Project:GNU Emacs/Developer guide

Summary

This document describes how GNU Emacs and associated packages are made available in Gentoo. These guidelines also describe the philosophy how the packages are maintained.

What is Emacs?

Emacs is the extensible, customizable, self-documenting real-time display editor. Large parts are written in the Lisp dialect Emacs Lisp, with which extensions can be easily developed. Apart from its editing features, GNU Emacs provides a whole environment to manage your system, mail, IRC chats and texts (be it correspondence or programming).

Maintaining the editor itself

Available versions

Currently the following Emacs versions are available:

Package	Version	Type	SLOT	eselect name	Remarks
app-editors/emacs	18.59	released	18	emacs-18	Not covered by virtual/emacs, for quick editing purposes
app-editors/emacs	23.4	released	23	emacs-23
app-editors/emacs	24.5	released	24	emacs-24
app-editors/emacs	25.3	released	25	emacs-25
app-editors/emacs	26.1	released	26	emacs-26
app-editors/emacs-vcs	26.1.9999	Git (emacs-26 branch)	26	emacs-26-vcs
app-editors/emacs-vcs	27.0.9999	Git (master branch)	27	emacs-27-vcs

Locations of files

The following files are installed in different locations or under a different name (as compared to vanilla GNU Emacs):

Emacs executable: /usr/bin/emacs-suffix, where suffix is normally equal to the Emacs major version
Auxiliary programs (e.g., ctags): /usr/bin/ctags-emacs-suffix
Man pages are named accordingly
Info files of Emacs are installed in directory /usr/share/info/emacs-suffix/
Game score files are placed in directory /var/games/emacs/

The programs are symlinked to their original names by the Emacs eselect module, apart from ctags and etags which have their own modules.

USE flags

Emacs has many USE flags, most are easy to understand what they are good for, others have some hidden “features” one should know about. The slot for version 18 has no USE flags at all, so if nothing else is noted, version 23 onwards is described.

Flag	Description	Introduced in version	Notes
acl	Include support for Access Control Lists.	24.4
alsa	Determine if ALSA should be used.		Emacs autodetects ALSA. That test is removed by the ebuild if the flag is not set. This is for cases where ALSA is installed but the user does not wish support for it in Emacs.
aqua	Include support for the Aqua/Cocoa GUI.		Only available under Prefix on Max OS X.
athena	Use the MIT Athena widget set as windowing toolkit.
cairo	Compile with Cairo drawing.	25	Experimental, currently package.use.masked.
dbus	Make Emacs D-Bus aware.	23
dynamic-loading	Loading of dynamic libraries aka emacs modules, using dlopen at runtime.	25
games	Support shared score files for games.		This flag is passed on to app-emacs/emacs-common-gentoo which will create the `games` user and install score files.
gconf	Use `gconf` to read the system font name.	23
gfile	Use gfile (glib) for file notification.	24.4	This takes precedence over the inotify flag. Note that glib file notification on Linux uses inotify internally.
gif	Support for GIF images.
gmp	Link against dev-libs/gmp, instead of using the internal mini-gmp lib.	27	Enabled by IUSE default.
gpm	Support for console-based mouse driver.	23
gsettings	Use gsettings (glib) to read the system font name.	24
gtk	Use the GIMP Toolkit (GTK+) as windowing toolkit (menu bar etc.)		When this toolkit is activated along with alternative ones (see other USE flags), GTK+ is chosen. This is in sync with upstream’s wishes.
gtk2	Prefer GTK+ version 2 to version 3.	24
gzip-el	Zip up all `el` files.		The zip binary is autodetected. So even when this USE flag is disabled but the binary is found, all `el` files will be compressed. The ebuild takes care of that by confusing the configure script.
imagemagick	Use the imagemagick toolset for enhanced image manipulation.	24
inotify	Enable inotify file change notification support.	24.4
jpeg	Support for JPEG images.
json	Compile with native JSON support (dev-libs/jansson).	27
kerberos	Support for the Kerberos network authentication protocol.
lcms	Support for the Little CMS color management engine.	26
libxml2	Parse XML with libxml2 instead of built-in Elisp functions which gives a substantial speed-up.	24
livecd	Force the /usr/bin/emacs symlink to be set during livecd building.		Internal use, not intended to be set by users.
m17n-lib	Use the m17n-lib multilingual library for complex text layout, e.g. for Indic scripts.	23	Only available if `xft` is enabled too.
mailutils	Rely on GNU Mailutils for movemail support.	26
motif	Use Motif as windowing toolkit.
png	Support for PNG images.
selinux	Support the SELinux security extensions.	24
sound	Control the availability of sound support.
source	Install the C source files and make them available in the internal documentation system of GNU Emacs.
ssl	Support SSL/TLS for secure session initiation.	24
svg	Support for SVG images.	23
systemd	Compile with libsystemd support.	26
threads	Add elisp threading support.	26
tiff	Support for TIFF images.
toolkit-scroll-bars	Instead of the internal scroll bars, the ones from the windowing toolkit are used.		You will lose some functionality (split windows by clicking on the scroll bar for example).
wide-int	Prefer wide Emacs integers, typically 62-bit.	24	This option has an effect only on architectures where `long` and `long long` types have different size.
X	Let Emacs use an X session if available. Text mode can always be forced.
Xaw3d	Use the 3D Athena widget set as windowing toolkit.
xft	Choose an alternative font renderer.	23
xpm	Support for XPM images.		If disabled, all logos, icons etc. in Emacs are displayed in grayscale. Enabled by IUSE default.
xwidgets	Enable use of some GTK+ widgets in Emacs buffers.	25	This flag has an effect only when GTK+ is selected as the windowing toolkit.
zlib	Compile with zlib decompression support.	24.4

