1 This is ../info/internals.info, produced by makeinfo version 4.0 from
2 internals/internals.texi.
4 INFO-DIR-SECTION XEmacs Editor
6 * Internals: (internals). XEmacs Internals Manual.
9 Copyright (C) 1992 - 1996 Ben Wing. Copyright (C) 1996, 1997 Sun
10 Microsystems. Copyright (C) 1994 - 1998 Free Software Foundation.
11 Copyright (C) 1994, 1995 Board of Trustees, University of Illinois.
13 Permission is granted to make and distribute verbatim copies of this
14 manual provided the copyright notice and this permission notice are
15 preserved on all copies.
17 Permission is granted to copy and distribute modified versions of
18 this manual under the conditions for verbatim copying, provided that the
19 entire resulting derived work is distributed under the terms of a
20 permission notice identical to this one.
22 Permission is granted to copy and distribute translations of this
23 manual into another language, under the above conditions for modified
24 versions, except that this permission notice may be stated in a
25 translation approved by the Foundation.
27 Permission is granted to copy and distribute modified versions of
28 this manual under the conditions for verbatim copying, provided also
29 that the section entitled "GNU General Public License" is included
30 exactly as in the original, and provided that the entire resulting
31 derived work is distributed under the terms of a permission notice
32 identical to this one.
34 Permission is granted to copy and distribute translations of this
35 manual into another language, under the above conditions for modified
36 versions, except that the section entitled "GNU General Public License"
37 may be included in a translation approved by the Free Software
38 Foundation instead of in the original English.
41 File: internals.info, Node: Top, Next: A History of Emacs, Prev: (dir), Up: (dir)
43 This Info file contains v1.3 of the XEmacs Internals Manual, August
48 * A History of Emacs:: Times, dates, important events.
49 * XEmacs From the Outside:: A broad conceptual overview.
50 * The Lisp Language:: An overview.
51 * XEmacs From the Perspective of Building::
52 * XEmacs From the Inside::
53 * The XEmacs Object System (Abstractly Speaking)::
54 * How Lisp Objects Are Represented in C::
55 * Rules When Writing New C Code::
56 * A Summary of the Various XEmacs Modules::
57 * Allocation of Objects in XEmacs Lisp::
59 * Events and the Event Loop::
60 * Evaluation; Stack Frames; Bindings::
61 * Symbols and Variables::
62 * Buffers and Textual Representation::
63 * MULE Character Sets and Encodings::
64 * The Lisp Reader and Compiler::
66 * Consoles; Devices; Frames; Windows::
67 * The Redisplay Mechanism::
74 * Interface to the X Window System::
78 --- The Detailed Node Listing ---
82 * Through Version 18:: Unification prevails.
83 * Lucid Emacs:: One version 19 Emacs.
84 * GNU Emacs 19:: The other version 19 Emacs.
85 * GNU Emacs 20:: The other version 20 Emacs.
86 * XEmacs:: The continuation of Lucid Emacs.
88 Rules When Writing New C Code
90 * General Coding Rules::
91 * Writing Lisp Primitives::
92 * Adding Global Lisp Variables::
94 * Techniques for XEmacs Developers::
98 * Character-Related Data Types::
99 * Working With Character and Byte Positions::
100 * Conversion to and from External Data::
101 * General Guidelines for Writing Mule-Aware Code::
102 * An Example of Mule-Aware Code::
104 A Summary of the Various XEmacs Modules
106 * Low-Level Modules::
107 * Basic Lisp Modules::
108 * Modules for Standard Editing Operations::
109 * Editor-Level Control Flow Modules::
110 * Modules for the Basic Displayable Lisp Objects::
111 * Modules for other Display-Related Lisp Objects::
112 * Modules for the Redisplay Mechanism::
113 * Modules for Interfacing with the File System::
114 * Modules for Other Aspects of the Lisp Interpreter and Object System::
115 * Modules for Interfacing with the Operating System::
116 * Modules for Interfacing with X Windows::
117 * Modules for Internationalization::
119 Allocation of Objects in XEmacs Lisp
121 * Introduction to Allocation::
122 * Garbage Collection::
124 * Garbage Collection - Step by Step::
125 * Integers and Characters::
126 * Allocation from Frob Blocks::
128 * Low-level allocation::
135 * Compiled Function::
137 Garbage Collection - Step by Step
140 * garbage_collect_1::
143 * sweep_lcrecords_1::
144 * compact_string_chars::
146 * sweep_bit_vectors_1::
151 * Data descriptions::
158 * Address allocation::
163 Events and the Event Loop
165 * Introduction to Events::
167 * Specifics of the Event Gathering Mechanism::
168 * Specifics About the Emacs Event::
169 * The Event Stream Callback Routines::
170 * Other Event Loop Functions::
171 * Converting Events::
172 * Dispatching Events; The Command Builder::
174 Evaluation; Stack Frames; Bindings
177 * Dynamic Binding; The specbinding Stack; Unwind-Protects::
178 * Simple Special Forms::
181 Symbols and Variables
183 * Introduction to Symbols::
187 Buffers and Textual Representation
189 * Introduction to Buffers:: A buffer holds a block of text such as a file.
190 * The Text in a Buffer:: Representation of the text in a buffer.
