-This is ../info/internals.info, produced by makeinfo version 4.0 from
+This is ../info/internals.info, produced by makeinfo version 4.6 from
internals/internals.texi.
INFO-DIR-SECTION XEmacs Editor
END-INFO-DIR-ENTRY
Copyright (C) 1992 - 1996 Ben Wing. Copyright (C) 1996, 1997 Sun
-Microsystems. Copyright (C) 1994 - 1998 Free Software Foundation.
-Copyright (C) 1994, 1995 Board of Trustees, University of Illinois.
+Microsystems. Copyright (C) 1994 - 1998, 2002, 2003 Free Software
+Foundation. Copyright (C) 1994, 1995 Board of Trustees, University of
+Illinois.
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
* The XEmacs Object System (Abstractly Speaking)::
* How Lisp Objects Are Represented in C::
* Rules When Writing New C Code::
+* Regression Testing XEmacs::
* A Summary of the Various XEmacs Modules::
* Allocation of Objects in XEmacs Lisp::
* Dumping::
* General Guidelines for Writing Mule-Aware Code::
* An Example of Mule-Aware Code::
+Regression Testing XEmacs
+
A Summary of the Various XEmacs Modules
* Low-Level Modules::
* Modules for Interfacing with the Operating System::
* Modules for Interfacing with X Windows::
* Modules for Internationalization::
+* Modules for Regression Testing::
Allocation of Objects in XEmacs Lisp
A History of Emacs
******************
- XEmacs is a powerful, customizable text editor and development
+XEmacs is a powerful, customizable text editor and development
environment. It began as Lucid Emacs, which was in turn derived from
GNU Emacs, a program written by Richard Stallman of the Free Software
Foundation. GNU Emacs dates back to the 1970's, and was modelled after
Through Version 18
==================
- Although the history of the early versions of GNU Emacs is unclear,
-the history is well-known from the middle of 1985. A time line is:
+Although the history of the early versions of GNU Emacs is unclear, the
+history is well-known from the middle of 1985. A time line is:
* GNU Emacs version 15 (15.34) was released sometime in 1984 or 1985
and shared some code with a version of Emacs written by James
Lucid Emacs
===========
- Lucid Emacs was developed by the (now-defunct) Lucid Inc., a maker of
+Lucid Emacs was developed by the (now-defunct) Lucid Inc., a maker of
C++ and Lisp development environments. It began when Lucid decided they
wanted to use Emacs as the editor and cornerstone of their C++
development environment (called "Energize"). They needed many features
GNU Emacs 19
============
- About a year after the initial release of Lucid Emacs, the FSF
-released a beta of their version of Emacs 19 (referred to here as "GNU
-Emacs"). By this time, the current version of Lucid Emacs was 19.6.
-(Strangely, the first released beta from the FSF was GNU Emacs 19.7.) A
-time line for GNU Emacs version 19 is
+About a year after the initial release of Lucid Emacs, the FSF released
+a beta of their version of Emacs 19 (referred to here as "GNU Emacs").
+By this time, the current version of Lucid Emacs was 19.6. (Strangely,
+the first released beta from the FSF was GNU Emacs 19.7.) A time line
+for GNU Emacs version 19 is
* version 19.8 (beta) released May 27, 1993.
GNU Emacs 20
============
- On February 2, 1997 work began on GNU Emacs to integrate Mule. The
+On February 2, 1997 work began on GNU Emacs to integrate Mule. The
first release was made in September of that year.
A timeline for Emacs 20 is
XEmacs
======
- Around the time that Lucid was developing Energize, Sun Microsystems
+Around the time that Lucid was developing Energize, Sun Microsystems
was developing their own development environment (called "SPARCWorks")
and also decided to use Emacs. They joined forces with the Epoch team
at the University of Illinois and later with Lucid. The maintainer of
XEmacs From the Outside
***********************
- XEmacs appears to the outside world as an editor, but it is really a
+XEmacs appears to the outside world as an editor, but it is really a
Lisp environment. At its heart is a Lisp interpreter; it also
"happens" to contain many specialized object types (e.g. buffers,
windows, frames, events) that are useful for implementing an editor.
The Lisp Language
*****************
- Lisp is a general-purpose language that is higher-level than C and in
+Lisp is a general-purpose language that is higher-level than C and in
many ways more powerful than C. Powerful dialects of Lisp such as
Common Lisp are probably much better languages for writing very large
applications than is C. (Unfortunately, for many non-technical reasons
XEmacs From the Perspective of Building
***************************************
- The heart of XEmacs is the Lisp environment, which is written in C.
+The heart of XEmacs is the Lisp environment, which is written in C.
This is contained in the `src/' subdirectory. Underneath `src/' are
two subdirectories of header files: `s/' (header files for particular
operating systems) and `m/' (header files for particular machine
XEmacs From the Inside
**********************
- Internally, XEmacs is quite complex, and can be very confusing. To
+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
told (by the event loop) to actually "do its job", i.e. snoop
around and see what the current state of the environment (mostly
of the currently-existing windows, frames, and buffers) is, and
- make sure that that state matches what's actually displayed. It
- keeps lots and lots of information around (such as what is
- actually being displayed currently, and what the environment was
- last time it checked) so that it can minimize the work it has to
- do. It is also helped along in that whenever a relevant change to
- the environment occurs, the redisplay mechanism is told about
- this, so it has a pretty good idea of where it has to look to find
- possible changes and doesn't have to look everywhere.
+ make sure that state matches what's actually displayed. It keeps
+ lots and lots of information around (such as what is actually
+ being displayed currently, and what the environment was last time
+ it checked) so that it can minimize the work it has to do. It is
+ also helped along in that whenever a relevant change to the
+ environment occurs, the redisplay mechanism is told about this, so
+ it has a pretty good idea of where it has to look to find possible
+ changes and doesn't have to look everywhere.
* The Lisp engine is responsible for executing the Lisp code in
which most user commands are written. It is entered through a
of obscure and unwanted interactions occurring than if they were to
change the C code.
+\1f
+File: internals.info, Node: The XEmacs Object System (Abstractly Speaking), Next: How Lisp Objects Are Represented in C, Prev: XEmacs From the Inside, Up: Top
+
+The XEmacs Object System (Abstractly Speaking)
+**********************************************
+
+At the heart of the Lisp interpreter is its management of objects.
+XEmacs Lisp contains many built-in objects, some of which are simple
+and others of which can be very complex; and some of which are very
+common, and others of which are rarely used or are only used
+internally. (Since the Lisp allocation system, with its automatic
+reclamation of unused storage, is so much more convenient than
+`malloc()' and `free()', the C code makes extensive use of it in its
+internal operations.)
+
+ The basic Lisp objects are
+
+`integer'
+ 28 or 31 bits of precision, or 60 or 63 bits on 64-bit machines;
+ the reason for this is described below when the internal Lisp
+ object representation is described.
+
+`float'
+ Same precision as a double in C.
+
+`cons'
+ A simple container for two Lisp objects, used to implement lists
+ and most other data structures in Lisp.
+
+`char'
+ An object representing a single character of text; chars behave
+ like integers in many ways but are logically considered text
+ rather than numbers and have a different read syntax. (the read
+ syntax for a char contains the char itself or some textual
+ encoding of it--for example, a Japanese Kanji character might be
+ encoded as `^[$(B#&^[(B' using the ISO-2022 encoding
+ standard--rather than the numerical representation of the char;
+ this way, if the mapping between chars and integers changes, which
+ is quite possible for Kanji characters and other extended
+ characters, the same character will still be created. Note that
+ some primitives confuse chars and integers. The worst culprit is
+ `eq', which makes a special exception and considers a char to be
+ `eq' to its integer equivalent, even though in no other case are
+ objects of two different types `eq'. The reason for this
+ monstrosity is compatibility with existing code; the separation of
+ char from integer came fairly recently.)
+
+`symbol'
+ An object that contains Lisp objects and is referred to by name;
+ symbols are used to implement variables and named functions and to
+ provide the equivalent of preprocessor constants in C.
+
+`vector'
+ A one-dimensional array of Lisp objects providing constant-time
+ access to any of the objects; access to an arbitrary object in a
+ vector is faster than for lists, but the operations that can be
+ done on a vector are more limited.
+
+`string'
+ Self-explanatory; behaves much like a vector of chars but has a
+ different read syntax and is stored and manipulated more compactly.
+
+`bit-vector'
+ A vector of bits; similar to a string in spirit.
+
+`compiled-function'
+ An object containing compiled Lisp code, known as "byte code".
+
+`subr'
+ A Lisp primitive, i.e. a Lisp-callable function implemented in C.
+
+ Note that there is no basic "function" type, as in more powerful
+versions of Lisp (where it's called a "closure"). XEmacs Lisp does not
+provide the closure semantics implemented by Common Lisp and Scheme.
+The guts of a function in XEmacs Lisp are represented in one of four
+ways: a symbol specifying another function (when one function is an
+alias for another), a list (whose first element must be the symbol
+`lambda') containing the function's source code, a compiled-function
+object, or a subr object. (In other words, given a symbol specifying
+the name of a function, calling `symbol-function' to retrieve the
+contents of the symbol's function cell will return one of these types
+of objects.)
+
+ XEmacs Lisp also contains numerous specialized objects used to
+implement the editor:
+
+`buffer'
+ Stores text like a string, but is optimized for insertion and
+ deletion and has certain other properties that can be set.
+
+`frame'
+ An object with various properties whose displayable representation
+ is a "window" in window-system parlance.
+
+`window'
+ A section of a frame that displays the contents of a buffer; often
+ called a "pane" in window-system parlance.
+
+`window-configuration'
+ An object that represents a saved configuration of windows in a
+ frame.
+
+`device'
+ An object representing a screen on which frames can be displayed;
+ equivalent to a "display" in the X Window System and a "TTY" in
+ character mode.
+
+`face'
+ An object specifying the appearance of text or graphics; it has
+ properties such as font, foreground color, and background color.
+
+`marker'
+ An object that refers to a particular position in a buffer and
+ moves around as text is inserted and deleted to stay in the same
+ relative position to the text around it.
+
+`extent'
+ Similar to a marker but covers a range of text in a buffer; can
+ also specify properties of the text, such as a face in which the
+ text is to be displayed, whether the text is invisible or
+ unmodifiable, etc.
+
+`event'
+ Generated by calling `next-event' and contains information
+ describing a particular event happening in the system, such as the
+ user pressing a key or a process terminating.
+
+`keymap'
+ An object that maps from events (described using lists, vectors,
+ and symbols rather than with an event object because the mapping
+ is for classes of events, rather than individual events) to
+ functions to execute or other events to recursively look up; the
+ functions are described by name, using a symbol, or using lists to
+ specify the function's code.
+
+`glyph'
+ An object that describes the appearance of an image (e.g. pixmap)
+ on the screen; glyphs can be attached to the beginning or end of
+ extents and in some future version of XEmacs will be able to be
+ inserted directly into a buffer.
+
+`process'
+ An object that describes a connection to an externally-running
+ process.
+
+ There are some other, less-commonly-encountered general objects:
+
+`hash-table'
+ An object that maps from an arbitrary Lisp object to another
+ arbitrary Lisp object, using hashing for fast lookup.
+
+`obarray'
+ A limited form of hash-table that maps from strings to symbols;
+ obarrays are used to look up a symbol given its name and are not
+ actually their own object type but are kludgily represented using
+ vectors with hidden fields (this representation derives from GNU
+ Emacs).
+
+`specifier'
+ A complex object used to specify the value of a display property; a
+ default value is given and different values can be specified for
+ particular frames, buffers, windows, devices, or classes of device.
+
+`char-table'
+ An object that maps from chars or classes of chars to arbitrary
+ Lisp objects; internally char tables use a complex nested-vector
+ representation that is optimized to the way characters are
+ represented as integers.
+
+`range-table'
+ An object that maps from ranges of integers to arbitrary Lisp
+ objects.
+
+ And some strange special-purpose objects:
+
+`charset'
+`coding-system'
+ Objects used when MULE, or multi-lingual/Asian-language, support is
+ enabled.
+
+`color-instance'
+`font-instance'
+`image-instance'
+ An object that encapsulates a window-system resource; instances are
+ mostly used internally but are exposed on the Lisp level for
+ cleanness of the specifier model and because it's occasionally
+ useful for Lisp program to create or query the properties of
+ instances.
+
+`subwindow'
+ An object that encapsulate a "subwindow" resource, i.e. a
+ window-system child window that is drawn into by an external
+ process; this object should be integrated into the glyph system
+ but isn't yet, and may change form when this is done.
+
+`tooltalk-message'
+`tooltalk-pattern'
+ Objects that represent resources used in the ToolTalk interprocess
+ communication protocol.
+
+`toolbar-button'
+ An object used in conjunction with the toolbar.
+
+ And objects that are only used internally:
+
+`opaque'
+ A generic object for encapsulating arbitrary memory; this allows
+ you the generality of `malloc()' and the convenience of the Lisp
+ object system.
+
+`lstream'
+ A buffering I/O stream, used to provide a unified interface to
+ anything that can accept output or provide input, such as a file
+ descriptor, a stdio stream, a chunk of memory, a Lisp buffer, a
+ Lisp string, etc.; it's a Lisp object to make its memory
+ management more convenient.
+
+`char-table-entry'
+ Subsidiary objects in the internal char-table representation.
+
+`extent-auxiliary'
+`menubar-data'
+`toolbar-data'
+ Various special-purpose objects that are basically just used to
+ encapsulate memory for particular subsystems, similar to the more
+ general "opaque" object.
+
+`symbol-value-forward'
+`symbol-value-buffer-local'
+`symbol-value-varalias'
+`symbol-value-lisp-magic'
+ Special internal-only objects that are placed in the value cell of
+ a symbol to indicate that there is something special with this
+ variable - e.g. it has no value, it mirrors another variable, or
+ it mirrors some C variable; there is really only one kind of
+ object, called a "symbol-value-magic", but it is sort-of halfway
+ kludged into semi-different object types.
+
+ Some types of objects are "permanent", meaning that once created,
+they do not disappear until explicitly destroyed, using a function such
+as `delete-buffer', `delete-window', `delete-frame', etc. Others will
+disappear once they are not longer used, through the garbage collection
+mechanism. Buffers, frames, windows, devices, and processes are among
+the objects that are permanent. Note that some objects can go both
+ways: Faces can be created either way; extents are normally permanent,
+but detached extents (extents not referring to any text, as happens to
+some extents when the text they are referring to is deleted) are
+temporary. Note that some permanent objects, such as faces and coding
+systems, cannot be deleted. Note also that windows are unique in that
+they can be _undeleted_ after having previously been deleted. (This
+happens as a result of restoring a window configuration.)
+
+ Note that many types of objects have a "read syntax", i.e. a way of
+specifying an object of that type in Lisp code. When you load a Lisp
+file, or type in code to be evaluated, what really happens is that the
+function `read' is called, which reads some text and creates an object
+based on the syntax of that text; then `eval' is called, which possibly
+does something special; then this loop repeats until there's no more
+text to read. (`eval' only actually does something special with
+symbols, which causes the symbol's value to be returned, similar to
+referencing a variable; and with conses [i.e. lists], which cause a
+function invocation. All other values are returned unchanged.)
+
+ The read syntax
+
+ 17297
+
+ converts to an integer whose value is 17297.
+
+ 1.983e-4
+
+ converts to a float whose value is 1.983e-4, or .0001983.
+
+ ?b
+
+ converts to a char that represents the lowercase letter b.
+
+ ?^[$(B#&^[(B
+
+ (where `^[' actually is an `ESC' character) converts to a particular
+Kanji character when using an ISO2022-based coding system for input.
+(To decode this goo: `ESC' begins an escape sequence; `ESC $ (' is a
+class of escape sequences meaning "switch to a 94x94 character set";
+`ESC $ ( B' means "switch to Japanese Kanji"; `#' and `&' collectively
+index into a 94-by-94 array of characters [subtract 33 from the ASCII
+value of each character to get the corresponding index]; `ESC (' is a
+class of escape sequences meaning "switch to a 94 character set"; `ESC
+(B' means "switch to US ASCII". It is a coincidence that the letter
+`B' is used to denote both Japanese Kanji and US ASCII. If the first
+`B' were replaced with an `A', you'd be requesting a Chinese Hanzi
+character from the GB2312 character set.)
+
+ "foobar"
+
+ converts to a string.
+
+ foobar
+
+ converts to a symbol whose name is `"foobar"'. This is done by
+looking up the string equivalent in the global variable `obarray',
+whose contents should be an obarray. If no symbol is found, a new
+symbol with the name `"foobar"' is automatically created and added to
+`obarray'; this process is called "interning" the symbol.
+
+ (foo . bar)
+
+ converts to a cons cell containing the symbols `foo' and `bar'.
+
+ (1 a 2.5)
+
+ converts to a three-element list containing the specified objects
+(note that a list is actually a set of nested conses; see the XEmacs
+Lisp Reference).
+
+ [1 a 2.5]
+
+ converts to a three-element vector containing the specified objects.
+
+ #[... ... ... ...]
+
+ converts to a compiled-function object (the actual contents are not
+shown since they are not relevant here; look at a file that ends with
+`.elc' for examples).
+
+ #*01110110
+
+ converts to a bit-vector.
+
+ #s(hash-table ... ...)
+
+ converts to a hash table (the actual contents are not shown).
+
+ #s(range-table ... ...)
+
+ converts to a range table (the actual contents are not shown).
+
+ #s(char-table ... ...)
+
+ converts to a char table (the actual contents are not shown).
+
+ Note that the `#s()' syntax is the general syntax for structures,
+which are not really implemented in XEmacs Lisp but should be.
+
+ When an object is printed out (using `print' or a related function),
+the read syntax is used, so that the same object can be read in again.
+
+ The other objects do not have read syntaxes, usually because it does
+not really make sense to create them in this fashion (i.e. processes,
+where it doesn't make sense to have a subprocess created as a side
+effect of reading some Lisp code), or because they can't be created at
+all (e.g. subrs). Permanent objects, as a rule, do not have a read
+syntax; nor do most complex objects, which contain too much state to be
+easily initialized through a read syntax.
+
+\1f
+File: internals.info, Node: How Lisp Objects Are Represented in C, Next: Rules When Writing New C Code, Prev: The XEmacs Object System (Abstractly Speaking), Up: Top
+
+How Lisp Objects Are Represented in C
+*************************************
+
+Lisp objects are represented in C using a 32-bit or 64-bit machine word
+(depending on the processor; i.e. DEC Alphas use 64-bit Lisp objects and
+most other processors use 32-bit Lisp objects). The representation
+stuffs a pointer together with a tag, as follows:
+
+ [ 3 3 2 2 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 ]
+ [ 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 ]
+
+ <---------------------------------------------------------> <->
+ a pointer to a structure, or an integer tag
+
+ A tag of 00 is used for all pointer object types, a tag of 10 is used
+for characters, and the other two tags 01 and 11 are joined together to
+form the integer object type. This representation gives us 31 bit
+integers and 30 bit characters, while pointers are represented directly
+without any bit masking or shifting. This representation, though,
+assumes that pointers to structs are always aligned to multiples of 4,
+so the lower 2 bits are always zero.
+
+ Lisp objects use the typedef `Lisp_Object', but the actual C type
+used for the Lisp object can vary. It can be either a simple type
+(`long' on the DEC Alpha, `int' on other machines) or a structure whose
+fields are bit fields that line up properly (actually, a union of
+structures is used). The choice of which type to use is determined by
+the preprocessor constant `USE_UNION_TYPE' which is defined via the
+`--use-union-type' option to `configure'.
+
+ Generally the simple integral type is preferable because it ensures
+that the compiler will actually use a machine word to represent the
+object (some compilers will use more general and less efficient code
+for unions and structs even if they can fit in a machine word). The
+union type, however, has the advantage of stricter _static_ type
+checking. Places where a `Lisp_Object' is mistakenly passed to a
+routine expecting an `int' (or vice-versa), or a check is written `if
+(foo)' (instead of `if (!NILP (foo))', will be flagged as errors. None
+of these lead to the expected results! `Qnil' is not represented as 0
+(so `if (foo)' will *ALWAYS* be true for a `Lisp_Object'), and the
+representation of an integer as a `Lisp_Object' is not just the
+integer's numeric value, but usually 2x the integer +/- 1.)
+
+ There used to be a claim that the union type simplified debugging.
+There may have been a grain of truth to this pre-19.8, when there was no
+`lrecord' type and all objects had a separate type appearing in the
+tag. Nowadays, however, there is no debugging gain, and in fact
+frequent debugging *_loss_*, since many debuggers don't handle unions
+very well, and usually there is no way to directly specify a union from
+a debugging prompt.
+
+ Furthermore, release builds should *_not_* be done with union type
+because (a) you may get less efficiency, with compilers that can't
+figure out how to optimize the union into a machine word; (b) even
+worse, the union type often triggers miscompilation, especially when
+combined with Mule and error-checking. This has been the case at
+various times when using GCC and MS VC, at least with `--pdump'.
+Therefore, be warned!
+
+ As of 2002 4Q, miscompilation is known to happen with current
+versions of *Microsoft VC++* and *GCC in combination with Mule, pdump,
+and KKCC* (no error checking).
+
+ Various macros are used to convert between Lisp_Objects and the
+corresponding C type. Macros of the form `XINT()', `XCHAR()',
+`XSTRING()', `XSYMBOL()', do any required bit shifting and/or masking
+and cast it to the appropriate type. `XINT()' needs to be a bit tricky
+so that negative numbers are properly sign-extended. Since integers
+are stored left-shifted, if the right-shift operator does an arithmetic
+shift (i.e. it leaves the most-significant bit as-is rather than
+shifting in a zero, so that it mimics a divide-by-two even for negative
+numbers) the shift to remove the tag bit is enough. This is the case
+on all the systems we support.
+
+ Note that when `ERROR_CHECK_TYPECHECK' is defined, the converter
+macros become more complicated--they check the tag bits and/or the type
+field in the first four bytes of a record type to ensure that the
+object is really of the correct type. This is great for catching places
+where an incorrect type is being dereferenced--this typically results
+in a pointer being dereferenced as the wrong type of structure, with
+unpredictable (and sometimes not easily traceable) results.
+
+ There are similar `XSETTYPE()' macros that construct a Lisp object.
+These macros are of the form `XSETTYPE (LVALUE, RESULT)', i.e. they
+have to be a statement rather than just used in an expression. The
+reason for this is that standard C doesn't let you "construct" a
+structure (but GCC does). Granted, this sometimes isn't too
+convenient; for the case of integers, at least, you can use the
+function `make_int()', which constructs and _returns_ an integer Lisp
+object. Note that the `XSETTYPE()' macros are also affected by
+`ERROR_CHECK_TYPECHECK' and make sure that the structure is of the
+right type in the case of record types, where the type is contained in
+the structure.
+
+ The C programmer is responsible for *guaranteeing* that a
+Lisp_Object is the correct type before using the `XTYPE' macros. This
+is especially important in the case of lists. Use `XCAR' and `XCDR' if
+a Lisp_Object is certainly a cons cell, else use `Fcar()' and `Fcdr()'.
+Trust other C code, but not Lisp code. On the other hand, if XEmacs
+has an internal logic error, it's better to crash immediately, so
+sprinkle `assert()'s and "unreachable" `abort()'s liberally about the
+source code. Where performance is an issue, use `type_checking_assert',
+`bufpos_checking_assert', and `gc_checking_assert', which do nothing
+unless the corresponding configure error checking flag was specified.
+
+\1f
+File: internals.info, Node: Rules When Writing New C Code, Next: Regression Testing XEmacs, Prev: How Lisp Objects Are Represented in C, Up: Top
+
+Rules When Writing New C Code
+*****************************
+
+The XEmacs C Code is extremely complex and intricate, and there are many
+rules that are more or less consistently followed throughout the code.
+Many of these rules are not obvious, so they are explained here. It is
+of the utmost importance that you follow them. If you don't, you may
+get something that appears to work, but which will crash in odd
+situations, often in code far away from where the actual breakage is.
+
+* Menu:
+
+* A Reader's Guide to XEmacs Coding Conventions::
+* General Coding Rules::
+* Writing Lisp Primitives::
+* Writing Good Comments::
+* Adding Global Lisp Variables::
+* Proper Use of Unsigned Types::
+* Coding for Mule::
+* Techniques for XEmacs Developers::
+
+\1f
+File: internals.info, Node: A Reader's Guide to XEmacs Coding Conventions, Next: General Coding Rules, Up: Rules When Writing New C Code
+
+A Reader's Guide to XEmacs Coding Conventions
+=============================================
+
+Of course the low-level implementation language of XEmacs is C, but much
+of that uses the Lisp engine to do its work. However, because the code
+is "inside" of the protective containment shell around the "reactor
+core," you'll see lots of complex "plumbing" needed to do the work and
+"safety mechanisms," whose failure results in a meltdown. This section
+provides a quick overview (or review) of the various components of the
+implementation of Lisp objects.
+
+ Two typographic conventions help to identify C objects that implement
+Lisp objects. The first is that capitalized identifiers, especially
+beginning with the letters `Q', `V', `F', and `S', for C variables and
+functions, and C macros with beginning with the letter `X', are used to
+implement Lisp. The second is that where Lisp uses the hyphen `-' in
+symbol names, the corresponding C identifiers use the underscore `_'.
+Of course, since XEmacs Lisp contains interfaces to many external
+libraries, those external names will follow the coding conventions
+their authors chose, and may overlap the "XEmacs name space." However
+these cases are usually pretty obvious.
+
+ All Lisp objects are handled indirectly. The `Lisp_Object' type is
+usually a pointer to a structure, except for a very small number of
+types with immediate representations (currently characters and
+integers). However, these types cannot be directly operated on in C
+code, either, so they can also be considered indirect. Types that do
+not have an immediate representation always have a C typedef
+`Lisp_TYPE' for a corresponding structure.
+
+ In older code, it was common practice to pass around pointers to
+`Lisp_TYPE', but this is now deprecated in favor of using `Lisp_Object'
+for all function arguments and return values that are Lisp objects.
+The `XTYPE' macro is used to extract the pointer and cast it to
+`(Lisp_TYPE *)' for the desired type.
+
+ *Convention*: macros whose names begin with `X' operate on
+`Lisp_Object's and do no type-checking. Many such macros are type
+extractors, but others implement Lisp operations in C (_e.g._, `XCAR'
+implements the Lisp `car' function). These are unsafe, and must only
+be used where types of all data have already been checked. Such macros
+are only applied to `Lisp_Object's. In internal implementations where
+the pointer has already been converted, the structure is operated on
+directly using the C `->' member access operator.
+
+ The `TYPEP', `CHECK_TYPE', and `CONCHECK_TYPE' macros are used to
+test types. The first returns a Boolean value, and the latter signal
+errors. (The `CONCHECK' variety allows execution to be CONtinued under
+some circumstances, thus the name.) Functions which expect to be
+passed user data invariably call `CHECK' macros on arguments.
+
+ There are many types of specialized Lisp objects implemented in C,
+but the most pervasive type is the "symbol". Symbols are used as
+identifiers, variables, and functions.
+
+ *Convention*: Global variables whose names begin with `Q' are
+constants whose value is a symbol. The name of the variable should be
+derived from the name of the symbol using the same rules as for Lisp
+primitives. Such variables allow the C code to check whether a
+particular `Lisp_Object' is equal to a given symbol. Symbols are Lisp
+objects, so these variables may be passed to Lisp primitives. (An
+alternative to the use of `Q...' variables is to call the `intern'
+function at initialization in the `vars_of_MODULE' function, which is
+hardly less efficient.)
+
+ *Convention*: Global variables whose names begin with `V' are
+variables that contain Lisp objects. The convention here is that all
+global variables of type `Lisp_Object' begin with `V', and no others do
+(not even integer and boolean variables that have Lisp equivalents).
+Most of the time, these variables have equivalents in Lisp, which are
+defined via the `DEFVAR' family of macros, but some don't. Since the
+variable's value is a `Lisp_Object', it can be passed to Lisp
+primitives.
+
+ The implementation of Lisp primitives is more complex.
