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 v1.0 of the XEmacs Internals Manual.
47 * A History of Emacs:: Times, dates, important events.
48 * XEmacs From the Outside:: A broad conceptual overview.
49 * The Lisp Language:: An overview.
50 * XEmacs From the Perspective of Building::
51 * XEmacs From the Inside::
52 * The XEmacs Object System (Abstractly Speaking)::
53 * How Lisp Objects Are Represented in C::
54 * Rules When Writing New C Code::
55 * A Summary of the Various XEmacs Modules::
56 * Allocation of Objects in XEmacs Lisp::
58 * Events and the Event Loop::
59 * Evaluation; Stack Frames; Bindings::
60 * Symbols and Variables::
61 * Buffers and Textual Representation::
62 * MULE Character Sets and Encodings::
63 * The Lisp Reader and Compiler::
65 * Consoles; Devices; Frames; Windows::
66 * The Redisplay Mechanism::
73 * Interface to X Windows::
77 --- The Detailed Node Listing ---
81 * Through Version 18:: Unification prevails.
82 * Lucid Emacs:: One version 19 Emacs.
83 * GNU Emacs 19:: The other version 19 Emacs.
84 * GNU Emacs 20:: The other version 20 Emacs.
85 * XEmacs:: The continuation of Lucid Emacs.
87 Rules When Writing New C Code
89 * General Coding Rules::
90 * Writing Lisp Primitives::
91 * Adding Global Lisp Variables::
93 * Techniques for XEmacs Developers::
97 * Character-Related Data Types::
98 * Working With Character and Byte Positions::
99 * Conversion to and from External Data::
100 * General Guidelines for Writing Mule-Aware Code::
101 * An Example of Mule-Aware Code::
103 A Summary of the Various XEmacs Modules
105 * Low-Level Modules::
106 * Basic Lisp Modules::
107 * Modules for Standard Editing Operations::
108 * Editor-Level Control Flow Modules::
109 * Modules for the Basic Displayable Lisp Objects::
110 * Modules for other Display-Related Lisp Objects::
111 * Modules for the Redisplay Mechanism::
112 * Modules for Interfacing with the File System::
113 * Modules for Other Aspects of the Lisp Interpreter and Object System::
114 * Modules for Interfacing with the Operating System::
115 * Modules for Interfacing with X Windows::
116 * Modules for Internationalization::
118 Allocation of Objects in XEmacs Lisp
120 * Introduction to Allocation::
121 * Garbage Collection::
123 * Garbage Collection - Step by Step::
124 * Integers and Characters::
125 * Allocation from Frob Blocks::
127 * Low-level allocation::
134 * Compiled Function::
136 Garbage Collection - Step by Step
139 * garbage_collect_1::
142 * sweep_lcrecords_1::
143 * compact_string_chars::
145 * sweep_bit_vectors_1::
150 * Data descriptions::
157 * Address allocation::
162 Events and the Event Loop
164 * Introduction to Events::
166 * Specifics of the Event Gathering Mechanism::
167 * Specifics About the Emacs Event::
168 * The Event Stream Callback Routines::
169 * Other Event Loop Functions::
170 * Converting Events::
171 * Dispatching Events; The Command Builder::
173 Evaluation; Stack Frames; Bindings
176 * Dynamic Binding; The specbinding Stack; Unwind-Protects::
177 * Simple Special Forms::
180 Symbols and Variables
182 * Introduction to Symbols::
186 Buffers and Textual Representation
188 * Introduction to Buffers:: A buffer holds a block of text such as a file.
189 * The Text in a Buffer:: Representation of the text in a buffer.
190 * Buffer Lists:: Keeping track of all buffers.
191 * Markers and Extents:: Tagging locations within a buffer.
192 * Bufbytes and Emchars:: Representation of individual characters.
193 * The Buffer Object:: The Lisp object corresponding to a buffer.
195 MULE Character Sets and Encodings
199 * Internal Mule Encodings::
204 * Japanese EUC (Extended Unix Code)::
207 Internal Mule Encodings
209 * Internal String Encoding::
210 * Internal Character Encoding::
214 * Creating an Lstream:: Creating an lstream object.
215 * Lstream Types:: Different sorts of things that are streamed.
216 * Lstream Functions:: Functions for working with lstreams.
217 * Lstream Methods:: Creating new lstream types.
219 Consoles; Devices; Frames; Windows
221 * Introduction to Consoles; Devices; Frames; Windows::
224 * The Window Object::
226 The Redisplay Mechanism
228 * Critical Redisplay Sections::
230 * Redisplay Piece by Piece::
234 * Introduction to Extents:: Extents are ranges over text, with properties.
235 * Extent Ordering:: How extents are ordered internally.
236 * Format of the Extent Info:: The extent information in a buffer or string.
237 * Zero-Length Extents:: A weird special case.
238 * Mathematics of Extent Ordering:: A rigorous foundation.
239 * Extent Fragments:: Cached information useful for redisplay.
