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### 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.

Building Emacs requires GNU Make version 3.81 or later.

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`’.

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