E.1 Building Emacs

This section explains the steps involved in building the Emacs executable. You don’t have to know this material to build and install Emacs, since the makefiles do all these things automatically. This information is pertinent to Emacs developers.

Compilation of the C source files in the src directory produces an executable file called temacs, also called a bare impure Emacs. It contains the Emacs Lisp interpreter and I/O routines, but not the editing commands.

The command temacs -l loadup would run temacs and direct it to load loadup.el. The loadup library loads additional Lisp libraries, which set up the normal Emacs editing environment. After this step, the Emacs executable is no longer bare.

Because it takes some time to load the standard Lisp files, the temacs executable usually isn’t run directly by users. Instead, as one of the last steps of building Emacs, the command ‘temacs -batch -l loadup dump’ is run. The special ‘dump’ argument causes temacs to dump out an executable program, called emacs, which has all the standard Lisp files preloaded. (The ‘-batch’ argument prevents temacs from trying to initialize any of its data on the terminal, so that the tables of terminal information are empty in the dumped Emacs.)

The dumped emacs executable (also called a pure Emacs) is the one which is installed. The variable preloaded-file-list stores a list of the Lisp files preloaded into the dumped Emacs. If you port Emacs to a new operating system, and are not able to implement dumping, then Emacs must load loadup.el each time it starts.

You can specify additional files to preload by writing a library named site-load.el that loads them. You may need to rebuild Emacs with an added definition

#define SITELOAD_PURESIZE_EXTRA n

to make n added bytes of pure space to hold the additional files; see src/puresize.h. (Try adding increments of 20000 until it is big enough.) However, the advantage of preloading additional files decreases as machines get faster. On modern machines, it is usually not advisable.

After loadup.el reads site-load.el, it finds the documentation strings for primitive and preloaded functions (and variables) in the file etc/DOC where they are stored, by calling Snarf-documentation (see Accessing Documentation).

You can specify other Lisp expressions to execute just before dumping by putting them in a library named site-init.el. This file is executed after the documentation strings are found.

If you want to preload function or variable definitions, there are three ways you can do this and make their documentation strings accessible when you subsequently run Emacs:

• Arrange to scan these files when producing the etc/DOC file, and load them with site-load.el.
• Load the files with site-init.el, then copy the files into the installation directory for Lisp files when you install Emacs.
• Specify a nil value for byte-compile-dynamic-docstrings as a local variable in each of these files, and load them with either site-load.el or site-init.el. (This method has the drawback that the documentation strings take up space in Emacs all the time.)

It is not advisable to put anything in site-load.el or site-init.el that would alter any of the features that users expect in an ordinary unmodified Emacs. If you feel you must override normal features for your site, do it with default.el, so that users can override your changes if they wish. See Startup Summary. Note that if either site-load.el or site-init.el changes load-path, the changes will be lost after dumping. See Library Search. To make a permanent change to load-path, use the --enable-locallisppath option of configure.

In a package that can be preloaded, it is sometimes necessary (or useful) to delay certain evaluations until Emacs subsequently starts up. The vast majority of such cases relate to the values of customizable variables. For example, tutorial-directory is a variable defined in startup.el, which is preloaded. The default value is set based on data-directory. The variable needs to access the value of data-directory when Emacs starts, not when it is dumped, because the Emacs executable has probably been installed in a different location since it was dumped.

Function: custom-initialize-delay symbol value

This function delays the initialization of symbol to the next Emacs start. You normally use this function by specifying it as the :initialize property of a customizable variable. (The argument value is unused, and is provided only for compatibility with the form Custom expects.)

In the unlikely event that you need a more general functionality than custom-initialize-delay provides, you can use before-init-hook (see Startup Summary).

Function: dump-emacs to-file from-file

This function dumps the current state of Emacs into an executable file to-file. It takes symbols from from-file (this is normally the executable file temacs).

If you want to use this function in an Emacs that was already dumped, you must run Emacs with ‘-batch’.