242 File: internals.info, Node: A History of Emacs, Next: XEmacs From the Outside, Prev: Top, Up: Top
247 XEmacs is a powerful, customizable text editor and development
248 environment. It began as Lucid Emacs, which was in turn derived from
249 GNU Emacs, a program written by Richard Stallman of the Free Software
250 Foundation. GNU Emacs dates back to the 1970's, and was modelled after
251 a package called "Emacs", written in 1976, that was a set of macros on
252 top of TECO, an old, old text editor written at MIT on the DEC PDP 10
253 under one of the earliest time-sharing operating systems, ITS
254 (Incompatible Timesharing System). (ITS dates back well before Unix.)
255 ITS, TECO, and Emacs were products of a group of people at MIT who
256 called themselves "hackers", who shared an idealistic belief system
257 about the free exchange of information and were fanatical in their
258 devotion to and time spent with computers. (The hacker subculture dates
259 back to the late 1950's at MIT and is described in detail in Steven
260 Levy's book `Hackers'. This book also includes a lot of information
261 about Stallman himself and the development of Lisp, a programming
262 language developed at MIT that underlies Emacs.)
266 * Through Version 18:: Unification prevails.
267 * Lucid Emacs:: One version 19 Emacs.
268 * GNU Emacs 19:: The other version 19 Emacs.
269 * GNU Emacs 20:: The other version 20 Emacs.
270 * XEmacs:: The continuation of Lucid Emacs.
273 File: internals.info, Node: Through Version 18, Next: Lucid Emacs, Prev: A History of Emacs, Up: A History of Emacs
278 Although the history of the early versions of GNU Emacs is unclear,
279 the history is well-known from the middle of 1985. A time line is:
281 * GNU Emacs version 15 (15.34) was released sometime in 1984 or 1985
282 and shared some code with a version of Emacs written by James
283 Gosling (the same James Gosling who later created the Java
286 * GNU Emacs version 16 (first released version was 16.56) was
287 released on July 15, 1985. All Gosling code was removed due to
288 potential copyright problems with the code.
290 * version 16.57: released on September 16, 1985.
292 * versions 16.58, 16.59: released on September 17, 1985.
294 * version 16.60: released on September 19, 1985. These later
295 version 16's incorporated patches from the net, esp. for getting
296 Emacs to work under System V.
298 * version 17.36 (first official v17 release) released on December 20,
299 1985. Included a TeX-able user manual. First official unpatched
300 version that worked on vanilla System V machines.
302 * version 17.43 (second official v17 release) released on January 25,
305 * version 17.45 released on January 30, 1986.
307 * version 17.46 released on February 4, 1986.
309 * version 17.48 released on February 10, 1986.
311 * version 17.49 released on February 12, 1986.
313 * version 17.55 released on March 18, 1986.
315 * version 17.57 released on March 27, 1986.
317 * version 17.58 released on April 4, 1986.
319 * version 17.61 released on April 12, 1986.
321 * version 17.63 released on May 7, 1986.
323 * version 17.64 released on May 12, 1986.
325 * version 18.24 (a beta version) released on October 2, 1986.
327 * version 18.30 (a beta version) released on November 15, 1986.
329 * version 18.31 (a beta version) released on November 23, 1986.
331 * version 18.32 (a beta version) released on December 7, 1986.
333 * version 18.33 (a beta version) released on December 12, 1986.
335 * version 18.35 (a beta version) released on January 5, 1987.
337 * version 18.36 (a beta version) released on January 21, 1987.
339 * January 27, 1987: The Great Usenet Renaming. net.emacs is now
342 * version 18.37 (a beta version) released on February 12, 1987.
344 * version 18.38 (a beta version) released on March 3, 1987.
346 * version 18.39 (a beta version) released on March 14, 1987.
348 * version 18.40 (a beta version) released on March 18, 1987.
350 * version 18.41 (the first "official" release) released on March 22,
353 * version 18.45 released on June 2, 1987.
355 * version 18.46 released on June 9, 1987.
357 * version 18.47 released on June 18, 1987.
359 * version 18.48 released on September 3, 1987.
361 * version 18.49 released on September 18, 1987.
363 * version 18.50 released on February 13, 1988.
365 * version 18.51 released on May 7, 1988.
367 * version 18.52 released on September 1, 1988.
369 * version 18.53 released on February 24, 1989.
371 * version 18.54 released on April 26, 1989.
373 * version 18.55 released on August 23, 1989. This is the earliest
374 version that is still available by FTP.
376 * version 18.56 released on January 17, 1991.
378 * version 18.57 released late January, 1991.
380 * version 18.58 released ?????.
382 * version 18.59 released October 31, 1992.