+*Convention*: Global variables with names beginning with `S' contain a
+structure that allows the Lisp engine to identify and call a C
+function. In modern versions of XEmacs, these identifiers are almost
+always completely hidden in the `DEFUN' and `SUBR' macros, but you will
+encounter them if you look at very old versions of XEmacs or at GNU
+Emacs. *Convention*: Functions with names beginning with `F' implement
+Lisp primitives. Of course all their arguments and their return values
+must be Lisp_Objects. (This is hidden in the `DEFUN' macro.)
+
+\1f
+File: internals.info, Node: General Coding Rules, Next: Writing Lisp Primitives, Prev: A Reader's Guide to XEmacs Coding Conventions, Up: Rules When Writing New C Code
+
+General Coding Rules
+====================
+
+The C code is actually written in a dialect of C called "Clean C",
+meaning that it can be compiled, mostly warning-free, with either a C or
+C++ compiler. Coding in Clean C has several advantages over plain C.
+C++ compilers are more nit-picking, and a number of coding errors have
+been found by compiling with C++. The ability to use both C and C++
+tools means that a greater variety of development tools are available to
+the developer.
+
+ Every module includes `<config.h>' (angle brackets so that
+`--srcdir' works correctly; `config.h' may or may not be in the same
+directory as the C sources) and `lisp.h'. `config.h' must always be
+included before any other header files (including system header files)
+to ensure that certain tricks played by various `s/' and `m/' files
+work out correctly.
+
+ When including header files, always use angle brackets, not double
+quotes, except when the file to be included is always in the same
+directory as the including file. If either file is a generated file,
+then that is not likely to be the case. In order to understand why we
+have this rule, imagine what happens when you do a build in the source
+directory using `./configure' and another build in another directory
+using `../work/configure'. There will be two different `config.h'
+files. Which one will be used if you `#include "config.h"'?
+
+ Almost every module contains a `syms_of_*()' function and a
+`vars_of_*()' function. The former declares any Lisp primitives you
+have defined and defines any symbols you will be using. The latter
+declares any global Lisp variables you have added and initializes global
+C variables in the module. *Important*: There are stringent
+requirements on exactly what can go into these functions. See the
+comment in `emacs.c'. The reason for this is to avoid obscure unwanted
+interactions during initialization. If you don't follow these rules,
+you'll be sorry! If you want to do anything that isn't allowed, create
+a `complex_vars_of_*()' function for it. Doing this is tricky, though:
+you have to make sure your function is called at the right time so that
+all the initialization dependencies work out.
+
+ Declare each function of these kinds in `symsinit.h'. Make sure
+it's called in the appropriate place in `emacs.c'. You never need to
+include `symsinit.h' directly, because it is included by `lisp.h'.
+
+ *All global and static variables that are to be modifiable must be
+declared uninitialized.* This means that you may not use the "declare
+with initializer" form for these variables, such as `int some_variable
+= 0;'. The reason for this has to do with some kludges done during the
+dumping process: If possible, the initialized data segment is re-mapped
+so that it becomes part of the (unmodifiable) code segment in the
+dumped executable. This allows this memory to be shared among multiple
+running XEmacs processes. XEmacs is careful to place as much constant
+data as possible into initialized variables during the `temacs' phase.
+
+ *Please note:* This kludge only works on a few systems nowadays, and
+is rapidly becoming irrelevant because most modern operating systems
+provide "copy-on-write" semantics. All data is initially shared
+between processes, and a private copy is automatically made (on a
+page-by-page basis) when a process first attempts to write to a page of
+memory.
+
+ Formerly, there was a requirement that static variables not be
+declared inside of functions. This had to do with another hack along
+the same vein as what was just described: old USG systems put
+statically-declared variables in the initialized data space, so those
+header files had a `#define static' declaration. (That way, the
+data-segment remapping described above could still work.) This fails
+badly on static variables inside of functions, which suddenly become
+automatic variables; therefore, you weren't supposed to have any of
+them. This awful kludge has been removed in XEmacs because
+
+ 1. almost all of the systems that used this kludge ended up having to
+ disable the data-segment remapping anyway;
+
+ 2. the only systems that didn't were extremely outdated ones;
+
+ 3. this hack completely messed up inline functions.
+
+ The C source code makes heavy use of C preprocessor macros. One
+popular macro style is:
+
+ #define FOO(var, value) do { \
+ Lisp_Object FOO_value = (value); \
+ ... /* compute using FOO_value */ \
+ (var) = bar; \
+ } while (0)
+
+ The `do {...} while (0)' is a standard trick to allow FOO to have
+statement semantics, so that it can safely be used within an `if'
+statement in C, for example. Multiple evaluation is prevented by
+copying a supplied argument into a local variable, so that
+`FOO(var,fun(1))' only calls `fun' once.
+
+ Lisp lists are popular data structures in the C code as well as in
+Elisp. There are two sets of macros that iterate over lists.
+`EXTERNAL_LIST_LOOP_N' should be used when the list has been supplied
+by the user, and cannot be trusted to be acyclic and `nil'-terminated.
+A `malformed-list' or `circular-list' error will be generated if the
+list being iterated over is not entirely kosher. `LIST_LOOP_N', on the
+other hand, is faster and less safe, and can be used only on trusted
+lists.
+
+ Related macros are `GET_EXTERNAL_LIST_LENGTH' and `GET_LIST_LENGTH',
+which calculate the length of a list, and in the case of
+`GET_EXTERNAL_LIST_LENGTH', validating the properness of the list. The
+macros `EXTERNAL_LIST_LOOP_DELETE_IF' and `LIST_LOOP_DELETE_IF' delete
+elements from a lisp list satisfying some predicate.
+
+\1f
+File: internals.info, Node: Writing Lisp Primitives, Next: Writing Good Comments, Prev: General Coding Rules, Up: Rules When Writing New C Code
+
+Writing Lisp Primitives
+=======================
+
+Lisp primitives are Lisp functions implemented in C. The details of
+interfacing the C function so that Lisp can call it are handled by a few
+C macros. The only way to really understand how to write new C code is
+to read the source, but we can explain some things here.
+
+ An example of a special form is the definition of `prog1', from
+`eval.c'. (An ordinary function would have the same general
+appearance.)
+
+ DEFUN ("prog1", Fprog1, 1, UNEVALLED, 0, /*
+ Similar to `progn', but the value of the first form is returned.
+ \(prog1 FIRST BODY...): All the arguments are evaluated sequentially.
+ The value of FIRST is saved during evaluation of the remaining args,
+ whose values are discarded.
+ */
+ (args))
+ {
+ /* This function can GC */
+ REGISTER Lisp_Object val, form, tail;
+ struct gcpro gcpro1;
+
+ val = Feval (XCAR (args));
+
+ GCPRO1 (val);
+
+ LIST_LOOP_3 (form, XCDR (args), tail)
+ Feval (form);
+
+ UNGCPRO;
+ return val;
+ }
+
+ Let's start with a precise explanation of the arguments to the
+`DEFUN' macro. Here is a template for them:
+
+ DEFUN (LNAME, FNAME, MIN_ARGS, MAX_ARGS, INTERACTIVE, /*
+ DOCSTRING
+ */
+ (ARGLIST))
+
+LNAME
+ This string is the name of the Lisp symbol to define as the
+ function name; in the example above, it is `"prog1"'.
+
+FNAME
+ This is the C function name for this function. This is the name
+ that is used in C code for calling the function. The name is, by
+ convention, `F' prepended to the Lisp name, with all dashes (`-')
+ in the Lisp name changed to underscores. Thus, to call this
+ function from C code, call `Fprog1'. Remember that the arguments
+ are of type `Lisp_Object'; various macros and functions for
+ creating values of type `Lisp_Object' are declared in the file
+ `lisp.h'.
+
+ Primitives whose names are special characters (e.g. `+' or `<')
+ are named by spelling out, in some fashion, the special character:
+ e.g. `Fplus()' or `Flss()'. Primitives whose names begin with
+ normal alphanumeric characters but also contain special characters
+ are spelled out in some creative way, e.g. `let*' becomes
+ `FletX()'.
+
+ Each function also has an associated structure that holds the data
+ for the subr object that represents the function in Lisp. This
+ structure conveys the Lisp symbol name to the initialization
+ routine that will create the symbol and store the subr object as
+ its definition. The C variable name of this structure is always
+ `S' prepended to the FNAME. You hardly ever need to be aware of
+ the existence of this structure, since `DEFUN' plus `DEFSUBR'
+ takes care of all the details.
+
+MIN_ARGS
+ This is the minimum number of arguments that the function
+ requires. The function `prog1' allows a minimum of one argument.
+
+MAX_ARGS
+ This is the maximum number of arguments that the function accepts,
+ if there is a fixed maximum. Alternatively, it can be `UNEVALLED',
+ indicating a special form that receives unevaluated arguments, or
+ `MANY', indicating an unlimited number of evaluated arguments (the
+ C equivalent of `&rest'). Both `UNEVALLED' and `MANY' are macros.
+ If MAX_ARGS is a number, it may not be less than MIN_ARGS and it
+ may not be greater than 8. (If you need to add a function with
+ more than 8 arguments, use the `MANY' form. Resist the urge to
+ edit the definition of `DEFUN' in `lisp.h'. If you do it anyways,
+ make sure to also add another clause to the switch statement in
+ `primitive_funcall().')
+
+INTERACTIVE
+ This is an interactive specification, a string such as might be
+ used as the argument of `interactive' in a Lisp function. In the
+ case of `prog1', it is 0 (a null pointer), indicating that `prog1'
+ cannot be called interactively. A value of `""' indicates a
+ function that should receive no arguments when called
+ interactively.
+
+DOCSTRING
+ This is the documentation string. It is written just like a
+ documentation string for a function defined in Lisp; in
+ particular, the first line should be a single sentence. Note how
+ the documentation string is enclosed in a comment, none of the
+ documentation is placed on the same lines as the comment-start and
+ comment-end characters, and the comment-start characters are on
+ the same line as the interactive specification. `make-docfile',
+ which scans the C files for documentation strings, is very
+ particular about what it looks for, and will not properly extract
+ the doc string if it's not in this exact format.
+
+ In order to make both `etags' and `make-docfile' happy, make sure
+ that the `DEFUN' line contains the LNAME and FNAME, and that the
+ comment-start characters for the doc string are on the same line
+ as the interactive specification, and put a newline directly after
+ them (and before the comment-end characters).
+
+ARGLIST
+ This is the comma-separated list of arguments to the C function.
+ For a function with a fixed maximum number of arguments, provide a
+ C argument for each Lisp argument. In this case, unlike regular C
+ functions, the types of the arguments are not declared; they are
+ simply always of type `Lisp_Object'.
+
+ The names of the C arguments will be used as the names of the
+ arguments to the Lisp primitive as displayed in its documentation,
+ modulo the same concerns described above for `F...' names (in
+ particular, underscores in the C arguments become dashes in the
+ Lisp arguments).
+
+ There is one additional kludge: A trailing `_' on the C argument is
+ discarded when forming the Lisp argument. This allows C language
+ reserved words (like `default') or global symbols (like `dirname')
+ to be used as argument names without compiler warnings or errors.
+
+ A Lisp function with MAX_ARGS = `UNEVALLED' is a "special form";
+ its arguments are not evaluated. Instead it receives one argument
+ of type `Lisp_Object', a (Lisp) list of the unevaluated arguments,
+ conventionally named `(args)'.
+
+ When a Lisp function has no upper limit on the number of arguments,
+ specify MAX_ARGS = `MANY'. In this case its implementation in C
+ actually receives exactly two arguments: the number of Lisp
+ arguments (an `int') and the address of a block containing their
+ values (a `Lisp_Object *'). In this case only are the C types
+ specified in the ARGLIST: `(int nargs, Lisp_Object *args)'.
+
+
+ Within the function `Fprog1' itself, note the use of the macros
+`GCPRO1' and `UNGCPRO'. `GCPRO1' is used to "protect" a variable from
+garbage collection--to inform the garbage collector that it must look
+in that variable and regard the object pointed at by its contents as an
+accessible object. This is necessary whenever you call `Feval' or
+anything that can directly or indirectly call `Feval' (this includes
+the `QUIT' macro!). At such a time, any Lisp object that you intend to
+refer to again must be protected somehow. `UNGCPRO' cancels the
+protection of the variables that are protected in the current function.
+It is necessary to do this explicitly.
+
+ The macro `GCPRO1' protects just one local variable. If you want to
+protect two, use `GCPRO2' instead; repeating `GCPRO1' will not work.
+Macros `GCPRO3' and `GCPRO4' also exist.
+
+ These macros implicitly use local variables such as `gcpro1'; you
+must declare these explicitly, with type `struct gcpro'. Thus, if you
+use `GCPRO2', you must declare `gcpro1' and `gcpro2'.
+
+ Note also that the general rule is "caller-protects"; i.e. you are
+only responsible for protecting those Lisp objects that you create. Any
+objects passed to you as arguments should have been protected by whoever
+created them, so you don't in general have to protect them.
+
+ In particular, the arguments to any Lisp primitive are always
+automatically `GCPRO'ed, when called "normally" from Lisp code or
+bytecode. So only a few Lisp primitives that are called frequently from
+C code, such as `Fprogn' protect their arguments as a service to their
+caller. You don't need to protect your arguments when writing a new
+`DEFUN'.
+
+ `GCPRO'ing is perhaps the trickiest and most error-prone part of
+XEmacs coding. It is *extremely* important that you get this right and
+use a great deal of discipline when writing this code. *Note
+`GCPRO'ing: GCPROing, for full details on how to do this.
+
+ What `DEFUN' actually does is declare a global structure of type
+`Lisp_Subr' whose name begins with capital `SF' and which contains
+information about the primitive (e.g. a pointer to the function, its
+minimum and maximum allowed arguments, a string describing its Lisp
+name); `DEFUN' then begins a normal C function declaration using the
+`F...' name. The Lisp subr object that is the function definition of a
+primitive (i.e. the object in the function slot of the symbol that
+names the primitive) actually points to this `SF' structure; when
+`Feval' encounters a subr, it looks in the structure to find out how to
+call the C function.
+
+ Defining the C function is not enough to make a Lisp primitive
+available; you must also create the Lisp symbol for the primitive (the
+symbol is "interned"; *note Obarrays::) and store a suitable subr
+object in its function cell. (If you don't do this, the primitive won't
+be seen by Lisp code.) The code looks like this:
+
+ DEFSUBR (FNAME);
+
+Here FNAME is the same name you used as the second argument to `DEFUN'.
+
+ This call to `DEFSUBR' should go in the `syms_of_*()' function at
+the end of the module. If no such function exists, create it and make
+sure to also declare it in `symsinit.h' and call it from the
+appropriate spot in `main()'. *Note General Coding Rules::.
+
+ Note that C code cannot call functions by name unless they are
+defined in C. The way to call a function written in Lisp from C is to
+use `Ffuncall', which embodies the Lisp function `funcall'. Since the
+Lisp function `funcall' accepts an unlimited number of arguments, in C
+it takes two: the number of Lisp-level arguments, and a one-dimensional
+array containing their values. The first Lisp-level argument is the
+Lisp function to call, and the rest are the arguments to pass to it.
+Since `Ffuncall' can call the evaluator, you must protect pointers from
+garbage collection around the call to `Ffuncall'. (However, `Ffuncall'
+explicitly protects all of its parameters, so you don't have to protect
+any pointers passed as parameters to it.)
+
+ The C functions `call0', `call1', `call2', and so on, provide handy
+ways to call a Lisp function conveniently with a fixed number of
+arguments. They work by calling `Ffuncall'.
+
+ `eval.c' is a very good file to look through for examples; `lisp.h'
+contains the definitions for important macros and functions.
+
+\1f
+File: internals.info, Node: Writing Good Comments, Next: Adding Global Lisp Variables, Prev: Writing Lisp Primitives, Up: Rules When Writing New C Code
+
+Writing Good Comments
+=====================
+
+Comments are a lifeline for programmers trying to understand tricky
+code. In general, the less obvious it is what you are doing, the more
+you need a comment, and the more detailed it needs to be. You should
+always be on guard when you're writing code for stuff that's tricky, and
+should constantly be putting yourself in someone else's shoes and asking
+if that person could figure out without much difficulty what's going
+on. (Assume they are a competent programmer who understands the
+essentials of how the XEmacs code is structured but doesn't know much
+about the module you're working on or any algorithms you're using.) If
+you're not sure whether they would be able to, add a comment. Always
+err on the side of more comments, rather than less.
+
+ Generally, when making comments, there is no need to attribute them
+with your name or initials. This especially goes for small,
+easy-to-understand, non-opinionated ones. Also, comments indicating
+where, when, and by whom a file was changed are _strongly_ discouraged,
+and in general will be removed as they are discovered. This is exactly
+what `ChangeLogs' are there for. However, it can occasionally be
+useful to mark exactly where (but not when or by whom) changes are
+made, particularly when making small changes to a file imported from
+elsewhere. These marks help when later on a newer version of the file
+is imported and the changes need to be merged. (If everything were
+always kept in CVS, there would be no need for this. But in practice,
+this often doesn't happen, or the CVS repository is later on lost or
+unavailable to the person doing the update.)
+
+ When putting in an explicit opinion in a comment, you should
+_always_ attribute it with your name, and optionally the date. This
+also goes for long, complex comments explaining in detail the workings
+of something - by putting your name there, you make it possible for
+someone who has questions about how that thing works to determine who
+wrote the comment so they can write to them. Preferably, use your
+actual name and not your initials, unless your initials are generally
+recognized (e.g. `jwz'). You can use only your first name if it's
+obvious who you are; otherwise, give first and last name. If you're
+not a regular contributor, you might consider putting your email
+address in - it may be in the ChangeLog, but after awhile ChangeLogs
+have a tendency of disappearing or getting muddled. (E.g. your comment
+may get copied somewhere else or even into another program, and
+tracking down the proper ChangeLog may be very difficult.)
+
+ If you come across an opinion that is not or no longer valid, or you
+come across any comment that no longer applies but you want to keep it
+around, enclose it in `[[ ' and ` ]]' marks and add a comment
+afterwards explaining why the preceding comment is no longer valid. Put
+your name on this comment, as explained above.
+
+ Just as comments are a lifeline to programmers, incorrect comments
+are death. If you come across an incorrect comment, *immediately*
+correct it or flag it as incorrect, as described in the previous
+paragraph. Whenever you work on a section of code, _always_ make sure
+to update any comments to be correct - or, at the very least, flag them
+as incorrect.
+
+ To indicate a "todo" or other problem, use four pound signs - i.e.
+`####'.
+
+\1f
+File: internals.info, Node: Adding Global Lisp Variables, Next: Proper Use of Unsigned Types, Prev: Writing Good Comments, Up: Rules When Writing New C Code
+
+Adding Global Lisp Variables
+============================
+
+Global variables whose names begin with `Q' are constants whose value
+is a symbol of a particular name. The name of the variable should be
+derived from the name of the symbol using the same rules as for Lisp
+primitives. These variables are initialized using a call to
+`defsymbol()' in the `syms_of_*()' function. (This call interns a
+symbol, sets the C variable to the resulting Lisp object, and calls
+`staticpro()' on the C variable to tell the garbage-collection
+mechanism about this variable. What `staticpro()' does is add a
+pointer to the variable to a large global array; when
+garbage-collection happens, all pointers listed in the array are used
+as starting points for marking Lisp objects. This is important because
+it's quite possible that the only current reference to the object is
+the C variable. In the case of symbols, the `staticpro()' doesn't
+matter all that much because the symbol is contained in `obarray',
+which is itself `staticpro()'ed. However, it's possible that a naughty
+user could do something like uninterning the symbol out of `obarray' or
+even setting `obarray' to a different value [although this is likely to
+make XEmacs crash!].)
+
+ *Please note:* It is potentially deadly if you declare a `Q...'
+variable in two different modules. The two calls to `defsymbol()' are
+no problem, but some linkers will complain about multiply-defined
+symbols. The most insidious aspect of this is that often the link will
+succeed anyway, but then the resulting executable will sometimes crash
+in obscure ways during certain operations!
+
+ To avoid this problem, declare any symbols with common names (such as
+`text') that are not obviously associated with this particular module
+in the file `general-slots.h'. The "-slots" suffix indicates that this
+is a file that is included multiple times in `general.c'. Redefinition
+of preprocessor macros allows the effects to be different in each
+context, so this is actually more convenient and less error-prone than
+doing it in your module.
+
+ Global variables whose names begin with `V' are variables that
+contain Lisp objects. The convention here is that all global variables
+of type `Lisp_Object' begin with `V', and all others don't (including
+integer and boolean variables that have Lisp equivalents). Most of the
+time, these variables have equivalents in Lisp, but some don't. Those
+that do are declared this way by a call to `DEFVAR_LISP()' in the
+`vars_of_*()' initializer for the module. What this does is create a
+special "symbol-value-forward" Lisp object that contains a pointer to
+the C variable, intern a symbol whose name is as specified in the call
+to `DEFVAR_LISP()', and set its value to the symbol-value-forward Lisp
+object; it also calls `staticpro()' on the C variable to tell the
+garbage-collection mechanism about the variable. When `eval' (or
+actually `symbol-value') encounters this special object in the process
+of retrieving a variable's value, it follows the indirection to the C
+variable and gets its value. `setq' does similar things so that the C
+variable gets changed.
+
+ Whether or not you `DEFVAR_LISP()' a variable, you need to
+initialize it in the `vars_of_*()' function; otherwise it will end up
+as all zeroes, which is the integer 0 (_not_ `nil'), and this is
+probably not what you want. Also, if the variable is not
+`DEFVAR_LISP()'ed, *you must call* `staticpro()' on the C variable in
+the `vars_of_*()' function. Otherwise, the garbage-collection
+mechanism won't know that the object in this variable is in use, and
+will happily collect it and reuse its storage for another Lisp object,
+and you will be the one who's unhappy when you can't figure out how
+your variable got overwritten.
+
+\1f
+File: internals.info, Node: Proper Use of Unsigned Types, Next: Coding for Mule, Prev: Adding Global Lisp Variables, Up: Rules When Writing New C Code
+
+Proper Use of Unsigned Types
+============================
+
+Avoid using `unsigned int' and `unsigned long' whenever possible.
+Unsigned types are viral - any arithmetic or comparisons involving
+mixed signed and unsigned types are automatically converted to
+unsigned, which is almost certainly not what you want. Many subtle and
+hard-to-find bugs are created by careless use of unsigned types. In
+general, you should almost _never_ use an unsigned type to hold a
+regular quantity of any sort. The only exceptions are
+
+ 1. When there's a reasonable possibility you will actually need all
+ 32 or 64 bits to store the quantity.
+
+ 2. When calling existing API's that require unsigned types. In this
+ case, you should still do all manipulation using signed types, and
+ do the conversion at the very threshold of the API call.
+
+ 3. In existing code that you don't want to modify because you don't
+ maintain it.
+
+ 4. In bit-field structures.
+
+ Other reasonable uses of `unsigned int' and `unsigned long' are
+representing non-quantities - e.g. bit-oriented flags and such.
+
+\1f
+File: internals.info, Node: Coding for Mule, Next: Techniques for XEmacs Developers, Prev: Proper Use of Unsigned Types, Up: Rules When Writing New C Code
+
+Coding for Mule
+===============
+
+Although Mule support is not compiled by default in XEmacs, many people
+are using it, and we consider it crucial that new code works correctly
+with multibyte characters. This is not hard; it is only a matter of
+following several simple user-interface guidelines. Even if you never
+compile with Mule, with a little practice you will find it quite easy
+to code Mule-correctly.
+
+ Note that these guidelines are not necessarily tied to the current
+Mule implementation; they are also a good idea to follow on the grounds
+of code generalization for future I18N work.
+
+* Menu:
+
+* Character-Related Data Types::
+* Working With Character and Byte Positions::
+* Conversion to and from External Data::
+* General Guidelines for Writing Mule-Aware Code::
+* An Example of Mule-Aware Code::
+
+\1f
+File: internals.info, Node: Character-Related Data Types, Next: Working With Character and Byte Positions, Up: Coding for Mule
+
+Character-Related Data Types
+----------------------------
+
+First, let's review the basic character-related datatypes used by
+XEmacs. Note that the separate `typedef's are not mandatory in the
+current implementation (all of them boil down to `unsigned char' or
+`int'), but they improve clarity of code a great deal, because one
+glance at the declaration can tell the intended use of the variable.
+
+`Emchar'
+ An `Emchar' holds a single Emacs character.
+
+ Obviously, the equality between characters and bytes is lost in
+ the Mule world. Characters can be represented by one or more
+ bytes in the buffer, and `Emchar' is the C type large enough to
+ hold any character.
+
+ Without Mule support, an `Emchar' is equivalent to an `unsigned
+ char'.
+
+`Bufbyte'
+ The data representing the text in a buffer or string is logically
+ a set of `Bufbyte's.
+
+ XEmacs does not work with the same character formats all the time;
+ when reading characters from the outside, it decodes them to an
+ internal format, and likewise encodes them when writing.
+ `Bufbyte' (in fact `unsigned char') is the basic unit of XEmacs
+ internal buffers and strings format. A `Bufbyte *' is the type
+ that points at text encoded in the variable-width internal
+ encoding.
+
+ One character can correspond to one or more `Bufbyte's. In the
+ current Mule implementation, an ASCII character is represented by
+ the same `Bufbyte', and other characters are represented by a
+ sequence of two or more `Bufbyte's.
+
+ Without Mule support, there are exactly 256 characters, implicitly
+ Latin-1, and each character is represented using one `Bufbyte', and
+ there is a one-to-one correspondence between `Bufbyte's and
+ `Emchar's.
+
+`Bufpos'
+`Charcount'
+ A `Bufpos' represents a character position in a buffer or string.
+ A `Charcount' represents a number (count) of characters.
+ Logically, subtracting two `Bufpos' values yields a `Charcount'
+ value. Although all of these are `typedef'ed to `EMACS_INT', we
+ use them in preference to `EMACS_INT' to make it clear what sort
+ of position is being used.
+
+ `Bufpos' and `Charcount' values are the only ones that are ever
+ visible to Lisp.
+
+`Bytind'
+`Bytecount'
+ A `Bytind' represents a byte position in a buffer or string. A
+ `Bytecount' represents the distance between two positions, in
+ bytes. The relationship between `Bytind' and `Bytecount' is the
+ same as the relationship between `Bufpos' and `Charcount'.
+
+`Extbyte'
+`Extcount'
+ When dealing with the outside world, XEmacs works with `Extbyte's,
+ which are equivalent to `unsigned char'. Obviously, an `Extcount'
+ is the distance between two `Extbyte's. Extbytes and Extcounts
+ are not all that frequent in XEmacs code.
+
+\1f
+File: internals.info, Node: Working With Character and Byte Positions, Next: Conversion to and from External Data, Prev: Character-Related Data Types, Up: Coding for Mule
+
+Working With Character and Byte Positions
+-----------------------------------------
+
+Now that we have defined the basic character-related types, we can look
+at the macros and functions designed for work with them and for
+conversion between them. Most of these macros are defined in
+`buffer.h', and we don't discuss all of them here, but only the most
+important ones. Examining the existing code is the best way to learn
+about them.
+
+`MAX_EMCHAR_LEN'
+ This preprocessor constant is the maximum number of buffer bytes to
+ represent an Emacs character in the variable width internal
+ encoding. It is useful when allocating temporary strings to keep
+ a known number of characters. For instance:
+
+ {
+ Charcount cclen;
+ ...
+ {
+ /* Allocate place for CCLEN characters. */
+ Bufbyte *buf = (Bufbyte *)alloca (cclen * MAX_EMCHAR_LEN);
+ ...
+
+ If you followed the previous section, you can guess that,
+ logically, multiplying a `Charcount' value with `MAX_EMCHAR_LEN'
+ produces a `Bytecount' value.
+
+ In the current Mule implementation, `MAX_EMCHAR_LEN' equals 4.
+ Without Mule, it is 1.
+
+`charptr_emchar'
+`set_charptr_emchar'
+ The `charptr_emchar' macro takes a `Bufbyte' pointer and returns
+ the `Emchar' stored at that position. If it were a function, its
+ prototype would be:
+
+ Emchar charptr_emchar (Bufbyte *p);
+
+ `set_charptr_emchar' stores an `Emchar' to the specified byte
+ position. It returns the number of bytes stored:
+
+ Bytecount set_charptr_emchar (Bufbyte *p, Emchar c);
+
+ It is important to note that `set_charptr_emchar' is safe only for
+ appending a character at the end of a buffer, not for overwriting a
+ character in the middle. This is because the width of characters
+ varies, and `set_charptr_emchar' cannot resize the string if it
+ writes, say, a two-byte character where a single-byte character
+ used to reside.
