Preparing Lisp code for distribution
************************************

Emacs provides a standard way to distribute Emacs Lisp code to users.  A package is a collection of one or more files, formatted and bundled in such a way that users can easily download, install, uninstall, and upgrade it.

The following sections describe how to create a package, and how to put it in a package archive for others to download. See Packages (The GNU Emacs Manual), for a description of user-level features of the packaging system.

These sections are mostly directed towards package archive maintainers---much of this information is not relevant for package authors (i.e., people who write code that will be distributed via these archives).

Packaging Basics
================

A package is either a simple package or a multi-file package.  A simple package is stored in a package archive as a single Emacs Lisp file, while a multi-file package is stored as a tar file (containing multiple Lisp files, and possibly non-Lisp files such as a manual).

In ordinary usage, the difference between simple packages and multi-file packages is relatively unimportant; the Package Menu interface makes no distinction between them.  However, the procedure for creating them differs, as explained in the following sections.

Each package (whether simple or multi-file) has certain attributes:

Name
     A short word (e.g., auctex).  This is usually also the symbol prefix used in the program (see Coding Conventions).

Version
     A version number, in a form that the function version-to-list understands (e.g., 11.86).  Each release of a package should be accompanied by an increase in the version number so that it will be recognized as an upgrade by users querying the package archive.

Brief description
     This is shown when the package is listed in the Package Menu.  It should occupy a single line, ideally in 36 characters or less.

Long description
     This is shown in the buffer created by C-h P (describe-package), following the package's brief description and installation status.  It normally spans multiple lines, and should fully describe the package's capabilities and how to begin using it once it is installed.

Dependencies
     A list of other packages (possibly including minimal acceptable version numbers) on which this package depends.  The list may be empty, meaning this package has no dependencies.  Otherwise, installing this package also automatically installs its dependencies, recursively; if any dependency cannot be found, the package cannot be installed.

Installing a package, either via the command package-install-file, or via the Package Menu, creates a subdirectory of package-user-dir named NAME-VERSION, where NAME is the package's name and VERSION its version (e.g., ~/.emacs.d/elpa/auctex-11.86/).  We call this the package's content directory.  It is where Emacs puts the package's contents (the single Lisp file for a simple package, or the files extracted from a multi-file package).

Emacs then searches every Lisp file in the content directory for autoload magic comments (see Autoload).  These autoload definitions are saved to a file named NAME-autoloads.el in the content directory.  They are typically used to autoload the principal user commands defined in the package, but they can also perform other tasks, such as adding an element to auto-mode-alist (see Auto Major Mode).  Note that a package typically does not autoload every function and variable defined within it---only the handful of commands typically called to begin using the package.  Emacs then byte-compiles every Lisp file in the package.

After installation, the installed package is loaded: Emacs adds the package's content directory to load-path, and evaluates the autoload definitions in NAME-autoloads.el.

Whenever Emacs starts up, it automatically calls the function package-activate-all to make installed packages available to the current session.  This is done after loading the early init file, but before loading the regular init file (see Startup Summary). Packages are not automatically made available if the user option package-enable-at-startup is set to nil in the early init file.

 -- Function: package-activate-all
     This function makes the packages available to the current session. The user option package-load-list specifies which packages to make available; by default, all installed packages are made available. See Package Installation (The GNU Emacs Manual).

     In most cases, you should not need to call package-activate-all, as this is done automatically during startup.  Simply make sure to put any code that should run before package-activate-all in the early init file, and any code that should run after it in the primary init file (see Init File (The GNU Emacs Manual)).

 -- Command: package-initialize &optional no-activate
     This function initializes Emacs's internal record of which packages are installed, and then calls package-activate-all.

     The optional argument NO-ACTIVATE, if non-nil, causes Emacs to update its record of installed packages without actually making them available.

Simple Packages
===============

A simple package consists of a single Emacs Lisp source file.  The file must conform to the Emacs Lisp library header conventions (see Library Headers).  The package's attributes are taken from the various headers, as illustrated by the following example:

     ;;; superfrobnicator.el --- Frobnicate and bifurcate flanges  -*- lexical-binding:t -*-

     ;; Copyright (C) 2022, 2025 Free Software Foundation, Inc.