191 * Buffer Lists:: Keeping track of all buffers.
192 * Markers and Extents:: Tagging locations within a buffer.
193 * Bufbytes and Emchars:: Representation of individual characters.
194 * The Buffer Object:: The Lisp object corresponding to a buffer.
196 MULE Character Sets and Encodings
200 * Internal Mule Encodings::
205 * Japanese EUC (Extended Unix Code)::
208 Internal Mule Encodings
210 * Internal String Encoding::
211 * Internal Character Encoding::
215 * Creating an Lstream:: Creating an lstream object.
216 * Lstream Types:: Different sorts of things that are streamed.
217 * Lstream Functions:: Functions for working with lstreams.
218 * Lstream Methods:: Creating new lstream types.
220 Consoles; Devices; Frames; Windows
222 * Introduction to Consoles; Devices; Frames; Windows::
225 * The Window Object::
227 The Redisplay Mechanism
229 * Critical Redisplay Sections::
231 * Redisplay Piece by Piece::
235 * Introduction to Extents:: Extents are ranges over text, with properties.
236 * Extent Ordering:: How extents are ordered internally.
237 * Format of the Extent Info:: The extent information in a buffer or string.
238 * Zero-Length Extents:: A weird special case.
239 * Mathematics of Extent Ordering:: A rigorous foundation.
240 * Extent Fragments:: Cached information useful for redisplay.
243 File: internals.info, Node: A History of Emacs, Next: XEmacs From the Outside, Prev: Top, Up: Top
248 XEmacs is a powerful, customizable text editor and development
249 environment. It began as Lucid Emacs, which was in turn derived from
250 GNU Emacs, a program written by Richard Stallman of the Free Software
251 Foundation. GNU Emacs dates back to the 1970's, and was modelled after
252 a package called "Emacs", written in 1976, that was a set of macros on
253 top of TECO, an old, old text editor written at MIT on the DEC PDP 10
254 under one of the earliest time-sharing operating systems, ITS
255 (Incompatible Timesharing System). (ITS dates back well before Unix.)
256 ITS, TECO, and Emacs were products of a group of people at MIT who
257 called themselves "hackers", who shared an idealistic belief system
258 about the free exchange of information and were fanatical in their
259 devotion to and time spent with computers. (The hacker subculture dates
260 back to the late 1950's at MIT and is described in detail in Steven
261 Levy's book `Hackers'. This book also includes a lot of information
262 about Stallman himself and the development of Lisp, a programming
263 language developed at MIT that underlies Emacs.)
267 * Through Version 18:: Unification prevails.
268 * Lucid Emacs:: One version 19 Emacs.
269 * GNU Emacs 19:: The other version 19 Emacs.
270 * GNU Emacs 20:: The other version 20 Emacs.
271 * XEmacs:: The continuation of Lucid Emacs.
274 File: internals.info, Node: Through Version 18, Next: Lucid Emacs, Up: A History of Emacs
279 Although the history of the early versions of GNU Emacs is unclear,
280 the history is well-known from the middle of 1985. A time line is:
282 * GNU Emacs version 15 (15.34) was released sometime in 1984 or 1985
283 and shared some code with a version of Emacs written by James
284 Gosling (the same James Gosling who later created the Java
287 * GNU Emacs version 16 (first released version was 16.56) was
288 released on July 15, 1985. All Gosling code was removed due to
289 potential copyright problems with the code.
291 * version 16.57: released on September 16, 1985.
293 * versions 16.58, 16.59: released on September 17, 1985.
295 * version 16.60: released on September 19, 1985. These later
296 version 16's incorporated patches from the net, esp. for getting
297 Emacs to work under System V.
299 * version 17.36 (first official v17 release) released on December 20,
300 1985. Included a TeX-able user manual. First official unpatched
301 version that worked on vanilla System V machines.
303 * version 17.43 (second official v17 release) released on January 25,
306 * version 17.45 released on January 30, 1986.
308 * version 17.46 released on February 4, 1986.
310 * version 17.48 released on February 10, 1986.
312 * version 17.49 released on February 12, 1986.
314 * version 17.55 released on March 18, 1986.
316 * version 17.57 released on March 27, 1986.
318 * version 17.58 released on April 4, 1986.
320 * version 17.61 released on April 12, 1986.
322 * version 17.63 released on May 7, 1986.
324 * version 17.64 released on May 12, 1986.
326 * version 18.24 (a beta version) released on October 2, 1986.
328 * version 18.30 (a beta version) released on November 15, 1986.
330 * version 18.31 (a beta version) released on November 23, 1986.
332 * version 18.32 (a beta version) released on December 7, 1986.
334 * version 18.33 (a beta version) released on December 12, 1986.
336 * version 18.35 (a beta version) released on January 5, 1987.
338 * version 18.36 (a beta version) released on January 21, 1987.
340 * January 27, 1987: The Great Usenet Renaming. net.emacs is now
343 * version 18.37 (a beta version) released on February 12, 1987.
345 * version 18.38 (a beta version) released on March 3, 1987.
347 * version 18.39 (a beta version) released on March 14, 1987.
349 * version 18.40 (a beta version) released on March 18, 1987.
351 * version 18.41 (the first "official" release) released on March 22,
354 * version 18.45 released on June 2, 1987.