+
+ A typical use of `set_charptr_emchar' can be demonstrated by this
+ example, which copies characters from buffer BUF to a temporary
+ string of Bufbytes.
+
+ {
+ Bufpos pos;
+ for (pos = beg; pos < end; pos++)
+ {
+ Emchar c = BUF_FETCH_CHAR (buf, pos);
+ p += set_charptr_emchar (buf, c);
+ }
+ }
+
+ Note how `set_charptr_emchar' is used to store the `Emchar' and
+ increment the counter, at the same time.
+
+`INC_CHARPTR'
+`DEC_CHARPTR'
+ These two macros increment and decrement a `Bufbyte' pointer,
+ respectively. They will adjust the pointer by the appropriate
+ number of bytes according to the byte length of the character
+ stored there. Both macros assume that the memory address is
+ located at the beginning of a valid character.
+
+ Without Mule support, `INC_CHARPTR (p)' and `DEC_CHARPTR (p)'
+ simply expand to `p++' and `p--', respectively.
+
+`bytecount_to_charcount'
+ Given a pointer to a text string and a length in bytes, return the
+ equivalent length in characters.
+
+ Charcount bytecount_to_charcount (Bufbyte *p, Bytecount bc);
+
+`charcount_to_bytecount'
+ Given a pointer to a text string and a length in characters,
+ return the equivalent length in bytes.
+
+ Bytecount charcount_to_bytecount (Bufbyte *p, Charcount cc);
+
+`charptr_n_addr'
+ Return a pointer to the beginning of the character offset CC (in
+ characters) from P.
+
+ Bufbyte *charptr_n_addr (Bufbyte *p, Charcount cc);
+
+\1f
+File: internals.info, Node: Conversion to and from External Data, Next: General Guidelines for Writing Mule-Aware Code, Prev: Working With Character and Byte Positions, Up: Coding for Mule
+
+Conversion to and from External Data
+------------------------------------
+
+When an external function, such as a C library function, returns a
+`char' pointer, you should almost never treat it as `Bufbyte'. This is
+because these returned strings may contain 8bit characters which can be
+misinterpreted by XEmacs, and cause a crash. Likewise, when exporting
+a piece of internal text to the outside world, you should always
+convert it to an appropriate external encoding, lest the internal stuff
+(such as the infamous \201 characters) leak out.
+
+ The interface to conversion between the internal and external
+representations of text are the numerous conversion macros defined in
+`buffer.h'. There used to be a fixed set of external formats supported
+by these macros, but now any coding system can be used with these
+macros. The coding system alias mechanism is used to create the
+following logical coding systems, which replace the fixed external
+formats. The (dontusethis-set-symbol-value-handler) mechanism was
+enhanced to make this possible (more work on that is needed - like
+remove the `dontusethis-' prefix).
+
+`Qbinary'
+ This is the simplest format and is what we use in the absence of a
+ more appropriate format. This converts according to the `binary'
+ coding system:
+
+ a. On input, bytes 0-255 are converted into (implicitly Latin-1)
+ characters 0-255. A non-Mule xemacs doesn't really know about
+ different character sets and the fonts to display them, so
+ the bytes can be treated as text in different 1-byte
+ encodings by simply setting the appropriate fonts. So in a
+ sense, non-Mule xemacs is a multi-lingual editor if, for
+ example, different fonts are used to display text in
+ different buffers, faces, or windows. The specifier
+ mechanism gives the user complete control over this kind of
+ behavior.
+
+ b. On output, characters 0-255 are converted into bytes 0-255
+ and other characters are converted into `~'.
+
+`Qfile_name'
+ Format used for filenames. This is user-definable via either the
+ `file-name-coding-system' or `pathname-coding-system' (now
+ obsolete) variables.
+
+`Qnative'
+ Format used for the external Unix environment--`argv[]', stuff
+ from `getenv()', stuff from the `/etc/passwd' file, etc.
+ Currently this is the same as Qfile_name. The two should be
+ distinguished for clarity and possible future separation.
+
+`Qctext'
+ Compound-text format. This is the standard X11 format used for
+ data stored in properties, selections, and the like. This is an
+ 8-bit no-lock-shift ISO2022 coding system. This is a real coding
+ system, unlike Qfile_name, which is user-definable.
+
+ There are two fundamental macros to convert between external and
+internal format.
+
+ `TO_INTERNAL_FORMAT' converts external data to internal format, and
+`TO_EXTERNAL_FORMAT' converts the other way around. The arguments each
+of these receives are a source type, a source, a sink type, a sink, and
+a coding system (or a symbol naming a coding system).
+
+ A typical call looks like
+ TO_EXTERNAL_FORMAT (LISP_STRING, str, C_STRING_MALLOC, ptr, Qfile_name);
+
+ which means that the contents of the lisp string `str' are written
+to a malloc'ed memory area which will be pointed to by `ptr', after the
+function returns. The conversion will be done using the `file-name'
+coding system, which will be controlled by the user indirectly by
+setting or binding the variable `file-name-coding-system'.
+
+ Some sources and sinks require two C variables to specify. We use
+some preprocessor magic to allow different source and sink types, and
+even different numbers of arguments to specify different types of
+sources and sinks.
+
+ So we can have a call that looks like
+ TO_INTERNAL_FORMAT (DATA, (ptr, len),
+ MALLOC, (ptr, len),
+ coding_system);
+
+ The parenthesized argument pairs are required to make the
+preprocessor magic work.
+
+ Here are the different source and sink types:
+
+``DATA, (ptr, len),''
+ input data is a fixed buffer of size LEN at address PTR
+
+``ALLOCA, (ptr, len),''
+ output data is placed in an alloca()ed buffer of size LEN pointed
+ to by PTR
+
+``MALLOC, (ptr, len),''
+ output data is in a malloc()ed buffer of size LEN pointed to by PTR
+
+``C_STRING_ALLOCA, ptr,''
+ equivalent to `ALLOCA (ptr, len_ignored)' on output.
+
+``C_STRING_MALLOC, ptr,''
+ equivalent to `MALLOC (ptr, len_ignored)' on output
+
+``C_STRING, ptr,''
+ equivalent to `DATA, (ptr, strlen (ptr) + 1)' on input
+
+``LISP_STRING, string,''
+ input or output is a Lisp_Object of type string
+
+``LISP_BUFFER, buffer,''
+ output is written to `(point)' in lisp buffer BUFFER
+
+``LISP_LSTREAM, lstream,''
+ input or output is a Lisp_Object of type lstream
+
+``LISP_OPAQUE, object,''
+ input or output is a Lisp_Object of type opaque
+
+ Often, the data is being converted to a '\0'-byte-terminated string,
+which is the format required by many external system C APIs. For these
+purposes, a source type of `C_STRING' or a sink type of
+`C_STRING_ALLOCA' or `C_STRING_MALLOC' is appropriate. Otherwise, we
+should try to keep XEmacs '\0'-byte-clean, which means using (ptr, len)
+pairs.
+
+ The sinks to be specified must be lvalues, unless they are the lisp
+object types `LISP_LSTREAM' or `LISP_BUFFER'.
+
+ For the sink types `ALLOCA' and `C_STRING_ALLOCA', the resulting
+text is stored in a stack-allocated buffer, which is automatically
+freed on returning from the function. However, the sink types `MALLOC'
+and `C_STRING_MALLOC' return `xmalloc()'ed memory. The caller is
+responsible for freeing this memory using `xfree()'.
+
+ Note that it doesn't make sense for `LISP_STRING' to be a source for
+`TO_INTERNAL_FORMAT' or a sink for `TO_EXTERNAL_FORMAT'. You'll get an
+assertion failure if you try.
+
+\1f
+File: internals.info, Node: General Guidelines for Writing Mule-Aware Code, Next: An Example of Mule-Aware Code, Prev: Conversion to and from External Data, Up: Coding for Mule
+
+General Guidelines for Writing Mule-Aware Code
+----------------------------------------------
+
+This section contains some general guidance on how to write Mule-aware
+code, as well as some pitfalls you should avoid.
+
+_Never use `char' and `char *'._
+ In XEmacs, the use of `char' and `char *' is almost always a
+ mistake. If you want to manipulate an Emacs character from "C",
+ use `Emchar'. If you want to examine a specific octet in the
+ internal format, use `Bufbyte'. If you want a Lisp-visible
+ character, use a `Lisp_Object' and `make_char'. If you want a
+ pointer to move through the internal text, use `Bufbyte *'. Also
+ note that you almost certainly do not need `Emchar *'.
+
+_Be careful not to confuse `Charcount', `Bytecount', and `Bufpos'._
+ The whole point of using different types is to avoid confusion
+ about the use of certain variables. Lest this effect be
+ nullified, you need to be careful about using the right types.
+
+_Always convert external data_
+ It is extremely important to always convert external data, because
+ XEmacs can crash if unexpected 8bit sequences are copied to its
+ internal buffers literally.
+
+ This means that when a system function, such as `readdir', returns
+ a string, you may need to convert it using one of the conversion
+ macros described in the previous chapter, before passing it
+ further to Lisp.
+
+ Actually, most of the basic system functions that accept
+ '\0'-terminated string arguments, like `stat()' and `open()', have
+ been *encapsulated* so that they are they `always' do internal to
+ external conversion themselves. This means you must pass
+ internally encoded data, typically the `XSTRING_DATA' of a
+ Lisp_String to these functions. This is actually a design bug,
+ since it unexpectedly changes the semantics of the system
+ functions. A better design would be to provide separate versions
+ of these system functions that accepted Lisp_Objects which were
+ lisp strings in place of their current `char *' arguments.
+
+ int stat_lisp (Lisp_Object path, struct stat *buf); /* Implement me */
+
+ Also note that many internal functions, such as `make_string',
+ accept Bufbytes, which removes the need for them to convert the
+ data they receive. This increases efficiency because that way
+ external data needs to be decoded only once, when it is read.
+ After that, it is passed around in internal format.
+
+\1f
+File: internals.info, Node: An Example of Mule-Aware Code, Prev: General Guidelines for Writing Mule-Aware Code, Up: Coding for Mule
+
+An Example of Mule-Aware Code
+-----------------------------
+
+As an example of Mule-aware code, we will analyze the `string'
+function, which conses up a Lisp string from the character arguments it
+receives. Here is the definition, pasted from `alloc.c':
+
+ DEFUN ("string", Fstring, 0, MANY, 0, /*
+ Concatenate all the argument characters and make the result a string.
+ */
+ (int nargs, Lisp_Object *args))
+ {
+ Bufbyte *storage = alloca_array (Bufbyte, nargs * MAX_EMCHAR_LEN);
+ Bufbyte *p = storage;
+
+ for (; nargs; nargs--, args++)
+ {
+ Lisp_Object lisp_char = *args;
+ CHECK_CHAR_COERCE_INT (lisp_char);
+ p += set_charptr_emchar (p, XCHAR (lisp_char));
+ }
+ return make_string (storage, p - storage);
+ }
+
+ Now we can analyze the source line by line.
+
+ Obviously, string will be as long as there are arguments to the
+function. This is why we allocate `MAX_EMCHAR_LEN' * NARGS bytes on
+the stack, i.e. the worst-case number of bytes for NARGS `Emchar's to
+fit in the string.
+
+ Then, the loop checks that each element is a character, converting
+integers in the process. Like many other functions in XEmacs, this
+function silently accepts integers where characters are expected, for
+historical and compatibility reasons. Unless you know what you are
+doing, `CHECK_CHAR' will also suffice. `XCHAR (lisp_char)' extracts
+the `Emchar' from the `Lisp_Object', and `set_charptr_emchar' stores it
+to storage, increasing `p' in the process.
+
+ Other instructive examples of correct coding under Mule can be found
+all over the XEmacs code. For starters, I recommend
+`Fnormalize_menu_item_name' in `menubar.c'. After you have understood
+this section of the manual and studied the examples, you can proceed
+writing new Mule-aware code.
+
+\1f
+File: internals.info, Node: Techniques for XEmacs Developers, Prev: Coding for Mule, Up: Rules When Writing New C Code
+
+Techniques for XEmacs Developers
+================================
+
+To make a purified XEmacs, do: `make puremacs'. To make a quantified
+XEmacs, do: `make quantmacs'.
+
+ You simply can't dump Quantified and Purified images (unless using
+the portable dumper). Purify gets confused when xemacs frees memory in
+one process that was allocated in a _different_ process on a different
+machine!. Run it like so:
+ temacs -batch -l loadup.el run-temacs XEMACS-ARGS...
+
+ Before you go through the trouble, are you compiling with all
+debugging and error-checking off? If not, try that first. Be warned
+that while Quantify is directly responsible for quite a few
+optimizations which have been made to XEmacs, doing a run which
+generates results which can be acted upon is not necessarily a trivial
+task.
+
+ Also, if you're still willing to do some runs make sure you configure
+with the `--quantify' flag. That will keep Quantify from starting to
+record data until after the loadup is completed and will shut off
+recording right before it shuts down (which generates enough bogus data
+to throw most results off). It also enables three additional elisp
+commands: `quantify-start-recording-data',
+`quantify-stop-recording-data' and `quantify-clear-data'.
+
+ If you want to make XEmacs faster, target your favorite slow
+benchmark, run a profiler like Quantify, `gprof', or `tcov', and figure
+out where the cycles are going. In many cases you can localize the
+problem (because a particular new feature or even a single patch
+elicited it). Don't hesitate to use brute force techniques like a
+global counter incremented at strategic places, especially in
+combination with other performance indications (_e.g._, degree of
+buffer fragmentation into extents).
+
+ Specific projects:
+
+ * Make the garbage collector faster. Figure out how to write an
+ incremental garbage collector.
+
+ * Write a compiler that takes bytecode and spits out C code.
+ Unfortunately, you will then need a C compiler and a more fully
+ developed module system.
+
+ * Speed up redisplay.
+
+ * Speed up syntax highlighting. It was suggested that "maybe moving
+ some of the syntax highlighting capabilities into C would make a
+ difference." Wrong idea, I think. When processing one large file
+ a particular low-level routine was being called 40 _million_ times
+ simply for _one_ call to `newline-and-indent'. Syntax
+ highlighting needs to be rewritten to use a reliable, fast parser,
+ then to trust the pre-parsed structure, and only do
+ re-highlighting locally to a text change. Modern machines are
+ fast enough to implement such parsers in Lisp; but no machine will
+ ever be fast enough to deal with quadratic (or worse) algorithms!
+
+ * Implement tail recursion in Emacs Lisp (hard!).
+
+ Unfortunately, Emacs Lisp is slow, and is going to stay slow.
+Function calls in elisp are especially expensive. Iterating over a
+long list is going to be 30 times faster implemented in C than in Elisp.
+
+ Heavily used small code fragments need to be fast. The traditional
+way to implement such code fragments in C is with macros. But macros
+in C are known to be broken.
+
+ Macro arguments that are repeatedly evaluated may suffer from
+repeated side effects or suboptimal performance.
+
+ Variable names used in macros may collide with caller's variables,
+causing (at least) unwanted compiler warnings.
+
+ In order to solve these problems, and maintain statement semantics,
+one should use the `do { ... } while (0)' trick while trying to
+reference macro arguments exactly once using local variables.
+
+ Let's take a look at this poor macro definition:
+
+ #define MARK_OBJECT(obj) \
+ if (!marked_p (obj)) mark_object (obj), did_mark = 1
+
+ This macro evaluates its argument twice, and also fails if used like
+this:
+ if (flag) MARK_OBJECT (obj); else do_something();
+
+ A much better definition is
+
+ #define MARK_OBJECT(obj) do { \
+ Lisp_Object mo_obj = (obj); \
+ if (!marked_p (mo_obj)) \
+ { \
+ mark_object (mo_obj); \
+ did_mark = 1; \
+ } \
+ } while (0)
+
+ Notice the elimination of double evaluation by using the local
+variable with the obscure name. Writing safe and efficient macros
+requires great care. The one problem with macros that cannot be
+portably worked around is, since a C block has no value, a macro used
+as an expression rather than a statement cannot use the techniques just
+described to avoid multiple evaluation.
+
+ In most cases where a macro has function semantics, an inline
+function is a better implementation technique. Modern compiler
+optimizers tend to inline functions even if they have no `inline'
+keyword, and configure magic ensures that the `inline' keyword can be
+safely used as an additional compiler hint. Inline functions used in a
+single .c files are easy. The function must already be defined to be
+`static'. Just add another `inline' keyword to the definition.
+
+ inline static int
+ heavily_used_small_function (int arg)
+ {
+ ...
+ }
+
+ Inline functions in header files are trickier, because we would like
+to make the following optimization if the function is _not_ inlined
+(for example, because we're compiling for debugging). We would like the
+function to be defined externally exactly once, and each calling
+translation unit would create an external reference to the function,
+instead of including a definition of the inline function in the object
+code of every translation unit that uses it. This optimization is
+currently only available for gcc. But you don't have to worry about the
+trickiness; just define your inline functions in header files using this
+pattern:
+
+ INLINE_HEADER int
+ i_used_to_be_a_crufty_macro_but_look_at_me_now (int arg);
+ INLINE_HEADER int
+ i_used_to_be_a_crufty_macro_but_look_at_me_now (int arg)
+ {
+ ...
+ }
+
+ The declaration right before the definition is to prevent warnings
+when compiling with `gcc -Wmissing-declarations'. I consider issuing
+this warning for inline functions a gcc bug, but the gcc maintainers
+disagree.
+
+ Every header which contains inline functions, either directly by
+using `INLINE_HEADER' or indirectly by using `DECLARE_LRECORD' must be
+added to `inline.c''s includes to make the optimization described above
+work. (Optimization note: if all INLINE_HEADER functions are in fact
+inlined in all translation units, then the linker can just discard
+`inline.o', since it contains only unreferenced code).
+
+ To get started debugging XEmacs, take a look at the `.gdbinit' and
+`.dbxrc' files in the `src' directory. See the section in the XEmacs
+FAQ on How to Debug an XEmacs problem with a debugger.
+
+ After making source code changes, run `make check' to ensure that
+you haven't introduced any regressions. If you want to make xemacs more
+reliable, please improve the test suite in `tests/automated'.
+
+ Did you make sure you didn't introduce any new compiler warnings?
+
+ Before submitting a patch, please try compiling at least once with
+
+ configure --with-mule --use-union-type --error-checking=all
+
+ Here are things to know when you create a new source file:
+
+ * All `.c' files should `#include <config.h>' first. Almost all
+ `.c' files should `#include "lisp.h"' second.
+
+ * Generated header files should be included using the `#include
+ <...>' syntax, not the `#include "..."' syntax. The generated
+ headers are:
+
+ `config.h sheap-adjust.h paths.h Emacs.ad.h'
+
+ The basic rule is that you should assume builds using `--srcdir'
+ and the `#include <...>' syntax needs to be used when the
+ to-be-included generated file is in a potentially different
+ directory _at compile time_. The non-obvious C rule is that
+ `#include "..."' means to search for the included file in the same
+ directory as the including file, _not_ in the current directory.
+
+ * Header files should _not_ include `<config.h>' and `"lisp.h"'. It
+ is the responsibility of the `.c' files that use it to do so.
+
+
+ Here is a checklist of things to do when creating a new lisp object
+type named FOO:
+
+ 1. create FOO.h
+
+ 2. create FOO.c
+
+ 3. add definitions of `syms_of_FOO', etc. to `FOO.c'
+
+ 4. add declarations of `syms_of_FOO', etc. to `symsinit.h'
+
+ 5. add calls to `syms_of_FOO', etc. to `emacs.c'
+
+ 6. add definitions of macros like `CHECK_FOO' and `FOOP' to `FOO.h'
+
+ 7. add the new type index to `enum lrecord_type'
+
+ 8. add a DEFINE_LRECORD_IMPLEMENTATION call to `FOO.c'
+
+ 9. add an INIT_LRECORD_IMPLEMENTATION call to `syms_of_FOO.c'
+
+\1f
+File: internals.info, Node: Regression Testing XEmacs, Next: A Summary of the Various XEmacs Modules, Prev: Rules When Writing New C Code, Up: Top
+
+Regression Testing XEmacs
+*************************
+
+The source directory `tests/automated' contains XEmacs' automated test
+suite. The usual way of running all the tests is running `make check'
+from the top-level source directory.
+
+ The test suite is unfinished and it's still lacking some essential
+features. It is nevertheless recommended that you run the tests to
+confirm that XEmacs behaves correctly.
+
+ If you want to run a specific test case, you can do it from the
+command-line like this:
+
+ $ xemacs -batch -l test-harness.elc -f batch-test-emacs TEST-FILE
+
+ If something goes wrong, you can run the test suite interactively by
+loading `test-harness.el' into a running XEmacs and typing `M-x
+test-emacs-test-file RET <filename> RET'. You will see a log of passed
+and failed tests, which should allow you to investigate the source of
+the error and ultimately fix the bug.
+
+ Adding a new test file is trivial: just create a new file here and it
+will be run. There is no need to byte-compile any of the files in this
+directory--the test-harness will take care of any necessary
+byte-compilation.
+
+ Look at the existing test cases for the examples of coding test
+cases. It all boils down to your imagination and judicious use of the
+macros `Assert', `Check-Error', `Check-Error-Message', and
+`Check-Message'.
+
+ Here's a simple example checking case-sensitive and case-insensitive
+comparisons from `case-tests.el'.
+
+ (with-temp-buffer
+ (insert "Test Buffer")
+ (let ((case-fold-search t))
+ (goto-char (point-min))
+ (Assert (eq (search-forward "test buffer" nil t) 12))
+ (goto-char (point-min))
+ (Assert (eq (search-forward "Test buffer" nil t) 12))
+ (goto-char (point-min))
+ (Assert (eq (search-forward "Test Buffer" nil t) 12))
+
+ (setq case-fold-search nil)
+ (goto-char (point-min))
+ (Assert (not (search-forward "test buffer" nil t)))
+ (goto-char (point-min))
+ (Assert (not (search-forward "Test buffer" nil t)))
+ (goto-char (point-min))
+ (Assert (eq (search-forward "Test Buffer" nil t) 12))))
+
+ This example could be inserted in a file in `tests/automated', and
+it would be a complete test, automatically executed when you run `make
+check' after building XEmacs. More complex tests may require
+substantial temporary scaffolding to create the environment that elicits
+the bugs, but the top-level Makefile and `test-harness.el' handle the
+running and collection of results from the `Assert', `Check-Error',
+`Check-Error-Message', and `Check-Message' macros.
+
+ In general, you should avoid using functionality from packages in
+your tests, because you can't be sure that everyone will have the
+required package. However, if you've got a test that works, by all
+means add it. Simply wrap the test in an appropriate test, add a
+notice that the test was skipped, and update the `skipped-test-reasons'
+hashtable. Here's an example from `syntax-tests.el':
+
+ ;; Test forward-comment at buffer boundaries
+ (with-temp-buffer
+
+ ;; try to use exactly what you need: featurep, boundp, fboundp
+ (if (not (fboundp 'c-mode))
+
+ ;; We should provide a standard function for this boilerplate,
+ ;; probably called `Skip-Test' -- check for that API with C-h f
+ (let* ((reason "c-mode unavailable")
+ (count (gethash reason skipped-test-reasons)))
+ (puthash reason (if (null count) 1 (1+ count))
+ skipped-test-reasons)
+ (Print-Skip "comment and parse-partial-sexp tests" reason))
+
+ ;; and here's the test code
+ (c-mode)
+ (insert "// comment\n")
+ (forward-comment -2)
+ (Assert (eq (point) (point-min)))
+ (let ((point (point)))
+ (insert "/* comment */")
+ (goto-char point)
+ (forward-comment 2)
+ (Assert (eq (point) (point-max)))
+ (parse-partial-sexp point (point-max)))))
+
+ `Skip-Test' is intended for use with features that are normally
+present in typical configurations. For truly optional features, or
+tests that apply to one of several alternative implementations (eg, to
+GTK widgets, but not Athena, Motif, MS Windows, or Carbon), simply
+silently omit the test.
+
+\1f
+File: internals.info, Node: A Summary of the Various XEmacs Modules, Next: Allocation of Objects in XEmacs Lisp, Prev: Regression Testing XEmacs, Up: Top
+
+A Summary of the Various XEmacs Modules
+***************************************
+
+This is accurate as of XEmacs 20.0.
+
+* Menu:
+
+* Low-Level Modules::
+* Basic Lisp Modules::
+* Modules for Standard Editing Operations::
+* Editor-Level Control Flow Modules::
+* Modules for the Basic Displayable Lisp Objects::
+* Modules for other Display-Related Lisp Objects::
+* Modules for the Redisplay Mechanism::
+* Modules for Interfacing with the File System::
+* Modules for Other Aspects of the Lisp Interpreter and Object System::
+* Modules for Interfacing with the Operating System::
+* Modules for Interfacing with X Windows::
+* Modules for Internationalization::
+* Modules for Regression Testing::
+
+\1f
+File: internals.info, Node: Low-Level Modules, Next: Basic Lisp Modules, Up: A Summary of the Various XEmacs Modules
+
+Low-Level Modules
+=================
+
+ config.h
+
+ This is automatically generated from `config.h.in' based on the
+results of configure tests and user-selected optional features and
+contains preprocessor definitions specifying the nature of the
+environment in which XEmacs is being compiled.
+
+ paths.h
+
+ This is automatically generated from `paths.h.in' based on supplied
+configure values, and allows for non-standard installed configurations
+of the XEmacs directories. It's currently broken, though.
+
+ emacs.c
+ signal.c
+
+ `emacs.c' contains `main()' and other code that performs the most
+basic environment initializations and handles shutting down the XEmacs
+process (this includes `kill-emacs', the normal way that XEmacs is
+exited; `dump-emacs', which is used during the build process to write
+out the XEmacs executable; `run-emacs-from-temacs', which can be used
+to start XEmacs directly when temacs has finished loading all the Lisp
+code; and emergency code to handle crashes [XEmacs tries to auto-save
+all files before it crashes]).
+
+ Low-level code that directly interacts with the Unix signal
+mechanism, however, is in `signal.c'. Note that this code does not
+handle system dependencies in interfacing to signals; that is handled
+using the `syssignal.h' header file, described in section J below.
+
+ unexaix.c
+ unexalpha.c
+ unexapollo.c
+ unexconvex.c
+ unexec.c
+ unexelf.c
+ unexelfsgi.c
+ unexencap.c
+ unexenix.c
+ unexfreebsd.c
+ unexfx2800.c
+ unexhp9k3.c
+ unexhp9k800.c
+ unexmips.c
+ unexnext.c
+ unexsol2.c
+ unexsunos4.c
+
+ These modules contain code dumping out the XEmacs executable on
+various different systems. (This process is highly machine-specific and
+requires intimate knowledge of the executable format and the memory map
+of the process.) Only one of these modules is actually used; this is
+chosen by `configure'.
+
+ ecrt0.c
+ lastfile.c
+ pre-crt0.c
+
+ These modules are used in conjunction with the dump mechanism. On
+some systems, an alternative version of the C startup code (the actual
+code that receives control from the operating system when the process is
+started, and which calls `main()') is required so that the dumping
+process works properly; `crt0.c' provides this.
+
+ `pre-crt0.c' and `lastfile.c' should be the very first and very last
+file linked, respectively. (Actually, this is not really true.
+`lastfile.c' should be after all Emacs modules whose initialized data
+should be made constant, and before all other Emacs files and all
+libraries. In particular, the allocation modules `gmalloc.c',
+`alloca.c', etc. are normally placed past `lastfile.c', and all of the
+files that implement Xt widget classes _must_ be placed after
+`lastfile.c' because they contain various structures that must be
+statically initialized and into which Xt writes at various times.)
+`pre-crt0.c' and `lastfile.c' contain exported symbols that are used to
+determine the start and end of XEmacs' initialized data space when
+dumping.
+
+ alloca.c
+ free-hook.c
+ getpagesize.h
+ gmalloc.c
+ malloc.c
+ mem-limits.h
+ ralloc.c
+ vm-limit.c
+
+ These handle basic C allocation of memory. `alloca.c' is an
+emulation of the stack allocation function `alloca()' on machines that
+lack this. (XEmacs makes extensive use of `alloca()' in its code.)