385 File: internals.info, Node: Lucid Emacs, Next: GNU Emacs 19, Prev: Through Version 18, Up: A History of Emacs
390 Lucid Emacs was developed by the (now-defunct) Lucid Inc., a maker of
391 C++ and Lisp development environments. It began when Lucid decided they
392 wanted to use Emacs as the editor and cornerstone of their C++
393 development environment (called "Energize"). They needed many features
394 that were not available in the existing version of GNU Emacs (version
395 18.5something), in particular good and integrated support for GUI
396 elements such as mouse support, multiple fonts, multiple window-system
397 windows, etc. A branch of GNU Emacs called Epoch, written at the
398 University of Illinois, existed that supplied many of these features;
399 however, Lucid needed more than what existed in Epoch. At the time, the
400 Free Software Foundation was working on version 19 of Emacs (this was
401 sometime around 1991), which was planned to have similar features, and
402 so Lucid decided to work with the Free Software Foundation. Their plan
403 was to add features that they needed, and coordinate with the FSF so
404 that the features would get included back into Emacs version 19.
406 Delays in the release of version 19 occurred, however (resulting in
407 it finally being released more than a year after what was initially
408 planned), and Lucid encountered unexpected technical resistance in
409 getting their changes merged back into version 19, so they decided to
410 release their own version of Emacs, which became Lucid Emacs 19.0.
412 The initial authors of Lucid Emacs were Matthieu Devin, Harlan
413 Sexton, and Eric Benson, and the work was later taken over by Jamie
414 Zawinski, who became "Mr. Lucid Emacs" for many releases.
416 A time line for Lucid Emacs/XEmacs is
418 * version 19.0 shipped with Energize 1.0, April 1992.
420 * version 19.1 released June 4, 1992.
422 * version 19.2 released June 19, 1992.
424 * version 19.3 released September 9, 1992.
426 * version 19.4 released January 21, 1993.
428 * version 19.5 was a repackaging of 19.4 with a few bug fixes and
429 shipped with Energize 2.0. Never released to the net.
431 * version 19.6 released April 9, 1993.
433 * version 19.7 was a repackaging of 19.6 with a few bug fixes and
434 shipped with Energize 2.1. Never released to the net.
436 * version 19.8 released September 6, 1993.
438 * version 19.9 released January 12, 1994.
440 * version 19.10 released May 27, 1994.
442 * version 19.11 (first XEmacs) released September 13, 1994.
444 * version 19.12 released June 23, 1995.
446 * version 19.13 released September 1, 1995.
448 * version 19.14 released June 23, 1996.
450 * version 20.0 released February 9, 1997.
452 * version 19.15 released March 28, 1997.
454 * version 20.1 (not released to the net) April 15, 1997.
456 * version 20.2 released May 16, 1997.
458 * version 19.16 released October 31, 1997.
460 * version 20.3 (the first stable version of XEmacs 20.x) released
461 November 30, 1997. version 20.4 released February 28, 1998.
464 File: internals.info, Node: GNU Emacs 19, Next: GNU Emacs 20, Prev: Lucid Emacs, Up: A History of Emacs
469 About a year after the initial release of Lucid Emacs, the FSF
470 released a beta of their version of Emacs 19 (referred to here as "GNU
471 Emacs"). By this time, the current version of Lucid Emacs was 19.6.
472 (Strangely, the first released beta from the FSF was GNU Emacs 19.7.) A
473 time line for GNU Emacs version 19 is
475 * version 19.8 (beta) released May 27, 1993.
477 * version 19.9 (beta) released May 27, 1993.
479 * version 19.10 (beta) released May 30, 1993.
481 * version 19.11 (beta) released June 1, 1993.
483 * version 19.12 (beta) released June 2, 1993.
485 * version 19.13 (beta) released June 8, 1993.
487 * version 19.14 (beta) released June 17, 1993.
489 * version 19.15 (beta) released June 19, 1993.
491 * version 19.16 (beta) released July 6, 1993.
493 * version 19.17 (beta) released late July, 1993.
495 * version 19.18 (beta) released August 9, 1993.
497 * version 19.19 (beta) released August 15, 1993.
499 * version 19.20 (beta) released November 17, 1993.
501 * version 19.21 (beta) released November 17, 1993.
503 * version 19.22 (beta) released November 28, 1993.
505 * version 19.23 (beta) released May 17, 1994.
507 * version 19.24 (beta) released May 16, 1994.
509 * version 19.25 (beta) released June 3, 1994.
511 * version 19.26 (beta) released September 11, 1994.
513 * version 19.27 (beta) released September 14, 1994.
515 * version 19.28 (first "official" release) released November 1, 1994.
517 * version 19.29 released June 21, 1995.
519 * version 19.30 released November 24, 1995.
521 * version 19.31 released May 25, 1996.
523 * version 19.32 released July 31, 1996.
525 * version 19.33 released August 11, 1996.
527 * version 19.34 released August 21, 1996.
529 * version 19.34b released September 6, 1996.
531 In some ways, GNU Emacs 19 was better than Lucid Emacs; in some ways,
532 worse. Lucid soon began incorporating features from GNU Emacs 19 into
533 Lucid Emacs; the work was mostly done by Richard Mlynarik, who had been
534 working on and using GNU Emacs for a long time (back as far as version
538 File: internals.info, Node: GNU Emacs 20, Next: XEmacs, Prev: GNU Emacs 19, Up: A History of Emacs
543 On February 2, 1997 work began on GNU Emacs to integrate Mule. The
544 first release was made in September of that year.