356 * version 18.46 released on June 9, 1987.
358 * version 18.47 released on June 18, 1987.
360 * version 18.48 released on September 3, 1987.
362 * version 18.49 released on September 18, 1987.
364 * version 18.50 released on February 13, 1988.
366 * version 18.51 released on May 7, 1988.
368 * version 18.52 released on September 1, 1988.
370 * version 18.53 released on February 24, 1989.
372 * version 18.54 released on April 26, 1989.
374 * version 18.55 released on August 23, 1989. This is the earliest
375 version that is still available by FTP.
377 * version 18.56 released on January 17, 1991.
379 * version 18.57 released late January, 1991.
381 * version 18.58 released ?????.
383 * version 18.59 released October 31, 1992.
386 File: internals.info, Node: Lucid Emacs, Next: GNU Emacs 19, Prev: Through Version 18, Up: A History of Emacs
391 Lucid Emacs was developed by the (now-defunct) Lucid Inc., a maker of
392 C++ and Lisp development environments. It began when Lucid decided they
393 wanted to use Emacs as the editor and cornerstone of their C++
394 development environment (called "Energize"). They needed many features
395 that were not available in the existing version of GNU Emacs (version
396 18.5something), in particular good and integrated support for GUI
397 elements such as mouse support, multiple fonts, multiple window-system
398 windows, etc. A branch of GNU Emacs called Epoch, written at the
399 University of Illinois, existed that supplied many of these features;
400 however, Lucid needed more than what existed in Epoch. At the time, the
401 Free Software Foundation was working on version 19 of Emacs (this was
402 sometime around 1991), which was planned to have similar features, and
403 so Lucid decided to work with the Free Software Foundation. Their plan
404 was to add features that they needed, and coordinate with the FSF so
405 that the features would get included back into Emacs version 19.
407 Delays in the release of version 19 occurred, however (resulting in
408 it finally being released more than a year after what was initially
409 planned), and Lucid encountered unexpected technical resistance in
410 getting their changes merged back into version 19, so they decided to
411 release their own version of Emacs, which became Lucid Emacs 19.0.
413 The initial authors of Lucid Emacs were Matthieu Devin, Harlan
414 Sexton, and Eric Benson, and the work was later taken over by Jamie
415 Zawinski, who became "Mr. Lucid Emacs" for many releases.
417 A time line for Lucid Emacs/XEmacs is
419 * version 19.0 shipped with Energize 1.0, April 1992.
421 * version 19.1 released June 4, 1992.
423 * version 19.2 released June 19, 1992.
425 * version 19.3 released September 9, 1992.
427 * version 19.4 released January 21, 1993.
429 * version 19.5 was a repackaging of 19.4 with a few bug fixes and
430 shipped with Energize 2.0. Never released to the net.
432 * version 19.6 released April 9, 1993.
434 * version 19.7 was a repackaging of 19.6 with a few bug fixes and
435 shipped with Energize 2.1. Never released to the net.
437 * version 19.8 released September 6, 1993.
439 * version 19.9 released January 12, 1994.
441 * version 19.10 released May 27, 1994.
443 * version 19.11 (first XEmacs) released September 13, 1994.
445 * version 19.12 released June 23, 1995.
447 * version 19.13 released September 1, 1995.
449 * version 19.14 released June 23, 1996.
451 * version 20.0 released February 9, 1997.
453 * version 19.15 released March 28, 1997.
455 * version 20.1 (not released to the net) April 15, 1997.
457 * version 20.2 released May 16, 1997.
459 * version 19.16 released October 31, 1997.
461 * version 20.3 (the first stable version of XEmacs 20.x) released
462 November 30, 1997. version 20.4 released February 28, 1998.
465 File: internals.info, Node: GNU Emacs 19, Next: GNU Emacs 20, Prev: Lucid Emacs, Up: A History of Emacs
470 About a year after the initial release of Lucid Emacs, the FSF
471 released a beta of their version of Emacs 19 (referred to here as "GNU
472 Emacs"). By this time, the current version of Lucid Emacs was 19.6.
473 (Strangely, the first released beta from the FSF was GNU Emacs 19.7.) A
474 time line for GNU Emacs version 19 is
476 * version 19.8 (beta) released May 27, 1993.
478 * version 19.9 (beta) released May 27, 1993.
480 * version 19.10 (beta) released May 30, 1993.
482 * version 19.11 (beta) released June 1, 1993.
484 * version 19.12 (beta) released June 2, 1993.
486 * version 19.13 (beta) released June 8, 1993.
488 * version 19.14 (beta) released June 17, 1993.
490 * version 19.15 (beta) released June 19, 1993.
492 * version 19.16 (beta) released July 6, 1993.
494 * version 19.17 (beta) released late July, 1993.
496 * version 19.18 (beta) released August 9, 1993.
498 * version 19.19 (beta) released August 15, 1993.
500 * version 19.20 (beta) released November 17, 1993.
502 * version 19.21 (beta) released November 17, 1993.
504 * version 19.22 (beta) released November 28, 1993.
506 * version 19.23 (beta) released May 17, 1994.
508 * version 19.24 (beta) released May 16, 1994.
