\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