546 A timeline for Emacs 20 is
548 * version 20.1 released September 17, 1997.
550 * version 20.2 released September 20, 1997.
552 * version 20.3 released August 19, 1998.
555 File: internals.info, Node: XEmacs, Prev: GNU Emacs 20, Up: A History of Emacs
560 Around the time that Lucid was developing Energize, Sun Microsystems
561 was developing their own development environment (called "SPARCWorks")
562 and also decided to use Emacs. They joined forces with the Epoch team
563 at the University of Illinois and later with Lucid. The maintainer of
564 the last-released version of Epoch was Marc Andreessen, but he dropped
565 out and the Epoch project, headed by Simon Kaplan, lured Chuck Thompson
566 away from a system administration job to become the primary Lucid Emacs
567 author for Epoch and Sun. Chuck's area of specialty became the
568 redisplay engine (he replaced the old Lucid Emacs redisplay engine with
569 a ported version from Epoch and then later rewrote it from scratch).
570 Sun also hired Ben Wing (the author of Win-Emacs, a port of Lucid Emacs
571 to Microsoft Windows 3.1) in 1993, for what was initially a one-month
572 contract to fix some event problems but later became a many-year
573 involvement, punctuated by a six-month contract with Amdahl Corporation.
575 In 1994, Sun and Lucid agreed to rename Lucid Emacs to XEmacs (a name
576 not favorable to either company); the first release called XEmacs was
577 version 19.11. In June 1994, Lucid folded and Jamie quit to work for
578 the newly formed Mosaic Communications Corp., later Netscape
579 Communications Corp. (co-founded by the same Marc Andreessen, who had
580 quit his Epoch job to work on a graphical browser for the World Wide
581 Web). Chuck then become the primary maintainer of XEmacs, and put out
582 versions 19.11 through 19.14 in conjunction with Ben. For 19.12 and
583 19.13, Chuck added the new redisplay and many other display improvements
584 and Ben added MULE support (support for Asian and other languages) and
585 redesigned most of the internal Lisp subsystems to better support the
586 MULE work and the various other features being added to XEmacs. After
587 19.14 Chuck retired as primary maintainer and Steve Baur stepped in.
589 Soon after 19.13 was released, work began in earnest on the MULE
590 internationalization code and the source tree was divided into two
591 development paths. The MULE version was initially called 19.20, but was
592 soon renamed to 20.0. In 1996 Martin Buchholz of Sun Microsystems took
593 over the care and feeding of it and worked on it in parallel with the
594 19.14 development that was occurring at the same time. After much work
595 by Martin, it was decided to release 20.0 ahead of 19.15 in February
596 1997. The source tree remained divided until 20.2 when the version 19
597 source was finally retired at version 19.16.
599 In 1997, Sun finally dropped all pretense of support for XEmacs and
600 Martin Buchholz left the company in November. Since then, and mostly
601 for the previous year, because Steve Baur was never paid to work on
602 XEmacs, XEmacs has existed solely on the contributions of volunteers
603 from the Free Software Community. Starting from 1997, Hrvoje Niksic and
604 Kyle Jones have figured prominently in XEmacs development.
606 Many attempts have been made to merge XEmacs and GNU Emacs, but they
607 have consistently failed.
609 A more detailed history is contained in the XEmacs About page.
612 File: internals.info, Node: XEmacs From the Outside, Next: The Lisp Language, Prev: A History of Emacs, Up: Top
614 XEmacs From the Outside
615 ***********************
617 XEmacs appears to the outside world as an editor, but it is really a
618 Lisp environment. At its heart is a Lisp interpreter; it also
619 "happens" to contain many specialized object types (e.g. buffers,
620 windows, frames, events) that are useful for implementing an editor.
621 Some of these objects (in particular windows and frames) have
622 displayable representations, and XEmacs provides a function
623 `redisplay()' that ensures that the display of all such objects matches
624 their internal state. Most of the time, a standard Lisp environment is
625 in a "read-eval-print" loop--i.e. "read some Lisp code, execute it, and
626 print the results". XEmacs has a similar loop:
630 * dispatch the event (i.e. "do it")
634 Reading an event is done using the Lisp function `next-event', which
635 waits for something to happen (typically, the user presses a key or
636 moves the mouse) and returns an event object describing this.
637 Dispatching an event is done using the Lisp function `dispatch-event',
638 which looks up the event in a keymap object (a particular kind of
639 object that associates an event with a Lisp function) and calls that
640 function. The function "does" what the user has requested by changing
641 the state of particular frame objects, buffer objects, etc. Finally,
642 `redisplay()' is called, which updates the display to reflect those
643 changes just made. Thus is an "editor" born.
645 Note that you do not have to use XEmacs as an editor; you could just
646 as well make it do your taxes, compute pi, play bridge, etc. You'd just
647 have to write functions to do those operations in Lisp.