+
+ `gmalloc.c' and `malloc.c' are two implementations of the standard C
+functions `malloc()', `realloc()' and `free()'. They are often used in
+place of the standard system-provided `malloc()' because they usually
+provide a much faster implementation, at the expense of additional
+memory use. `gmalloc.c' is a newer implementation that is much more
+memory-efficient for large allocations than `malloc.c', and should
+always be preferred if it works. (At one point, `gmalloc.c' didn't work
+on some systems where `malloc.c' worked; but this should be fixed now.)
+
+ `ralloc.c' is the "relocating allocator". It provides functions
+similar to `malloc()', `realloc()' and `free()' that allocate memory
+that can be dynamically relocated in memory. The advantage of this is
+that allocated memory can be shuffled around to place all the free
+memory at the end of the heap, and the heap can then be shrunk,
+releasing the memory back to the operating system. The use of this can
+be controlled with the configure option `--rel-alloc'; if enabled,
+memory allocated for buffers will be relocatable, so that if a very
+large file is visited and the buffer is later killed, the memory can be
+released to the operating system. (The disadvantage of this mechanism
+is that it can be very slow. On systems with the `mmap()' system call,
+the XEmacs version of `ralloc.c' uses this to move memory around
+without actually having to block-copy it, which can speed things up;
+but it can still cause noticeable performance degradation.)
+
+ `free-hook.c' contains some debugging functions for checking for
+invalid arguments to `free()'.
+
+ `vm-limit.c' contains some functions that warn the user when memory
+is getting low. These are callback functions that are called by
+`gmalloc.c' and `malloc.c' at appropriate times.
+
+ `getpagesize.h' provides a uniform interface for retrieving the size
+of a page in virtual memory. `mem-limits.h' provides a uniform
+interface for retrieving the total amount of available virtual memory.
+Both are similar in spirit to the `sys*.h' files described in section
+J, below.
+
+ blocktype.c
+ blocktype.h
+ dynarr.c
+
+ These implement a couple of basic C data types to facilitate memory
+allocation. The `Blocktype' type efficiently manages the allocation of
+fixed-size blocks by minimizing the number of times that `malloc()' and
+`free()' are called. It allocates memory in large chunks, subdivides
+the chunks into blocks of the proper size, and returns the blocks as
+requested. When blocks are freed, they are placed onto a linked list,
+so they can be efficiently reused. This data type is not much used in
+XEmacs currently, because it's a fairly new addition.
+
+ The `Dynarr' type implements a "dynamic array", which is similar to
+a standard C array but has no fixed limit on the number of elements it
+can contain. Dynamic arrays can hold elements of any type, and when
+you add a new element, the array automatically resizes itself if it
+isn't big enough. Dynarrs are extensively used in the redisplay
+mechanism.
+
+ inline.c
+
+ This module is used in connection with inline functions (available in
+some compilers). Often, inline functions need to have a corresponding
+non-inline function that does the same thing. This module is where they
+reside. It contains no actual code, but defines some special flags that
+cause inline functions defined in header files to be rendered as actual
+functions. It then includes all header files that contain any inline
+function definitions, so that each one gets a real function equivalent.
+
+ debug.c
+ debug.h
+
+ These functions provide a system for doing internal consistency
+checks during code development. This system is not currently used;
+instead the simpler `assert()' macro is used along with the various
+checks provided by the `--error-check-*' configuration options.
+
+ universe.h
+
+ This is not currently used.
+
+\1f
+File: internals.info, Node: Basic Lisp Modules, Next: Modules for Standard Editing Operations, Prev: Low-Level Modules, Up: A Summary of the Various XEmacs Modules
+
+Basic Lisp Modules
+==================
+
+ lisp-disunion.h
+ lisp-union.h
+ lisp.h
+ lrecord.h
+ symsinit.h
+
+ These are the basic header files for all XEmacs modules. Each module
+includes `lisp.h', which brings the other header files in. `lisp.h'
+contains the definitions of the structures and extractor and
+constructor macros for the basic Lisp objects and various other basic
+definitions for the Lisp environment, as well as some general-purpose
+definitions (e.g. `min()' and `max()'). `lisp.h' includes either
+`lisp-disunion.h' or `lisp-union.h', depending on whether
+`USE_UNION_TYPE' is defined. These files define the typedef of the
+Lisp object itself (as described above) and the low-level macros that
+hide the actual implementation of the Lisp object. All extractor and
+constructor macros for particular types of Lisp objects are defined in
+terms of these low-level macros.
+
+ As a general rule, all typedefs should go into the typedefs section
+of `lisp.h' rather than into a module-specific header file even if the
+structure is defined elsewhere. This allows function prototypes that
+use the typedef to be placed into other header files. Forward structure
+declarations (i.e. a simple declaration like `struct foo;' where the
+structure itself is defined elsewhere) should be placed into the
+typedefs section as necessary.
+
+ `lrecord.h' contains the basic structures and macros that implement
+all record-type Lisp objects--i.e. all objects whose type is a field in
+their C structure, which includes all objects except the few most basic
+ones.
+
+ `lisp.h' contains prototypes for most of the exported functions in
+the various modules. Lisp primitives defined using `DEFUN' that need
+to be called by C code should be declared using `EXFUN'. Other
+function prototypes should be placed either into the appropriate
+section of `lisp.h', or into a module-specific header file, depending
+on how general-purpose the function is and whether it has
+special-purpose argument types requiring definitions not in `lisp.h'.)
+All initialization functions are prototyped in `symsinit.h'.
+
+ alloc.c
+
+ The large module `alloc.c' implements all of the basic allocation and
+garbage collection for Lisp objects. The most commonly used Lisp
+objects are allocated in chunks, similar to the Blocktype data type
+described above; others are allocated in individually `malloc()'ed
+blocks. This module provides the foundation on which all other aspects
+of the Lisp environment sit, and is the first module initialized at
+startup.
+
+ Note that `alloc.c' provides a series of generic functions that are
+not dependent on any particular object type, and interfaces to
+particular types of objects using a standardized interface of
+type-specific methods. This scheme is a fundamental principle of
+object-oriented programming and is heavily used throughout XEmacs. The
+great advantage of this is that it allows for a clean separation of
+functionality into different modules--new classes of Lisp objects, new
+event interfaces, new device types, new stream interfaces, etc. can be
+added transparently without affecting code anywhere else in XEmacs.
+Because the different subsystems are divided into general and specific
+code, adding a new subtype within a subsystem will in general not
+require changes to the generic subsystem code or affect any of the other
+subtypes in the subsystem; this provides a great deal of robustness to
+the XEmacs code.
+
+ eval.c
+ backtrace.h
+
+ This module contains all of the functions to handle the flow of
+control. This includes the mechanisms of defining functions, calling
+functions, traversing stack frames, and binding variables; the control
+primitives and other special forms such as `while', `if', `eval',
+`let', `and', `or', `progn', etc.; handling of non-local exits,
+unwind-protects, and exception handlers; entering the debugger; methods
+for the subr Lisp object type; etc. It does _not_ include the `read'
+function, the `print' function, or the handling of symbols and obarrays.
+
+ `backtrace.h' contains some structures related to stack frames and
+the flow of control.
+
+ lread.c
+
+ This module implements the Lisp reader and the `read' function,
+which converts text into Lisp objects, according to the read syntax of
+the objects, as described above. This is similar to the parser that is
+a part of all compilers.
+
+ print.c
+
+ This module implements the Lisp print mechanism and the `print'
+function and related functions. This is the inverse of the Lisp reader
+- it converts Lisp objects to a printed, textual representation.
+(Hopefully something that can be read back in using `read' to get an
+equivalent object.)
+
+ general.c
+ symbols.c
+ symeval.h
+
+ `symbols.c' implements the handling of symbols, obarrays, and
+retrieving the values of symbols. Much of the code is devoted to
+handling the special "symbol-value-magic" objects that define special
+types of variables--this includes buffer-local variables, variable
+aliases, variables that forward into C variables, etc. This module is
+initialized extremely early (right after `alloc.c'), because it is here
+that the basic symbols `t' and `nil' are created, and those symbols are
+used everywhere throughout XEmacs.
+
+ `symeval.h' contains the definitions of symbol structures and the
+`DEFVAR_LISP()' and related macros for declaring variables.
+
+ data.c
+ floatfns.c
+ fns.c
+
+ These modules implement the methods and standard Lisp primitives for
+all the basic Lisp object types other than symbols (which are described
+above). `data.c' contains all the predicates (primitives that return
+whether an object is of a particular type); the integer arithmetic
+functions; and the basic accessor and mutator primitives for the various
+object types. `fns.c' contains all the standard predicates for working
+with sequences (where, abstractly speaking, a sequence is an ordered set
+of objects, and can be represented by a list, string, vector, or
+bit-vector); it also contains `equal', perhaps on the grounds that bulk
+of the operation of `equal' is comparing sequences. `floatfns.c'
+contains methods and primitives for floats and floating-point
+arithmetic.
+
+ bytecode.c
+ bytecode.h
+
+ `bytecode.c' implements the byte-code interpreter and
+compiled-function objects, and `bytecode.h' contains associated
+structures. Note that the byte-code _compiler_ is written in Lisp.
+
+\1f
+File: internals.info, Node: Modules for Standard Editing Operations, Next: Editor-Level Control Flow Modules, Prev: Basic Lisp Modules, Up: A Summary of the Various XEmacs Modules
+
+Modules for Standard Editing Operations
+=======================================
+
+ buffer.c
+ buffer.h
+ bufslots.h
+
+ `buffer.c' implements the "buffer" Lisp object type. This includes
+functions that create and destroy buffers; retrieve buffers by name or
+by other properties; manipulate lists of buffers (remember that buffers
+are permanent objects and stored in various ordered lists); retrieve or
+change buffer properties; etc. It also contains the definitions of all
+the built-in buffer-local variables (which can be viewed as buffer
+properties). It does _not_ contain code to manipulate buffer-local
+variables (that's in `symbols.c', described above); or code to
+manipulate the text in a buffer.
+
+ `buffer.h' defines the structures associated with a buffer and the
+various macros for retrieving text from a buffer and special buffer
+positions (e.g. `point', the default location for text insertion). It
+also contains macros for working with buffer positions and converting
+between their representations as character offsets and as byte offsets
+(under MULE, they are different, because characters can be multi-byte).
+It is one of the largest header files.
+
+ `bufslots.h' defines the fields in the buffer structure that
+correspond to the built-in buffer-local variables. It is its own
+header file because it is included many times in `buffer.c', as a way
+of iterating over all the built-in buffer-local variables.
+
+ insdel.c
+ insdel.h
+
+ `insdel.c' contains low-level functions for inserting and deleting
+text in a buffer, keeping track of changed regions for use by
+redisplay, and calling any before-change and after-change functions
+that may have been registered for the buffer. It also contains the
+actual functions that convert between byte offsets and character
+offsets.
+
+ `insdel.h' contains associated headers.
+
+ marker.c
+
+ This module implements the "marker" Lisp object type, which
+conceptually is a pointer to a text position in a buffer that moves
+around as text is inserted and deleted, so as to remain in the same
+relative position. This module doesn't actually move the markers around
+- that's handled in `insdel.c'. This module just creates them and
+implements the primitives for working with them. As markers are simple
+objects, this does not entail much.
+
+ Note that the standard arithmetic primitives (e.g. `+') accept
+markers in place of integers and automatically substitute the value of
+`marker-position' for the marker, i.e. an integer describing the
+current buffer position of the marker.
+
+ extents.c
+ extents.h
+
+ This module implements the "extent" Lisp object type, which is like
+a marker that works over a range of text rather than a single position.
+Extents are also much more complex and powerful than markers and have a
+more efficient (and more algorithmically complex) implementation. The
+implementation is described in detail in comments in `extents.c'.
+
+ The code in `extents.c' works closely with `insdel.c' so that
+extents are properly moved around as text is inserted and deleted.
+There is also code in `extents.c' that provides information needed by
+the redisplay mechanism for efficient operation. (Remember that extents
+can have display properties that affect [sometimes drastically, as in
+the `invisible' property] the display of the text they cover.)
+
+ editfns.c
+
+ `editfns.c' contains the standard Lisp primitives for working with a
+buffer's text, and calls the low-level functions in `insdel.c'. It
+also contains primitives for working with `point' (the default buffer
+insertion location).
+
+ `editfns.c' also contains functions for retrieving various
+characteristics from the external environment: the current time, the
+process ID of the running XEmacs process, the name of the user who ran
+this XEmacs process, etc. It's not clear why this code is in
+`editfns.c'.
+
+ callint.c
+ cmds.c
+ commands.h
+
+ These modules implement the basic "interactive" commands, i.e.
+user-callable functions. Commands, as opposed to other functions, have
+special ways of getting their parameters interactively (by querying the
+user), as opposed to having them passed in a normal function
+invocation. Many commands are not really meant to be called from other
+Lisp functions, because they modify global state in a way that's often
+undesired as part of other Lisp functions.
+
+ `callint.c' implements the mechanism for querying the user for
+parameters and calling interactive commands. The bulk of this module is
+code that parses the interactive spec that is supplied with an
+interactive command.
+
+ `cmds.c' implements the basic, most commonly used editing commands:
+commands to move around the current buffer and insert and delete
+characters. These commands are implemented using the Lisp primitives
+defined in `editfns.c'.
+
+ `commands.h' contains associated structure definitions and
+prototypes.
+
+ regex.c
+ regex.h
+ search.c
+
+ `search.c' implements the Lisp primitives for searching for text in
+a buffer, and some of the low-level algorithms for doing this. In
+particular, the fast fixed-string Boyer-Moore search algorithm is
+implemented in `search.c'. The low-level algorithms for doing
+regular-expression searching, however, are implemented in `regex.c' and
+`regex.h'. These two modules are largely independent of XEmacs, and
+are similar to (and based upon) the regular-expression routines used in
+`grep' and other GNU utilities.
+
+ doprnt.c
+
+ `doprnt.c' implements formatted-string processing, similar to
+`printf()' command in C.
+
+ undo.c
+
+ This module implements the undo mechanism for tracking buffer
+changes. Most of this could be implemented in Lisp.
+
+\1f
+File: internals.info, Node: Editor-Level Control Flow Modules, Next: Modules for the Basic Displayable Lisp Objects, Prev: Modules for Standard Editing Operations, Up: A Summary of the Various XEmacs Modules
+
+Editor-Level Control Flow Modules
+=================================
+
+ event-Xt.c
+ event-msw.c
+ event-stream.c
+ event-tty.c
+ events-mod.h
+ gpmevent.c
+ gpmevent.h
+ events.c
+ events.h
+
+ These implement the handling of events (user input and other system
+notifications).
+
+ `events.c' and `events.h' define the "event" Lisp object type and
+primitives for manipulating it.
+
+ `event-stream.c' implements the basic functions for working with
+event queues, dispatching an event by looking it up in relevant keymaps
+and such, and handling timeouts; this includes the primitives
+`next-event' and `dispatch-event', as well as related primitives such
+as `sit-for', `sleep-for', and `accept-process-output'.
+(`event-stream.c' is one of the hairiest and trickiest modules in
+XEmacs. Beware! You can easily mess things up here.)
+
+ `event-Xt.c' and `event-tty.c' implement the low-level interfaces
+onto retrieving events from Xt (the X toolkit) and from TTY's (using
+`read()' and `select()'), respectively. The event interface enforces a
+clean separation between the specific code for interfacing with the
+operating system and the generic code for working with events, by
+defining an API of basic, low-level event methods; `event-Xt.c' and
+`event-tty.c' are two different implementations of this API. To add
+support for a new operating system (e.g. NeXTstep), one merely needs to
+provide another implementation of those API functions.
+
+ Note that the choice of whether to use `event-Xt.c' or `event-tty.c'
+is made at compile time! Or at the very latest, it is made at startup
+time. `event-Xt.c' handles events for _both_ X and TTY frames;
+`event-tty.c' is only used when X support is not compiled into XEmacs.
+The reason for this is that there is only one event loop in XEmacs:
+thus, it needs to be able to receive events from all different kinds of
+frames.
+
+ keymap.c
+ keymap.h
+
+ `keymap.c' and `keymap.h' define the "keymap" Lisp object type and
+associated methods and primitives. (Remember that keymaps are objects
+that associate event descriptions with functions to be called to
+"execute" those events; `dispatch-event' looks up events in the
+relevant keymaps.)
+
+ cmdloop.c
+
+ `cmdloop.c' contains functions that implement the actual editor
+command loop--i.e. the event loop that cyclically retrieves and
+dispatches events. This code is also rather tricky, just like
+`event-stream.c'.
+
+ macros.c
+ macros.h
+
+ These two modules contain the basic code for defining keyboard
+macros. These functions don't actually do much; most of the code that
+handles keyboard macros is mixed in with the event-handling code in
+`event-stream.c'.
+
+ minibuf.c
+
+ This contains some miscellaneous code related to the minibuffer
+(most of the minibuffer code was moved into Lisp by Richard Mlynarik).
+This includes the primitives for completion (although filename
+completion is in `dired.c'), the lowest-level interface to the
+minibuffer (if the command loop were cleaned up, this too could be in
+Lisp), and code for dealing with the echo area (this, too, was mostly
+moved into Lisp, and the only code remaining is code to call out to
+Lisp or provide simple bootstrapping implementations early in temacs,
+before the echo-area Lisp code is loaded).
+
+\1f
+File: internals.info, Node: Modules for the Basic Displayable Lisp Objects, Next: Modules for other Display-Related Lisp Objects, Prev: Editor-Level Control Flow Modules, Up: A Summary of the Various XEmacs Modules
+
+Modules for the Basic Displayable Lisp Objects
+==============================================
+
+ console-msw.c
+ console-msw.h
+ console-stream.c
+ console-stream.h
+ console-tty.c
+ console-tty.h
+ console-x.c
+ console-x.h
+ console.c
+ console.h
+
+ These modules implement the "console" Lisp object type. A console
+contains multiple display devices, but only one keyboard and mouse.
+Most of the time, a console will contain exactly one device.
+
+ Consoles are the top of a lisp object inclusion hierarchy. Consoles
+contain devices, which contain frames, which contain windows.
+
+ device-msw.c
+ device-tty.c
+ device-x.c
+ device.c
+ device.h
+
+ These modules implement the "device" Lisp object type. This
+abstracts a particular screen or connection on which frames are
+displayed. As with Lisp objects, event interfaces, and other
+subsystems, the device code is separated into a generic component that
+contains a standardized interface (in the form of a set of methods) onto
+particular device types.
+
+ The device subsystem defines all the methods and provides method
+services for not only device operations but also for the frame, window,
+menubar, scrollbar, toolbar, and other displayable-object subsystems.
+The reason for this is that all of these subsystems have the same
+subtypes (X, TTY, NeXTstep, Microsoft Windows, etc.) as devices do.
+
+ frame-msw.c
+ frame-tty.c
+ frame-x.c
+ frame.c
+ frame.h
+
+ Each device contains one or more frames in which objects (e.g. text)
+are displayed. A frame corresponds to a window in the window system;
+usually this is a top-level window but it could potentially be one of a
+number of overlapping child windows within a top-level window, using the
+MDI (Multiple Document Interface) protocol in Microsoft Windows or a
+similar scheme.
+
+ The `frame-*' files implement the "frame" Lisp object type and
+provide the generic and device-type-specific operations on frames (e.g.
+raising, lowering, resizing, moving, etc.).
+
+ window.c
+ window.h
+
+ Each frame consists of one or more non-overlapping "windows" (better
+known as "panes" in standard window-system terminology) in which a
+buffer's text can be displayed. Windows can also have scrollbars
+displayed around their edges.
+
+ `window.c' and `window.h' implement the "window" Lisp object type
+and provide code to manage windows. Since windows have no associated
+resources in the window system (the window system knows only about the
+frame; no child windows or anything are used for XEmacs windows), there
+is no device-type-specific code here; all of that code is part of the
+redisplay mechanism or the code for particular object types such as
+scrollbars.
+
+\1f
+File: internals.info, Node: Modules for other Display-Related Lisp Objects, Next: Modules for the Redisplay Mechanism, Prev: Modules for the Basic Displayable Lisp Objects, Up: A Summary of the Various XEmacs Modules
+
+Modules for other Display-Related Lisp Objects
+==============================================
+
+ faces.c
+ faces.h
+
+ bitmaps.h
+ glyphs-eimage.c
+ glyphs-msw.c
+ glyphs-msw.h
+ glyphs-widget.c
+ glyphs-x.c
+ glyphs-x.h
+ glyphs.c
+ glyphs.h
+
+ objects-msw.c
+ objects-msw.h
+ objects-tty.c
+ objects-tty.h
+ objects-x.c
+ objects-x.h
+ objects.c
+ objects.h
+
+ menubar-msw.c
+ menubar-msw.h
+ menubar-x.c
+ menubar.c
+ menubar.h
+
+ scrollbar-msw.c
+ scrollbar-msw.h
+ scrollbar-x.c
+ scrollbar-x.h
+ scrollbar.c
+ scrollbar.h
+
+ toolbar-msw.c
+ toolbar-x.c
+ toolbar.c
+ toolbar.h
+
+ font-lock.c
+
+ This file provides C support for syntax highlighting--i.e.
+highlighting different syntactic constructs of a source file in
+different colors, for easy reading. The C support is provided so that
+this is fast.
+
+ As of 21.4.10, bugs introduced at the very end of the 21.2 series in
+the "syntax properties" code were fixed, and highlighting is acceptably
+quick again. However, presumably more improvements are possible, and
+the places to look are probably here, in the defun-traversing code, and
+in `syntax.c', in the comment-traversing code.
+
+ dgif_lib.c
+ gif_err.c
+ gif_lib.h
+ gifalloc.c
+
+ These modules decode GIF-format image files, for use with glyphs.
+These files were removed due to Unisys patent infringement concerns.
+
+\1f
+File: internals.info, Node: Modules for the Redisplay Mechanism, Next: Modules for Interfacing with the File System, Prev: Modules for other Display-Related Lisp Objects, Up: A Summary of the Various XEmacs Modules
+
+Modules for the Redisplay Mechanism
+===================================
+
+ redisplay-output.c
+ redisplay-msw.c
+ redisplay-tty.c
+ redisplay-x.c
+ redisplay.c
+ redisplay.h
+
+ These files provide the redisplay mechanism. As with many other
+subsystems in XEmacs, there is a clean separation between the general
+and device-specific support.
+
+ `redisplay.c' contains the bulk of the redisplay engine. These
+functions update the redisplay structures (which describe how the screen
+is to appear) to reflect any changes made to the state of any
+displayable objects (buffer, frame, window, etc.) since the last time
+that redisplay was called. These functions are highly optimized to
+avoid doing more work than necessary (since redisplay is called
+extremely often and is potentially a huge time sink), and depend heavily
+on notifications from the objects themselves that changes have occurred,
+so that redisplay doesn't explicitly have to check each possible object.
+The redisplay mechanism also contains a great deal of caching to further
+speed things up; some of this caching is contained within the various
+displayable objects.
+
+ `redisplay-output.c' goes through the redisplay structures and
+converts them into calls to device-specific methods to actually output
+the screen changes.
+
+ `redisplay-x.c' and `redisplay-tty.c' are two implementations of
+these redisplay output methods, for X frames and TTY frames,
+respectively.
+
+ indent.c
+
+ This module contains various functions and Lisp primitives for
+converting between buffer positions and screen positions. These
+functions call the redisplay mechanism to do most of the work, and then
+examine the redisplay structures to get the necessary information. This
+module needs work.
+
+ termcap.c
+ terminfo.c
+ tparam.c
+
+ These files contain functions for working with the termcap
+(BSD-style) and terminfo (System V style) databases of terminal
+capabilities and escape sequences, used when XEmacs is displaying in a
+TTY.
+
+ cm.c
+ cm.h
+
+ These files provide some miscellaneous TTY-output functions and
+should probably be merged into `redisplay-tty.c'.
+
+\1f
+File: internals.info, Node: Modules for Interfacing with the File System, Next: Modules for Other Aspects of the Lisp Interpreter and Object System, Prev: Modules for the Redisplay Mechanism, Up: A Summary of the Various XEmacs Modules
+
+Modules for Interfacing with the File System
+============================================
+
+ lstream.c
+ lstream.h
+
+ These modules implement the "stream" Lisp object type. This is an
+internal-only Lisp object that implements a generic buffering stream.
+The idea is to provide a uniform interface onto all sources and sinks of
+data, including file descriptors, stdio streams, chunks of memory, Lisp
+buffers, Lisp strings, etc. That way, I/O functions can be written to
+the stream interface and can transparently handle all possible sources
+and sinks. (For example, the `read' function can read data from a
+file, a string, a buffer, or even a function that is called repeatedly
+to return data, without worrying about where the data is coming from or
+what-size chunks it is returned in.)
+
+ Note that in the C code, streams are called "lstreams" (for "Lisp
+streams") to distinguish them from other kinds of streams, e.g. stdio
+streams and C++ I/O streams.
+
+ Similar to other subsystems in XEmacs, lstreams are separated into
+generic functions and a set of methods for the different types of
+lstreams. `lstream.c' provides implementations of many different types
+of streams; others are provided, e.g., in `file-coding.c'.
+
+ fileio.c
+
+ This implements the basic primitives for interfacing with the file
+system. This includes primitives for reading files into buffers,
+writing buffers into files, checking for the presence or accessibility
+of files, canonicalizing file names, etc. Note that these primitives
+are usually not invoked directly by the user: There is a great deal of
+higher-level Lisp code that implements the user commands such as
+`find-file' and `save-buffer'. This is similar to the distinction
+between the lower-level primitives in `editfns.c' and the higher-level
+user commands in `commands.c' and `simple.el'.
+
+ filelock.c
+
+ This file provides functions for detecting clashes between different
+processes (e.g. XEmacs and some external process, or two different
+XEmacs processes) modifying the same file. (XEmacs can optionally use
+the `lock/' subdirectory to provide a form of "locking" between
+different XEmacs processes.) This module is also used by the low-level
+functions in `insdel.c' to ensure that, if the first modification is
+being made to a buffer whose corresponding file has been externally
+modified, the user is made aware of this so that the buffer can be
+synched up with the external changes if necessary.
+
+ filemode.c
+
+ This file provides some miscellaneous functions that construct a
+`rwxr-xr-x'-type permissions string (as might appear in an `ls'-style
+directory listing) given the information returned by the `stat()'
+system call.
+
+ dired.c
+ ndir.h
+
+ These files implement the XEmacs interface to directory searching.
+This includes a number of primitives for determining the files in a
+directory and for doing filename completion. (Remember that generic
+completion is handled by a different mechanism, in `minibuf.c'.)
+
+ `ndir.h' is a header file used for the directory-searching emulation
+functions provided in `sysdep.c' (see section J below), for systems
+that don't provide any directory-searching functions. (On those
+systems, directories can be read directly as files, and parsed.)
+
+ realpath.c
+
+ This file provides an implementation of the `realpath()' function
+for expanding symbolic links, on systems that don't implement it or have
+a broken implementation.
+
+\1f
+File: internals.info, Node: Modules for Other Aspects of the Lisp Interpreter and Object System, Next: Modules for Interfacing with the Operating System, Prev: Modules for Interfacing with the File System, Up: A Summary of the Various XEmacs Modules
+
+Modules for Other Aspects of the Lisp Interpreter and Object System
+===================================================================
+
+ elhash.c
+ elhash.h
+ hash.c
+ hash.h
+
+ These files provide two implementations of hash tables. Files
+`hash.c' and `hash.h' provide a generic C implementation of hash tables
+which can stand independently of XEmacs. Files `elhash.c' and
+`elhash.h' provide a separate implementation of hash tables that can
+store only Lisp objects, and knows about Lispy things like garbage
+collection, and implement the "hash-table" Lisp object type.
+
+ specifier.c
+ specifier.h
+
+ This module implements the "specifier" Lisp object type. This is
+primarily used for displayable properties, and allows for values that
+are specific to a particular buffer, window, frame, device, or device
+class, as well as a default value existing. This is used, for example,
+to control the height of the horizontal scrollbar or the appearance of
+the `default', `bold', or other faces. The specifier object consists
+of a number of specifications, each of which maps from a buffer,
+window, etc. to a value. The function `specifier-instance' looks up a
+value given a window (from which a buffer, frame, and device can be
+derived).