ChangeLog files

ChangeLog* files for app-editors/emacs are not being installed, because their combined size would be several tens of megabytes (e.g., 23 MB for Emacs 24.4).

Packages

Depending on a specific Emacs version

Note
The documentation of the functions provided is to be found in the eclasses itself.

A minimum version of GNU Emacs required by a package can be specified by assigning NEED_EMACS=version_number before inheriting elisp.eclass. The given version number must be available as an ebuild in virtual/emacs. Without such an assignment, the package will by default depend on >=virtual/emacs-23. Packages that have optional support for GNU Emacs (via USE flags) can check for a minimum version of GNU Emacs at build time with the elisp-need-emacs() function.

The site-lisp directory and package loading

The regular location for Emacs lisp packages in Gentoo is /usr/share/emacs/site-lisp/package/. All Elisp files (.el) and compiled Elisp files (.elc) should go there.

Emacs packages normally need to be activated or loaded when a certain condition is met (like c-mode for C source files).

In Gentoo every package has a site initialisation file that holds the needed commands. The file is located in ${FILESDIR} and starts with a two-digit number, followed by the package name and -gentoo. The elisp-install() function puts this file in the directory /usr/share/emacs/site-lisp/site-gentoo.d/.

When calling elisp-site-regen() in an ebuild, the global site file /usr/share/emacs/site-lisp/site-gentoo.el is regenerated, which holds the contents of all individual site init files in one. They are read in alphabetical order, so the numbers determine which comes first, the lowest is to be found at the beginning. That means: Packages depending on each other need to have rising order for site initialisation, too.

Formerly, all those initialisations were directly added to site-start.el, which is executed at Emacs start-up. Today there is another level of indirection, i.e. initialisations are in /usr/share/emacs/site-lisp/site-gentoo.el which can either be loaded from /etc/emacs/site-start.el (the default), or it can be individually loaded from users’ ~/.emacs files by adding a line like:

CODE

(require 'site-gentoo)

Important
Site initialisation means that the package should be ready for use, but not that the package should be imposed on all users by loading or even starting it at Emacs startup time. (Think of systems with multiple users.) A load command for site initialisations is only acceptable for a few packages. If used, it always loads the whole package and makes Emacs start-up really slow, so the autoload mechanism is the preferred way. The elisp-common eclass has functions to generate an autoload file if none is shipped with the package but the functionality is available in the source nonetheless. For more information about the mechanism see the Elisp manual. Also manual keybindings directly in the site-file are discouraged to not disturb the user as he could have bound exactly that keys himself. Keep pollution low but provide sane default settings out of the box so even a novice can start working fast.

Eclasses

Packages that have support for or rely on GNU Emacs can use two eclasses to do some recurring tasks in a simple way. The documentation of the functions are provided in the eclasses, so they are not repeated here! Format of documentation is to allow man-page generation from source with the package app-portage/eclass-manpages.

Name	Purpose
elisp-common.eclass	Provides functions to handle Elisp files. Compilation, installation and generation of autoloads is covered here in a general way. Packages with optional Emacs support must rely on this eclass.
elisp.eclass	The functions from elisp-common.eclass are used to construct the default functions (`src_unpack`, `src_compile` and friends) for packages in the `app-emacs` hierarchy of ebuilds. It pulls in an appropriate version of Emacs (meaning a version of virtual/emacs, controlled by the `NEED_EMACS` variable) and is not intended for packages with optional (via USE flag) support for GNU Emacs.

Emacs eselect module

Having several Emacs versions simultaneously installed on a system, needs some caution by maintainers. Usual pitfalls are file collisions and installations of one slot using data from another. As described earlier, the executables are suffixed with their corresponding version number. All data files go to similar directories, also distinguished by a version suffix.

