emacs/doc/lispref/package.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 2010-2011  Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@setfilename ../../info/package
@node Packaging, Antinews, System Interface, Top
@chapter Preparing Lisp code for distribution
@cindex packaging

  Emacs provides a standard way for Emacs Lisp code to be distributed
to users.  This approach lets users easily download, install,
uninstall, and upgrade Lisp code that they might want to use.

  A @dfn{package} is simply one or more files, formatted and bundled
in a particular way.  Typically a package includes primarily Emacs
Lisp code, but it is possible to create other kinds of packages as
well.

@menu
* Packaging Basics::        The basic concepts of Emacs Lisp packages.
* Simple Packages::         How to package a single .el file.
* Multi-file Packages::     How to package multiple files.
@end menu

@node Packaging Basics
@section Packaging Basics
@cindex packaging basics

  A package has a few attributes:
@cindex package attributes

@table @asis
@item Name
A string, the name of the package.  This attribute is mandatory.  If
it does not exist, the package cannot be installed by the package
manager.

@item Version
A version number, which is anything that can be parsed by
@code{version-to-list}.  This attribute is mandatory.  If it does not
exist, the package cannot be installed by the package manager.

@item Brief description
This is shown to the user in the package menu buffer.  It is just a
single line.  On a terminal with 80 characters per line, there are
only 36 characters available in the package menu mode for showing the
brief description, so it is best to keep it very brief.  If no brief
name is given, an empty string is used.

@item Long description
This can be a @file{README} file or the like.  This is available to
the user before the package is installed, via the package menu.  It
should more fully describe the package and its capabilities, so a user
can read it to decide whether he wants to install the package.  This
attribute is optional.

@item Dependencies
This is a list of other packages and their minimal acceptable
versions.  This is used both at download time (to make sure all the
needed code is available) and at activation time (to ensure a package
is only activated if all its dependencies have been successfully
activated).  This attribute is optional.

@item Manual
A package can optionally include an Info manual.
@end table

  Conceptually, a package goes through several state transitions (in
reality some of these transitions are grouped together):

@table @asis
@item Download
Fetch the package from somewhere.

@item Install
Unpack the package, or write a @file{.el} file into the appropriate
install directory.  This step also includes extracting autoloads and
byte-compiling the Emacs Lisp code.

@item Activate
Update @code{load-path} and @code{Info-directory-list} and evaluate
the autoloads, so that the package is ready for the user to use.
@end table

  It is best for users if packages do not do too much work at
activation time.  The best approach is to have activation consist of
some autoloads and little more.

@node Simple Packages
@section Simple Packages
@cindex single file packages

  The simplest package consists of a single Emacs Lisp source file.
In this case, all the attributes of the package (@pxref{Packaging
Basics}) are taken from this file.

  The package system expects this @file{.el} file to conform to the
Emacs Lisp library header conventions.  @xref{Library Headers}.

  The name of the package is the same as the base name of the
@file{.el} file, as written in the first comment line.  For example,
given the header line:

@smallexample
;;; superfrobnicator.el --- frobnicate and bifurcate flanges
@end smallexample

the package name will be @samp{superfrobnicator}.

  The short description of the package is also taken from the first
line of the file.

  If the file has a ``Commentary'' header, then it is used as the long
description.

  The version of the package comes either from the ``Package-Version''
header, if it exists, or from the ``Version'' header.  A package is
required to have a version number.  Each release of a package must be
accompanied by an increase in the version number.

  If the file has a ``Package-Requires'' header, then that is used as
the package dependencies.  Otherwise, the package is assumed not to
have any dependencies.

  A single-file package cannot have an Info manual.

  The file will be scanned for autoload cookies at install time.
@xref{Autoload}.

@node Multi-file Packages
@section Multi-file Packages
@cindex multi-file packages

  A multi-file package is just a @file{.tar} file.  While less
convenient to create than a single-file package, a multi-file package
also offers more features: it can include an Info manual, multiple
Emacs Lisp files, and also other data files needed by a package.

  The contents of the @file{.tar} file must all appear beneath a
single directory, named after the package and version.  Files can
appear in subdirectories of this top-most directory, but Emacs Lisp
code will only be found (and thus byte-compiled) at the top-most
level.  Also, the @file{.tar} file is typically also given this same
name.  For example, if you are distributing version 1.3 of the
superfrobnicator, the package file would be named
``superfrobnicator-1.3.tar'' and the contents would all appear in the
directory @file{superfrobnicator-1.3} in that @file{.tar}.

  The package must include a @file{-pkg.el} file, named after the
package.  In our example above, this file would be called
@file{superfrobnicator-pkg.el}.  This file must have a single form in
it, a call to @code{define-package}.  The package dependencies and
brief description are taken from this form.

@defun define-package name version &optional docstring requirements
Define a package.  @var{name} is the name of the package, a string.
@var{version} is the package's version, a string.  It must be in a
form that can be understood by @code{version-to-list}.
@var{docstring} is the short description of the package.
@var{requirements} is a list of required packages and their versions.
@end defun

  If a @file{README} file exists in the content directory, then it is
used as the long description.

  If the package has an Info manual, you should distribute the needed
info files, plus a @file{dir} file made with @command{install-info}.
@xref{Invoking install-info, Invoking install-info, Invoking
install-info, texinfo, Texinfo}.

  Do not include any @file{.elc} files in the package.  Those will be
created at install time.  Note that there is no way to control the
order in which files are byte-compiled; your package must be robust
here.

  The installation process will scan all the @file{.el} files in the
package for autoload cookies.  @xref{Autoload}.  They are extracted
into a @file{-autoloads.el} file (e.g.,
@file{superfrobnicator-autoloads.el}), so do not include a file of
that name in your package.

  Any other files in the @file{.tar} file are simply unpacked when the
package is installed.  This can be useful if your package needs
auxiliary data files --- e.g., icons or sounds.

  Emacs Lisp code installed via the package manager must take special
care to be location-independent.  One easy way to do this is to make
references to auxiliary data files relative to @var{load-file-name}.
For example:

@smallexample
(defconst superfrobnicator-base (file-name-directory load-file-name))

(defun superfrobnicator-fetch-image (file)
  (expand-file-name file superfrobnicator-base))
@end smallexample