+
+ chartab.c
+ chartab.h
+ casetab.c
+
+ `chartab.c' and `chartab.h' implement the "char table" Lisp object
+type, which maps from characters or certain sorts of character ranges
+to Lisp objects. The implementation of this object type is optimized
+for the internal representation of characters. Char tables come in
+different types, which affect the allowed object types to which a
+character can be mapped and also dictate certain other properties of
+the char table.
+
+ `casetab.c' implements one sort of char table, the "case table",
+which maps characters to other characters of possibly different case.
+These are used by XEmacs to implement case-changing primitives and to
+do case-insensitive searching.
+
+ syntax.c
+ syntax.h
+
+ This module implements "syntax tables", another sort of char table
+that maps characters into syntax classes that define the syntax of these
+characters (e.g. a parenthesis belongs to a class of `open' characters
+that have corresponding `close' characters and can be nested). This
+module also implements the Lisp "scanner", a set of primitives for
+scanning over text based on syntax tables. This is used, for example,
+to find the matching parenthesis in a command such as `forward-sexp',
+and by `font-lock.c' to locate quoted strings, comments, etc.
+
+ Syntax codes are implemented as bitfields in an int. Bits 0-6
+contain the syntax code itself, bit 7 is a special prefix flag used for
+Lisp, and bits 16-23 contain comment syntax flags. From the Lisp
+programmer's point of view, there are 11 flags: 2 styles X 2 characters
+X {start, end} flags for two-character comment delimiters, 2 style
+flags for one-character comment delimiters, and the prefix flag.
+
+ Internally, however, the characters used in multi-character
+delimiters will have non-comment-character syntax classes (_e.g._, the
+`/' in C's `/*' comment-start delimiter has "punctuation" (here meaning
+"operator-like") class in C modes). Thus in a mixed comment style,
+such as C++'s `//' to end of line, is represented by giving `/' the
+"punctuation" class and the "style b first character of start sequence"
+and "style b second character of start sequence" flags. The fact that
+class is _not_ punctuation allows the syntax scanner to recognize that
+this is a multi-character delimiter. The `newline' character is given
+(single-character) "comment-end" _class_ and the "style b first
+character of end sequence" _flag_. The "comment-end" class allows the
+scanner to determine that no second character is needed to terminate
+the comment.
+
+ casefiddle.c
+
+ This module implements various Lisp primitives for upcasing,
+downcasing and capitalizing strings or regions of buffers.
+
+ rangetab.c
+
+ This module implements the "range table" Lisp object type, which
+provides for a mapping from ranges of integers to arbitrary Lisp
+objects.
+
+ opaque.c
+ opaque.h
+
+ This module implements the "opaque" Lisp object type, an
+internal-only Lisp object that encapsulates an arbitrary block of memory
+so that it can be managed by the Lisp allocation system. To create an
+opaque object, you call `make_opaque()', passing a pointer to a block
+of memory. An object is created that is big enough to hold the memory,
+which is copied into the object's storage. The object will then stick
+around as long as you keep pointers to it, after which it will be
+automatically reclaimed.
+
+ Opaque objects can also have an arbitrary "mark method" associated
+with them, in case the block of memory contains other Lisp objects that
+need to be marked for garbage-collection purposes. (If you need other
+object methods, such as a finalize method, you should just go ahead and
+create a new Lisp object type--it's not hard.)
+
+ abbrev.c
+
+ This function provides a few primitives for doing dynamic
+abbreviation expansion. In XEmacs, most of the code for this has been
+moved into Lisp. Some C code remains for speed and because the
+primitive `self-insert-command' (which is executed for all
+self-inserting characters) hooks into the abbrev mechanism.
+(`self-insert-command' is itself in C only for speed.)
+
+ doc.c
+
+ This function provides primitives for retrieving the documentation
+strings of functions and variables. These documentation strings contain
+certain special markers that get dynamically expanded (e.g. a
+reverse-lookup is performed on some named functions to retrieve their
+current key bindings). Some documentation strings (in particular, for
+the built-in primitives and pre-loaded Lisp functions) are stored
+externally in a file `DOC' in the `lib-src/' directory and need to be
+fetched from that file. (Part of the build stage involves building this
+file, and another part involves constructing an index for this file and
+embedding it into the executable, so that the functions in `doc.c' do
+not have to search the entire `DOC' file to find the appropriate
+documentation string.)
+
+ md5.c
+
+ This function provides a Lisp primitive that implements the MD5
+secure hashing scheme, used to create a large hash value of a string of
+data such that the data cannot be derived from the hash value. This is
+used for various security applications on the Internet.
+
+\1f
+File: internals.info, Node: Modules for Interfacing with the Operating System, Next: Modules for Interfacing with X Windows, Prev: Modules for Other Aspects of the Lisp Interpreter and Object System, Up: A Summary of the Various XEmacs Modules
+
+Modules for Interfacing with the Operating System
+=================================================
+
+ callproc.c
+ process.c
+ process.h
+
+ These modules allow XEmacs to spawn and communicate with subprocesses
+and network connections.
+
+ `callproc.c' implements (through the `call-process' primitive) what
+are called "synchronous subprocesses". This means that XEmacs runs a
+program, waits till it's done, and retrieves its output. A typical
+example might be calling the `ls' program to get a directory listing.
+
+ `process.c' and `process.h' implement "asynchronous subprocesses".
+This means that XEmacs starts a program and then continues normally,
+not waiting for the process to finish. Data can be sent to the process
+or retrieved from it as it's running. This is used for the `shell'
+command (which provides a front end onto a shell program such as
+`csh'), the mail and news readers implemented in XEmacs, etc. The
+result of calling `start-process' to start a subprocess is a process
+object, a particular kind of object used to communicate with the
+subprocess. You can send data to the process by passing the process
+object and the data to `send-process', and you can specify what happens
+to data retrieved from the process by setting properties of the process
+object. (When the process sends data, XEmacs receives a process event,
+which says that there is data ready. When `dispatch-event' is called
+on this event, it reads the data from the process and does something
+with it, as specified by the process object's properties. Typically,
+this means inserting the data into a buffer or calling a function.)
+Another property of the process object is called the "sentinel", which
+is a function that is called when the process terminates.
+
+ Process objects are also used for network connections (connections
+to a process running on another machine). Network connections are
+started with `open-network-stream' but otherwise work just like
+subprocesses.
+
+ sysdep.c
+ sysdep.h
+
+ These modules implement most of the low-level, messy operating-system
+interface code. This includes various device control (ioctl) operations
+for file descriptors, TTY's, pseudo-terminals, etc. (usually this stuff
+is fairly system-dependent; thus the name of this module), and emulation
+of standard library functions and system calls on systems that don't
+provide them or have broken versions.
+
+ sysdir.h
+ sysfile.h
+ sysfloat.h
+ sysproc.h
+ syspwd.h
+ syssignal.h
+ systime.h
+ systty.h
+ syswait.h
+
+ These header files provide consistent interfaces onto
+system-dependent header files and system calls. The idea is that,
+instead of including a standard header file like `<sys/param.h>' (which
+may or may not exist on various systems) or having to worry about
+whether all system provide a particular preprocessor constant, or
+having to deal with the four different paradigms for manipulating
+signals, you just include the appropriate `sys*.h' header file, which
+includes all the right system header files, defines and missing
+preprocessor constants, provides a uniform interface onto system calls,
+etc.
+
+ `sysdir.h' provides a uniform interface onto directory-querying
+functions. (In some cases, this is in conjunction with emulation
+functions in `sysdep.c'.)
+
+ `sysfile.h' includes all the necessary header files for standard
+system calls (e.g. `read()'), ensures that all necessary `open()' and
+`stat()' preprocessor constants are defined, and possibly (usually)
+substitutes sugared versions of `read()', `write()', etc. that
+automatically restart interrupted I/O operations.
+
+ `sysfloat.h' includes the necessary header files for floating-point
+operations.
+
+ `sysproc.h' includes the necessary header files for calling
+`select()', `fork()', `execve()', socket operations, and the like, and
+ensures that the `FD_*()' macros for descriptor-set manipulations are
+available.
+
+ `syspwd.h' includes the necessary header files for obtaining
+information from `/etc/passwd' (the functions are emulated under VMS).
+
+ `syssignal.h' includes the necessary header files for
+signal-handling and provides a uniform interface onto the different
+signal-handling and signal-blocking paradigms.
+
+ `systime.h' includes the necessary header files and provides uniform
+interfaces for retrieving the time of day, setting file
+access/modification times, getting the amount of time used by the XEmacs
+process, etc.
+
+ `systty.h' buffers against the infinitude of different ways of
+controlling TTY's.
+
+ `syswait.h' provides a uniform way of retrieving the exit status
+from a `wait()'ed-on process (some systems use a union, others use an
+int).
+
+ hpplay.c
+ libsst.c
+ libsst.h
+ libst.h
+ linuxplay.c
+ nas.c
+ sgiplay.c
+ sound.c
+ sunplay.c
+
+ These files implement the ability to play various sounds on some
+types of computers. You have to configure your XEmacs with sound
+support in order to get this capability.
+
+ `sound.c' provides the generic interface. It implements various
+Lisp primitives and variables that let you specify which sounds should
+be played in certain conditions. (The conditions are identified by
+symbols, which are passed to `ding' to make a sound. Various standard
+functions call this function at certain times; if sound support does
+not exist, a simple beep results.
+
+ `sgiplay.c', `sunplay.c', `hpplay.c', and `linuxplay.c' interface to
+the machine's speaker for various different kind of machines. This is
+called "native" sound.
+
+ `nas.c' interfaces to a computer somewhere else on the network using
+the NAS (Network Audio Server) protocol, playing sounds on that
+machine. This allows you to run XEmacs on a remote machine, with its
+display set to your local machine, and have the sounds be made on your
+local machine, provided that you have a NAS server running on your local
+machine.
+
+ `libsst.c', `libsst.h', and `libst.h' provide some additional
+functions for playing sound on a Sun SPARC but are not currently in use.
+
+ tooltalk.c
+ tooltalk.h
+
+ These two modules implement an interface to the ToolTalk protocol,
+which is an interprocess communication protocol implemented on some
+versions of Unix. ToolTalk is a high-level protocol that allows
+processes to register themselves as providers of particular services;
+other processes can then request a service without knowing or caring
+exactly who is providing the service. It is similar in spirit to the
+DDE protocol provided under Microsoft Windows. ToolTalk is a part of
+the new CDE (Common Desktop Environment) specification and is used to
+connect the parts of the SPARCWorks development environment.
+
+ getloadavg.c
+
+ This module provides the ability to retrieve the system's current
+load average. (The way to do this is highly system-specific,
+unfortunately, and requires a lot of special-case code.)
+
+ sunpro.c
+
+ This module provides a small amount of code used internally at Sun to
+keep statistics on the usage of XEmacs.
+
+ broken-sun.h
+ strcmp.c
+ strcpy.c
+ sunOS-fix.c
+
+ These files provide replacement functions and prototypes to fix
+numerous bugs in early releases of SunOS 4.1.
+
+ hftctl.c
+
+ This module provides some terminal-control code necessary on
+versions of AIX prior to 4.1.
+
+\1f
+File: internals.info, Node: Modules for Interfacing with X Windows, Next: Modules for Internationalization, Prev: Modules for Interfacing with the Operating System, Up: A Summary of the Various XEmacs Modules
+
+Modules for Interfacing with X Windows
+======================================
+
+ Emacs.ad.h
+
+ A file generated from `Emacs.ad', which contains XEmacs-supplied
+fallback resources (so that XEmacs has pretty defaults).
+
+ EmacsFrame.c
+ EmacsFrame.h
+ EmacsFrameP.h
+
+ These modules implement an Xt widget class that encapsulates a frame.
+This is for ease in integrating with Xt. The EmacsFrame widget covers
+the entire X window except for the menubar; the scrollbars are
+positioned on top of the EmacsFrame widget.
+
+ *Warning:* Abandon hope, all ye who enter here. This code took an
+ungodly amount of time to get right, and is likely to fall apart
+mercilessly at the slightest change. Such is life under Xt.
+
+ EmacsManager.c
+ EmacsManager.h
+ EmacsManagerP.h
+
+ These modules implement a simple Xt manager (i.e. composite) widget
+class that simply lets its children set whatever geometry they want.
+It's amazing that Xt doesn't provide this standardly, but on second
+thought, it makes sense, considering how amazingly broken Xt is.
+
+ EmacsShell-sub.c
+ EmacsShell.c
+ EmacsShell.h
+ EmacsShellP.h
+
+ These modules implement two Xt widget classes that are subclasses of
+the TopLevelShell and TransientShell classes. This is necessary to deal
+with more brokenness that Xt has sadistically thrust onto the backs of
+developers.
+
+ xgccache.c
+ xgccache.h
+
+ These modules provide functions for maintenance and caching of GC's
+(graphics contexts) under the X Window System. This code is junky and
+needs to be rewritten.
+
+ select-msw.c
+ select-x.c
+ select.c
+ select.h
+
+ This module provides an interface to the X Window System's concept of
+"selections", the standard way for X applications to communicate with
+each other.
+
+ xintrinsic.h
+ xintrinsicp.h
+ xmmanagerp.h
+ xmprimitivep.h
+
+ These header files are similar in spirit to the `sys*.h' files and
+buffer against different implementations of Xt and Motif.
+
+ * `xintrinsic.h' should be included in place of `<Intrinsic.h>'.
+
+ * `xintrinsicp.h' should be included in place of `<IntrinsicP.h>'.
+
+ * `xmmanagerp.h' should be included in place of `<XmManagerP.h>'.
+
+ * `xmprimitivep.h' should be included in place of `<XmPrimitiveP.h>'.
+
+ xmu.c
+ xmu.h
+
+ These files provide an emulation of the Xmu library for those systems
+(i.e. HPUX) that don't provide it as a standard part of X.
+
+ ExternalClient-Xlib.c
+ ExternalClient.c
+ ExternalClient.h
+ ExternalClientP.h
+ ExternalShell.c
+ ExternalShell.h
+ ExternalShellP.h
+ extw-Xlib.c
+ extw-Xlib.h
+ extw-Xt.c
+ extw-Xt.h
+
+ These files provide the "external widget" interface, which allows an
+XEmacs frame to appear as a widget in another application. To do this,
+you have to configure with `--external-widget'.
+
+ `ExternalShell*' provides the server (XEmacs) side of the connection.
+
+ `ExternalClient*' provides the client (other application) side of
+the connection. These files are not compiled into XEmacs but are
+compiled into libraries that are then linked into your application.
+
+ `extw-*' is common code that is used for both the client and server.
+
+ Don't touch this code; something is liable to break if you do.
+
+\1f
+File: internals.info, Node: Modules for Internationalization, Next: Modules for Regression Testing, Prev: Modules for Interfacing with X Windows, Up: A Summary of the Various XEmacs Modules
+
+Modules for Internationalization
+================================
+
+ mule-canna.c
+ mule-ccl.c
+ mule-charset.c
+ mule-charset.h
+ file-coding.c
+ file-coding.h
+ mule-mcpath.c
+ mule-mcpath.h
+ mule-wnnfns.c
+ mule.c
+
+ These files implement the MULE (Asian-language) support. Note that
+MULE actually provides a general interface for all sorts of languages,
+not just Asian languages (although they are generally the most
+complicated to support). This code is still in beta.
+
+ `mule-charset.*' and `file-coding.*' provide the heart of the XEmacs
+MULE support. `mule-charset.*' implements the "charset" Lisp object
+type, which encapsulates a character set (an ordered one- or
+two-dimensional set of characters, such as US ASCII or JISX0208 Japanese
+Kanji).
+
+ `file-coding.*' implements the "coding-system" Lisp object type,
+which encapsulates a method of converting between different encodings.
+An encoding is a representation of a stream of characters, possibly
+from multiple character sets, using a stream of bytes or words, and
+defines (e.g.) which escape sequences are used to specify particular
+character sets, how the indices for a character are converted into bytes
+(sometimes this involves setting the high bit; sometimes complicated
+rearranging of the values takes place, as in the Shift-JIS encoding),
+etc.
+
+ `mule-ccl.c' provides the CCL (Code Conversion Language)
+interpreter. CCL is similar in spirit to Lisp byte code and is used to
+implement converters for custom encodings.
+
+ `mule-canna.c' and `mule-wnnfns.c' implement interfaces to external
+programs used to implement the Canna and WNN input methods,
+respectively. This is currently in beta.
+
+ `mule-mcpath.c' provides some functions to allow for pathnames
+containing extended characters. This code is fragmentary, obsolete, and
+completely non-working. Instead, `pathname-coding-system' is used to
+specify conversions of names of files and directories. The standard C
+I/O functions like `open()' are wrapped so that conversion occurs
+automatically.
+
+ `mule.c' provides a few miscellaneous things that should probably be
+elsewhere.
+
+ intl.c
+
+ This provides some miscellaneous internationalization code for
+implementing message translation and interfacing to the Ximp input
+method. None of this code is currently working.
+
+ iso-wide.h
+
+ This contains leftover code from an earlier implementation of
+Asian-language support, and is not currently used.
+
+\1f
+File: internals.info, Node: Modules for Regression Testing, Prev: Modules for Internationalization, Up: A Summary of the Various XEmacs Modules
+
+Modules for Regression Testing
+==============================
+
+ test-harness.el
+ base64-tests.el
+ byte-compiler-tests.el
+ case-tests.el
+ ccl-tests.el
+ c-tests.el
+ database-tests.el
+ extent-tests.el
+ hash-table-tests.el
+ lisp-tests.el
+ md5-tests.el
+ mule-tests.el
+ regexp-tests.el
+ symbol-tests.el
+ syntax-tests.el
+
+ `test-harness.el' defines the macros `Assert', `Check-Error',
+`Check-Error-Message', and `Check-Message'. The other files are test
+files, testing various XEmacs modules.
+
+\1f
+File: internals.info, Node: Allocation of Objects in XEmacs Lisp, Next: Dumping, Prev: A Summary of the Various XEmacs Modules, Up: Top
+
+Allocation of Objects in XEmacs Lisp
+************************************
+
+* Menu:
+
+* Introduction to Allocation::
+* Garbage Collection::
+* GCPROing::
+* Garbage Collection - Step by Step::
+* Integers and Characters::
+* Allocation from Frob Blocks::
+* lrecords::
+* Low-level allocation::
+* Cons::
+* Vector::
+* Bit Vector::
+* Symbol::
+* Marker::
+* String::
+* Compiled Function::
+
+\1f
+File: internals.info, Node: Introduction to Allocation, Next: Garbage Collection, Up: Allocation of Objects in XEmacs Lisp
+
+Introduction to Allocation
+==========================
+
+Emacs Lisp, like all Lisps, has garbage collection. This means that
+the programmer never has to explicitly free (destroy) an object; it
+happens automatically when the object becomes inaccessible. Most
+experts agree that garbage collection is a necessity in a modern,
+high-level language. Its omission from C stems from the fact that C was
+originally designed to be a nice abstract layer on top of assembly
+language, for writing kernels and basic system utilities rather than
+large applications.
+
+ Lisp objects can be created by any of a number of Lisp primitives.
+Most object types have one or a small number of basic primitives for
+creating objects. For conses, the basic primitive is `cons'; for
+vectors, the primitives are `make-vector' and `vector'; for symbols,
+the primitives are `make-symbol' and `intern'; etc. Some Lisp objects,
+especially those that are primarily used internally, have no
+corresponding Lisp primitives. Every Lisp object, though, has at least
+one C primitive for creating it.
+
+ Recall from section (VII) that a Lisp object, as stored in a 32-bit
+or 64-bit word, has a few tag bits, and a "value" that occupies the
+remainder of the bits. We can separate the different Lisp object types
+into three broad categories:
+
+ * (a) Those for whom the value directly represents the contents of
+ the Lisp object. Only two types are in this category: integers and
+ characters. No special allocation or garbage collection is
+ necessary for such objects. Lisp objects of these types do not
+ need to be `GCPRO'ed.
+
+ In the remaining two categories, the type is stored in the object
+itself. The tag for all such objects is the generic "lrecord"
+(Lisp_Type_Record) tag. The first bytes of the object's structure are
+an integer (actually a char) characterising the object's type and some
+flags, in particular the mark bit used for garbage collection. A
+structure describing the type is accessible thru the
+lrecord_implementation_table indexed with said integer. This structure
+includes the method pointers and a pointer to a string naming the type.
+
+ * (b) Those lrecords that are allocated in frob blocks (see above).
+ This includes the objects that are most common and relatively
+ small, and includes conses, strings, subrs, floats, compiled
+ functions, symbols, extents, events, and markers. With the
+ cleanup of frob blocks done in 19.12, it's not terribly hard to
+ add more objects to this category, but it's a bit trickier than
+ adding an object type to type (c) (esp. if the object needs a
+ finalization method), and is not likely to save much space unless
+ the object is small and there are many of them. (In fact, if there
+ are very few of them, it might actually waste space.)
+
+ * (c) Those lrecords that are individually `malloc()'ed. These are
+ called "lcrecords". All other types are in this category. Adding
+ a new type to this category is comparatively easy, and all types
+ added since 19.8 (when the current allocation scheme was devised,
+ by Richard Mlynarik), with the exception of the character type,
+ have been in this category.
+
+ Note that bit vectors are a bit of a special case. They are simple
+lrecords as in category (b), but are individually `malloc()'ed like
+vectors. You can basically view them as exactly like vectors except
+that their type is stored in lrecord fashion rather than in
+directly-tagged fashion.
+
+\1f
+File: internals.info, Node: Garbage Collection, Next: GCPROing, Prev: Introduction to Allocation, Up: Allocation of Objects in XEmacs Lisp
+
+Garbage Collection
+==================
+
+Garbage collection is simple in theory but tricky to implement. Emacs
+Lisp uses the oldest garbage collection method, called "mark and
+sweep". Garbage collection begins by starting with all accessible
+locations (i.e. all variables and other slots where Lisp objects might
+occur) and recursively traversing all objects accessible from those
+slots, marking each one that is found. We then go through all of
+memory and free each object that is not marked, and unmarking each
+object that is marked. Note that "all of memory" means all currently
+allocated objects. Traversing all these objects means traversing all
+frob blocks, all vectors (which are chained in one big list), and all
+lcrecords (which are likewise chained).
+
+ Garbage collection can be invoked explicitly by calling
+`garbage-collect' but is also called automatically by `eval', once a
+certain amount of memory has been allocated since the last garbage
+collection (according to `gc-cons-threshold').
+
+\1f
+File: internals.info, Node: GCPROing, Next: Garbage Collection - Step by Step, Prev: Garbage Collection, Up: Allocation of Objects in XEmacs Lisp
+
+`GCPRO'ing
+==========
+
+`GCPRO'ing is one of the ugliest and trickiest parts of Emacs
+internals. The basic idea is that whenever garbage collection occurs,
+all in-use objects must be reachable somehow or other from one of the
+roots of accessibility. The roots of accessibility are:
+
+ 1. All objects that have been `staticpro()'d or
+ `staticpro_nodump()'ed. This is used for any global C variables
+ that hold Lisp objects. A call to `staticpro()' happens implicitly
+ as a result of any symbols declared with `defsymbol()' and any
+ variables declared with `DEFVAR_FOO()'. You need to explicitly
+ call `staticpro()' (in the `vars_of_foo()' method of a module) for
+ other global C variables holding Lisp objects. (This typically
+ includes internal lists and such things.). Use
+ `staticpro_nodump()' only in the rare cases when you do not want
+ the pointed variable to be saved at dump time but rather recompute
+ it at startup.
+
+ Note that `obarray' is one of the `staticpro()'d things.
+ Therefore, all functions and variables get marked through this.
+
+ 2. Any shadowed bindings that are sitting on the `specpdl' stack.
+
+ 3. Any objects sitting in currently active (Lisp) stack frames,
+ catches, and condition cases.
+
+ 4. A couple of special-case places where active objects are located.
+
+ 5. Anything currently marked with `GCPRO'.
+
+ Marking with `GCPRO' is necessary because some C functions (quite a
+lot, in fact), allocate objects during their operation. Quite
+frequently, there will be no other pointer to the object while the
+function is running, and if a garbage collection occurs and the object
+needs to be referenced again, bad things will happen. The solution is
+to mark those objects with `GCPRO'. Unfortunately this is easy to
+forget, and there is basically no way around this problem. Here are
+some rules, though:
+
+ 1. For every `GCPRON', there have to be declarations of `struct gcpro
+ gcpro1, gcpro2', etc.
+
+ 2. You _must_ `UNGCPRO' anything that's `GCPRO'ed, and you _must not_
+ `UNGCPRO' if you haven't `GCPRO'ed. Getting either of these wrong
+ will lead to crashes, often in completely random places unrelated
+ to where the problem lies.
+
+ 3. The way this actually works is that all currently active `GCPRO's
+ are chained through the `struct gcpro' local variables, with the
+ variable `gcprolist' pointing to the head of the list and the nth
+ local `gcpro' variable pointing to the first `gcpro' variable in
+ the next enclosing stack frame. Each `GCPRO'ed thing is an
+ lvalue, and the `struct gcpro' local variable contains a pointer to
+ this lvalue. This is why things will mess up badly if you don't
+ pair up the `GCPRO's and `UNGCPRO's--you will end up with
+ `gcprolist's containing pointers to `struct gcpro's or local
+ `Lisp_Object' variables in no-longer-active stack frames.
+
+ 4. It is actually possible for a single `struct gcpro' to protect a
+ contiguous array of any number of values, rather than just a
+ single lvalue. To effect this, call `GCPRON' as usual on the
+ first object in the array and then set `gcproN.nvars'.
+
+ 5. *Strings are relocated.* What this means in practice is that the
+ pointer obtained using `XSTRING_DATA()' is liable to change at any
+ time, and you should never keep it around past any function call,
+ or pass it as an argument to any function that might cause a
+ garbage collection. This is why a number of functions accept
+ either a "non-relocatable" `char *' pointer or a relocatable Lisp
+ string, and only access the Lisp string's data at the very last
+ minute. In some cases, you may end up having to `alloca()' some
+ space and copy the string's data into it.
+
+ 6. By convention, if you have to nest `GCPRO''s, use `NGCPRON' (along
+ with `struct gcpro ngcpro1, ngcpro2', etc.), `NNGCPRON', etc.
+ This avoids compiler warnings about shadowed locals.
+
+ 7. It is _always_ better to err on the side of extra `GCPRO's rather
+ than too few. The extra cycles spent on this are almost never
+ going to make a whit of difference in the speed of anything.
+
+ 8. The general rule to follow is that caller, not callee, `GCPRO's.
+ That is, you should not have to explicitly `GCPRO' any Lisp objects
+ that are passed in as parameters.
+
+ One exception from this rule is if you ever plan to change the
+ parameter value, and store a new object in it. In that case, you
+ _must_ `GCPRO' the parameter, because otherwise the new object
+ will not be protected.
+
+ So, if you create any Lisp objects (remember, this happens in all
+ sorts of circumstances, e.g. with `Fcons()', etc.), you are
+ responsible for `GCPRO'ing them, unless you are _absolutely sure_
+ that there's no possibility that a garbage-collection can occur
+ while you need to use the object. Even then, consider `GCPRO'ing.
+
+ 9. A garbage collection can occur whenever anything calls `Feval', or
+ whenever a QUIT can occur where execution can continue past this.
+ (Remember, this is almost anywhere.)
+
+ 10. If you have the _least smidgeon of doubt_ about whether you need
+ to `GCPRO', you should `GCPRO'.
+
+ 11. Beware of `GCPRO'ing something that is uninitialized. If you have
+ any shade of doubt about this, initialize all your variables to
+ `Qnil'.
+
+ 12. Be careful of traps, like calling `Fcons()' in the argument to
+ another function. By the "caller protects" law, you should be
+ `GCPRO'ing the newly-created cons, but you aren't. A certain
+ number of functions that are commonly called on freshly created
+ stuff (e.g. `nconc2()', `Fsignal()'), break the "caller protects"
+ law and go ahead and `GCPRO' their arguments so as to simplify
+ things, but make sure and check if it's OK whenever doing
+ something like this.
