--- /dev/null
+\input texinfo @c -*-texinfo-*-
+
+@c %**start of header
+@setfilename ../info/emodules.info
+@settitle Extending Emacs using C Modules
+@c %**end of header
+
+@c
+@c Use some macros so that we can format for either XEmacs
+@c or (shudder) GNU Emacs.
+@c
+
+@ifset XEMACS
+@set emacs XEmacs
+@clear EMACS
+@set HAVE-EMACS
+@end ifset
+
+@ifset EMACS
+@set emacs Emacs
+@clear XEMACS
+@set HAVE-EMACS
+@end ifset
+
+@ifclear HAVE-EMACS
+@set XEMACS
+@set emacs XEmacs
+@end ifclear
+
+@ifinfo
+This file documents the module loading technology of @value{emacs}.
+
+Copyright @copyright{} 1998 J. Kean Johnston.
+
+Permission is granted to make and distribute verbatim copies of this
+manual provided the copyright notice and this permission notice are
+preserved on all copies.
+
+@ignore
+Permission is granted to process this file through TeX and print the
+results, provided the printed document carries copying permission notice
+identical to this one except for the removal of this paragraph (this
+paragraph not being relevant to the printed manual).
+
+@end ignore
+Permission is granted to copy and distribute modified versions of this
+manual under the conditions for verbatim copying, provided that the
+entire resulting derived work is distributed under the terms of a
+permission notice identical to this one.
+
+Permission is granted to copy and distribute translations of this manual
+into another language, under the above conditions for modified versions,
+except that this permission notice may be stated in a translation
+approved by the Foundation.
+
+Permission is granted to copy and distribute modified versions of this
+manual under the conditions for verbatim copying, provided also that the
+section entitled ``GNU General Public License'' is included exactly as
+in the original, and provided that the entire resulting derived work is
+distributed under the terms of a permission notice identical to this
+one.
+
+Permission is granted to copy and distribute translations of this manual
+into another language, under the above conditions for modified versions,
+except that the section entitled ``GNU General Public License'' may be
+included in a translation approved by the Free Software Foundation
+instead of in the original English.
+@end ifinfo
+
+@c Combine indices.
+@syncodeindex fn cp
+@syncodeindex vr cp
+@syncodeindex ky cp
+@syncodeindex pg cp
+@syncodeindex tp cp
+
+@setchapternewpage odd
+@finalout
+
+@titlepage
+@title Extending @value{emacs} using C and C++
+@subtitle Version 1.0, September 1998
+
+@author J. Kean Johnston
+@page
+@vskip 0pt plus 1fill
+
+@noindent
+Copyright @copyright{} 1998 J. Kean Johnston. @*
+
+@sp 2
+Version 1.0 @*
+September, 1998.@*
+
+Permission is granted to make and distribute verbatim copies of this
+manual provided the copyright notice and this permission notice are
+preserved on all copies.
+
+Permission is granted to copy and distribute modified versions of this
+manual under the conditions for verbatim copying, provided also that the
+section entitled ``GNU General Public License'' is included
+exactly as in the original, and provided that the entire resulting
+derived work is distributed under the terms of a permission notice
+identical to this one.
+
+Permission is granted to copy and distribute translations of this manual
+into another language, under the above conditions for modified versions,
+except that the section entitled ``GNU General Public License'' may be
+included in a translation approved by the Free Software Foundation
+instead of in the original English.
+@end titlepage
+@page
+
+@ifinfo
+@node Top, Introduction, (dir), (dir)
+This Info file contains v1.0 of the @value{emacs} dynamic loadable module
+support documentation.
+@menu
+* Introduction:: Introducing Emacs Modules
+* Anatomy of a Module:: Basic module layout and technology
+* Using ellcc:: How to use the module compiler
+* Defining Functions:: Creating new Lisp primitives
+* Defining Variables:: Creating new Lisp variables
+* Index:: Concept Index
+
+ --- The Detailed Node Listing ---
+
+Anatomy of a Module
+
+* Required Header File:: Always include <emodules.h>
+* Required Functions:: Functions you must always provide
+* Required Variables:: Variables whose values you must provide
+* Loading other Modules:: How to load dependent modules
+
+Using @code{ellcc}
+
+* Compile Mode:: Compiling modules using ellcc
+* Initialization Mode:: Generating documentation and variables
+* Link Mode:: Creating the final loadable module
+* Other ellcc options:: Other useful options
+* Environment Variables:: How to control ellcc
+
+Defining Functions
+
+* Using DEFUN:: Using the DEFUN macro to define functions
+* Declaring Functions:: Declaring functions to the Lisp reader
+@end menu
+
+@end ifinfo
+
+@node Introduction, Anatomy of a Module, Top, Top
+@chapter Introduction
+
+ @value{emacs} is a powerful, extensible editor. The traditional way of
+extending the functionality of @value{emacs} is to use its built-in Lisp
+language (called Emacs Lisp, or Elisp for short). However, while Elisp
+is a full programming language and capable of extending @value{emacs} in more
+ways than you can imagine, it does have its short-comings.