650 File: internals.info, Node: The Lisp Language, Next: XEmacs From the Perspective of Building, Prev: XEmacs From the Outside, Up: Top
655 Lisp is a general-purpose language that is higher-level than C and in
656 many ways more powerful than C. Powerful dialects of Lisp such as
657 Common Lisp are probably much better languages for writing very large
658 applications than is C. (Unfortunately, for many non-technical reasons
659 C and its successor C++ have become the dominant languages for
660 application development. These languages are both inadequate for
661 extremely large applications, which is evidenced by the fact that newer,
662 larger programs are becoming ever harder to write and are requiring ever
663 more programmers despite great increases in C development environments;
664 and by the fact that, although hardware speeds and reliability have been
665 growing at an exponential rate, most software is still generally
666 considered to be slow and buggy.)
668 The new Java language holds promise as a better general-purpose
669 development language than C. Java has many features in common with
670 Lisp that are not shared by C (this is not a coincidence, since Java
671 was designed by James Gosling, a former Lisp hacker). This will be
672 discussed more later.
674 For those used to C, here is a summary of the basic differences
677 1. Lisp has an extremely regular syntax. Every function, expression,
678 and control statement is written in the form
682 This is as opposed to C, which writes functions as
684 func(ARG1, ARG2, ...)
686 but writes expressions involving operators as (e.g.)
690 and writes control statements as (e.g.)
692 while (EXPR) { STATEMENT1; STATEMENT2; ... }
694 Lisp equivalents of the latter two would be
700 (while EXPR STATEMENT1 STATEMENT2 ...)
702 2. Lisp is a safe language. Assuming there are no bugs in the Lisp
703 interpreter/compiler, it is impossible to write a program that
704 "core dumps" or otherwise causes the machine to execute an illegal
705 instruction. This is very different from C, where perhaps the most
706 common outcome of a bug is exactly such a crash. A corollary of
707 this is that the C operation of casting a pointer is impossible
708 (and unnecessary) in Lisp, and that it is impossible to access
709 memory outside the bounds of an array.
711 3. Programs and data are written in the same form. The
712 parenthesis-enclosing form described above for statements is the
713 same form used for the most common data type in Lisp, the list.
714 Thus, it is possible to represent any Lisp program using Lisp data
715 types, and for one program to construct Lisp statements and then
716 dynamically "evaluate" them, or cause them to execute.
718 4. All objects are "dynamically typed". This means that part of every
719 object is an indication of what type it is. A Lisp program can
720 manipulate an object without knowing what type it is, and can
721 query an object to determine its type. This means that,
722 correspondingly, variables and function parameters can hold
723 objects of any type and are not normally declared as being of any
724 particular type. This is opposed to the "static typing" of C,
725 where variables can hold exactly one type of object and must be
726 declared as such, and objects do not contain an indication of
727 their type because it's implicit in the variables they are stored
728 in. It is possible in C to have a variable hold different types
729 of objects (e.g. through the use of `void *' pointers or
730 variable-argument functions), but the type information must then be
731 passed explicitly in some other fashion, leading to additional
734 5. Allocated memory is automatically reclaimed when it is no longer
735 in use. This operation is called "garbage collection" and
736 involves looking through all variables to see what memory is being
737 pointed to, and reclaiming any memory that is not pointed to and
738 is thus "inaccessible" and out of use. This is as opposed to C,
739 in which allocated memory must be explicitly reclaimed using
740 `free()'. If you simply drop all pointers to memory without
741 freeing it, it becomes "leaked" memory that still takes up space.
742 Over a long period of time, this can cause your program to grow
743 and grow until it runs out of memory.
745 6. Lisp has built-in facilities for handling errors and exceptions.
746 In C, when an error occurs, usually either the program exits
747 entirely or the routine in which the error occurs returns a value
748 indicating this. If an error occurs in a deeply-nested routine,
749 then every routine currently called must unwind itself normally
750 and return an error value back up to the next routine. This means
751 that every routine must explicitly check for an error in all the
752 routines it calls; if it does not do so, unexpected and often
753 random behavior results. This is an extremely common source of
754 bugs in C programs. An alternative would be to do a non-local
755 exit using `longjmp()', but that is often very dangerous because
756 the routines that were exited past had no opportunity to clean up
757 after themselves and may leave things in an inconsistent state,
758 causing a crash shortly afterwards.
760 Lisp provides mechanisms to make such non-local exits safe. When
761 an error occurs, a routine simply signals that an error of a
762 particular class has occurred, and a non-local exit takes place.
763 Any routine can trap errors occurring in routines it calls by
764 registering an error handler for some or all classes of errors.
765 (If no handler is registered, a default handler, generally
766 installed by the top-level event loop, is executed; this prints
767 out the error and continues.) Routines can also specify cleanup
768 code (called an "unwind-protect") that will be called when control
769 exits from a block of code, no matter how that exit occurs--i.e.
770 even if a function deeply nested below it causes a non-local exit
771 back to the top level.