510 * version 19.25 (beta) released June 3, 1994.
512 * version 19.26 (beta) released September 11, 1994.
514 * version 19.27 (beta) released September 14, 1994.
516 * version 19.28 (first "official" release) released November 1, 1994.
518 * version 19.29 released June 21, 1995.
520 * version 19.30 released November 24, 1995.
522 * version 19.31 released May 25, 1996.
524 * version 19.32 released July 31, 1996.
526 * version 19.33 released August 11, 1996.
528 * version 19.34 released August 21, 1996.
530 * version 19.34b released September 6, 1996.
532 In some ways, GNU Emacs 19 was better than Lucid Emacs; in some ways,
533 worse. Lucid soon began incorporating features from GNU Emacs 19 into
534 Lucid Emacs; the work was mostly done by Richard Mlynarik, who had been
535 working on and using GNU Emacs for a long time (back as far as version
539 File: internals.info, Node: GNU Emacs 20, Next: XEmacs, Prev: GNU Emacs 19, Up: A History of Emacs
544 On February 2, 1997 work began on GNU Emacs to integrate Mule. The
545 first release was made in September of that year.
547 A timeline for Emacs 20 is
549 * version 20.1 released September 17, 1997.
551 * version 20.2 released September 20, 1997.
553 * version 20.3 released August 19, 1998.
556 File: internals.info, Node: XEmacs, Prev: GNU Emacs 20, Up: A History of Emacs
561 Around the time that Lucid was developing Energize, Sun Microsystems
562 was developing their own development environment (called "SPARCWorks")
563 and also decided to use Emacs. They joined forces with the Epoch team
564 at the University of Illinois and later with Lucid. The maintainer of
565 the last-released version of Epoch was Marc Andreessen, but he dropped
566 out and the Epoch project, headed by Simon Kaplan, lured Chuck Thompson
567 away from a system administration job to become the primary Lucid Emacs
568 author for Epoch and Sun. Chuck's area of specialty became the
569 redisplay engine (he replaced the old Lucid Emacs redisplay engine with
570 a ported version from Epoch and then later rewrote it from scratch).
571 Sun also hired Ben Wing (the author of Win-Emacs, a port of Lucid Emacs
572 to Microsoft Windows 3.1) in 1993, for what was initially a one-month
573 contract to fix some event problems but later became a many-year
574 involvement, punctuated by a six-month contract with Amdahl Corporation.
576 In 1994, Sun and Lucid agreed to rename Lucid Emacs to XEmacs (a name
577 not favorable to either company); the first release called XEmacs was
578 version 19.11. In June 1994, Lucid folded and Jamie quit to work for
579 the newly formed Mosaic Communications Corp., later Netscape
580 Communications Corp. (co-founded by the same Marc Andreessen, who had
581 quit his Epoch job to work on a graphical browser for the World Wide
582 Web). Chuck then become the primary maintainer of XEmacs, and put out
583 versions 19.11 through 19.14 in conjunction with Ben. For 19.12 and
584 19.13, Chuck added the new redisplay and many other display improvements
585 and Ben added MULE support (support for Asian and other languages) and
586 redesigned most of the internal Lisp subsystems to better support the
587 MULE work and the various other features being added to XEmacs. After
588 19.14 Chuck retired as primary maintainer and Steve Baur stepped in.
590 Soon after 19.13 was released, work began in earnest on the MULE
591 internationalization code and the source tree was divided into two
592 development paths. The MULE version was initially called 19.20, but was
593 soon renamed to 20.0. In 1996 Martin Buchholz of Sun Microsystems took
594 over the care and feeding of it and worked on it in parallel with the
595 19.14 development that was occurring at the same time. After much work
596 by Martin, it was decided to release 20.0 ahead of 19.15 in February
597 1997. The source tree remained divided until 20.2 when the version 19
598 source was finally retired at version 19.16.
600 In 1997, Sun finally dropped all pretense of support for XEmacs and
601 Martin Buchholz left the company in November. Since then, and mostly
602 for the previous year, because Steve Baur was never paid to work on
603 XEmacs, XEmacs has existed solely on the contributions of volunteers
604 from the Free Software Community. Starting from 1997, Hrvoje Niksic and
605 Kyle Jones have figured prominently in XEmacs development.
607 Many attempts have been made to merge XEmacs and GNU Emacs, but they
608 have consistently failed.
610 A more detailed history is contained in the XEmacs About page.
613 File: internals.info, Node: XEmacs From the Outside, Next: The Lisp Language, Prev: A History of Emacs, Up: Top
615 XEmacs From the Outside
616 ***********************
618 XEmacs appears to the outside world as an editor, but it is really a
619 Lisp environment. At its heart is a Lisp interpreter; it also
620 "happens" to contain many specialized object types (e.g. buffers,
621 windows, frames, events) that are useful for implementing an editor.
622 Some of these objects (in particular windows and frames) have
623 displayable representations, and XEmacs provides a function
624 `redisplay()' that ensures that the display of all such objects matches
625 their internal state. Most of the time, a standard Lisp environment is
626 in a "read-eval-print" loop--i.e. "read some Lisp code, execute it, and
627 print the results". XEmacs has a similar loop:
631 * dispatch the event (i.e. "do it")
635 Reading an event is done using the Lisp function `next-event', which
636 waits for something to happen (typically, the user presses a key or
637 moves the mouse) and returns an event object describing this.