+
+ Firstly, Elisp is an interpreted language, and this has serious speed
+implications. Like all other interpreted languages (like Java), Elisp
+is often suitable only for certain types of application or extension.
+So although Elisp is a general purpose language, and very high level,
+there are times when it is desirable to descend to a lower level compiled
+language for speed purposes.
+
+ Secondly, Elisp (or Lisp in general) is not a very common language any
+more, except for certain circles in the computer industry. C is a far
+more commonly known language, and because it is compiled, more suited to
+a wider range of applications, especially those that require low level
+access to a system or need to be as quick as possible.
+
+@cindex Emacs Modules
+@cindex DLL
+@cindex DSO
+@cindex shared object
+ This manual describes a new way of extending @value{emacs}, by using dynamic
+loadable modules (also known as dynamically loadable libraries (DLLs),
+dynamic shared objects (DSOs) or just simply shared objects), which can
+be written in C or C++ and loaded into @value{emacs} at any time. I sometimes
+refer to this technology as @dfn{CEmacs}, which is short for @dfn{C
+Extensible Emacs}.
+
+ @value{emacs} modules are configured into and installed with @value{emacs} by
+default on all systems that support loading of shared objects. From a
+users perspective, the internals of @value{emacs} modules are irrelevant.
+All a user will ever need to know about shared objects is the name of
+the shared object when they want to load a given module. From a
+developers perspective though, a lot more is provided.
+
+@itemize @bullet
+@item
+@pindex ellcc
+@cindex compiler
+@cindex linker
+ Of primary interest is the @code{ellcc} program. This program is
+created during compile time, and is intended to abstract compiler
+specific characteristics from the developer. This program is called to
+compile and link all objects that will make up the final shared object,
+and accepts all common C compiler flags. @code{ellcc} also sets up the
+correct environment for compiling modules by enabling any special
+compiler modes (such as PIC mode), setting the correct include paths for
+the location of @value{emacs} internal header files etc. The program will also
+invoke the linker correctly to created the final shared object which is
+loaded into @value{emacs}.
+
+@item
+@cindex header files
+ CEmacs also makes all of the relevant @value{emacs} internal header files
+available for module authors to use. This is often required to get data
+structure definitions and external variable declarations. The header
+files installed include the module specific header file
+@file{emodules.h}. Due to the nature of dynamic modules, most of the
+internals of @value{emacs} are exposed.
+@xref{Top,,,internals,@value{emacs} Internals Manual}, for a
+more complete discussion on how to extend and understand @value{emacs}. All of
+the rules for C modules are discussed there.
+
+@item
+@cindex samples
+ Part of the @value{emacs} distribution is a set of sample modules. These are
+not installed when @value{emacs} is, but remain in the @value{emacs} source tree.
+These modules live in the directory @file{modules}, which is a
+sub-directory of the main @value{emacs} source code directory. Please look at
+the samples carefully, and maybe even use them as a basis for making
+your own modules. Most of the concepts required for writing extension
+modules are covered in the samples.
+
+@item
+@cindex documentation
+@cindex help
+ Last, but not least is this manual. This can be viewed from within
+@value{emacs}, and it can be printed out as well. It is the intention of this
+document that it will describe everything you need to know about
+extending @value{emacs} in C. If you do not find this to be the case, please
+contact the author(s).
+@end itemize
+
+ The rest of this document will discuss the actual mechanics of
+@value{emacs} modules and work through several of the samples. Please be
+sure that you have read the @value{emacs} Internals Manual and understand
+everything in it. The concepts there apply to all modules. This
+document may have some overlap, but it is the internals manual which
+should be considered the final authority. It will also help a great
+deal to look at the actual @value{emacs} source code to see how things are
+done.
+
+@node Anatomy of a Module, Using ellcc, Introduction, Top
+@chapter Anatomy of a Module
+@cindex anatomy
+@cindex module skeleton
+@cindex skeleton, module
+@cindex module format
+@cindex format, module
+
+ Each dynamically loadable @value{emacs} extension (hereafter referred to as a
+module) has a certain compulsory format, and must contain several
+pieces of information and several mandatory functions. This chapter
+describes the basic layout of a module, and provides a very simple
+sample. The source for this sample can be found in the file
+@file{modules/simple/sample.c} in the main @value{emacs} source code tree.