773 Note that this facility has appeared in some recent vintages of C,
774 in particular Visual C++ and other PC compilers written for the
777 7. In Emacs Lisp, local variables are "dynamically scoped". This
778 means that if you declare a local variable in a particular
779 function, and then call another function, that subfunction can
780 "see" the local variable you declared. This is actually
781 considered a bug in Emacs Lisp and in all other early dialects of
782 Lisp, and was corrected in Common Lisp. (In Common Lisp, you can
783 still declare dynamically scoped variables if you want to--they
784 are sometimes useful--but variables by default are "lexically
787 For those familiar with Lisp, Emacs Lisp is modelled after MacLisp,
788 an early dialect of Lisp developed at MIT (no relation to the Macintosh
789 computer). There is a Common Lisp compatibility package available for
790 Emacs that provides many of the features of Common Lisp.
792 The Java language is derived in many ways from C, and shares a
793 similar syntax, but has the following features in common with Lisp (and
796 1. Java is a safe language, like Lisp.
798 2. Java provides garbage collection, like Lisp.
800 3. Java has built-in facilities for handling errors and exceptions,
803 4. Java has a type system that combines the best advantages of both
804 static and dynamic typing. Objects (except very simple types) are
805 explicitly marked with their type, as in dynamic typing; but there
806 is a hierarchy of types and functions are declared to accept only
807 certain types, thus providing the increased compile-time
808 error-checking of static typing.
810 The Java language also has some negative attributes:
812 1. Java uses the edit/compile/run model of software development. This
813 makes it hard to use interactively. For example, to use Java like
814 `bc' it is necessary to write a special purpose, albeit tiny,
815 application. In Emacs Lisp, a calculator comes built-in without
816 any effort - one can always just type an expression in the
819 2. Java tries too hard to enforce, not merely enable, portability,
820 making ordinary access to standard OS facilities painful. Java
821 has an "agenda". I think this is why `chdir' is not part of
822 standard Java, which is inexcusable.
824 Unfortunately, there is no perfect language. Static typing allows a
825 compiler to catch programmer errors and produce more efficient code, but
826 makes programming more tedious and less fun. For the foreseeable
827 future, an Ideal Editing and Programming Environment (and that is what
828 XEmacs aspires to) will be programmable in multiple languages: high
829 level ones like Lisp for user customization and prototyping, and lower
830 level ones for infrastructure and industrial strength applications. If
831 I had my way, XEmacs would be friendly towards the Python, Scheme, C++,
832 ML, etc... communities. But there are serious technical difficulties to
835 The word "application" in the previous paragraph was used
836 intentionally. XEmacs implements an API for programs written in Lisp
837 that makes it a full-fledged application platform, very much like an OS
841 File: internals.info, Node: XEmacs From the Perspective of Building, Next: XEmacs From the Inside, Prev: The Lisp Language, Up: Top
843 XEmacs From the Perspective of Building
844 ***************************************
846 The heart of XEmacs is the Lisp environment, which is written in C.
847 This is contained in the `src/' subdirectory. Underneath `src/' are
848 two subdirectories of header files: `s/' (header files for particular
849 operating systems) and `m/' (header files for particular machine
850 types). In practice the distinction between the two types of header
851 files is blurred. These header files define or undefine certain
852 preprocessor constants and macros to indicate particular
853 characteristics of the associated machine or operating system. As part
854 of the configure process, one `s/' file and one `m/' file is identified
855 for the particular environment in which XEmacs is being built.
857 XEmacs also contains a great deal of Lisp code. This implements the
858 operations that make XEmacs useful as an editor as well as just a Lisp
859 environment, and also contains many add-on packages that allow XEmacs to
860 browse directories, act as a mail and Usenet news reader, compile Lisp
861 code, etc. There is actually more Lisp code than C code associated with
862 XEmacs, but much of the Lisp code is peripheral to the actual operation
863 of the editor. The Lisp code all lies in subdirectories underneath the
866 The `lwlib/' directory contains C code that implements a generalized
867 interface onto different X widget toolkits and also implements some
868 widgets of its own that behave like Motif widgets but are faster, free,
869 and in some cases more powerful. The code in this directory compiles
870 into a library and is mostly independent from XEmacs.
872 The `etc/' directory contains various data files associated with
873 XEmacs. Some of them are actually read by XEmacs at startup; others
874 merely contain useful information of various sorts.
876 The `lib-src/' directory contains C code for various auxiliary
877 programs that are used in connection with XEmacs. Some of them are used
878 during the build process; others are used to perform certain functions
879 that cannot conveniently be placed in the XEmacs executable (e.g. the
880 `movemail' program for fetching mail out of `/var/spool/mail', which
881 must be setgid to `mail' on many systems; and the `gnuclient' program,
882 which allows an external script to communicate with a running XEmacs
885 The `man/' directory contains the sources for the XEmacs
886 documentation. It is mostly in a form called Texinfo, which can be
887 converted into either a printed document (by passing it through TeX) or
888 into on-line documentation called "info files".
890 The `info/' directory contains the results of formatting the XEmacs
891 documentation as "info files", for on-line use. These files are used
892 when you enter the Info system using `C-h i' or through the Help menu.