638 Dispatching an event is done using the Lisp function `dispatch-event',
639 which looks up the event in a keymap object (a particular kind of
640 object that associates an event with a Lisp function) and calls that
641 function. The function "does" what the user has requested by changing
642 the state of particular frame objects, buffer objects, etc. Finally,
643 `redisplay()' is called, which updates the display to reflect those
644 changes just made. Thus is an "editor" born.
646 Note that you do not have to use XEmacs as an editor; you could just
647 as well make it do your taxes, compute pi, play bridge, etc. You'd just
648 have to write functions to do those operations in Lisp.
651 File: internals.info, Node: The Lisp Language, Next: XEmacs From the Perspective of Building, Prev: XEmacs From the Outside, Up: Top
656 Lisp is a general-purpose language that is higher-level than C and in
657 many ways more powerful than C. Powerful dialects of Lisp such as
658 Common Lisp are probably much better languages for writing very large
659 applications than is C. (Unfortunately, for many non-technical reasons
660 C and its successor C++ have become the dominant languages for
661 application development. These languages are both inadequate for
662 extremely large applications, which is evidenced by the fact that newer,
663 larger programs are becoming ever harder to write and are requiring ever
664 more programmers despite great increases in C development environments;
665 and by the fact that, although hardware speeds and reliability have been
666 growing at an exponential rate, most software is still generally
667 considered to be slow and buggy.)
669 The new Java language holds promise as a better general-purpose
670 development language than C. Java has many features in common with
671 Lisp that are not shared by C (this is not a coincidence, since Java
672 was designed by James Gosling, a former Lisp hacker). This will be
673 discussed more later.
675 For those used to C, here is a summary of the basic differences
678 1. Lisp has an extremely regular syntax. Every function, expression,
679 and control statement is written in the form
683 This is as opposed to C, which writes functions as
685 func(ARG1, ARG2, ...)
687 but writes expressions involving operators as (e.g.)
691 and writes control statements as (e.g.)
693 while (EXPR) { STATEMENT1; STATEMENT2; ... }
695 Lisp equivalents of the latter two would be
701 (while EXPR STATEMENT1 STATEMENT2 ...)
703 2. Lisp is a safe language. Assuming there are no bugs in the Lisp
704 interpreter/compiler, it is impossible to write a program that
705 "core dumps" or otherwise causes the machine to execute an illegal
706 instruction. This is very different from C, where perhaps the most
707 common outcome of a bug is exactly such a crash. A corollary of
708 this is that the C operation of casting a pointer is impossible
709 (and unnecessary) in Lisp, and that it is impossible to access
710 memory outside the bounds of an array.
712 3. Programs and data are written in the same form. The
713 parenthesis-enclosing form described above for statements is the
714 same form used for the most common data type in Lisp, the list.
715 Thus, it is possible to represent any Lisp program using Lisp data
716 types, and for one program to construct Lisp statements and then
717 dynamically "evaluate" them, or cause them to execute.
719 4. All objects are "dynamically typed". This means that part of every
720 object is an indication of what type it is. A Lisp program can
721 manipulate an object without knowing what type it is, and can
722 query an object to determine its type. This means that,
723 correspondingly, variables and function parameters can hold
724 objects of any type and are not normally declared as being of any
725 particular type. This is opposed to the "static typing" of C,
726 where variables can hold exactly one type of object and must be
727 declared as such, and objects do not contain an indication of
728 their type because it's implicit in the variables they are stored
729 in. It is possible in C to have a variable hold different types
730 of objects (e.g. through the use of `void *' pointers or
731 variable-argument functions), but the type information must then be
732 passed explicitly in some other fashion, leading to additional
735 5. Allocated memory is automatically reclaimed when it is no longer
736 in use. This operation is called "garbage collection" and
737 involves looking through all variables to see what memory is being
738 pointed to, and reclaiming any memory that is not pointed to and
739 is thus "inaccessible" and out of use. This is as opposed to C,
740 in which allocated memory must be explicitly reclaimed using
741 `free()'. If you simply drop all pointers to memory without
742 freeing it, it becomes "leaked" memory that still takes up space.
743 Over a long period of time, this can cause your program to grow
744 and grow until it runs out of memory.
746 6. Lisp has built-in facilities for handling errors and exceptions.
747 In C, when an error occurs, usually either the program exits
748 entirely or the routine in which the error occurs returns a value
749 indicating this. If an error occurs in a deeply-nested routine,
750 then every routine currently called must unwind itself normally
751 and return an error value back up to the next routine. This means
752 that every routine must explicitly check for an error in all the
753 routines it calls; if it does not do so, unexpected and often
754 random behavior results. This is an extremely common source of
755 bugs in C programs. An alternative would be to do a non-local
756 exit using `longjmp()', but that is often very dangerous because
757 the routines that were exited past had no opportunity to clean up
758 after themselves and may leave things in an inconsistent state,
759 causing a crash shortly afterwards.
761 Lisp provides mechanisms to make such non-local exits safe. When
762 an error occurs, a routine simply signals that an error of a
763 particular class has occurred, and a non-local exit takes place.