+
+@menu
+* Required Header File:: Always include <emodules.h>
+* Required Functions:: Functions you must always provide
+* Required Variables:: Variables whose values you must provide
+* Loading other Modules:: How to load dependent modules
+@end menu
+
+@node Required Header File, Required Functions, Anatomy of a Module, Anatomy of a Module
+@section Required Header File
+@cindex required header
+@cindex include files
+
+@cindex emodules.h
+@cindex config.h
+ Every module must include the file @file{<emodules.h>}. This
+will include several other @value{emacs} internal header files, and will set up
+certain vital macros. One of the most important files included by
+@file{emodules.h} is the generated @file{config.h} file, which contains
+all of the required system abstraction macros and definitions. Most
+modules will probably require some pre-processor conditionals based on
+constants defined in @file{config.h}. Please read that file to
+familiarize yourself with the macros defined there.
+
+ Depending on exactly what your module will be doing, you will probably
+need to include one or more of the @value{emacs} internal header files. When
+you @code{#include <emodules.h>}, you will get a few of the most important
+@value{emacs} header files included automatically for you. The files included
+are:
+
+@table @file
+@item lisp.h
+This file contains most of the macros required for declaring Lisp object
+types, macros for accessing Lisp objects, and global variable
+declarations.
+
+@item sysdep.h
+All system dependent declarations and abstraction macros live here. You
+should never call low level system functions directly. Rather, you
+should use the abstraction macros provided in this header file.
+
+@item window.h
+This header file defines the window structures and Lisp types, and
+provides functions and macros for manipulating multiple @value{emacs} windows.
+
+@item buffer.h
+All macros and function declarations for manipulating internal and user
+visible buffers appear in this file.
+
+@item insdel.h
+This header provides the information required for performing text
+insertion and deletion.
+
+@item frame.h
+Provides the required structure, macro and function definitions for
+manipulating @value{emacs} frames.
+@end table
+
+@node Required Functions, Required Variables, Required Header File, Anatomy of a Module
+@section Required Functions
+@cindex initialization
+@cindex functions, required
+@cindex required functions
+
+Every module requires several initialization functions. It is the
+responsibility of these functions to load in any dependent modules, and to
+declare all variables and functions which are to be made visible to the
+@value{emacs} Lisp reader. Each of these functions performs a very specific
+task, and they are executed in the correct order by @value{emacs}. All of
+these functions are @code{void} functions which take no arguments.
+Here, briefly, are the required module functions. Note that the actual
+function names do not end with the string @code{_module}, but rather
+they end with the abbreviated module name by which the module is known.
+More on the module name and its importance later. Just bear in mind
+that the text @code{_module} in the functions below is simply a
+place-holder, not an actual function name.
+
+@table @code
+@item syms_of_module
+@findex syms_of_module
+This required function is responsible for introducing to the Lisp reader
+all functions that you have defined in your module using
+@code{DEFUN()}. Note that @emph{only} functions are declared here, using
+the @code{DEFSUBR()} macro. No variables are declared.
+
+@item vars_of_module
+@findex vars_of_module
+This required function contains calls to macros such as
+@code{DEFVAR_LISP()}, @code{DEFVAR_BOOL()} etc, and its purpose is to
+declare and initialize all and any variables that your module defines.
+They syntax for declaring variables is identical to the syntax used for
+all internal @value{emacs} source code. If the module is intended to be
+usable statically linked into XEmacs, the actions of this function are
+severely restricted. @xref{General Coding Rules,,,internals,
+@value{emacs} Internals Manual}. Also see the comments in
+@file{src/emacs.c} (@code{main_1}). Modules which perform
+initializations not permitted by these rules will probably work, but
+dual-use (dynamic loading and static linking) modules will require very
+careful, and possibly fragile, coding.
+
+@item modules_of_module
+@findex modules_of_module
+This optional function should be used to load in any modules which your
+module depends on. The @value{emacs} module loading code makes sure that the
+same module is not loaded twice, so several modules can safely call the
+module load function for the same module. Only one copy of each module
+(at a given version) will ever be loaded.
+
+@item docs_of_module
+@findex docs_of_module
+This is a required function, but not one which you need ever write.
+This function is created automatically by @code{ellcc} when the module
+initialization code is produced. It is required to document all
+functions and variables declared in your module.
+@end table
+
+@node Required Variables, Loading other Modules, Required Functions, Anatomy of a Module
+@section Required Variables
+@cindex initialization
+@cindex variables, required
+@cindex required variables
+
+Not only does a module need to declare the initialization functions
+mentioned above, it is also required to provide certain variables which
+the module loading code searches for in order to determine the viability
+of a module. You are @emph{not} required to provide these variables in
+your source files. They are automatically set up in the module
+initialization file by the @code{ellcc} compiler. These variables are
+discussed here simply for the sake of completeness.
+
+@table @code
+@item emodules_compiler
+This is a variable of type @code{long}, and is used to indicate the
+version of the @value{emacs} loading technology that was used to produce the
+module being loaded. This version number is completely unrelated to
+the @value{emacs} version number, as a given module may quite well work
+regardless of the version of @value{emacs} that was installed at the time the
+module was created.
+
+The @value{emacs} modules version is used to differentiate between major
+changes in the module loading technology, not versions of @value{emacs}.