894 The `dynodump/' directory contains auxiliary code used to build
895 XEmacs on Solaris platforms.
897 The other directories contain various miscellaneous code and
898 information that is not normally used or needed.
900 The first step of building involves running the `configure' program
901 and passing it various parameters to specify any optional features you
902 want and compiler arguments and such, as described in the `INSTALL'
903 file. This determines what the build environment is, chooses the
904 appropriate `s/' and `m/' file, and runs a series of tests to determine
905 many details about your environment, such as which library functions
906 are available and exactly how they work. The reason for running these
907 tests is that it allows XEmacs to be compiled on a much wider variety
908 of platforms than those that the XEmacs developers happen to be
909 familiar with, including various sorts of hybrid platforms. This is
910 especially important now that many operating systems give you a great
911 deal of control over exactly what features you want installed, and allow
912 for easy upgrading of parts of a system without upgrading the rest. It
913 would be impossible to pre-determine and pre-specify the information for
914 all possible configurations.
916 In fact, the `s/' and `m/' files are basically _evil_, since they
917 contain unmaintainable platform-specific hard-coded information.
918 XEmacs has been moving in the direction of having all system-specific
919 information be determined dynamically by `configure'. Perhaps someday
920 we can `rm -rf src/s src/m'.
922 When configure is done running, it generates `Makefile's and
923 `GNUmakefile's and the file `src/config.h' (which describes the
924 features of your system) from template files. You then run `make',
925 which compiles the auxiliary code and programs in `lib-src/' and
926 `lwlib/' and the main XEmacs executable in `src/'. The result of
927 compiling and linking is an executable called `temacs', which is _not_
928 the final XEmacs executable. `temacs' by itself is not intended to
929 function as an editor or even display any windows on the screen, and if
930 you simply run it, it will exit immediately. The `Makefile' runs
931 `temacs' with certain options that cause it to initialize itself, read
932 in a number of basic Lisp files, and then dump itself out into a new
933 executable called `xemacs'. This new executable has been
934 pre-initialized and contains pre-digested Lisp code that is necessary
935 for the editor to function (this includes most basic editing functions,
936 e.g. `kill-line', that can be defined in terms of other Lisp
937 primitives; some initialization code that is called when certain
938 objects, such as frames, are created; and all of the standard
939 keybindings and code for the actions they result in). This executable,
940 `xemacs', is the executable that you run to use the XEmacs editor.
942 Although `temacs' is not intended to be run as an editor, it can, by
943 using the incantation `temacs -batch -l loadup.el run-temacs'. This is
944 useful when the dumping procedure described above is broken, or when
945 using certain program debugging tools such as Purify. These tools get
946 mighty confused by the tricks played by the XEmacs build process, such
947 as allocation memory in one process, and freeing it in the next.
950 File: internals.info, Node: XEmacs From the Inside, Next: The XEmacs Object System (Abstractly Speaking), Prev: XEmacs From the Perspective of Building, Up: Top
952 XEmacs From the Inside
953 **********************
955 Internally, XEmacs is quite complex, and can be very confusing. To
956 simplify things, it can be useful to think of XEmacs as containing an
957 event loop that "drives" everything, and a number of other subsystems,
958 such as a Lisp engine and a redisplay mechanism. Each of these other
959 subsystems exists simultaneously in XEmacs, and each has a certain
960 state. The flow of control continually passes in and out of these
961 different subsystems in the course of normal operation of the editor.
963 It is important to keep in mind that, most of the time, the editor is
964 "driven" by the event loop. Except during initialization and batch
965 mode, all subsystems are entered directly or indirectly through the
966 event loop, and ultimately, control exits out of all subsystems back up
967 to the event loop. This cycle of entering a subsystem, exiting back out
968 to the event loop, and starting another iteration of the event loop
969 occurs once each keystroke, mouse motion, etc.
971 If you're trying to understand a particular subsystem (other than the
972 event loop), think of it as a "daemon" process or "servant" that is
973 responsible for one particular aspect of a larger system, and
974 periodically receives commands or environment changes that cause it to
975 do something. Ultimately, these commands and environment changes are
976 always triggered by the event loop. For example:
978 * The window and frame mechanism is responsible for keeping track of
979 what windows and frames exist, what buffers are in them, etc. It
980 is periodically given commands (usually from the user) to make a
981 change to the current window/frame state: i.e. create a new frame,
982 delete a window, etc.
984 * The buffer mechanism is responsible for keeping track of what
985 buffers exist and what text is in them. It is periodically given
986 commands (usually from the user) to insert or delete text, create
987 a buffer, etc. When it receives a text-change command, it
988 notifies the redisplay mechanism.
