programs in external Lisps.
You can select this mode with @kbd{M-x lisp-mode}.  It is used automatically
for files whose names end in @file{.l} or @file{.lisp}, as most Lisp
systems usually expect.

@kindex C-M-x
@findex lisp-send-defun
  When you edit a function in a Lisp program you are running, the easiest
way to send the changed definition to the inferior Lisp process is the key
@kbd{C-M-x}.  In Lisp mode, this key runs the function @code{lisp-send-defun},
which finds the defun around or following point and sends it as input to
the Lisp process.  (Emacs can send input to any inferior process regardless
of what buffer is current.)

  Contrast the meanings of @kbd{C-M-x} in Lisp mode (for editing programs
to be run in another Lisp system) and Emacs-Lisp mode (for editing Lisp
programs to be run in Emacs): in both modes it has the effect of installing
the function definition that point is in, but the way of doing so is
different according to where the relevant Lisp environment is found.
@xref{Lisp Modes}.