764 Any routine can trap errors occurring in routines it calls by
765 registering an error handler for some or all classes of errors.
766 (If no handler is registered, a default handler, generally
767 installed by the top-level event loop, is executed; this prints
768 out the error and continues.) Routines can also specify cleanup
769 code (called an "unwind-protect") that will be called when control
770 exits from a block of code, no matter how that exit occurs--i.e.
771 even if a function deeply nested below it causes a non-local exit
772 back to the top level.
774 Note that this facility has appeared in some recent vintages of C,
775 in particular Visual C++ and other PC compilers written for the
778 7. In Emacs Lisp, local variables are "dynamically scoped". This
779 means that if you declare a local variable in a particular
780 function, and then call another function, that subfunction can
781 "see" the local variable you declared. This is actually
782 considered a bug in Emacs Lisp and in all other early dialects of
783 Lisp, and was corrected in Common Lisp. (In Common Lisp, you can
784 still declare dynamically scoped variables if you want to--they
785 are sometimes useful--but variables by default are "lexically
788 For those familiar with Lisp, Emacs Lisp is modelled after MacLisp,
789 an early dialect of Lisp developed at MIT (no relation to the Macintosh
790 computer). There is a Common Lisp compatibility package available for
791 Emacs that provides many of the features of Common Lisp.
793 The Java language is derived in many ways from C, and shares a
794 similar syntax, but has the following features in common with Lisp (and
797 1. Java is a safe language, like Lisp.
799 2. Java provides garbage collection, like Lisp.
801 3. Java has built-in facilities for handling errors and exceptions,
804 4. Java has a type system that combines the best advantages of both
805 static and dynamic typing. Objects (except very simple types) are
806 explicitly marked with their type, as in dynamic typing; but there
807 is a hierarchy of types and functions are declared to accept only
808 certain types, thus providing the increased compile-time
809 error-checking of static typing.
811 The Java language also has some negative attributes:
813 1. Java uses the edit/compile/run model of software development. This
814 makes it hard to use interactively. For example, to use Java like
815 `bc' it is necessary to write a special purpose, albeit tiny,
816 application. In Emacs Lisp, a calculator comes built-in without
817 any effort - one can always just type an expression in the
820 2. Java tries too hard to enforce, not merely enable, portability,
821 making ordinary access to standard OS facilities painful. Java
822 has an "agenda". I think this is why `chdir' is not part of
823 standard Java, which is inexcusable.
825 Unfortunately, there is no perfect language. Static typing allows a
826 compiler to catch programmer errors and produce more efficient code, but
827 makes programming more tedious and less fun. For the foreseeable
828 future, an Ideal Editing and Programming Environment (and that is what
829 XEmacs aspires to) will be programmable in multiple languages: high
830 level ones like Lisp for user customization and prototyping, and lower
831 level ones for infrastructure and industrial strength applications. If
832 I had my way, XEmacs would be friendly towards the Python, Scheme, C++,
833 ML, etc... communities. But there are serious technical difficulties to
836 The word "application" in the previous paragraph was used
837 intentionally. XEmacs implements an API for programs written in Lisp
838 that makes it a full-fledged application platform, very much like an OS
842 File: internals.info, Node: XEmacs From the Perspective of Building, Next: XEmacs From the Inside, Prev: The Lisp Language, Up: Top
844 XEmacs From the Perspective of Building
845 ***************************************
847 The heart of XEmacs is the Lisp environment, which is written in C.
848 This is contained in the `src/' subdirectory. Underneath `src/' are
849 two subdirectories of header files: `s/' (header files for particular
850 operating systems) and `m/' (header files for particular machine
851 types). In practice the distinction between the two types of header
852 files is blurred. These header files define or undefine certain
853 preprocessor constants and macros to indicate particular
854 characteristics of the associated machine or operating system. As part
855 of the configure process, one `s/' file and one `m/' file is identified
856 for the particular environment in which XEmacs is being built.
858 XEmacs also contains a great deal of Lisp code. This implements the
859 operations that make XEmacs useful as an editor as well as just a Lisp
860 environment, and also contains many add-on packages that allow XEmacs to
861 browse directories, act as a mail and Usenet news reader, compile Lisp
862 code, etc. There is actually more Lisp code than C code associated with
863 XEmacs, but much of the Lisp code is peripheral to the actual operation
864 of the editor. The Lisp code all lies in subdirectories underneath the
867 The `lwlib/' directory contains C code that implements a generalized
868 interface onto different X widget toolkits and also implements some
869 widgets of its own that behave like Motif widgets but are faster, free,
870 and in some cases more powerful. The code in this directory compiles
871 into a library and is mostly independent from XEmacs.
873 The `etc/' directory contains various data files associated with
874 XEmacs. Some of them are actually read by XEmacs at startup; others
875 merely contain useful information of various sorts.
877 The `lib-src/' directory contains C code for various auxiliary
878 programs that are used in connection with XEmacs. Some of them are used
879 during the build process; others are used to perform certain functions
880 that cannot conveniently be placed in the XEmacs executable (e.g. the
881 `movemail' program for fetching mail out of `/var/spool/mail', which
882 must be setgid to `mail' on many systems; and the `gnuclient' program,
883 which allows an external script to communicate with a running XEmacs
886 The `man/' directory contains the sources for the XEmacs
887 documentation. It is mostly in a form called Texinfo, which can be
888 converted into either a printed document (by passing it through TeX) or
889 into on-line documentation called "info files".