990 * The redisplay mechanism is responsible for making sure that
991 windows and frames are displayed correctly. It is periodically
992 told (by the event loop) to actually "do its job", i.e. snoop
993 around and see what the current state of the environment (mostly
994 of the currently-existing windows, frames, and buffers) is, and
995 make sure that that state matches what's actually displayed. It
996 keeps lots and lots of information around (such as what is
997 actually being displayed currently, and what the environment was
998 last time it checked) so that it can minimize the work it has to
999 do. It is also helped along in that whenever a relevant change to
1000 the environment occurs, the redisplay mechanism is told about
1001 this, so it has a pretty good idea of where it has to look to find
1002 possible changes and doesn't have to look everywhere.
1004 * The Lisp engine is responsible for executing the Lisp code in
1005 which most user commands are written. It is entered through a
1006 call to `eval' or `funcall', which occurs as a result of
1007 dispatching an event from the event loop. The functions it calls
1008 issue commands to the buffer mechanism, the window/frame
1011 * The Lisp allocation subsystem is responsible for keeping track of
1012 Lisp objects. It is given commands from the Lisp engine to
1013 allocate objects, garbage collect, etc.
1017 The important idea here is that there are a number of independent
1018 subsystems each with its own responsibility and persistent state, just
1019 like different employees in a company, and each subsystem is
1020 periodically given commands from other subsystems. Commands can flow
1021 from any one subsystem to any other, but there is usually some sort of
1022 hierarchy, with all commands originating from the event subsystem.
1024 XEmacs is entered in `main()', which is in `emacs.c'. When this is
1025 called the first time (in a properly-invoked `temacs'), it does the
1028 1. It does some very basic environment initializations, such as
1029 determining where it and its directories (e.g. `lisp/' and `etc/')
1030 reside and setting up signal handlers.
1032 2. It initializes the entire Lisp interpreter.
1034 3. It sets the initial values of many built-in variables (including
1035 many variables that are visible to Lisp programs), such as the
1036 global keymap object and the built-in faces (a face is an object
1037 that describes the display characteristics of text). This
1038 involves creating Lisp objects and thus is dependent on step (2).
1040 4. It performs various other initializations that are relevant to the
1041 particular environment it is running in, such as retrieving
1042 environment variables, determining the current date and the user
1043 who is running the program, examining its standard input, creating
1044 any necessary file descriptors, etc.
1046 5. At this point, the C initialization is complete. A Lisp program
1047 that was specified on the command line (usually `loadup.el') is
1048 called (temacs is normally invoked as `temacs -batch -l loadup.el
1049 dump'). `loadup.el' loads all of the other Lisp files that are
1050 needed for the operation of the editor, calls the `dump-emacs'
1051 function to write out `xemacs', and then kills the temacs process.
1053 When `xemacs' is then run, it only redoes steps (1) and (4) above;
1054 all variables already contain the values they were set to when the
1055 executable was dumped, and all memory that was allocated with
1056 `malloc()' is still around. (XEmacs knows whether it is being run as
1057 `xemacs' or `temacs' because it sets the global variable `initialized'
1058 to 1 after step (4) above.) At this point, `xemacs' calls a Lisp
1059 function to do any further initialization, which includes parsing the
1060 command-line (the C code can only do limited command-line parsing,
1061 which includes looking for the `-batch' and `-l' flags and a few other
1062 flags that it needs to know about before initialization is complete),
1063 creating the first frame (or "window" in standard window-system
1064 parlance), running the user's init file (usually the file `.emacs' in
1065 the user's home directory), etc. The function to do this is usually
1066 called `normal-top-level'; `loadup.el' tells the C code about this
1067 function by setting its name as the value of the Lisp variable
1070 When the Lisp initialization code is done, the C code enters the
1071 event loop, and stays there for the duration of the XEmacs process.
1072 The code for the event loop is contained in `keyboard.c', and is called
1073 `Fcommand_loop_1()'. Note that this event loop could very well be
1074 written in Lisp, and in fact a Lisp version exists; but apparently,
1075 doing this makes XEmacs run noticeably slower.
1077 Notice how much of the initialization is done in Lisp, not in C. In
1078 general, XEmacs tries to move as much code as is possible into Lisp.
1079 Code that remains in C is code that implements the Lisp interpreter
1080 itself, or code that needs to be very fast, or code that needs to do
1081 system calls or other such stuff that needs to be done in C, or code
1082 that needs to have access to "forbidden" structures. (One conscious
1083 aspect of the design of Lisp under XEmacs is a clean separation between
1084 the external interface to a Lisp object's functionality and its internal
1085 implementation. Part of this design is that Lisp programs are
1086 forbidden from accessing the contents of the object other than through
1087 using a standard API. In this respect, XEmacs Lisp is similar to
1088 modern Lisp dialects but differs from GNU Emacs, which tends to expose
1089 the implementation and allow Lisp programs to look at it directly. The
1090 major advantage of hiding the implementation is that it allows the
1091 implementation to be redesigned without affecting any Lisp programs,
1092 including those that might want to be "clever" by looking directly at
1093 the object's contents and possibly manipulating them.)
1095 Moving code into Lisp makes the code easier to debug and maintain and
1096 makes it much easier for people who are not XEmacs developers to
1097 customize XEmacs, because they can make a change with much less chance
1098 of obscure and unwanted interactions occurring than if they were to
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