+
+ 13. Once again, remember to `GCPRO'! Bugs resulting from insufficient
+ `GCPRO'ing are intermittent and extremely difficult to track down,
+ often showing up in crashes inside of `garbage-collect' or in
+ weirdly corrupted objects or even in incorrect values in a totally
+ different section of code.
+
+ If you don't understand whether to `GCPRO' in a particular instance,
+ask on the mailing lists. A general hint is that `prog1' is the
+canonical example.
+
+ Given the extremely error-prone nature of the `GCPRO' scheme, and
+the difficulties in tracking down, it should be considered a deficiency
+in the XEmacs code. A solution to this problem would involve
+implementing so-called "conservative" garbage collection for the C
+stack. That involves looking through all of stack memory and treating
+anything that looks like a reference to an object as a reference. This
+will result in a few objects not getting collected when they should, but
+it obviates the need for `GCPRO'ing, and allows garbage collection to
+happen at any point at all, such as during object allocation.
+
+\1f
+File: internals.info, Node: Garbage Collection - Step by Step, Next: Integers and Characters, Prev: GCPROing, Up: Allocation of Objects in XEmacs Lisp
+
+Garbage Collection - Step by Step
+=================================
+
+* Menu:
+
+* Invocation::
+* garbage_collect_1::
+* mark_object::
+* gc_sweep::
+* sweep_lcrecords_1::
+* compact_string_chars::
+* sweep_strings::
+* sweep_bit_vectors_1::
+
+\1f
+File: internals.info, Node: Invocation, Next: garbage_collect_1, Up: Garbage Collection - Step by Step
+
+Invocation
+----------
+
+The first thing that anyone should know about garbage collection is:
+when and how the garbage collector is invoked. One might think that this
+could happen every time new memory is allocated, e.g. new objects are
+created, but this is _not_ the case. Instead, we have the following
+situation:
+
+ The entry point of any process of garbage collection is an invocation
+of the function `garbage_collect_1' in file `alloc.c'. The invocation
+can occur _explicitly_ by calling the function `Fgarbage_collect' (in
+addition this function provides information about the freed memory), or
+can occur _implicitly_ in four different situations:
+ 1. In function `main_1' in file `emacs.c'. This function is called at
+ each startup of xemacs. The garbage collection is invoked after all
+ initial creations are completed, but only if a special internal
+ error checking-constant `ERROR_CHECK_GC' is defined.
+
+ 2. In function `disksave_object_finalization' in file `alloc.c'. The
+ only purpose of this function is to clear the objects from memory
+ which need not be stored with xemacs when we dump out an
+ executable. This is only done by `Fdump_emacs' or by
+ `Fdump_emacs_data' respectively (both in `emacs.c'). The actual
+ clearing is accomplished by making these objects unreachable and
+ starting a garbage collection. The function is only used while
+ building xemacs.
+
+ 3. In function `Feval / eval' in file `eval.c'. Each time the well
+ known and often used function eval is called to evaluate a form,
+ one of the first things that could happen, is a potential call of
+ `garbage_collect_1'. There exist three global variables,
+ `consing_since_gc' (counts the created cons-cells since the last
+ garbage collection), `gc_cons_threshold' (a specified threshold
+ after which a garbage collection occurs) and `always_gc'. If
+ `always_gc' is set or if the threshold is exceeded, the garbage
+ collection will start.
+
+ 4. In function `Ffuncall / funcall' in file `eval.c'. This function
+ evaluates calls of elisp functions and works according to `Feval'.
+
+ The upshot is that garbage collection can basically occur everywhere
+`Feval', respectively `Ffuncall', is used - either directly or through
+another function. Since calls to these two functions are hidden in
+various other functions, many calls to `garbage_collect_1' are not
+obviously foreseeable, and therefore unexpected. Instances where they
+are used that are worth remembering are various elisp commands, as for
+example `or', `and', `if', `cond', `while', `setq', etc., miscellaneous
+`gui_item_...' functions, everything related to `eval' (`Feval_buffer',
+`call0', ...) and inside `Fsignal'. The latter is used to handle
+signals, as for example the ones raised by every `QUIT'-macro triggered
+after pressing Ctrl-g.
+
+\1f
+File: internals.info, Node: garbage_collect_1, Next: mark_object, Prev: Invocation, Up: Garbage Collection - Step by Step
+
+`garbage_collect_1'
+-------------------
+
+We can now describe exactly what happens after the invocation takes
+place.
+ 1. There are several cases in which the garbage collector is left
+ immediately: when we are already garbage collecting
+ (`gc_in_progress'), when the garbage collection is somehow
+ forbidden (`gc_currently_forbidden'), when we are currently
+ displaying something (`in_display') or when we are preparing for
+ the armageddon of the whole system (`preparing_for_armageddon').
+
+ 2. Next the correct frame in which to put all the output occurring
+ during garbage collecting is determined. In order to be able to
+ restore the old display's state after displaying the message, some
+ data about the current cursor position has to be saved. The
+ variables `pre_gc_cursor' and `cursor_changed' take care of that.
+
+ 3. The state of `gc_currently_forbidden' must be restored after the
+ garbage collection, no matter what happens during the process. We
+ accomplish this by `record_unwind_protect'ing the suitable function
+ `restore_gc_inhibit' together with the current value of
+ `gc_currently_forbidden'.
+
+ 4. If we are concurrently running an interactive xemacs session, the
+ next step is simply to show the garbage collector's cursor/message.
+
+ 5. The following steps are the intrinsic steps of the garbage
+ collector, therefore `gc_in_progress' is set.
+
+ 6. For debugging purposes, it is possible to copy the current C stack
+ frame. However, this seems to be a currently unused feature.
+
+ 7. Before actually starting to go over all live objects, references to
+ objects that are no longer used are pruned. We only have to do
+ this for events (`clear_event_resource') and for specifiers
+ (`cleanup_specifiers').
+
+ 8. Now the mark phase begins and marks all accessible elements. In
+ order to start from all slots that serve as roots of
+ accessibility, the function `mark_object' is called for each root
+ individually to go out from there to mark all reachable objects.
+ All roots that are traversed are shown in their processed order:
+ * all constant symbols and static variables that are registered
+ via `staticpro' in the dynarr `staticpros'. *Note Adding
+ Global Lisp Variables::.
+
+ * all Lisp objects that are created in C functions and that
+ must be protected from freeing them. They are registered in
+ the global list `gcprolist'. *Note GCPROing::.
+
+ * all local variables (i.e. their name fields `symbol' and old
+ values `old_values') that are bound during the evaluation by
+ the Lisp engine. They are stored in `specbinding' structs
+ pushed on a stack called `specpdl'. *Note Dynamic Binding;
+ The specbinding Stack; Unwind-Protects::.
+
+ * all catch blocks that the Lisp engine encounters during the
+ evaluation cause the creation of structs `catchtag' inserted
+ in the list `catchlist'. Their tag (`tag') and value (`val'
+ fields are freshly created objects and therefore have to be
+ marked. *Note Catch and Throw::.
+
+ * every function application pushes new structs `backtrace' on
+ the call stack of the Lisp engine (`backtrace_list'). The
+ unique parts that have to be marked are the fields for each
+ function (`function') and all their arguments (`args').
+ *Note Evaluation::.
+
+ * all objects that are used by the redisplay engine that must
+ not be freed are marked by a special function called
+ `mark_redisplay' (in `redisplay.c').
+
+ * all objects created for profiling purposes are allocated by C
+ functions instead of using the lisp allocation mechanisms. In
+ order to receive the right ones during the sweep phase, they
+ also have to be marked manually. That is done by the function
+ `mark_profiling_info'
+
+ 9. Hash tables in XEmacs belong to a kind of special objects that
+ make use of a concept often called 'weak pointers'. To make a
+ long story short, these kind of pointers are not followed during
+ the estimation of the live objects during garbage collection. Any
+ object referenced only by weak pointers is collected anyway, and
+ the reference to it is cleared. In hash tables there are different
+ usage patterns of them, manifesting in different types of hash
+ tables, namely 'non-weak', 'weak', 'key-weak' and 'value-weak'
+ (internally also 'key-car-weak' and 'value-car-weak') hash tables,
+ each clearing entries depending on different conditions. More
+ information can be found in the documentation to the function
+ `make-hash-table'.
+
+ Because there are complicated dependency rules about when and what
+ to mark while processing weak hash tables, the standard `marker'
+ method is only active if it is marking non-weak hash tables. As
+ soon as a weak component is in the table, the hash table entries
+ are ignored while marking. Instead their marking is done each
+ separately by the function `finish_marking_weak_hash_tables'. This
+ function iterates over each hash table entry `hentries' for each
+ weak hash table in `Vall_weak_hash_tables'. Depending on the type
+ of a table, the appropriate action is performed. If a table is
+ acting as `HASH_TABLE_KEY_WEAK', and a key already marked,
+ everything reachable from the `value' component is marked. If it is
+ acting as a `HASH_TABLE_VALUE_WEAK' and the value component is
+ already marked, the marking starts beginning only from the `key'
+ component. If it is a `HASH_TABLE_KEY_CAR_WEAK' and the car of
+ the key entry is already marked, we mark both the `key' and
+ `value' components. Finally, if the table is of the type
+ `HASH_TABLE_VALUE_CAR_WEAK' and the car of the value components is
+ already marked, again both the `key' and the `value' components
+ get marked.
+
+ Again, there are lists with comparable properties called weak
+ lists. There exist different peculiarities of their types called
+ `simple', `assoc', `key-assoc' and `value-assoc'. You can find
+ further details about them in the description to the function
+ `make-weak-list'. The scheme of their marking is similar: all weak
+ lists are listed in `Qall_weak_lists', therefore we iterate over
+ them. The marking is advanced until we hit an already marked pair.
+ Then we know that during a former run all the rest has been marked
+ completely. Again, depending on the special type of the weak list,
+ our jobs differ. If it is a `WEAK_LIST_SIMPLE' and the elem is
+ marked, we mark the `cons' part. If it is a `WEAK_LIST_ASSOC' and
+ not a pair or a pair with both marked car and cdr, we mark the
+ `cons' and the `elem'. If it is a `WEAK_LIST_KEY_ASSOC' and not a
+ pair or a pair with a marked car of the elem, we mark the `cons'
+ and the `elem'. Finally, if it is a `WEAK_LIST_VALUE_ASSOC' and
+ not a pair or a pair with a marked cdr of the elem, we mark both
+ the `cons' and the `elem'.
+
+ Since, by marking objects in reach from weak hash tables and weak
+ lists, other objects could get marked, this perhaps implies
+ further marking of other weak objects, both finishing functions
+ are redone as long as yet unmarked objects get freshly marked.
+
+ 10. After completing the special marking for the weak hash tables and
+ for the weak lists, all entries that point to objects that are
+ going to be swept in the further process are useless, and
+ therefore have to be removed from the table or the list.
+
+ The function `prune_weak_hash_tables' does the job for weak hash
+ tables. Totally unmarked hash tables are removed from the list
+ `Vall_weak_hash_tables'. The other ones are treated more carefully
+ by scanning over all entries and removing one as soon as one of
+ the components `key' and `value' is unmarked.
+
+ The same idea applies to the weak lists. It is accomplished by
+ `prune_weak_lists': An unmarked list is pruned from
+ `Vall_weak_lists' immediately. A marked list is treated more
+ carefully by going over it and removing just the unmarked pairs.
+
+ 11. The function `prune_specifiers' checks all listed specifiers held
+ in `Vall_specifiers' and removes the ones from the lists that are
+ unmarked.
+
+ 12. All syntax tables are stored in a list called
+ `Vall_syntax_tables'. The function `prune_syntax_tables' walks
+ through it and unlinks the tables that are unmarked.
+
+ 13. Next, we will attack the complete sweeping - the function
+ `gc_sweep' which holds the predominance.
+
+ 14. First, all the variables with respect to garbage collection are
+ reset. `consing_since_gc' - the counter of the created cells since
+ the last garbage collection - is set back to 0, and
+ `gc_in_progress' is not `true' anymore.
+
+ 15. In case the session is interactive, the displayed cursor and
+ message are removed again.
+
+ 16. The state of `gc_inhibit' is restored to the former value by
+ unwinding the stack.
+
+ 17. A small memory reserve is always held back that can be reached by
+ `breathing_space'. If nothing more is left, we create a new reserve
+ and exit.
+
+\1f
+File: internals.info, Node: mark_object, Next: gc_sweep, Prev: garbage_collect_1, Up: Garbage Collection - Step by Step
+
+`mark_object'
+-------------
+
+The first thing that is checked while marking an object is whether the
+object is a real Lisp object `Lisp_Type_Record' or just an integer or a
+character. Integers and characters are the only two types that are
+stored directly - without another level of indirection, and therefore
+they don't have to be marked and collected. *Note How Lisp Objects Are
+Represented in C::.
+
+ The second case is the one we have to handle. It is the one when we
+are dealing with a pointer to a Lisp object. But, there exist also three
+possibilities, that prevent us from doing anything while marking: The
+object is read only which prevents it from being garbage collected,
+i.e. marked (`C_READONLY_RECORD_HEADER'). The object in question is
+already marked, and need not be marked for the second time (checked by
+`MARKED_RECORD_HEADER_P'). If it is a special, unmarkable object
+(`UNMARKABLE_RECORD_HEADER_P', apparently, these are objects that sit
+in some const space, and can therefore not be marked, see
+`this_one_is_unmarkable' in `alloc.c').
+
+ Now, the actual marking is feasible. We do so by once using the macro
+`MARK_RECORD_HEADER' to mark the object itself (actually the special
+flag in the lrecord header), and calling its special marker "method"
+`marker' if available. The marker method marks every other object that
+is in reach from our current object. Note, that these marker methods
+should not call `mark_object' recursively, but instead should return
+the next object from where further marking has to be performed.
+
+ In case another object was returned, as mentioned before, we
+reiterate the whole `mark_object' process beginning with this next
+object.
+
+\1f
+File: internals.info, Node: gc_sweep, Next: sweep_lcrecords_1, Prev: mark_object, Up: Garbage Collection - Step by Step
+
+`gc_sweep'
+----------
+
+The job of this function is to free all unmarked records from memory. As
+we know, there are different types of objects implemented and managed,
+and consequently different ways to free them from memory. *Note
+Introduction to Allocation::.
+
+ We start with all objects stored through `lcrecords'. All bulkier
+objects are allocated and handled using that scheme of `lcrecords'.
+Each object is `malloc'ed separately instead of placing it in one of
+the contiguous frob blocks. All types that are currently stored using
+`lcrecords''s `alloc_lcrecord' and `make_lcrecord_list' are the types:
+vectors, buffers, char-table, char-table-entry, console, weak-list,
+database, device, ldap, hash-table, command-builder, extent-auxiliary,
+extent-info, face, coding-system, frame, image-instance, glyph,
+popup-data, gui-item, keymap, charset, color_instance, font_instance,
+opaque, opaque-list, process, range-table, specifier,
+symbol-value-buffer-local, symbol-value-lisp-magic,
+symbol-value-varalias, toolbar-button, tooltalk-message,
+tooltalk-pattern, window, and window-configuration. We take care of
+them in the fist place in order to be able to handle and to finalize
+items stored in them more easily. The function `sweep_lcrecords_1' as
+described below is doing the whole job for us. For a description about
+the internals: *Note lrecords::.
+
+ Our next candidates are the other objects that behave quite
+differently than everything else: the strings. They consists of two
+parts, a fixed-size portion (`struct Lisp_String') holding the string's
+length, its property list and a pointer to the second part, and the
+actual string data, which is stored in string-chars blocks comparable to
+frob blocks. In this block, the data is not only freed, but also a
+compression of holes is made, i.e. all strings are relocated together.
+*Note String::. This compacting phase is performed by the function
+`compact_string_chars', the actual sweeping by the function
+`sweep_strings' is described below.
+
+ After that, the other types are swept step by step using functions
+`sweep_conses', `sweep_bit_vectors_1', `sweep_compiled_functions',
+`sweep_floats', `sweep_symbols', `sweep_extents', `sweep_markers' and
+`sweep_extents'. They are the fixed-size types cons, floats,
+compiled-functions, symbol, marker, extent, and event stored in
+so-called "frob blocks", and therefore we can basically do the same on
+every type objects, using the same macros, especially defined only to
+handle everything with respect to fixed-size blocks. The only fixed-size
+type that is not handled here are the fixed-size portion of strings,
+because we took special care of them earlier.
+
+ The only big exceptions are bit vectors stored differently and
+therefore treated differently by the function `sweep_bit_vectors_1'
+described later.
+
+ At first, we need some brief information about how these fixed-size
+types are managed in general, in order to understand how the sweeping
+is done. They have all a fixed size, and are therefore stored in big
+blocks of memory - allocated at once - that can hold a certain amount
+of objects of one type. The macro `DECLARE_FIXED_TYPE_ALLOC' creates
+the suitable structures for every type. More precisely, we have the
+block struct (holding a pointer to the previous block `prev' and the
+objects in `block[]'), a pointer to current block
+(`current_..._block)') and its last index (`current_..._block_index'),
+and a pointer to the free list that will be created. Also a macro
+`FIXED_TYPE_FROM_BLOCK' plus some related macros exists that are used
+to obtain a new object, either from the free list
+`ALLOCATE_FIXED_TYPE_1' if there is an unused object of that type
+stored or by allocating a completely new block using
+`ALLOCATE_FIXED_TYPE_FROM_BLOCK'.
+
+ The rest works as follows: all of them define a macro `UNMARK_...'
+that is used to unmark the object. They define a macro
+`ADDITIONAL_FREE_...' that defines additional work that has to be done
+when converting an object from in use to not in use (so far, only
+markers use it in order to unchain them). Then, they all call the macro
+`SWEEP_FIXED_TYPE_BLOCK' instantiated with their type name and their
+struct name.
+
+ This call in particular does the following: we go over all blocks
+starting with the current moving towards the oldest. For each block,
+we look at every object in it. If the object already freed (checked
+with `FREE_STRUCT_P' using the first pointer of the object), or if it is
+set to read only (`C_READONLY_RECORD_HEADER_P', nothing must be done.
+If it is unmarked (checked with `MARKED_RECORD_HEADER_P'), it is put in
+the free list and set free (using the macro `FREE_FIXED_TYPE',
+otherwise it stays in the block, but is unmarked (by `UNMARK_...').
+While going through one block, we note if the whole block is empty. If
+so, the whole block is freed (using `xfree') and the free list state is
+set to the state it had before handling this block.
+
+\1f
+File: internals.info, Node: sweep_lcrecords_1, Next: compact_string_chars, Prev: gc_sweep, Up: Garbage Collection - Step by Step
+
+`sweep_lcrecords_1'
+-------------------
+
+After nullifying the complete lcrecord statistics, we go over all
+lcrecords two separate times. They are all chained together in a list
+with a head called `all_lcrecords'.
+
+ The first loop calls for each object its `finalizer' method, but only
+in the case that it is not read only (`C_READONLY_RECORD_HEADER_P)', it
+is not already marked (`MARKED_RECORD_HEADER_P'), it is not already in
+a free list (list of freed objects, field `free') and finally it owns a
+finalizer method.
+
+ The second loop actually frees the appropriate objects again by
+iterating through the whole list. In case an object is read only or
+marked, it has to persist, otherwise it is manually freed by calling
+`xfree'. During this loop, the lcrecord statistics are kept up to date
+by calling `tick_lcrecord_stats' with the right arguments,
+
+\1f
+File: internals.info, Node: compact_string_chars, Next: sweep_strings, Prev: sweep_lcrecords_1, Up: Garbage Collection - Step by Step
+
+`compact_string_chars'
+----------------------
+
+The purpose of this function is to compact all the data parts of the
+strings that are held in so-called `string_chars_block', i.e. the
+strings that do not exceed a certain maximal length.
+
+ The procedure with which this is done is as follows. We are keeping
+two positions in the `string_chars_block's using two pointer/integer
+pairs, namely `from_sb'/`from_pos' and `to_sb'/`to_pos'. They stand for
+the actual positions, from where to where, to copy the actually handled
+string.
+
+ While going over all chained `string_char_block's and their held
+strings, staring at `first_string_chars_block', both pointers are
+advanced and eventually a string is copied from `from_sb' to `to_sb',
+depending on the status of the pointed at strings.
+
+ More precisely, we can distinguish between the following actions.
+ * The string at `from_sb''s position could be marked as free, which
+ is indicated by an invalid pointer to the pointer that should
+ point back to the fixed size string object, and which is checked by
+ `FREE_STRUCT_P'. In this case, the `from_sb'/`from_pos' is
+ advanced to the next string, and nothing has to be copied.
+
+ * Also, if a string object itself is unmarked, nothing has to be
+ copied. We likewise advance the `from_sb'/`from_pos' pair as
+ described above.
+
+ * In all other cases, we have a marked string at hand. The string
+ data must be moved from the from-position to the to-position. In
+ case there is not enough space in the actual `to_sb'-block, we
+ advance this pointer to the beginning of the next block before
+ copying. In case the from and to positions are different, we
+ perform the actual copying using the library function `memmove'.
+
+ After compacting, the pointer to the current `string_chars_block',
+sitting in `current_string_chars_block', is reset on the last block to
+which we moved a string, i.e. `to_block', and all remaining blocks (we
+know that they just carry garbage) are explicitly `xfree'd.
+
+\1f
+File: internals.info, Node: sweep_strings, Next: sweep_bit_vectors_1, Prev: compact_string_chars, Up: Garbage Collection - Step by Step
+
+`sweep_strings'
+---------------
+
+The sweeping for the fixed sized string objects is essentially exactly
+the same as it is for all other fixed size types. As before, the freeing
+into the suitable free list is done by using the macro
+`SWEEP_FIXED_SIZE_BLOCK' after defining the right macros
+`UNMARK_string' and `ADDITIONAL_FREE_string'. These two definitions are
+a little bit special compared to the ones used for the other fixed size
+types.
+
+ `UNMARK_string' is defined the same way except some additional code
+used for updating the bookkeeping information.
+
+ For strings, `ADDITIONAL_FREE_string' has to do something in
+addition: in case, the string was not allocated in a
+`string_chars_block' because it exceeded the maximal length, and
+therefore it was `malloc'ed separately, we know also `xfree' it
+explicitly.
+
+\1f
+File: internals.info, Node: sweep_bit_vectors_1, Prev: sweep_strings, Up: Garbage Collection - Step by Step
+
+`sweep_bit_vectors_1'
+---------------------
+
+Bit vectors are also one of the rare types that are `malloc'ed
+individually. Consequently, while sweeping, all further needless bit
+vectors must be freed by hand. This is done, as one might imagine, the
+expected way: since they are all registered in a list called
+`all_bit_vectors', all elements of that list are traversed, all
+unmarked bit vectors are unlinked by calling `xfree' and all of them
+become unmarked. In addition, the bookkeeping information used for
+garbage collector's output purposes is updated.
+
+\1f
+File: internals.info, Node: Integers and Characters, Next: Allocation from Frob Blocks, Prev: Garbage Collection - Step by Step, Up: Allocation of Objects in XEmacs Lisp
+
+Integers and Characters
+=======================
+
+Integer and character Lisp objects are created from integers using the
+macros `XSETINT()' and `XSETCHAR()' or the equivalent functions
+`make_int()' and `make_char()'. (These are actually macros on most
+systems.) These functions basically just do some moving of bits
+around, since the integral value of the object is stored directly in
+the `Lisp_Object'.
+
+ `XSETINT()' and the like will truncate values given to them that are
+too big; i.e. you won't get the value you expected but the tag bits
+will at least be correct.
+
+\1f
+File: internals.info, Node: Allocation from Frob Blocks, Next: lrecords, Prev: Integers and Characters, Up: Allocation of Objects in XEmacs Lisp
+
+Allocation from Frob Blocks
+===========================
+
+The uninitialized memory required by a `Lisp_Object' of a particular
+type is allocated using `ALLOCATE_FIXED_TYPE()'. This only occurs
+inside of the lowest-level object-creating functions in `alloc.c':
+`Fcons()', `make_float()', `Fmake_byte_code()', `Fmake_symbol()',
+`allocate_extent()', `allocate_event()', `Fmake_marker()', and
+`make_uninit_string()'. The idea is that, for each type, there are a
+number of frob blocks (each 2K in size); each frob block is divided up
+into object-sized chunks. Each frob block will have some of these
+chunks that are currently assigned to objects, and perhaps some that are
+free. (If a frob block has nothing but free chunks, it is freed at the
+end of the garbage collection cycle.) The free chunks are stored in a
+free list, which is chained by storing a pointer in the first four bytes
+of the chunk. (Except for the free chunks at the end of the last frob
+block, which are handled using an index which points past the end of the
+last-allocated chunk in the last frob block.) `ALLOCATE_FIXED_TYPE()'
+first tries to retrieve a chunk from the free list; if that fails, it
+calls `ALLOCATE_FIXED_TYPE_FROM_BLOCK()', which looks at the end of the
+last frob block for space, and creates a new frob block if there is
+none. (There are actually two versions of these macros, one of which is
+more defensive but less efficient and is used for error-checking.)
+
+\1f
+File: internals.info, Node: lrecords, Next: Low-level allocation, Prev: Allocation from Frob Blocks, Up: Allocation of Objects in XEmacs Lisp
+
+lrecords
+========
+
+[see `lrecord.h']
+
+ All lrecords have at the beginning of their structure a `struct
+lrecord_header'. This just contains a type number and some flags,
+including the mark bit. All builtin type numbers are defined as
+constants in `enum lrecord_type', to allow the compiler to generate
+more efficient code for `TYPEP'. The type number, thru the
+`lrecord_implementation_table', gives access to a `struct
+lrecord_implementation', which is a structure containing method pointers
+and such. There is one of these for each type, and it is a global,
+constant, statically-declared structure that is declared in the
+`DEFINE_LRECORD_IMPLEMENTATION()' macro.
+
+ Simple lrecords (of type (b) above) just have a `struct
+lrecord_header' at their beginning. lcrecords, however, actually have a
+`struct lcrecord_header'. This, in turn, has a `struct lrecord_header'
+at its beginning, so sanity is preserved; but it also has a pointer
+used to chain all lcrecords together, and a special ID field used to
+distinguish one lcrecord from another. (This field is used only for
+debugging and could be removed, but the space gain is not significant.)
+
+ Simple lrecords are created using `ALLOCATE_FIXED_TYPE()', just like
+for other frob blocks. The only change is that the implementation
+pointer must be initialized correctly. (The implementation structure for
+an lrecord, or rather the pointer to it, is named `lrecord_float',
+`lrecord_extent', `lrecord_buffer', etc.)
+
+ lcrecords are created using `alloc_lcrecord()'. This takes a size
+to allocate and an implementation pointer. (The size needs to be passed
+because some lcrecords, such as window configurations, are of variable
+size.) This basically just `malloc()'s the storage, initializes the
+`struct lcrecord_header', and chains the lcrecord onto the head of the
+list of all lcrecords, which is stored in the variable `all_lcrecords'.
+The calls to `alloc_lcrecord()' generally occur in the lowest-level
+allocation function for each lrecord type.
+
+ Whenever you create an lrecord, you need to call either
+`DEFINE_LRECORD_IMPLEMENTATION()' or
+`DEFINE_LRECORD_SEQUENCE_IMPLEMENTATION()'. This needs to be specified
+in a `.c' file, at the top level. What this actually does is define
+and initialize the implementation structure for the lrecord. (And
+possibly declares a function `error_check_foo()' that implements the
+`XFOO()' macro when error-checking is enabled.) The arguments to the
+macros are the actual type name (this is used to construct the C
+variable name of the lrecord implementation structure and related
+structures using the `##' macro concatenation operator), a string that
+names the type on the Lisp level (this may not be the same as the C
+type name; typically, the C type name has underscores, while the Lisp
+string has dashes), various method pointers, and the name of the C
+structure that contains the object. The methods are used to
+encapsulate type-specific information about the object, such as how to
+print it or mark it for garbage collection, so that it's easy to add
+new object types without having to add a specific case for each new
+type in a bunch of different places.
+
+ The difference between `DEFINE_LRECORD_IMPLEMENTATION()' and
+`DEFINE_LRECORD_SEQUENCE_IMPLEMENTATION()' is that the former is used
+for fixed-size object types and the latter is for variable-size object
+types. Most object types are fixed-size; some complex types, however
+(e.g. window configurations), are variable-size. Variable-size object
+types have an extra method, which is called to determine the actual
+size of a particular object of that type. (Currently this is only used
+for keeping allocation statistics.)
