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Internally, XEmacs is quite complex, and can be very confusing. To simplify things, it can be useful to think of XEmacs as containing an event loop that "drives" everything, and a number of other subsystems, such as a Lisp engine and a redisplay mechanism. Each of these other subsystems exists simultaneously in XEmacs, and each has a certain state. The flow of control continually passes in and out of these different subsystems in the course of normal operation of the editor.
It is important to keep in mind that, most of the time, the editor is "driven" by the event loop. Except during initialization and batch mode, all subsystems are entered directly or indirectly through the event loop, and ultimately, control exits out of all subsystems back up to the event loop. This cycle of entering a subsystem, exiting back out to the event loop, and starting another iteration of the event loop occurs once each keystroke, mouse motion, etc.
If you're trying to understand a particular subsystem (other than the event loop), think of it as a "daemon" process or "servant" that is responsible for one particular aspect of a larger system, and periodically receives commands or environment changes that cause it to do something. Ultimately, these commands and environment changes are always triggered by the event loop. For example:
funcall, which occurs as a result of dispatching an event from the event loop. The functions it calls issue commands to the buffer mechanism, the window/frame subsystem, etc.
The important idea here is that there are a number of independent subsystems each with its own responsibility and persistent state, just like different employees in a company, and each subsystem is periodically given commands from other subsystems. Commands can flow from any one subsystem to any other, but there is usually some sort of hierarchy, with all commands originating from the event subsystem.
XEmacs is entered in
main(), which is in `emacs.c'. When
this is called the first time (in a properly-invoked `temacs'), it
does the following:
temacs -batch -l loadup.el dump). `loadup.el' loads all of the other Lisp files that are needed for the operation of the editor, calls the
dump-emacsfunction to write out `xemacs', and then kills the temacs process.
When `xemacs' is then run, it only redoes steps (1) and (4)
above; all variables already contain the values they were set to when
the executable was dumped, and all memory that was allocated with
malloc() is still around. (XEmacs knows whether it is being run
as `xemacs' or `temacs' because it sets the global variable
initialized to 1 after step (4) above.) At this point,
`xemacs' calls a Lisp function to do any further initialization,
which includes parsing the command-line (the C code can only do limited
command-line parsing, which includes looking for the `-batch' and
`-l' flags and a few other flags that it needs to know about before
initialization is complete), creating the first frame (or window
in standard window-system parlance), running the user's init file
(usually the file `.emacs' in the user's home directory), etc. The
function to do this is usually called
`loadup.el' tells the C code about this function by setting its
name as the value of the Lisp variable
When the Lisp initialization code is done, the C code enters the event
loop, and stays there for the duration of the XEmacs process. The code
for the event loop is contained in `cmdloop.c', and is called
Fcommand_loop_1(). Note that this event loop could very well be
written in Lisp, and in fact a Lisp version exists; but apparently,
doing this makes XEmacs run noticeably slower.
Notice how much of the initialization is done in Lisp, not in C. In general, XEmacs tries to move as much code as is possible into Lisp. Code that remains in C is code that implements the Lisp interpreter itself, or code that needs to be very fast, or code that needs to do system calls or other such stuff that needs to be done in C, or code that needs to have access to "forbidden" structures. (One conscious aspect of the design of Lisp under XEmacs is a clean separation between the external interface to a Lisp object's functionality and its internal implementation. Part of this design is that Lisp programs are forbidden from accessing the contents of the object other than through using a standard API. In this respect, XEmacs Lisp is similar to modern Lisp dialects but differs from GNU Emacs, which tends to expose the implementation and allow Lisp programs to look at it directly. The major advantage of hiding the implementation is that it allows the implementation to be redesigned without affecting any Lisp programs, including those that might want to be "clever" by looking directly at the object's contents and possibly manipulating them.)
Moving code into Lisp makes the code easier to debug and maintain and makes it much easier for people who are not XEmacs developers to customize XEmacs, because they can make a change with much less chance of obscure and unwanted interactions occurring than if they were to change the C code.
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