+
+@item emodules_name
+This is a short (typically 10 characters or less) name for the module,
+and it is used as a suffix for all of the required functions. This is
+also the name by which the module is recognized when loading dependent
+modules. The name does not necessarily have to be the same as the
+physical file name, although keeping the two names in sync is a pretty
+good idea. The name must not be empty, and it must be a valid part of a
+C function name. The value of this variable is appended to the function
+names @code{syms_of_}, @code{vars_of_}, @code{modules_of_} and
+@code{docs_of_} to form the actual function names that the module
+loading code looks for when loading a module.
+
+This variable is set by the @code{--mod-name} argument to @code{ellcc}.
+
+@item emodules_version
+This string variable is used to load specific versions of a module.
+Rarely will two or more versions of a module be left lying around, but
+just in case this does happen, this variable can be used to control
+exactly which module should be loaded. See the Lisp function
+@code{load-module} for more details. This variable is set by the
+@code{--mod-version} argument to @code{ellcc}.
+
+@item emodules_title
+This is a string which describes the module, and can contain spaces or
+other special characters. It is used solely for descriptive purposes,
+and does not affect the loading of the module. The value is set by the
+@code{--mod-title} argument to @code{ellcc}.
+@end table
+
+@node Loading other Modules, , Required Variables, Anatomy of a Module
+@section Loading other Modules
+@cindex dependencies
+@findex modules_of_module
+@findex emodules_load
+
+During the loading of a module, it is the responsibility of the function
+@code{modules_of_module} to load in any modules which the current module
+depends on. If the module is stand-alone, and does not depend on other
+modules, then this function can be left empty or even undeclared.
+However, if it does have dependencies, it must call
+@code{emodules_load}:
+
+@example
+@cartouche
+int emodules_load (const char *module,
+ const char *modname,
+ const char *modver)
+@end cartouche
+@end example
+
+The first argument @var{module} is the name of the actual shared object
+or DLL. You can omit the @file{.so}, @file{.ell} or @file{.dll}
+extension of you wish. If you do not specify an absolute path name,
+then the same rules as apply to loading Lisp modules are applied when
+searching for the module. If the module cannot be found in any of the
+standard places, and an absolute path name was not specified,
+@code{emodules_load} will signal an error and loading of the module
+will stop.
+
+The second argument (@var{modname}) is the module name to load, and
+must match the contents of the variable @var{emodule_name} in the
+module to be loaded. A mis-match will cause the module load to fail. If
+this parameter is @code{NULL} or empty, then no checks are performed
+against the target module's @var{emodule_name} variable.
+
+The last argument, @var{modver}, is the desired version of the module
+to load, and is compared to the target module's
+@var{emodule_version} value. If this parameter is not @code{NULL}
+or empty, and the match fails, then the load of the module will fail.
+
+@code{emodules_load} can be called recursively. If, at any point
+during the loading of modules a failure is encountered, then all modules
+that were loaded since the top level call to @code{emodules_load}
+will be unloaded. This means that if any child modules fail to load,
+then their parents will also fail to load. This does not include
+previous successful calls to @code{emodules_load} at the top level.
+
+@strong{Warning:} Modules are @emph{not} loaded with the
+@code{RTLD_GLOBAL} flag. The practical upshot is that individual
+modules do not have access to each other's C symbols. One module cannot
+make a C function call to a function defined in another module, nor can
+it read or set a C variable in another module. All interaction between
+modules must, therefore, take place at the Lisp level. This is by
+design. Other projects have attempted to use @code{RTLD_GLOBAL}, only
+to find that spurious symbol name clashes were the result. Helper
+functions often have simple names, increasing the probability of such a
+clash. If you really need to share symbols between modules, create a
+shared library containing those symbols, and link your modules with
+that library. Otherwise, interactions between modules must take place
+via Lisp function calls and Lisp variables accesses.
+
+@node Using ellcc, Defining Functions, Anatomy of a Module, Top
+@chapter Using @code{ellcc}
+@cindex @code{ellcc}
+@cindex module compiler
+
+Before discussing the anatomy of a module in greater detail, you should
+be aware of the steps required in order to correctly compile and link a
+module for use within @value{emacs}. There is little difference between
+compiling normal C code and compiling a module. In fact, all that
+changes is the command used to compile the module, and a few extra
+arguments to the compiler.
+
+@value{emacs} now ships with a new user utility, called @code{ellcc}. This
+is the @dfn{Emacs Loadable Library C Compiler}. This is a wrapper
+program that will invoke the real C compiler with the correct arguments
+to compile and link your module. With the exception of a few command
+line options, this program can be considered a replacement for your C
+compiler. It accepts all of the same flags and arguments that your C
+compiler does, so in many cases you can simply set the @code{make}
+variable @code{CC} to @code{ellcc} and your code will be compiled as
+an Emacs module rather than a static C object.