+
+ For the purpose of keeping allocation statistics, the allocation
+engine keeps a list of all the different types that exist. Note that,
+since `DEFINE_LRECORD_IMPLEMENTATION()' is a macro that is specified at
+top-level, there is no way for it to initialize the global data
+structures containing type information, like
+`lrecord_implementations_table'. For this reason a call to
+`INIT_LRECORD_IMPLEMENTATION' must be added to the same source file
+containing `DEFINE_LRECORD_IMPLEMENTATION', but instead of to the top
+level, to one of the init functions, typically `syms_of_FOO.c'.
+`INIT_LRECORD_IMPLEMENTATION' must be called before an object of this
+type is used.
+
+ The type number is also used to index into an array holding the
+number of objects of each type and the total memory allocated for
+objects of that type. The statistics in this array are computed during
+the sweep stage. These statistics are returned by the call to
+`garbage-collect'.
+
+ Note that for every type defined with a `DEFINE_LRECORD_*()' macro,
+there needs to be a `DECLARE_LRECORD_IMPLEMENTATION()' somewhere in a
+`.h' file, and this `.h' file needs to be included by `inline.c'.
+
+ Furthermore, there should generally be a set of `XFOOBAR()',
+`FOOBARP()', etc. macros in a `.h' (or occasionally `.c') file. To
+create one of these, copy an existing model and modify as necessary.
+
+ *Please note:* If you define an lrecord in an external
+dynamically-loaded module, you must use `DECLARE_EXTERNAL_LRECORD',
+`DEFINE_EXTERNAL_LRECORD_IMPLEMENTATION', and
+`DEFINE_EXTERNAL_LRECORD_SEQUENCE_IMPLEMENTATION' instead of the
+non-EXTERNAL forms. These macros will dynamically add new type numbers
+to the global enum that records them, whereas the non-EXTERNAL forms
+assume that the programmer has already inserted the correct type numbers
+into the enum's code at compile-time.
+
+ The various methods in the lrecord implementation structure are:
+
+ 1. A "mark" method. This is called during the marking stage and
+ passed a function pointer (usually the `mark_object()' function),
+ which is used to mark an object. All Lisp objects that are
+ contained within the object need to be marked by applying this
+ function to them. The mark method should also return a Lisp
+ object, which should be either `nil' or an object to mark. (This
+ can be used in lieu of calling `mark_object()' on the object, to
+ reduce the recursion depth, and consequently should be the most
+ heavily nested sub-object, such as a long list.)
+
+ *Please note:* When the mark method is called, garbage collection
+ is in progress, and special precautions need to be taken when
+ accessing objects; see section (B) above.
+
+ If your mark method does not need to do anything, it can be `NULL'.
+
+ 2. A "print" method. This is called to create a printed
+ representation of the object, whenever `princ', `prin1', or the
+ like is called. It is passed the object, a stream to which the
+ output is to be directed, and an `escapeflag' which indicates
+ whether the object's printed representation should be "escaped" so
+ that it is readable. (This corresponds to the difference between
+ `princ' and `prin1'.) Basically, "escaped" means that strings will
+ have quotes around them and confusing characters in the strings
+ such as quotes, backslashes, and newlines will be backslashed; and
+ that special care will be taken to make symbols print in a
+ readable fashion (e.g. symbols that look like numbers will be
+ backslashed). Other readable objects should perhaps pass
+ `escapeflag' on when sub-objects are printed, so that readability
+ is preserved when necessary (or if not, always pass in a 1 for
+ `escapeflag'). Non-readable objects should in general ignore
+ `escapeflag', except that some use it as an indication that more
+ verbose output should be given.
+
+ Sub-objects are printed using `print_internal()', which takes
+ exactly the same arguments as are passed to the print method.
+
+ Literal C strings should be printed using `write_c_string()', or
+ `write_string_1()' for non-null-terminated strings.
+
+ Functions that do not have a readable representation should check
+ the `print_readably' flag and signal an error if it is set.
+
+ If you specify NULL for the print method, the
+ `default_object_printer()' will be used.
+
+ 3. A "finalize" method. This is called at the beginning of the sweep
+ stage on lcrecords that are about to be freed, and should be used
+ to perform any extra object cleanup. This typically involves
+ freeing any extra `malloc()'ed memory associated with the object,
+ releasing any operating-system and window-system resources
+ associated with the object (e.g. pixmaps, fonts), etc.
+
+ The finalize method can be NULL if nothing needs to be done.
+
+ WARNING #1: The finalize method is also called at the end of the
+ dump phase; this time with the for_disksave parameter set to
+ non-zero. The object is _not_ about to disappear, so you have to
+ make sure to _not_ free any extra `malloc()'ed memory if you're
+ going to need it later. (Also, signal an error if there are any
+ operating-system and window-system resources here, because they
+ can't be dumped.)
+
+ Finalize methods should, as a rule, set to zero any pointers after
+ they've been freed, and check to make sure pointers are not zero
+ before freeing. Although I'm pretty sure that finalize methods
+ are not called twice on the same object (except for the
+ `for_disksave' proviso), we've gotten nastily burned in some cases
+ by not doing this.
+
+ WARNING #2: The finalize method is _only_ called for lcrecords,
+ _not_ for simply lrecords. If you need a finalize method for
+ simple lrecords, you have to stick it in the
+ `ADDITIONAL_FREE_foo()' macro in `alloc.c'.
+
+ WARNING #3: Things are in an _extremely_ bizarre state when
+ `ADDITIONAL_FREE_foo()' is called, so you have to be incredibly
+ careful when writing one of these functions. See the comment in
+ `gc_sweep()'. If you ever have to add one of these, consider
+ using an lcrecord or dealing with the problem in a different
+ fashion.
+
+ 4. An "equal" method. This compares the two objects for similarity,
+ when `equal' is called. It should compare the contents of the
+ objects in some reasonable fashion. It is passed the two objects
+ and a "depth" value, which is used to catch circular objects. To
+ compare sub-Lisp-objects, call `internal_equal()' and bump the
+ depth value by one. If this value gets too high, a
+ `circular-object' error will be signaled.
+
+ If this is NULL, objects are `equal' only when they are `eq', i.e.
+ identical.
+
+ 5. A "hash" method. This is used to hash objects when they are to be
+ compared with `equal'. The rule here is that if two objects are
+ `equal', they _must_ hash to the same value; i.e. your hash
+ function should use some subset of the sub-fields of the object
+ that are compared in the "equal" method. If you specify this
+ method as `NULL', the object's pointer will be used as the hash,
+ which will _fail_ if the object has an `equal' method, so don't do
+ this.
+
+ To hash a sub-Lisp-object, call `internal_hash()'. Bump the depth
+ by one, just like in the "equal" method.
+
+ To convert a Lisp object directly into a hash value (using its
+ pointer), use `LISP_HASH()'. This is what happens when the hash
+ method is NULL.
+
+ To hash two or more values together into a single value, use
+ `HASH2()', `HASH3()', `HASH4()', etc.
+
+ 6. "getprop", "putprop", "remprop", and "plist" methods. These are
+ used for object types that have properties. I don't feel like
+ documenting them here. If you create one of these objects, you
+ have to use different macros to define them, i.e.
+ `DEFINE_LRECORD_IMPLEMENTATION_WITH_PROPS()' or
+ `DEFINE_LRECORD_SEQUENCE_IMPLEMENTATION_WITH_PROPS()'.
+
+ 7. A "size_in_bytes" method, when the object is of variable-size.
+ (i.e. declared with a `_SEQUENCE_IMPLEMENTATION' macro.) This
+ should simply return the object's size in bytes, exactly as you
+ might expect. For an example, see the methods for window
+ configurations and opaques.
+
+\1f
+File: internals.info, Node: Low-level allocation, Next: Cons, Prev: lrecords, Up: Allocation of Objects in XEmacs Lisp
+
+Low-level allocation
+====================
+
+Memory that you want to allocate directly should be allocated using
+`xmalloc()' rather than `malloc()'. This implements error-checking on
+the return value, and once upon a time did some more vital stuff (i.e.
+`BLOCK_INPUT', which is no longer necessary). Free using `xfree()',
+and realloc using `xrealloc()'. Note that `xmalloc()' will do a
+non-local exit if the memory can't be allocated. (Many functions,
+however, do not expect this, and thus XEmacs will likely crash if this
+happens. *This is a bug.* If you can, you should strive to make your
+function handle this OK. However, it's difficult in the general
+circumstance, perhaps requiring extra unwind-protects and such.)
+
+ Note that XEmacs provides two separate replacements for the standard
+`malloc()' library function. These are called "old GNU malloc"
+(`malloc.c') and "new GNU malloc" (`gmalloc.c'), respectively. New GNU
+malloc is better in pretty much every way than old GNU malloc, and
+should be used if possible. (It used to be that on some systems, the
+old one worked but the new one didn't. I think this was due
+specifically to a bug in SunOS, which the new one now works around; so
+I don't think the old one ever has to be used any more.) The primary
+difference between both of these mallocs and the standard system malloc
+is that they are much faster, at the expense of increased space. The
+basic idea is that memory is allocated in fixed chunks of powers of
+two. This allows for basically constant malloc time, since the various
+chunks can just be kept on a number of free lists. (The standard system
+malloc typically allocates arbitrary-sized chunks and has to spend some
+time, sometimes a significant amount of time, walking the heap looking
+for a free block to use and cleaning things up.) The new GNU malloc
+improves on things by allocating large objects in chunks of 4096 bytes
+rather than in ever larger powers of two, which results in ever larger
+wastage. There is a slight speed loss here, but it's of doubtful
+significance.
+
+ NOTE: Apparently there is a third-generation GNU malloc that is
+significantly better than the new GNU malloc, and should probably be
+included in XEmacs.
+
+ There is also the relocating allocator, `ralloc.c'. This actually
+moves blocks of memory around so that the `sbrk()' pointer shrunk and
+virtual memory released back to the system. On some systems, this is a
+big win. On all systems, it causes a noticeable (and sometimes huge)
+speed penalty, so I turn it off by default. `ralloc.c' only works with
+the new GNU malloc in `gmalloc.c'. There are also two versions of
+`ralloc.c', one that uses `mmap()' rather than block copies to move
+data around. This purports to be faster, although that depends on the
+amount of data that would have had to be block copied and the
+system-call overhead for `mmap()'. I don't know exactly how this
+works, except that the relocating-allocation routines are pretty much
+used only for the memory allocated for a buffer, which is the biggest
+consumer of space, esp. of space that may get freed later.
+
+ Note that the GNU mallocs have some "memory warning" facilities.
+XEmacs taps into them and issues a warning through the standard warning
+system, when memory gets to 75%, 85%, and 95% full. (On some systems,
+the memory warnings are not functional.)
+
+ Allocated memory that is going to be used to make a Lisp object is
+created using `allocate_lisp_storage()'. This just calls `xmalloc()'.
+It used to verify that the pointer to the memory can fit into a Lisp
+word, before the current Lisp object representation was introduced.
+`allocate_lisp_storage()' is called by `alloc_lcrecord()',
+`ALLOCATE_FIXED_TYPE()', and the vector and bit-vector creation
+routines. These routines also call `INCREMENT_CONS_COUNTER()' at the
+appropriate times; this keeps statistics on how much memory is
+allocated, so that garbage-collection can be invoked when the threshold
+is reached.
+
+\1f
+File: internals.info, Node: Cons, Next: Vector, Prev: Low-level allocation, Up: Allocation of Objects in XEmacs Lisp
+
+Cons
+====
+
+Conses are allocated in standard frob blocks. The only thing to note
+is that conses can be explicitly freed using `free_cons()' and
+associated functions `free_list()' and `free_alist()'. This
+immediately puts the conses onto the cons free list, and decrements the
+statistics on memory allocation appropriately. This is used to good
+effect by some extremely commonly-used code, to avoid generating extra
+objects and thereby triggering GC sooner. However, you have to be
+_extremely_ careful when doing this. If you mess this up, you will get
+BADLY BURNED, and it has happened before.
+
+\1f
+File: internals.info, Node: Vector, Next: Bit Vector, Prev: Cons, Up: Allocation of Objects in XEmacs Lisp
+
+Vector
+======
+
+As mentioned above, each vector is `malloc()'ed individually, and all
+are threaded through the variable `all_vectors'. Vectors are marked
+strangely during garbage collection, by kludging the size field. Note
+that the `struct Lisp_Vector' is declared with its `contents' field
+being a _stretchy_ array of one element. It is actually `malloc()'ed
+with the right size, however, and access to any element through the
+`contents' array works fine.
+
+\1f
+File: internals.info, Node: Bit Vector, Next: Symbol, Prev: Vector, Up: Allocation of Objects in XEmacs Lisp
+
+Bit Vector
+==========
+
+Bit vectors work exactly like vectors, except for more complicated code
+to access an individual bit, and except for the fact that bit vectors
+are lrecords while vectors are not. (The only difference here is that
+there's an lrecord implementation pointer at the beginning and the tag
+field in bit vector Lisp words is "lrecord" rather than "vector".)
+
+\1f
+File: internals.info, Node: Symbol, Next: Marker, Prev: Bit Vector, Up: Allocation of Objects in XEmacs Lisp
+
+Symbol
+======
+
+Symbols are also allocated in frob blocks. Symbols in the awful
+horrible obarray structure are chained through their `next' field.
+
+ Remember that `intern' looks up a symbol in an obarray, creating one
+if necessary.
+
+\1f
+File: internals.info, Node: Marker, Next: String, Prev: Symbol, Up: Allocation of Objects in XEmacs Lisp
+
+Marker
+======
+
+Markers are allocated in frob blocks, as usual. They are kept in a
+buffer unordered, but in a doubly-linked list so that they can easily
+be removed. (Formerly this was a singly-linked list, but in some cases
+garbage collection took an extraordinarily long time due to the O(N^2)
+time required to remove lots of markers from a buffer.) Markers are
+removed from a buffer in the finalize stage, in
+`ADDITIONAL_FREE_marker()'.
+
+\1f
+File: internals.info, Node: String, Next: Compiled Function, Prev: Marker, Up: Allocation of Objects in XEmacs Lisp
+
+String
+======
+
+As mentioned above, strings are a special case. A string is logically
+two parts, a fixed-size object (containing the length, property list,
+and a pointer to the actual data), and the actual data in the string.
+The fixed-size object is a `struct Lisp_String' and is allocated in
+frob blocks, as usual. The actual data is stored in special
+"string-chars blocks", which are 8K blocks of memory.
+Currently-allocated strings are simply laid end to end in these
+string-chars blocks, with a pointer back to the `struct Lisp_String'
+stored before each string in the string-chars block. When a new string
+needs to be allocated, the remaining space at the end of the last
+string-chars block is used if there's enough, and a new string-chars
+block is created otherwise.
+
+ There are never any holes in the string-chars blocks due to the
+string compaction and relocation that happens at the end of garbage
+collection. During the sweep stage of garbage collection, when objects
+are reclaimed, the garbage collector goes through all string-chars
+blocks, looking for unused strings. Each chunk of string data is
+preceded by a pointer to the corresponding `struct Lisp_String', which
+indicates both whether the string is used and how big the string is,
+i.e. how to get to the next chunk of string data. Holes are compressed
+by block-copying the next string into the empty space and relocating the
+pointer stored in the corresponding `struct Lisp_String'. *This means
+you have to be careful with strings in your code.* See the section
+above on `GCPRO'ing.
+
+ Note that there is one situation not handled: a string that is too
+big to fit into a string-chars block. Such strings, called "big
+strings", are all `malloc()'ed as their own block. (#### Although it
+would make more sense for the threshold for big strings to be somewhat
+lower, e.g. 1/2 or 1/4 the size of a string-chars block. It seems that
+this was indeed the case formerly--indeed, the threshold was set at
+1/8--but Mly forgot about this when rewriting things for 19.8.)
+
+ Note also that the string data in string-chars blocks is padded as
+necessary so that proper alignment constraints on the `struct
+Lisp_String' back pointers are maintained.
+
+ Finally, strings can be resized. This happens in Mule when a
+character is substituted with a different-length character, or during
+modeline frobbing. (You could also export this to Lisp, but it's not
+done so currently.) Resizing a string is a potentially tricky process.
+If the change is small enough that the padding can absorb it, nothing
+other than a simple memory move needs to be done. Keep in mind,
+however, that the string can't shrink too much because the offset to the
+next string in the string-chars block is computed by looking at the
+length and rounding to the nearest multiple of four or eight. If the
+string would shrink or expand beyond the correct padding, new string
+data needs to be allocated at the end of the last string-chars block and
+the data moved appropriately. This leaves some dead string data, which
+is marked by putting a special marker of 0xFFFFFFFF in the `struct
+Lisp_String' pointer before the data (there's no real `struct
+Lisp_String' to point to and relocate), and storing the size of the dead
+string data (which would normally be obtained from the now-non-existent
+`struct Lisp_String') at the beginning of the dead string data gap.
+The string compactor recognizes this special 0xFFFFFFFF marker and
+handles it correctly.
+
+\1f
+File: internals.info, Node: Compiled Function, Prev: String, Up: Allocation of Objects in XEmacs Lisp
+
+Compiled Function
+=================
+
+Not yet documented.
+
+\1f
+File: internals.info, Node: Dumping, Next: Events and the Event Loop, Prev: Allocation of Objects in XEmacs Lisp, Up: Top
+
+Dumping
+*******
+
+What is dumping and its justification
+=====================================
+
+The C code of XEmacs is just a Lisp engine with a lot of built-in
+primitives useful for writing an editor. The editor itself is written
+mostly in Lisp, and represents around 100K lines of code. Loading and
+executing the initialization of all this code takes a bit a time (five
+to ten times the usual startup time of current xemacs) and requires
+having all the lisp source files around. Having to reload them each
+time the editor is started would not be acceptable.
+
+ The traditional solution to this problem is called dumping: the build
+process first creates the lisp engine under the name `temacs', then
+runs it until it has finished loading and initializing all the lisp
+code, and eventually creates a new executable called `xemacs' including
+both the object code in `temacs' and all the contents of the memory
+after the initialization.
+
+ This solution, while working, has a huge problem: the creation of the
+new executable from the actual contents of memory is an extremely
+system-specific process, quite error-prone, and which interferes with a
+lot of system libraries (like malloc). It is even getting worse
+nowadays with libraries using constructors which are automatically
+called when the program is started (even before main()) which tend to
+crash when they are called multiple times, once before dumping and once
+after (IRIX 6.x libz.so pulls in some C++ image libraries thru
+dependencies which have this problem). Writing the dumper is also one
+of the most difficult parts of porting XEmacs to a new operating system.
+Basically, `dumping' is an operation that is just not officially
+supported on many operating systems.
+
+ The aim of the portable dumper is to solve the same problem as the
+system-specific dumper, that is to be able to reload quickly, using only
+a small number of files, the fully initialized lisp part of the editor,
+without any system-specific hacks.
+
+* Menu:
+
+* Overview::
+* Data descriptions::
+* Dumping phase::
+* Reloading phase::
+* Remaining issues::
+
+\1f
+File: internals.info, Node: Overview, Next: Data descriptions, Up: Dumping
+
+Overview
+========
+
+The portable dumping system has to:
+
+ 1. At dump time, write all initialized, non-quickly-rebuildable data
+ to a file [Note: currently named `xemacs.dmp', but the name will
+ change], along with all informations needed for the reloading.
+
+ 2. When starting xemacs, reload the dump file, relocate it to its new
+ starting address if needed, and reinitialize all pointers to this
+ data. Also, rebuild all the quickly rebuildable data.
+
+\1f
+File: internals.info, Node: Data descriptions, Next: Dumping phase, Prev: Overview, Up: Dumping
+
+Data descriptions
+=================
+
+The more complex task of the dumper is to be able to write lisp objects
+(lrecords) and C structs to disk and reload them at a different address,
+updating all the pointers they include in the process. This is done by
+using external data descriptions that give information about the layout
+of the structures in memory.
+
+ The specification of these descriptions is in lrecord.h. A
+description of an lrecord is an array of struct lrecord_description.
+Each of these structs include a type, an offset in the structure and
+some optional parameters depending on the type. For instance, here is
+the string description:
+
+ static const struct lrecord_description string_description[] = {
+ { XD_BYTECOUNT, offsetof (Lisp_String, size) },
+ { XD_OPAQUE_DATA_PTR, offsetof (Lisp_String, data), XD_INDIRECT(0, 1) },
+ { XD_LISP_OBJECT, offsetof (Lisp_String, plist) },
+ { XD_END }
+ };
+
+ The first line indicates a member of type Bytecount, which is used by
+the next, indirect directive. The second means "there is a pointer to
+some opaque data in the field `data'". The length of said data is
+given by the expression `XD_INDIRECT(0, 1)', which means "the value in
+the 0th line of the description (welcome to C) plus one". The third
+line means "there is a Lisp_Object member `plist' in the Lisp_String
+structure". `XD_END' then ends the description.
+
+ This gives us all the information we need to move around what is
+pointed to by a structure (C or lrecord) and, by transitivity,
+everything that it points to. The only missing information for dumping
+is the size of the structure. For lrecords, this is part of the
+lrecord_implementation, so we don't need to duplicate it. For C
+structures we use a struct struct_description, which includes a size
+field and a pointer to an associated array of lrecord_description.
+
+\1f
+File: internals.info, Node: Dumping phase, Next: Reloading phase, Prev: Data descriptions, Up: Dumping
+
+Dumping phase
+=============
+
+Dumping is done by calling the function pdump() (in dumper.c) which is
+invoked from Fdump_emacs (in emacs.c). This function performs a number
+of tasks.
+
+* Menu:
+
+* Object inventory::
+* Address allocation::
+* The header::
+* Data dumping::
+* Pointers dumping::
+
+\1f
+File: internals.info, Node: Object inventory, Next: Address allocation, Up: Dumping phase
+
+Object inventory
+----------------
+
+The first task is to build the list of the objects to dump. This
+includes:
+
+ * lisp objects
+
+ * C structures
+
+ We end up with one `pdump_entry_list_elmt' per object group (arrays
+of C structs are kept together) which includes a pointer to the first
+object of the group, the per-object size and the count of objects in the
+group, along with some other information which is initialized later.
+
+ These entries are linked together in `pdump_entry_list' structures
+and can be enumerated thru either:
+
+ 1. the `pdump_object_table', an array of `pdump_entry_list', one per
+ lrecord type, indexed by type number.
+
+ 2. the `pdump_opaque_data_list', used for the opaque data which does
+ not include pointers, and hence does not need descriptions.
+
+ 3. the `pdump_struct_table', which is a vector of
+ `struct_description'/`pdump_entry_list' pairs, used for non-opaque
+ C structures.
+
+ This uses a marking strategy similar to the garbage collector. Some
+differences though:
+
+ 1. We do not use the mark bit (which does not exist for C structures
+ anyway); we use a big hash table instead.
+
+ 2. We do not use the mark function of lrecords but instead rely on the
+ external descriptions. This happens essentially because we need to
+ follow pointers to C structures and opaque data in addition to
+ Lisp_Object members.
+
+ This is done by `pdump_register_object()', which handles Lisp_Object
+variables, and `pdump_register_struct()' which handles C structures,
+which both delegate the description management to
+`pdump_register_sub()'.
+
+ The hash table doubles as a map object to pdump_entry_list_elmt (i.e.
+allows us to look up a pdump_entry_list_elmt with the object it points
+to). Entries are added with `pdump_add_entry()' and looked up with
+`pdump_get_entry()'. There is no need for entry removal. The hash
+value is computed quite simply from the object pointer by
+`pdump_make_hash()'.
+
+ The roots for the marking are:
+
+ 1. the `staticpro''ed variables (there is a special
+ `staticpro_nodump()' call for protected variables we do not want
+ to dump).
+
+ 2. the variables registered via `dump_add_root_object' (`staticpro()'
+ is equivalent to `staticpro_nodump()' + `dump_add_root_object()').
+
+ 3. the variables registered via `dump_add_root_struct_ptr', each of
+ which points to a C structure.
+
+ This does not include the GCPRO'ed variables, the specbinds, the
+catchtags, the backlist, the redisplay or the profiling info, since we
+do not want to rebuild the actual chain of lisp calls which end up to
+the dump-emacs call, only the global variables.
+
+ Weak lists and weak hash tables are dumped as if they were their
+non-weak equivalent (without changing their type, of course). This has
+not yet been a problem.
+
+\1f
+File: internals.info, Node: Address allocation, Next: The header, Prev: Object inventory, Up: Dumping phase
+
+Address allocation
+------------------
+
+The next step is to allocate the offsets of each of the objects in the
+final dump file. This is done by `pdump_allocate_offset()' which is
+called indirectly by `pdump_scan_by_alignment()'.
+
+ The strategy to deal with alignment problems uses these facts:
+
+ 1. real world alignment requirements are powers of two.
+
+ 2. the C compiler is required to adjust the size of a struct so that
+ you can have an array of them next to each other. This means you
+ can have an upper bound of the alignment requirements of a given
+ structure by looking at which power of two its size is a multiple.
+
+ 3. the non-variant part of variable size lrecords has an alignment
+ requirement of 4.
+
+ Hence, for each lrecord type, C struct type or opaque data block the
+alignment requirement is computed as a power of two, with a minimum of
+2^2 for lrecords. `pdump_scan_by_alignment()' then scans all the
+`pdump_entry_list_elmt''s, the ones with the highest requirements
+first. This ensures the best packing.
+
+ The maximum alignment requirement we take into account is 2^8.
+
+ `pdump_allocate_offset()' only has to do a linear allocation,
+starting at offset 256 (this leaves room for the header and keeps the
+alignments happy).
+
+\1f
+File: internals.info, Node: The header, Next: Data dumping, Prev: Address allocation, Up: Dumping phase
+
+The header
+----------
+
+The next step creates the file and writes a header with a signature and
+some random information in it. The `reloc_address' field, which
+indicates at which address the file should be loaded if we want to avoid
+post-reload relocation, is set to 0. It then seeks to offset 256 (base
+offset for the objects).
+
+\1f
+File: internals.info, Node: Data dumping, Next: Pointers dumping, Prev: The header, Up: Dumping phase
+
+Data dumping
+------------
+
+The data is dumped in the same order as the addresses were allocated by
+`pdump_dump_data()', called from `pdump_scan_by_alignment()'. This
+function copies the data to a temporary buffer, relocates all pointers
+in the object to the addresses allocated in step Address Allocation,
+and writes it to the file. Using the same order means that, if we are
+careful with lrecords whose size is not a multiple of 4, we are ensured
+that the object is always written at the offset in the file allocated
+in step Address Allocation.
+
+\1f
+File: internals.info, Node: Pointers dumping, Prev: Data dumping, Up: Dumping phase
+
+Pointers dumping
+----------------
+
+A bunch of tables needed to reassign properly the global pointers are
+then written. They are:
+
+ 1. the pdump_root_struct_ptrs dynarr
+
+ 2. the pdump_opaques dynarr
+
+ 3. a vector of all the offsets to the objects in the file that
+ include a description (for faster relocation at reload time)
+
+ 4. the pdump_root_objects and pdump_weak_object_chains dynarrs.
+
+ For each of the dynarrs we write both the pointer to the variables
+and the relocated offset of the object they point to. Since these
+variables are global, the pointers are still valid when restarting the
+program and are used to regenerate the global pointers.
+
+ The `pdump_weak_object_chains' dynarr is a special case. The
+variables it points to are the head of weak linked lists of lisp objects
+of the same type. Not all objects of this list are dumped so the
+relocated pointer we associate with them points to the first dumped
+object of the list, or Qnil if none is available. This is also the
+reason why they are not used as roots for the purpose of object
+enumeration.
+
+ Some very important information like the `staticpros' and
+`lrecord_implementations_table' are handled indirectly using
+`dump_add_opaque' or `dump_add_root_struct_ptr'.
+
+ This is the end of the dumping part.
+
+\1f
+File: internals.info, Node: Reloading phase, Next: Remaining issues, Prev: Dumping phase, Up: Dumping
+
+Reloading phase
+===============
+
+File loading
+------------
+
+The file is mmap'ed in memory (which ensures a PAGESIZE alignment, at
+least 4096), or if mmap is unavailable or fails, a 256-bytes aligned
+malloc is done and the file is loaded.