891 The `info/' directory contains the results of formatting the XEmacs
892 documentation as "info files", for on-line use. These files are used
893 when you enter the Info system using `C-h i' or through the Help menu.
895 The `dynodump/' directory contains auxiliary code used to build
896 XEmacs on Solaris platforms.
898 The other directories contain various miscellaneous code and
899 information that is not normally used or needed.
901 The first step of building involves running the `configure' program
902 and passing it various parameters to specify any optional features you
903 want and compiler arguments and such, as described in the `INSTALL'
904 file. This determines what the build environment is, chooses the
905 appropriate `s/' and `m/' file, and runs a series of tests to determine
906 many details about your environment, such as which library functions
907 are available and exactly how they work. The reason for running these
908 tests is that it allows XEmacs to be compiled on a much wider variety
909 of platforms than those that the XEmacs developers happen to be
910 familiar with, including various sorts of hybrid platforms. This is
911 especially important now that many operating systems give you a great
912 deal of control over exactly what features you want installed, and allow
913 for easy upgrading of parts of a system without upgrading the rest. It
914 would be impossible to pre-determine and pre-specify the information for
915 all possible configurations.
917 In fact, the `s/' and `m/' files are basically _evil_, since they
918 contain unmaintainable platform-specific hard-coded information.
919 XEmacs has been moving in the direction of having all system-specific
920 information be determined dynamically by `configure'. Perhaps someday
921 we can `rm -rf src/s src/m'.
923 When configure is done running, it generates `Makefile's and
924 `GNUmakefile's and the file `src/config.h' (which describes the
925 features of your system) from template files. You then run `make',
926 which compiles the auxiliary code and programs in `lib-src/' and
927 `lwlib/' and the main XEmacs executable in `src/'. The result of
928 compiling and linking is an executable called `temacs', which is _not_
929 the final XEmacs executable. `temacs' by itself is not intended to
930 function as an editor or even display any windows on the screen, and if
931 you simply run it, it will exit immediately. The `Makefile' runs
932 `temacs' with certain options that cause it to initialize itself, read
933 in a number of basic Lisp files, and then dump itself out into a new
934 executable called `xemacs'. This new executable has been
935 pre-initialized and contains pre-digested Lisp code that is necessary
936 for the editor to function (this includes most basic editing functions,
937 e.g. `kill-line', that can be defined in terms of other Lisp
938 primitives; some initialization code that is called when certain
939 objects, such as frames, are created; and all of the standard
940 keybindings and code for the actions they result in). This executable,
941 `xemacs', is the executable that you run to use the XEmacs editor.
943 Although `temacs' is not intended to be run as an editor, it can, by
944 using the incantation `temacs -batch -l loadup.el run-temacs'. This is
945 useful when the dumping procedure described above is broken, or when
946 using certain program debugging tools such as Purify. These tools get
947 mighty confused by the tricks played by the XEmacs build process, such
948 as allocation memory in one process, and freeing it in the next.
951 File: internals.info, Node: XEmacs From the Inside, Next: The XEmacs Object System (Abstractly Speaking), Prev: XEmacs From the Perspective of Building, Up: Top
953 XEmacs From the Inside
954 **********************
956 Internally, XEmacs is quite complex, and can be very confusing. To
957 simplify things, it can be useful to think of XEmacs as containing an
958 event loop that "drives" everything, and a number of other subsystems,
959 such as a Lisp engine and a redisplay mechanism. Each of these other
960 subsystems exists simultaneously in XEmacs, and each has a certain
961 state. The flow of control continually passes in and out of these
962 different subsystems in the course of normal operation of the editor.
964 It is important to keep in mind that, most of the time, the editor is
965 "driven" by the event loop. Except during initialization and batch
966 mode, all subsystems are entered directly or indirectly through the
967 event loop, and ultimately, control exits out of all subsystems back up
968 to the event loop. This cycle of entering a subsystem, exiting back out
969 to the event loop, and starting another iteration of the event loop
970 occurs once each keystroke, mouse motion, etc.
972 If you're trying to understand a particular subsystem (other than the
973 event loop), think of it as a "daemon" process or "servant" that is
974 responsible for one particular aspect of a larger system, and
975 periodically receives commands or environment changes that cause it to
976 do something. Ultimately, these commands and environment changes are
977 always triggered by the event loop. For example:
979 * The window and frame mechanism is responsible for keeping track of
980 what windows and frames exist, what buffers are in them, etc. It
981 is periodically given commands (usually from the user) to make a
982 change to the current window/frame state: i.e. create a new frame,
983 delete a window, etc.
985 * The buffer mechanism is responsible for keeping track of what
986 buffers exist and what text is in them. It is periodically given
987 commands (usually from the user) to insert or delete text, create
988 a buffer, etc. When it receives a text-change command, it
989 notifies the redisplay mechanism.