+
+@code{ellcc} has three distinct modes of operation. It can be run in
+compile, link or initialization mode. These modes are discussed in more
+detail below. If you want @code{ellcc} to show the commands it is
+executing, you can specify the option @code{--mode=verbose} to
+@code{ellcc}. Specifying this option twice will enable certain extra
+debugging messages to be displayed on the standard output.
+
+@menu
+* Compile Mode:: Compiling modules using ellcc
+* Initialization Mode:: Generating documentation and variables
+* Link Mode:: Creating the final loadable module
+* Other ellcc options:: Other useful options
+* Environment Variables:: How to control ellcc
+@end menu
+
+@node Compile Mode, Initialization Mode, Using ellcc, Using ellcc
+@section Compile Mode
+@cindex compiling
+
+By default, @code{ellcc} is in @dfn{compile} mode. This means that it
+assumes that all of the command line arguments are C compiler arguments,
+and that you want to compile the specified source file or files. You
+can force compile mode by specifying the @code{--mode=compile} argument
+to @code{ellcc}.
+
+In this mode, @code{ellcc} is simply a front-end to the same C compiler
+that was used to create the @value{emacs} binary itself. All @code{ellcc}
+does in this mode is insert a few extra command line arguments before
+the arguments you specify to @code{ellcc} itself. @code{ellcc} will
+then invoke the C compiler to compile your module, and will return the
+same exit codes and messages that your C compiler does.
+
+By far the easiest way to compile modules is to construct a
+@file{Makefile} as you would for a normal program, and simply insert, at
+some appropriate place something similar to:
+
+@example
+@cartouche
+CC=ellcc --mode=compile
+
+.c.o:
+ $(CC) $(CFLAGS) -c $<
+@end cartouche
+@end example
+
+After this, all you need to do is provide simple @code{make} rules for
+compiling your module source files. Since modules are most useful when
+they are small and self-contained, most modules will have a single
+source file, aside from the module specific initialization file (see
+below for details).
+
+@node Initialization Mode, Link Mode, Compile Mode, Using ellcc
+@section Initialization Mode
+@cindex initialization
+@cindex documentation
+
+@value{emacs} uses a rather bizarre way of documenting variables and
+functions. Rather than have the documentation for compiled functions
+and variables passed as static strings in the source code, the
+documentation is included as a C comment. A special program, called
+@file{make-docfile}, is used to scan the source code files and extract
+the documentation from these comments, producing the @value{emacs} @file{DOC}
+file, which the internal help engine scans when the documentation for a
+function or variable is requested.
+
+Due to the internal construction of Lisp objects, subrs and other such
+things, adding documentation for a compiled function or variable in a
+compiled module, at any time after @value{emacs} has been @dfn{dumped} is
+somewhat problematic. Fortunately, as a module writer you are insulated
+from the difficulties thanks to your friend @code{ellcc} and some
+internal trickery in the module loading code. This is all done using
+the @dfn{initialization} mode of @code{ellcc}.
+
+The result of running @code{ellcc} in initialization mode is a C source
+file which you compile with (you guessed it) @code{ellcc} in compile
+mode. Initialization mode is where you set the module name, version,
+title and gather together all of the documentation strings for the
+functions and variables in your module. There are several options that
+you are required to pass @code{ellcc} in initialization mode, the first
+of which is the mode switch itself, @code{--mode=init}.
+
+Next, you need to specify the name of the C source code file that
+@code{ellcc} will produce, and you specify this using the
+@code{--mod-output=FILENAME} argument. @var{FILENAME} is the name of
+the C source code file that will contain the module variables and
+@code{docs_of_module} function.
+
+As discussed previously, each module requires a short @dfn{handle} or
+module name. This is specified with the @code{--mod-name=NAME} option,
+where @var{NAME} is the abbreviated module name. This @var{NAME} must
+consist only of characters that are valid in C function and variable
+names.
+
+The module version is specified using @code{--mod-version=VERSION}
+argument, with @var{VERSION} being any arbitrary version string. This
+version can be passed as an optional second argument to the Lisp
+function @code{load-module}, and as the third argument to the internal
+module loading command @code{emodules_load}. This version string is
+used to distinguish between different versions of the same module, and
+to ensure that the module is loaded at a specific version.
+
+Last, but not least, is the module title. Specified using the
+@code{--mod-title=TITLE} option, the specified @var{TITLE} is used when
+the list of loaded modules is displayed. The module title serves no
+purpose other than to inform the user of the function of the module.
+This string should be brief, as it has to be formatted to fit the
+screen.
+
+Following all of these parameters, you need to provide the list of all
+source code modules that make up your module. These are the files which
+are scanned by @file{make-docfile}, and provide the information required
+to populate the @code{docs_of_module} function. Below is a sample
+@file{Makefile} fragment which indicates how all of this is used.