     ;; Author: J. R. Hacker <jrh@example.com>
     ;; Version: 1.3
     ;; Package-Requires: ((flange "1.0"))
     ;; Keywords: multimedia, hypermedia
     ;; URL: https://example.com/jrhacker/superfrobnicate

     ...

     ;;; Commentary:

     ;; This package provides a minor mode to frobnicate and/or
     ;; bifurcate any flanges you desire.  To activate it, just type
     ...

     ;;;###autoload
     (define-minor-mode superfrobnicator-mode
     ...

The name of the package is the same as the base name of the file, as written on the first line.  Here, it is superfrobnicator.

The brief description is also taken from the first line.  Here, it is Frobnicate and bifurcate flanges.

The version number comes from the Package-Version header, if it exists, or from the Version header otherwise.  One or the other must be present.  Here, the version number is 1.3.

If the file has a ;;; Commentary: section, this section is used as the long description.  (When displaying the description, Emacs omits the ;;; Commentary: line, as well as the leading comment characters in the commentary itself.)

If the file has a Package-Requires header, that is used as the package dependencies.  In the above example, the package depends on the flange package, version 1.0 or higher.  See Library Headers, for a description of the Package-Requires header.  To depend on a specific version of Emacs, specify emacs as the package name.  If the header is omitted, the package has no dependencies.

The Keywords and URL headers are optional, but recommended. The command describe-package uses these to add links to its output.  The Keywords header should contain at least one standard keyword from the finder-known-keywords list.

The file ought to also contain one or more autoload magic comments, as explained in Packaging Basics.  In the above example, a magic comment autoloads superfrobnicator-mode.

See Package Archives, for an explanation of how to add a single-file package to a package archive.

Multi-file Packages
===================

A multi-file package is less convenient to create than a single-file package, but it offers more features: it can include multiple Emacs Lisp files, an Info manual, and other file types (such as images).

Prior to installation, a multi-file package is stored in a package archive as a tar file.  The tar file must be named NAME-VERSION.tar, where NAME is the package name and VERSION is the version number.  Its contents, once extracted, must all appear in a directory named NAME-VERSION, the content directory (see Packaging Basics).  Files may also extract into subdirectories of the content directory.

One of the files in the content directory must be named NAME-pkg.el.  It must contain a single Lisp form, consisting of a call to the function define-package, described below.  This defines the package's attributes: version, brief description, and requirements.

For example, if we distribute version 1.3 of the superfrobnicator as a multi-file package, the tar file would be superfrobnicator-1.3.tar.  Its contents would extract into the directory superfrobnicator-1.3, and one of these would be the file superfrobnicator-pkg.el.

 -- Function: define-package name version &optional docstring requirements
     This function defines a package.  NAME is the package name, a string.  VERSION is the version, as a string of a form that can be understood by the function version-to-list.  DOCSTRING is the brief description.

     REQUIREMENTS is a list of required packages and their versions. Each element in this list should have the form (DEP-NAME DEP-VERSION), where DEP-NAME is a symbol whose name is the dependency's package name, and DEP-VERSION is the dependency's version (a string).  The special value emacs means that the package depends on the given version of Emacs.

If the content directory contains a file named README, this file is used as the long description (overriding any ;;; Commentary: section).

If the content directory contains a file named dir, this is assumed to be an Info directory file made with install-info. See Invoking install-info (Texinfo).  The relevant Info files should also be present in the content directory.  In this case, Emacs will automatically add the content directory to Info-directory-list when the package is activated.

Do not include any .elc files in the package.  Those are created when the package is installed.  Note that there is no way to control the order in which files are byte-compiled.

Do not include any file named NAME-autoloads.el.  This file is reserved for the package's autoload definitions (see Packaging Basics).  It is created automatically when the package is installed, by searching all the Lisp files in the package for autoload magic comments.