991 * The redisplay mechanism is responsible for making sure that
992 windows and frames are displayed correctly. It is periodically
993 told (by the event loop) to actually "do its job", i.e. snoop
994 around and see what the current state of the environment (mostly
995 of the currently-existing windows, frames, and buffers) is, and
996 make sure that that state matches what's actually displayed. It
997 keeps lots and lots of information around (such as what is
998 actually being displayed currently, and what the environment was
999 last time it checked) so that it can minimize the work it has to
1000 do. It is also helped along in that whenever a relevant change to
1001 the environment occurs, the redisplay mechanism is told about
1002 this, so it has a pretty good idea of where it has to look to find
1003 possible changes and doesn't have to look everywhere.
1005 * The Lisp engine is responsible for executing the Lisp code in
1006 which most user commands are written. It is entered through a
1007 call to `eval' or `funcall', which occurs as a result of
1008 dispatching an event from the event loop. The functions it calls
1009 issue commands to the buffer mechanism, the window/frame
1012 * The Lisp allocation subsystem is responsible for keeping track of
1013 Lisp objects. It is given commands from the Lisp engine to
1014 allocate objects, garbage collect, etc.
1018 The important idea here is that there are a number of independent
1019 subsystems each with its own responsibility and persistent state, just
1020 like different employees in a company, and each subsystem is
1021 periodically given commands from other subsystems. Commands can flow
1022 from any one subsystem to any other, but there is usually some sort of
1023 hierarchy, with all commands originating from the event subsystem.
1025 XEmacs is entered in `main()', which is in `emacs.c'. When this is
1026 called the first time (in a properly-invoked `temacs'), it does the
1029 1. It does some very basic environment initializations, such as
1030 determining where it and its directories (e.g. `lisp/' and `etc/')
1031 reside and setting up signal handlers.
1033 2. It initializes the entire Lisp interpreter.
1035 3. It sets the initial values of many built-in variables (including
1036 many variables that are visible to Lisp programs), such as the
1037 global keymap object and the built-in faces (a face is an object
1038 that describes the display characteristics of text). This
1039 involves creating Lisp objects and thus is dependent on step (2).
1041 4. It performs various other initializations that are relevant to the
1042 particular environment it is running in, such as retrieving
1043 environment variables, determining the current date and the user
1044 who is running the program, examining its standard input, creating
1045 any necessary file descriptors, etc.
1047 5. At this point, the C initialization is complete. A Lisp program
1048 that was specified on the command line (usually `loadup.el') is
1049 called (temacs is normally invoked as `temacs -batch -l loadup.el
1050 dump'). `loadup.el' loads all of the other Lisp files that are
1051 needed for the operation of the editor, calls the `dump-emacs'
1052 function to write out `xemacs', and then kills the temacs process.
1054 When `xemacs' is then run, it only redoes steps (1) and (4) above;
1055 all variables already contain the values they were set to when the
1056 executable was dumped, and all memory that was allocated with
1057 `malloc()' is still around. (XEmacs knows whether it is being run as
1058 `xemacs' or `temacs' because it sets the global variable `initialized'
1059 to 1 after step (4) above.) At this point, `xemacs' calls a Lisp
1060 function to do any further initialization, which includes parsing the
1061 command-line (the C code can only do limited command-line parsing,
1062 which includes looking for the `-batch' and `-l' flags and a few other
1063 flags that it needs to know about before initialization is complete),
1064 creating the first frame (or "window" in standard window-system
1065 parlance), running the user's init file (usually the file `.emacs' in
1066 the user's home directory), etc. The function to do this is usually
1067 called `normal-top-level'; `loadup.el' tells the C code about this
1068 function by setting its name as the value of the Lisp variable
1071 When the Lisp initialization code is done, the C code enters the
1072 event loop, and stays there for the duration of the XEmacs process.
1073 The code for the event loop is contained in `cmdloop.c', and is called
1074 `Fcommand_loop_1()'. Note that this event loop could very well be
1075 written in Lisp, and in fact a Lisp version exists; but apparently,
1076 doing this makes XEmacs run noticeably slower.
1078 Notice how much of the initialization is done in Lisp, not in C. In
1079 general, XEmacs tries to move as much code as is possible into Lisp.
1080 Code that remains in C is code that implements the Lisp interpreter
1081 itself, or code that needs to be very fast, or code that needs to do
1082 system calls or other such stuff that needs to be done in C, or code
1083 that needs to have access to "forbidden" structures. (One conscious
1084 aspect of the design of Lisp under XEmacs is a clean separation between
1085 the external interface to a Lisp object's functionality and its internal
1086 implementation. Part of this design is that Lisp programs are
1087 forbidden from accessing the contents of the object other than through
1088 using a standard API. In this respect, XEmacs Lisp is similar to
1089 modern Lisp dialects but differs from GNU Emacs, which tends to expose
1090 the implementation and allow Lisp programs to look at it directly. The
1091 major advantage of hiding the implementation is that it allows the
1092 implementation to be redesigned without affecting any Lisp programs,
1093 including those that might want to be "clever" by looking directly at
1094 the object's contents and possibly manipulating them.)
1096 Moving code into Lisp makes the code easier to debug and maintain and
1097 makes it much easier for people who are not XEmacs developers to
1098 customize XEmacs, because they can make a change with much less chance
1099 of obscure and unwanted interactions occurring than if they were to
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