+
+@example
+@cartouche
+CC=ellcc --mode=compile
+LD=ellcc --mode=link
+MODINIT=ellcc --mode=init
+CFLAGS=-O2 -DSOME_STUFF
+
+.c.o:
+ $(CC) $(CFLAGS) -c $<
+
+MODNAME=sample
+MODVER=1.0.0
+MODTITLE="Small sample module"
+
+SRCS=modfile1.c modfile2.c modfile3.c
+OBJS=$(SRCS:.c=.o)
+
+all: sample.ell
+clean:
+ rm -f $(OBJS) sample_init.o sample.ell
+
+install: all
+ mkdir `ellcc --mod-location`/mymods > /dev/null
+ cp sample.ell `ellcc --mod-location`/mymods/sample.ell
+
+sample.ell: $(OBJS) sample_init.o
+ $(LD) --mod-output=$@ $(OBJS) sample_init.o
+
+sample_init.o: sample_init.c
+sample_init.c: $(SRCS)
+ $(MODINIT) --mod-name=$(MODNAME) --mod-version=$(MODVER) \
+ --mod-title=$(MODTITLE) --mod-output=$@ $(SRCS)
+@end cartouche
+@end example
+
+The above @file{Makefile} is, in fact, complete, and would compile the
+sample module, and optionally install it. The @code{--mod-location}
+argument to @code{ellcc} will produce, on the standard output, the base
+location of the @value{emacs} module directory. Each sub-directory of that
+directory is automatically searched for modules when they are loaded with
+@code{load-module}. An alternative location would be
+@file{/usr/local/lib/xemacs/site-modules}. That path can change depending
+on the options the person who compiled @value{emacs} chose, so you can
+always determine the correct site location using the
+@code{--mod-site-location} option. This directory is treated the same way
+as the main module directory. Each sub-directory within it is searched for
+a given module when the user attempts to load it. The valid extensions that
+the loader attempts to use are @file{.so}, @file{.ell} and @file{.dll}. You
+can use any of these extensions, although @file{.ell} is the preferred
+extension.
+
+@node Link Mode, Other ellcc options, Initialization Mode, Using ellcc
+@section Link Mode
+@cindex linking
+
+Once all of your source code files have been compiled (including the
+generated init file) you need to link them all together to create the
+loadable module. To do this, you invoke @code{ellcc} in link mode, by
+passing the @code{--mode=link} option. You need to specify the final
+output file using the @code{--mod-output=NAME} option, but other than
+that all other arguments are passed on directly to the system compiler
+or linker, along with any other required arguments to create the
+loadable module.
+
+The module has complete access to all symbols that were present in the
+dumped @value{emacs}, so you do not need to link against libraries that were
+linked in with the main executable. If your library uses some other
+extra libraries, you will need to link with those. There is nothing
+particularly complicated about link mode. All you need to do is make
+sure you invoke it correctly in the @file{Makefile}. See the sample
+@file{Makefile} above for an example of a well constructed
+@file{Makefile} that invoked the linker correctly.
+
+@node Other ellcc options, Environment Variables, Link Mode, Using ellcc
+@section Other @code{ellcc} options
+@cindex paths
+
+Aside from the three main @code{ellcc} modes described above,
+@code{ellcc} can accept several other options. These are typically used
+in a @file{Makefile} to determine installation paths. @code{ellcc} also
+allows you to over-ride several of its built-in compiler and linker
+options using environment variables. Here is the complete list of
+options that @code{ellcc} accepts.
+
+@table @code
+@item --mode=compile
+Enables compilation mode. Use this to compile source modules.
+
+@item --mode=link
+Enabled link edit mode. Use this to create the final module.
+
+@item --mode=init
+Used to create the documentation function and to initialize other
+required variables. Produces a C source file that must be compiled with
+@code{ellcc} in compile mode before linking the final module.
+
+@item --mode=verbose
+Enables verbose mode. This will show you the commands that are being
+executed, as well as the version number of @code{ellcc}. If you specify
+this option twice, then some extra debugging information is displayed.
+
+@item --mod-name=NAME
+Sets the short internal module @var{NAME} to the string specified,
+which must consist only of valid C identifiers. Required during
+initialization mode.
+
+@item --mod-version=VERSION
+Sets the internal module @var{VERSION} to the specified string.
+Required during initialization mode.
+
+@item --mod-title=TITLE
+Sets the module descriptive @var{TITLE} to the string specified. This
+string can contain any printable characters, but should not be too
+long. It is required during initialization mode.
+
+@item --mod-output=FILENAME
+Used to control the output file name. This is used during
+initialization mode to set the name of the C source file that will be
+created to @var{FILENAME}. During link mode, it sets the name of the
+final loadable module to @var{FILENAME}.
+
+@item --mod-location
+This will print the name of the standard module installation path on the
+standard output and immediately exit @code{ellcc}. Use this option to
+determine the directory prefix of where you should install your modules.