+
+ Some variables are reinitialized from the values found in the header.
+
+ The difference between the actual loading address and the
+reloc_address is computed and will be used for all the relocations.
+
+Putting back the pdump_opaques
+------------------------------
+
+The memory contents are restored in the obvious and trivial way.
+
+Putting back the pdump_root_struct_ptrs
+---------------------------------------
+
+The variables pointed to by pdump_root_struct_ptrs in the dump phase are
+reset to the right relocated object addresses.
+
+Object relocation
+-----------------
+
+All the objects are relocated using their description and their offset
+by `pdump_reloc_one'. This step is unnecessary if the reloc_address is
+equal to the file loading address.
+
+Putting back the pdump_root_objects and pdump_weak_object_chains
+----------------------------------------------------------------
+
+Same as Putting back the pdump_root_struct_ptrs.
+
+Reorganize the hash tables
+--------------------------
+
+Since some of the hash values in the lisp hash tables are
+address-dependent, their layout is now wrong. So we go through each of
+them and have them resorted by calling `pdump_reorganize_hash_table'.
+
+\1f
+File: internals.info, Node: Remaining issues, Prev: Reloading phase, Up: Dumping
+
+Remaining issues
+================
+
+The build process will have to start a post-dump xemacs, ask it the
+loading address (which will, hopefully, be always the same between
+different xemacs invocations) and relocate the file to the new address.
+This way the object relocation phase will not have to be done, which
+means no writes in the objects and that, because of the use of mmap, the
+dumped data will be shared between all the xemacs running on the
+computer.
+
+ Some executable signature will be necessary to ensure that a given
+dump file is really associated with a given executable, or random
+crashes will occur. Maybe a random number set at compile or configure
+time thru a define. This will also allow for having
+differently-compiled xemacsen on the same system (mule and no-mule
+comes to mind).
+
+ The DOC file contents should probably end up in the dump file.
+
+\1f
+File: internals.info, Node: Events and the Event Loop, Next: Evaluation; Stack Frames; Bindings, Prev: Dumping, Up: Top
+
+Events and the Event Loop
+*************************
+
+* Menu:
+
+* Introduction to Events::
+* Main Loop::
+* Specifics of the Event Gathering Mechanism::
+* Specifics About the Emacs Event::
+* The Event Stream Callback Routines::
+* Other Event Loop Functions::
+* Converting Events::
+* Dispatching Events; The Command Builder::
+
+\1f
+File: internals.info, Node: Introduction to Events, Next: Main Loop, Up: Events and the Event Loop
+
+Introduction to Events
+======================
+
+An event is an object that encapsulates information about an
+interesting occurrence in the operating system. Events are generated
+either by user action, direct (e.g. typing on the keyboard or moving
+the mouse) or indirect (moving another window, thereby generating an
+expose event on an Emacs frame), or as a result of some other typically
+asynchronous action happening, such as output from a subprocess being
+ready or a timer expiring. Events come into the system in an
+asynchronous fashion (typically through a callback being called) and
+are converted into a synchronous event queue (first-in, first-out) in a
+process that we will call "collection".
+
+ Note that each application has its own event queue. (It is
+immaterial whether the collection process directly puts the events in
+the proper application's queue, or puts them into a single system
+queue, which is later split up.)
+
+ The most basic level of event collection is done by the operating
+system or window system. Typically, XEmacs does its own event
+collection as well. Often there are multiple layers of collection in
+XEmacs, with events from various sources being collected into a queue,
+which is then combined with other sources to go into another queue
+(i.e. a second level of collection), with perhaps another level on top
+of this, etc.
+
+ XEmacs has its own types of events (called "Emacs events"), which
+provides an abstract layer on top of the system-dependent nature of the
+most basic events that are received. Part of the complex nature of the
+XEmacs event collection process involves converting from the
+operating-system events into the proper Emacs events--there may not be
+a one-to-one correspondence.
+
+ Emacs events are documented in `events.h'; I'll discuss them later.
+
+\1f
+File: internals.info, Node: Main Loop, Next: Specifics of the Event Gathering Mechanism, Prev: Introduction to Events, Up: Events and the Event Loop
+
+Main Loop
+=========
+
+The "command loop" is the top-level loop that the editor is always
+running. It loops endlessly, calling `next-event' to retrieve an event
+and `dispatch-event' to execute it. `dispatch-event' does the
+appropriate thing with non-user events (process, timeout, magic, eval,
+mouse motion); this involves calling a Lisp handler function, redrawing
+a newly-exposed part of a frame, reading subprocess output, etc. For
+user events, `dispatch-event' looks up the event in relevant keymaps or
+menubars; when a full key sequence or menubar selection is reached, the
+appropriate function is executed. `dispatch-event' may have to keep
+state across calls; this is done in the "command-builder" structure
+associated with each console (remember, there's usually only one
+console), and the engine that looks up keystrokes and constructs full
+key sequences is called the "command builder". This is documented
+elsewhere.
+
+ The guts of the command loop are in `command_loop_1()'. This
+function doesn't catch errors, though--that's the job of
+`command_loop_2()', which is a condition-case (i.e. error-trapping)
+wrapper around `command_loop_1()'. `command_loop_1()' never returns,
+but may get thrown out of.
+
+ When an error occurs, `cmd_error()' is called, which usually invokes
+the Lisp error handler in `command-error'; however, a default error
+handler is provided if `command-error' is `nil' (e.g. during startup).
+The purpose of the error handler is simply to display the error message
+and do associated cleanup; it does not need to throw anywhere. When
+the error handler finishes, the condition-case in `command_loop_2()'
+will finish and `command_loop_2()' will reinvoke `command_loop_1()'.
+
+ `command_loop_2()' is invoked from three places: from
+`initial_command_loop()' (called from `main()' at the end of internal
+initialization), from the Lisp function `recursive-edit', and from
+`call_command_loop()'.
+
+ `call_command_loop()' is called when a macro is started and when the
+minibuffer is entered; normal termination of the macro or minibuffer
+causes a throw out of the recursive command loop. (To
+`execute-kbd-macro' for macros and `exit' for minibuffers. Note also
+that the low-level minibuffer-entering function,
+`read-minibuffer-internal', provides its own error handling and does
+not need `command_loop_2()''s error encapsulation; so it tells
+`call_command_loop()' to invoke `command_loop_1()' directly.)
+
+ Note that both read-minibuffer-internal and recursive-edit set up a
+catch for `exit'; this is why `abort-recursive-edit', which throws to
+this catch, exits out of either one.
+
+ `initial_command_loop()', called from `main()', sets up a catch for
+`top-level' when invoking `command_loop_2()', allowing functions to
+throw all the way to the top level if they really need to. Before
+invoking `command_loop_2()', `initial_command_loop()' calls
+`top_level_1()', which handles all of the startup stuff (creating the
+initial frame, handling the command-line options, loading the user's
+`.emacs' file, etc.). The function that actually does this is in Lisp
+and is pointed to by the variable `top-level'; normally this function is
+`normal-top-level'. `top_level_1()' is just an error-handling wrapper
+similar to `command_loop_2()'. Note also that `initial_command_loop()'
+sets up a catch for `top-level' when invoking `top_level_1()', just
+like when it invokes `command_loop_2()'.
+
+\1f
+File: internals.info, Node: Specifics of the Event Gathering Mechanism, Next: Specifics About the Emacs Event, Prev: Main Loop, Up: Events and the Event Loop
+
+Specifics of the Event Gathering Mechanism
+==========================================
+
+Here is an approximate diagram of the collection processes at work in
+XEmacs, under TTY's (TTY's are simpler than X so we'll look at this
+first):
+
+ asynch. asynch. asynch. asynch. [Collectors in
+ kbd events kbd events process process the OS]
+ | | output output
+ | | | |
+ | | | | SIGINT, [signal handlers
+ | | | | SIGQUIT, in XEmacs]
+ V V V V SIGWINCH,
+ file file file file SIGALRM
+ desc. desc. desc. desc. |
+ (TTY) (TTY) (pipe) (pipe) |
+ | | | | fake timeouts
+ | | | | file |
+ | | | | desc. |
+ | | | | (pipe) |
+ | | | | | |
+ | | | | | |
+ | | | | | |
+ V V V V V V
+ ------>-----------<----------------<----------------
+ |
+ |
+ | [collected using select() in emacs_tty_next_event()
+ | and converted to the appropriate Emacs event]
+ |
+ |
+ V (above this line is TTY-specific)
+ Emacs -----------------------------------------------
+ event (below this line is the generic event mechanism)
+ |
+ |
+ was there if not, call
+ a SIGINT? emacs_tty_next_event()
+ | |
+ | |
+ | |
+ V V
+ --->------<----
+ |
+ | [collected in event_stream_next_event();
+ | SIGINT is converted using maybe_read_quit_event()]
+ V
+ Emacs
+ event
+ |
+ \---->------>----- maybe_kbd_translate() ---->---\
+ |
+ |
+ |
+ command event queue |
+ if not from command
+ (contains events that were event queue, call
+ read earlier but not processed, event_stream_next_event()
+ typically when waiting in a |
+ sit-for, sleep-for, etc. for |
+ a particular event to be received) |
+ | |
+ | |
+ V V
+ ---->------------------------------------<----
+ |
+ | [collected in
+ | next_event_internal()]
+ |
+ unread- unread- event from |
+ command- command- keyboard else, call
+ events event macro next_event_internal()
+ | | | |
+ | | | |
+ | | | |
+ V V V V
+ --------->----------------------<------------
+ |
+ | [collected in `next-event', which may loop
+ | more than once if the event it gets is on
+ | a dead frame, device, etc.]
+ |
+ |
+ V
+ feed into top-level event loop,
+ which repeatedly calls `next-event'
+ and then dispatches the event
+ using `dispatch-event'
+
+ Notice the separation between TTY-specific and generic event
+mechanism. When using the Xt-based event loop, the TTY-specific stuff
+is replaced but the rest stays the same.
+
+ It's also important to realize that only one different kind of
+system-specific event loop can be operating at a time, and must be able
+to receive all kinds of events simultaneously. For the two existing
+event loops (implemented in `event-tty.c' and `event-Xt.c',
+respectively), the TTY event loop _only_ handles TTY consoles, while
+the Xt event loop handles _both_ TTY and X consoles. This situation is
+different from all of the output handlers, where you simply have one
+per console type.
+
+ Here's the Xt Event Loop Diagram (notice that below a certain point,
+it's the same as the above diagram):
+
+ asynch. asynch. asynch. asynch. [Collectors in
+ kbd kbd process process the OS]
+ events events output output
+ | | | |
+ | | | | asynch. asynch. [Collectors in the
+ | | | | X X OS and X Window System]
+ | | | | events events
+ | | | | | |
+ | | | | | |
+ | | | | | | SIGINT, [signal handlers
+ | | | | | | SIGQUIT, in XEmacs]
+ | | | | | | SIGWINCH,
+ | | | | | | SIGALRM
+ | | | | | | |
+ | | | | | | |
+ | | | | | | | timeouts
+ | | | | | | | |
+ | | | | | | | |
+ | | | | | | V |
+ V V V V V V fake |
+ file file file file file file file |
+ desc. desc. desc. desc. desc. desc. desc. |
+ (TTY) (TTY) (pipe) (pipe) (socket) (socket) (pipe) |
+ | | | | | | | |
+ | | | | | | | |
+ | | | | | | | |
+ V V V V V V V V
+ --->----------------------------------------<---------<------
+ | | |
+ | | |[collected using select() in
+ | | | _XtWaitForSomething(), called
+ | | | from XtAppProcessEvent(), called
+ | | | in emacs_Xt_next_event();
+ | | | dispatched to various callbacks]
+ | | |
+ | | |
+ emacs_Xt_ p_s_callback(), | [popup_selection_callback]
+ event_handler() x_u_v_s_callback(),| [x_update_vertical_scrollbar_
+ | x_u_h_s_callback(),| callback]
+ | search_callback() | [x_update_horizontal_scrollbar_
+ | | | callback]
+ | | |
+ | | |
+ enqueue_Xt_ signal_special_ |
+ dispatch_event() Xt_user_event() |
+ [maybe multiple | |
+ times, maybe 0 | |
+ times] | |
+ | enqueue_Xt_ |
+ | dispatch_event() |
+ | | |
+ | | |
+ V V |
+ -->----------<-- |
+ | |
+ | |
+ dispatch Xt_what_callback()
+ event sets flags
+ queue |
+ | |
+ | |
+ | |
+ | |
+ ---->-----------<--------
+ |
+ |
+ | [collected and converted as appropriate in
+ | emacs_Xt_next_event()]
+ |
+ |
+ V (above this line is Xt-specific)
+ Emacs ------------------------------------------------
+ event (below this line is the generic event mechanism)
+ |
+ |
+ was there if not, call
+ a SIGINT? emacs_Xt_next_event()
+ | |
+ | |
+ | |
+ V V
+ --->-------<----
+ |
+ | [collected in event_stream_next_event();
+ | SIGINT is converted using maybe_read_quit_event()]
+ V
+ Emacs
+ event
+ |
+ \---->------>----- maybe_kbd_translate() -->-----\
+ |
+ |
+ |
+ command event queue |
+ if not from command
+ (contains events that were event queue, call
+ read earlier but not processed, event_stream_next_event()
+ typically when waiting in a |
+ sit-for, sleep-for, etc. for |
+ a particular event to be received) |
+ | |
+ | |
+ V V
+ ---->----------------------------------<------
+ |
+ | [collected in
+ | next_event_internal()]
+ |
+ unread- unread- event from |
+ command- command- keyboard else, call
+ events event macro next_event_internal()
+ | | | |
+ | | | |
+ | | | |
+ V V V V
+ --------->----------------------<------------
+ |
+ | [collected in `next-event', which may loop
+ | more than once if the event it gets is on
+ | a dead frame, device, etc.]
+ |
+ |
+ V
+ feed into top-level event loop,
+ which repeatedly calls `next-event'
+ and then dispatches the event
+ using `dispatch-event'
+
+\1f
+File: internals.info, Node: Specifics About the Emacs Event, Next: The Event Stream Callback Routines, Prev: Specifics of the Event Gathering Mechanism, Up: Events and the Event Loop
+
+Specifics About the Emacs Event
+===============================
+
+\1f
+File: internals.info, Node: The Event Stream Callback Routines, Next: Other Event Loop Functions, Prev: Specifics About the Emacs Event, Up: Events and the Event Loop
+
+The Event Stream Callback Routines
+==================================
+
+\1f
+File: internals.info, Node: Other Event Loop Functions, Next: Converting Events, Prev: The Event Stream Callback Routines, Up: Events and the Event Loop
+
+Other Event Loop Functions
+==========================
+
+`detect_input_pending()' and `input-pending-p' look for input by
+calling `event_stream->event_pending_p' and looking in
+`[V]unread-command-event' and the `command_event_queue' (they do not
+check for an executing keyboard macro, though).
+
+ `discard-input' cancels any command events pending (and any keyboard
+macros currently executing), and puts the others onto the
+`command_event_queue'. There is a comment about a "race condition",
+which is not a good sign.
+
+ `next-command-event' and `read-char' are higher-level interfaces to
+`next-event'. `next-command-event' gets the next "command" event (i.e.
+keypress, mouse event, menu selection, or scrollbar action), calling
+`dispatch-event' on any others. `read-char' calls `next-command-event'
+and uses `event_to_character()' to return the character equivalent.
+With the right kind of input method support, it is possible for
+(read-char) to return a Kanji character.
+
+\1f
+File: internals.info, Node: Converting Events, Next: Dispatching Events; The Command Builder, Prev: Other Event Loop Functions, Up: Events and the Event Loop
+
+Converting Events
+=================
+
+`character_to_event()', `event_to_character()', `event-to-character',
+and `character-to-event' convert between characters and keypress events
+corresponding to the characters. If the event was not a keypress,
+`event_to_character()' returns -1 and `event-to-character' returns
+`nil'. These functions convert between character representation and
+the split-up event representation (keysym plus mod keys).
+
+\1f
+File: internals.info, Node: Dispatching Events; The Command Builder, Prev: Converting Events, Up: Events and the Event Loop
+
+Dispatching Events; The Command Builder
+=======================================
+
+Not yet documented.
+
+\1f
+File: internals.info, Node: Evaluation; Stack Frames; Bindings, Next: Symbols and Variables, Prev: Events and the Event Loop, Up: Top
+
+Evaluation; Stack Frames; Bindings
+**********************************
+
+* Menu:
+
+* Evaluation::
+* Dynamic Binding; The specbinding Stack; Unwind-Protects::
+* Simple Special Forms::
+* Catch and Throw::
+
+\1f
+File: internals.info, Node: Evaluation, Next: Dynamic Binding; The specbinding Stack; Unwind-Protects, Up: Evaluation; Stack Frames; Bindings
+
+Evaluation
+==========
+
+`Feval()' evaluates the form (a Lisp object) that is passed to it.
+Note that evaluation is only non-trivial for two types of objects:
+symbols and conses. A symbol is evaluated simply by calling
+`symbol-value' on it and returning the value.
+
+ Evaluating a cons means calling a function. First, `eval' checks to
+see if garbage-collection is necessary, and calls `garbage_collect_1()'
+if so. It then increases the evaluation depth by 1 (`lisp_eval_depth',
+which is always less than `max_lisp_eval_depth') and adds an element to
+the linked list of `struct backtrace''s (`backtrace_list'). Each such
+structure contains a pointer to the function being called plus a list
+of the function's arguments. Originally these values are stored
+unevalled, and as they are evaluated, the backtrace structure is
+updated. Garbage collection pays attention to the objects pointed to
+in the backtrace structures (garbage collection might happen while a
+function is being called or while an argument is being evaluated, and
+there could easily be no other references to the arguments in the
+argument list; once an argument is evaluated, however, the unevalled
+version is not needed by eval, and so the backtrace structure is
+changed).
+
+ At this point, the function to be called is determined by looking at
+the car of the cons (if this is a symbol, its function definition is
+retrieved and the process repeated). The function should then consist
+of either a `Lisp_Subr' (built-in function written in C), a
+`Lisp_Compiled_Function' object, or a cons whose car is one of the
+symbols `autoload', `macro' or `lambda'.
+
+ If the function is a `Lisp_Subr', the lisp object points to a
+`struct Lisp_Subr' (created by `DEFUN()'), which contains a pointer to
+the C function, a minimum and maximum number of arguments (or possibly
+the special constants `MANY' or `UNEVALLED'), a pointer to the symbol
+referring to that subr, and a couple of other things. If the subr
+wants its arguments `UNEVALLED', they are passed raw as a list.
+Otherwise, an array of evaluated arguments is created and put into the
+backtrace structure, and either passed whole (`MANY') or each argument
+is passed as a C argument.
+
+ If the function is a `Lisp_Compiled_Function',
+`funcall_compiled_function()' is called. If the function is a lambda
+list, `funcall_lambda()' is called. If the function is a macro, [.....
+fill in] is done. If the function is an autoload, `do_autoload()' is
+called to load the definition and then eval starts over [explain this
+more].
+
+ When `Feval()' exits, the evaluation depth is reduced by one, the
+debugger is called if appropriate, and the current backtrace structure
+is removed from the list.
+
+ Both `funcall_compiled_function()' and `funcall_lambda()' need to go
+through the list of formal parameters to the function and bind them to
+the actual arguments, checking for `&rest' and `&optional' symbols in
+the formal parameters and making sure the number of actual arguments is
+correct. `funcall_compiled_function()' can do this a little more
+efficiently, since the formal parameter list can be checked for sanity
+when the compiled function object is created.
+
+ `funcall_lambda()' simply calls `Fprogn' to execute the code in the
+lambda list.
+
+ `funcall_compiled_function()' calls the real byte-code interpreter
+`execute_optimized_program()' on the byte-code instructions, which are
+converted into an internal form for faster execution.
+
+ When a compiled function is executed for the first time by
+`funcall_compiled_function()', or during the dump phase of building
+XEmacs, the byte-code instructions are converted from a `Lisp_String'
+(which is inefficient to access, especially in the presence of MULE)
+into a `Lisp_Opaque' object containing an array of unsigned char, which
+can be directly executed by the byte-code interpreter. At this time
+the byte code is also analyzed for validity and transformed into a more
+optimized form, so that `execute_optimized_program()' can really fly.
+
+ Here are some of the optimizations performed by the internal
+byte-code transformer:
+ 1. References to the `constants' array are checked for out-of-range
+ indices, so that the byte interpreter doesn't have to.
+
+ 2. References to the `constants' array that will be used as a Lisp
+ variable are checked for being correct non-constant (i.e. not `t',
+ `nil', or `keywordp') symbols, so that the byte interpreter
+ doesn't have to.
+
+ 3. The maximum number of variable bindings in the byte-code is
+ pre-computed, so that space on the `specpdl' stack can be
+ pre-reserved once for the whole function execution.
+
+ 4. All byte-code jumps are relative to the current program counter
+ instead of the start of the program, thereby saving a register.
+
+ 5. One-byte relative jumps are converted from the byte-code form of
+ unsigned chars offset by 127 to machine-friendly signed chars.
+
+ Of course, this transformation of the `instructions' should not be
+visible to the user, so `Fcompiled_function_instructions()' needs to
+know how to convert the optimized opaque object back into a Lisp string
+that is identical to the original string from the `.elc' file.
+(Actually, the resulting string may (rarely) contain slightly
+different, yet equivalent, byte code.)
+
+ `Ffuncall()' implements Lisp `funcall'. `(funcall fun x1 x2 x3
+...)' is equivalent to `(eval (list fun (quote x1) (quote x2) (quote
+x3) ...))'. `Ffuncall()' contains its own code to do the evaluation,
+however, and is very similar to `Feval()'.
+
+ From the performance point of view, it is worth knowing that most of
+the time in Lisp evaluation is spent executing `Lisp_Subr' and
+`Lisp_Compiled_Function' objects via `Ffuncall()' (not `Feval()').
+
+ `Fapply()' implements Lisp `apply', which is very similar to
+`funcall' except that if the last argument is a list, the result is the
+same as if each of the arguments in the list had been passed separately.
+`Fapply()' does some business to expand the last argument if it's a
+list, then calls `Ffuncall()' to do the work.
+
+ `apply1()', `call0()', `call1()', `call2()', and `call3()' call a
+function, passing it the argument(s) given (the arguments are given as
+separate C arguments rather than being passed as an array). `apply1()'
+uses `Fapply()' while the others use `Ffuncall()' to do the real work.
+
+\1f
+File: internals.info, Node: Dynamic Binding; The specbinding Stack; Unwind-Protects, Next: Simple Special Forms, Prev: Evaluation, Up: Evaluation; Stack Frames; Bindings
+
+Dynamic Binding; The specbinding Stack; Unwind-Protects
+=======================================================
+
+ struct specbinding
+ {
+ Lisp_Object symbol;
+ Lisp_Object old_value;
+ Lisp_Object (*func) (Lisp_Object); /* for unwind-protect */
+ };
+
+ `struct specbinding' is used for local-variable bindings and
+unwind-protects. `specpdl' holds an array of `struct specbinding''s,
+`specpdl_ptr' points to the beginning of the free bindings in the
+array, `specpdl_size' specifies the total number of binding slots in
+the array, and `max_specpdl_size' specifies the maximum number of
+bindings the array can be expanded to hold. `grow_specpdl()' increases
+the size of the `specpdl' array, multiplying its size by 2 but never
+exceeding `max_specpdl_size' (except that if this number is less than
+400, it is first set to 400).
+
+ `specbind()' binds a symbol to a value and is used for local
+variables and `let' forms. The symbol and its old value (which might
+be `Qunbound', indicating no prior value) are recorded in the specpdl
+array, and `specpdl_size' is increased by 1.
+
+ `record_unwind_protect()' implements an "unwind-protect", which,
+when placed around a section of code, ensures that some specified
+cleanup routine will be executed even if the code exits abnormally
+(e.g. through a `throw' or quit). `record_unwind_protect()' simply
+adds a new specbinding to the `specpdl' array and stores the
+appropriate information in it. The cleanup routine can either be a C
+function, which is stored in the `func' field, or a `progn' form, which
+is stored in the `old_value' field.
+
+ `unbind_to()' removes specbindings from the `specpdl' array until
+the specified position is reached. Each specbinding can be one of
+three types:
+
+ 1. an unwind-protect with a C cleanup function (`func' is not 0, and
+ `old_value' holds an argument to be passed to the function);
+
+ 2. an unwind-protect with a Lisp form (`func' is 0, `symbol' is
+ `nil', and `old_value' holds the form to be executed with
+ `Fprogn()'); or
+
+ 3. a local-variable binding (`func' is 0, `symbol' is not `nil', and
+ `old_value' holds the old value, which is stored as the symbol's
+ value).
+
+\1f
+File: internals.info, Node: Simple Special Forms, Next: Catch and Throw, Prev: Dynamic Binding; The specbinding Stack; Unwind-Protects, Up: Evaluation; Stack Frames; Bindings
+
+Simple Special Forms
+====================
+
+`or', `and', `if', `cond', `progn', `prog1', `prog2', `setq', `quote',
+`function', `let*', `let', `while'
+
+ All of these are very simple and work as expected, calling `Feval()'
+or `Fprogn()' as necessary and (in the case of `let' and `let*') using
+`specbind()' to create bindings and `unbind_to()' to undo the bindings
+when finished.
+
+ Note that, with the exception of `Fprogn', these functions are
+typically called in real life only in interpreted code, since the byte
+compiler knows how to convert calls to these functions directly into
+byte code.
+
+\1f
+File: internals.info, Node: Catch and Throw, Prev: Simple Special Forms, Up: Evaluation; Stack Frames; Bindings
+
+Catch and Throw
+===============
+
+ struct catchtag
+ {
+ Lisp_Object tag;
+ Lisp_Object val;
+ struct catchtag *next;
+ struct gcpro *gcpro;
+ jmp_buf jmp;
+ struct backtrace *backlist;
+ int lisp_eval_depth;
+ int pdlcount;
+ };
+
+ `catch' is a Lisp function that places a catch around a body of
+code. A catch is a means of non-local exit from the code. When a catch
+is created, a tag is specified, and executing a `throw' to this tag
+will exit from the body of code caught with this tag, and its value will
+be the value given in the call to `throw'. If there is no such call,
+the code will be executed normally.
+
+ Information pertaining to a catch is held in a `struct catchtag',
+which is placed at the head of a linked list pointed to by `catchlist'.
+`internal_catch()' is passed a C function to call (`Fprogn()' when
+Lisp `catch' is called) and arguments to give it, and places a catch
+around the function. Each `struct catchtag' is held in the stack frame
+of the `internal_catch()' instance that created the catch.
+
+ `internal_catch()' is fairly straightforward. It stores into the
+`struct catchtag' the tag name and the current values of
+`backtrace_list', `lisp_eval_depth', `gcprolist', and the offset into
+the `specpdl' array, sets a jump point with `_setjmp()' (storing the
+jump point into the `struct catchtag'), and calls the function.
+Control will return to `internal_catch()' either when the function
+exits normally or through a `_longjmp()' to this jump point. In the
+latter case, `throw' will store the value to be returned into the
+`struct catchtag' before jumping. When it's done, `internal_catch()'
+removes the `struct catchtag' from the catchlist and returns the proper
+value.
+
+ `Fthrow()' goes up through the catchlist until it finds one with a
+matching tag. It then calls `unbind_catch()' to restore everything to
+what it was when the appropriate catch was set, stores the return value
+in the `struct catchtag', and jumps (with `_longjmp()') to its jump
+point.
+
+ `unbind_catch()' removes all catches from the catchlist until it
+finds the correct one. Some of the catches might have been placed for
+error-trapping, and if so, the appropriate entries on the handlerlist
+must be removed (see "errors"). `unbind_catch()' also restores the
+values of `gcprolist', `backtrace_list', and `lisp_eval', and calls
+`unbind_to()' to undo any specbindings created since the catch.
+
+\1f
+File: internals.info, Node: Symbols and Variables, Next: Buffers and Textual Representation, Prev: Evaluation; Stack Frames; Bindings, Up: Top
+
+Symbols and Variables
+*********************
+
+* Menu:
+
+* Introduction to Symbols::
+* Obarrays::
+* Symbol Values::
+
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