If the multi-file package contains auxiliary data files (such as images), the package's Lisp code can refer to these files via the variable load-file-name (see Loading).  Here is an example:

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

     (defun superfrobnicator-fetch-image (file)
       (expand-file-name file superfrobnicator-base))

If your package contains files that you don't wish to distribute to users (e.g. regression tests), you can add them to an .elpaignore file.  In this file, each line lists a file or a wildcard matching files; those files should be ignored when producing your package's tarball on ELPA (see Package Archives).  (ELPA will pass this file to the tar command via the -X command-line option, when it prepares the package for download.)

Creating and Maintaining Package Archives
=========================================

Via the Package Menu, users may download packages from package archives.  Such archives are specified by the variable package-archives, whose default value lists the archives hosted on GNU ELPA (https://elpa.gnu.org) and NonGNU ELPA (https://elpa.nongnu.org).  This section describes how to set up and maintain a package archive.

A package archive is simply a directory in which the package files, and associated files, are stored.  If you want the archive to be reachable via HTTP, this directory must be accessible to a web server; see Archive Web Server.

A convenient way to set up and update a package archive is via the package-x library.  This is included with Emacs, but not loaded by default; type M-x load-library RET package-x RET to load it, or add (require 'package-x) to your init file. See Lisp Libraries (The GNU Emacs Manual).

After you create an archive, remember that it is not accessible in the Package Menu interface unless it is in package-archives.

Maintaining a public package archive entails a degree of responsibility. When Emacs users install packages from your archive, those packages can cause Emacs to run arbitrary code with the permissions of the installing user.  (This is true for Emacs code in general, not just for packages.)  So you should ensure that your archive is well-maintained and keep the hosting system secure.

One way to increase the security of your packages is to sign them using a cryptographic key.  If you have generated a private/public gpg key pair, you can use gpg to sign the package like this:

     gpg -ba -o FILE.sig FILE

For a single-file package, FILE is the package Lisp file; for a multi-file package, it is the package tar file. You can also sign the archive's contents file in the same way. Make the .sig files available in the same location as the packages. You should also make your public key available for people to download; e.g., by uploading it to a key server such as https://pgp.mit.edu/. When people install packages from your archive, they can use your public key to verify the signatures.

A full explanation of these matters is outside the scope of this manual.  For more information on cryptographic keys and signing, see GnuPG (The GNU Privacy Guard Manual).  Emacs comes with an interface to GNU Privacy Guard, see EasyPG (Emacs EasyPG Assistant Manual).

Interfacing to an archive web server
====================================

A web server providing access to a package archive must support the following queries:

archive-contents
     Return a lisp form describing the archive contents.  The form is a list of 'package-desc' structures (see package.el), except the first element of the list is the archive version.

<package name>-readme.txt
     Return the long description of the package.

<file name>.sig
     Return the signature for the file.

<file name>
     Return the file.  This will be the tarball for a multi-file package, or the single file for a simple package.

Supporting older versions of Emacs
==================================

Packages that wish to support older releases of Emacs, without giving up on newer functionality from recent Emacs releases, one can make use of the Compat package on GNU ELPA.  By depending on the package, Emacs can provide compatibility definitions for missing functionality.

The versioning of Compat follows that of Emacs, so next to the oldest version that a package relies on (via the emacs-package), one can also indicate what the newest version of Emacs is, that a package wishes to use definitions from:

     ;; Package-Requires: ((emacs "27.2") (compat "29.1"))

Note that Compat provides replacement functions with extended functionality for functions that are already defined (sort, assoc, ...).  These functions may have changed their calling convention (additional optional arguments) or may have changed their behavior.  These functions must be looked up explicitly with compat-function or called explicitly with compat-call. We call them Extended Definitions.  In contrast, newly Added Definitions can be called as usual.

 -- Macro: compat-call fun &rest args
     This macro calls the compatibility function FUN with ARGS. Many functions provided by Compat can be called directly without this macro.  However in the case where Compat provides an alternative version of an existing function, the function call has to go through compat-call.

 -- Macro: compat-function fun
     This macro returns the compatibility function symbol for FUN. See compat-call for a more convenient macro to directly call compatibility functions.

For further details on how to make use of the package, see Usage ("Compat" Manual).  In case you don't have the manual installed, you can also read the Online Compat manual (https://elpa.gnu.org/packages/doc/compat.html#Usage).