+
+@item --mod-site-location
+This will print the name of the site specific module location and exit.
+
+@item --mod-archdir
+Prints the name of the root of the architecture-dependent directory that
+@value{emacs} searches for architecture-dependent files.
+
+@item --mod-config
+Prints the name of the configuration for which @value{emacs} and @code{ellcc}
+were compiled.
+@end table
+
+@node Environment Variables, , Other ellcc options, Using ellcc
+@section Environment Variables
+@cindex environment variables
+
+During its normal operation, @code{ellcc} uses the compiler and linker
+flags that were determined at the time @value{emacs} was configured. In
+certain rare circumstances you may wish to over-ride the flags passed to
+the compiler or linker, and you can do so using environment variables.
+The table below lists all of the environment variables that @code{ellcc}
+recognizes.
+
+@table @code
+@item ELLCC
+@cindex @code{ELLCC}
+This is used to over-ride the name of the C compiler that is invoked by
+@code{ellcc}.
+
+@item ELLLD
+@cindex @code{ELLLD}
+Sets the name of the link editor to use to created the final module.
+
+@item ELLCFLAGS
+@cindex @code{ELLCFLAGS}
+Sets the compiler flags passed on when compiling source modules. This
+only sets the basic C compiler flags. There are certain hard-coded
+flags that will always be passed.
+
+@item ELLLDFLAGS
+@cindex @code{ELLLDFLAGS}
+Sets the flags passed on to the linker. This does @strong{not} include
+the flags for enabling PIC mode. This just sets basic linker flags.
+
+@item ELLDLLFLAGS
+@cindex @code{ELLDLLFLAGS}
+Sets the flags passed to the linker that are required to created shared
+and loadable objects.
+
+@item ELLPICFLAGS
+@cindex @code{ELLPICFLAGS}
+Sets the C compiler option required to produce an object file that is
+suitable for including in a shared library. This option should turn on
+PIC mode, or the moral equivalent thereof on the target system.
+
+@item ELLMAKEDOC
+@cindex @code{ELLMAKEDOC}
+Sets the name of the @file{make-docfile} program to use. Usually
+@code{ellcc} will use the version that was compiled and installed with
+@value{emacs}, but this option allows you to specify an alternative path.
+Used during the compile phase of @value{emacs} itself.
+@end table
+
+@node Defining Functions, Defining Variables, Using ellcc, Top
+@chapter Defining Functions
+@cindex defining functions
+
+ One of the main reasons you would ever write a module is to
+provide one or more @dfn{functions} for the user or the editor to use.
+The term
+@dfn{function} is a bit overloaded here, as it refers to both a C
+function and the way it appears to Lisp, which is a @dfn{subroutine}, or
+simply a @dfn{subr}. A Lisp subr is also known as a Lisp primitive, but
+that term applies less to dynamic modules. @xref{Writing Lisp
+Primitives,,,internals,@value{emacs} Internals Manual}, for details on how to
+declare functions. You should familiarize yourself with the
+instructions there. The format of the function declaration is identical
+in modules.
+
+ Normal Lisp primitives document the functions they defining by including
+the documentation as a C comment. During the build process, a program
+called @file{make-docfile} is run, which will extract all of these
+comments, build up a single large documentation file, and will store
+pointers to the start of each documentation entry in the dumped @value{emacs}.
+This, of course, will not work for dynamic modules, as they are loaded
+long after @value{emacs} has been dumped. For this reason, we require a
+special means for adding documentation for new subrs. This is what the
+macro @code{CDOCSUBR} is used for, and this is used extensively during
+@code{ellcc} initialization mode.
+
+ When using @code{DEFUN} in normal @value{emacs} C code, the sixth
+``parameter'' is a C comment which documents the function. For a
+dynamic module, we of course need to convert the C comment to a usable
+string, and we need to set the documentation pointer of the subr to this
+string. As a module programmer, you don't actually need to do any work
+for this to happen. It is all taken care of in the
+@code{docs_of_module} function created by @code{ellcc}.
+
+@menu
+* Using DEFUN:: Using the DEFUN macro to define functions
+* Declaring Functions:: Declaring functions to the Lisp reader
+@end menu
+
+@node Using DEFUN, Declaring Functions, Defining Functions, Defining Functions
+@section Using @code{DEFUN}
+@cindex subrs
+@findex DEFUN
+@cindex functions, Lisp
+@cindex functions, defining
+
+ Although the full syntax of a function declaration is discussed in the
+@value{emacs} internals manual in greater depth, what follows is a brief
+description of how to define and implement a new Lisp primitive in a
+module. This is done using the @code{DEFUN} macro. Here is a small
+example:
+
+@example
+@cartouche
+DEFUN ("my-function", Fmy_function, 1, 1, "FFile name: ", /*
+Sample Emacs primitive function.
+
+The specified FILE is frobnicated before it is fnozzled.