To be independent of the installed version, the eselect module from app-eselect/eselect-emacs guarantees that /usr/bin/emacs always points to the Emacs you want. All ebuilds for the editor check if the symlink is set, and change it to the highest available in the case where it does not exist. If no GNU Emacs is found, but XEmacs, all helper programs are symlinked to the variants shipped with XEmacs.

The module file has some comments about how the code works. For more information how an eselect module is set up, consult the eselect developer guide.

Virtuals

Sometimes the same functionality is available through different packages. To not force a subjective choice the maintainer made, virtuals check if one of the alternatives is installed on the user’s system.

virtual/editor: Just makes sure you have an editing capability available on your system, Emacs is one choice out of many.
virtual/emacs: This gives you the choice between several version of Emacs. Elisp packages can choose which virtual version is the minimum they need through the elisp.eclass.
virtual/emacs-flim: There are several libraries that provide encoding functions for other packages. If they are compatible to app-emacs/flim, they should provide the emacs-flim virtual.

Where Emacs team is upstream

Not all packages maintained by the Emacs team are developed by people from outside the Gentoo project (they are usually called upstream). Most of those exceptions are for proper operation of GNU Emacs in the Gentoo environment.

Sources of these packages are kept in Git (see below). They are released and brought to the Emacs overlay or to the Gentoo repository when they have proven stable.

Package name	Git repository	Purpose
app-admin/emacs-updater	proj/emacs-tools	Ships the `emacs-updater` script, which makes the transition from the old location of the site-init files to the new one. Another purpose is to byte-compile all installed Emacs support files again, e.g. after a major upgrade of Emacs.
app-emacs/ebuild-mode	proj/ebuild-mode	A collection of Emacs major modes that help you edit ebuilds and other Gentoo specific files. This is developed in cooperation with the XEmacs team, so we share the same source.
app-emacs/emacs-common-gentoo	proj/emacs-tools	Installs common files needed by all GNU Emacs versions. These include subdirs.el and a default site-start.el file. With `USE=X` also desktop files (which provide file associations for many desktop environments) and icons for Emacs and Emacsclient are installed.
app-emacs/emacs-daemon	proj/emacs-tools	Contains the init script to start Emacs as a background service in server mode. The `emacsclient` executable then uses this to connect to.
app-eselect/eselect-ctags	proj/emacs-tools	There are several implementations of the `ctags` binary, all with a different feature set. This eselect module lets you choose the variant you need.
app-eselect/eselect-emacs	proj/emacs-tools	Setting the correct man page locations, Info documentation paths and target for /usr/bin/emacs, see the separate section.

A sample ebuild

We present an ebuild that introduces the canonical form regarding variable ordering in global scope and implementation along an example.

CODE

# Copyright 1999-2018 Gentoo Foundation
# Distributed under the terms of the GNU General Public License v2

EAPI=6

inherit elisp

DESCRIPTION="ReStructuredText support for Emacs"
HOMEPAGE="http://www.emacswiki.org/cgi-bin/wiki/reStructuredText"
SRC_URI="mirror://sourceforge/docutils/docutils-${PV}.tar.gz"

LICENSE="GPL-2+"
SLOT="0"
KEYWORDS="alpha amd64 arm hppa ia64 ppc ppc64 s390 sh sparc ~sparc-fbsd x86 ~x86-fbsd"

S="${WORKDIR}/docutils-${PV}/tools/editors/emacs"
DOCS="README.txt"
ELISP_PATCHES="${P}-lazy-lock-mode-fix.patch"
SITEFILE="50${PN}-gentoo.el"

The first lines from inherit to KEYWORDS are standard Gentoo ebuild variables and thus outside the scope of this text.

Resources

Resource	Comment
Various authors: Gentoo Development Guide.	Extensive reference about how development is organised in Gentoo, plus style advises for ebuild authors.
Robert J. Chassell: An Introduction to Programming in Emacs Lisp. Revised 3rd edition. GNU Press, ISBN 1-882114-43-3.	A primer on Elisp programming (very basic, but very good).
Richard M. Stallman: GNU Emacs Manual. 17th edition (updated for Emacs version 24.5). GNU Press, ISBN 978-0-9831592-4-7.	The official handbook of Emacs, also shipped with the editor. This covers the usage of Emacs, not the programming or deep internals.
Various authors: GNU Emacs Lisp Reference Manual.	All information needed for Elisp programming, dense and extensive, but not for beginners.
Craig A. Finseth: The Craft of Text Editing: Emacs for the Modern World. Springer-Verlag, New York 1991, ISBN 0-387-97616-7.	Background information about user interfaces and the ergonomics of text editing.