+*/
+ (file))
+@{
+ char *filename;
+
+ if (NILP(file))
+ return Qnil;
+
+ filename = (char *)XSTRING_DATA(file);
+ frob(filename);
+ return Qt;
+@}
+@end cartouche
+@end example
+
+The first argument is the name of the function as it will appear to the
+Lisp reader. This must be provided as a string. The second argument is
+the name of the actual C function that will be created. This is
+typically the Lisp function name with a preceding capital @code{F}, with
+hyphens converted to underscores. This must be a valid C function
+name. Next come the minimum and maximum number of arguments,
+respectively. This is used to ensure that the correct number of
+arguments are passed to the function. Next is the @code{interactive}
+definition. If this function is meant to be run by a user
+interactively, then you need to specify the argument types and prompts
+in this string. Please consult the @value{emacs} Lisp manual for more
+details. Next comes a C comment that is the documentation for this
+function. This comment @strong{must} exist. Last comes the list of
+function argument names, if any.
+
+@node Declaring Functions, , Using DEFUN, Defining Functions
+@section Declaring Functions
+@findex DEFSUBR
+@cindex functions, declaring
+
+Simply writing the code for a function is not enough to make it
+available to the Lisp reader. You have to, during module
+initialization, let the Lisp reader know about the new function. This
+is done by calling @code{DEFSUBR} with the name of the function. This
+is the sole purpose of the initialization function
+@code{syms_of_module}. @xref{Required Functions}, for more details.
+
+Each call to @code{DEFSUBR} takes as its only argument the name of the
+function, which is the same as the second argument to the call to
+@code{DEFUN}. Using the example function above, you would insert the
+following code in the @code{syms_of_module} function:
+
+@example
+@cartouche
+DEFSUBR(Fmy_function);
+@end cartouche
+@end example
+
+This call will instruct @value{emacs} to make the function visible to the Lisp
+reader and will prepare for the insertion of the documentation into
+the right place. Once this is done, the user can call the Lisp
+function @code{my-function}, if it was defined as an interactive
+function (which in this case it was).
+
+Thats all there is to defining and announcing new functions. The rules
+for what goes inside the functions, and how to write good modules, is
+beyond the scope of this document. Please consult the @value{emacs}
+internals manual for more details.
+
+@node Defining Variables, Index, Defining Functions, Top
+@chapter Defining Variables
+@cindex defining variables
+@cindex defining objects
+@findex DEFVAR_LISP
+@findex DEFVAR_BOOL
+@findex DEFVAR_INT
+@cindex variables, Lisp
+@cindex variables, defining
+@cindex objects, defining
+@cindex objects, Lisp
+
+ Rarely will you write a module that only contains functions. It is
+common to also provide variables which can be used to control the
+behavior of the function, or store the results of the function being
+executed. The actual C variable types are the same for modules
+and internal @value{emacs} primitives, and the declaration of the variables
+is identical.
+
+ @xref{Adding Global Lisp Variables,,,internals,XEmacs Internals Manual},
+for more information on variables and naming conventions.
+
+ Once your variables are defined, you need to initialize them and make
+the Lisp reader aware of them. This is done in the
+@code{vars_of_module} initialization function using special @value{emacs}
+macros such as @code{DEFVAR_LISP}, @code{DEFVAR_BOOL}, @code{DEFVAR_INT}
+etc. The best way to see how to use these macros is to look at existing
+source code, or read the internals manual.
+
+ One @emph{very} important difference between @value{emacs} variables and
+module variables is how you use pure space. Simply put, you
+@strong{never} use pure space in @value{emacs} modules. The pure space
+storage is of a limited size, and is initialized properly during the
+dumping of @value{emacs}. Because variables are being added dynamically to
+an already running @value{emacs} when you load a module, you cannot use pure
+space. Be warned: @strong{do not use pure space in modules. Repeat, do
+not use pure space in modules.} Once again, to remove all doubts:
+@strong{DO NOT USE PURE SPACE IN MODULES!!!}
+
+ Below is a small example which declares and initializes two
+variables. You will note that this code takes into account the fact
+that this module may very well be compiled into @value{emacs} itself. This
+is a prudent thing to do.
+
+@example
+@cartouche
+Lisp_Object Vsample_string;
+int sample_boolean;
+
+void
+vars_of_module()
+@{
+ DEFVAR_LISP ("sample-string", &Vsample_string /*
+This is a sample string, declared in a module.
+
+Nothing magical about it.
+*/);
+
+ DEFVAR_BOOL("sample-boolean", &sample_boolean /*
+*Sample user-settable boolean.
+*/);
+
+ sample_boolean = 0;
+ Vsample_string = build_string("My string");
+@}
+@end cartouche
+@end example
+
+@c Print the tables of contents
+@contents
+@c That's all
+
+@node Index, , Defining Variables, Top
+@unnumbered Index
+
+@printindex cp
+
+@bye
+
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