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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, one of the last steps of building Emacs runs the command ‘temacs -batch -l loadup --temacs=dump-method’. The special option --temacs tells temacs how to record all the standard preloaded Lisp functions and variables, so that when you subsequently run Emacs, it will start much faster. The --temacs option requires an argument dump-method, which can be one of the following:

‘pdump’​

Record the preloaded Lisp data in a dump file. This method produces an additional data file which Emacs will load at startup. The produced dump file is usually called emacs.pdmp, and is installed in the Emacs exec-directory (see Help Functions). This method is the most preferred one, as it does not require Emacs to employ any special techniques of memory allocation, which might get in the way of various memory-layout techniques used by modern systems to enhance security and privacy.

‘pbootstrap’​

Like ‘pdump’, but used while bootstrapping Emacs, when no previous Emacs binary and no *.elc byte-compiled Lisp files are available. The produced dump file is usually named bootstrap-emacs.pdmp in this case.

‘dump’​

This method causes temacs to dump out an executable program, called emacs, which has all the standard Lisp files already preloaded into it. (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.) This method is also known as unexec, because it produces a program file from a running process, and thus is in some sense the opposite of executing a program to start a process. Although this method was the way that Emacs traditionally saved its state, it is now deprecated.

‘bootstrap’​

Like ‘dump’, but used when bootstrapping Emacs with the unexec method.

The dumped emacs executable (also called a pure Emacs) is the one which is installed. If the portable dumper was used to build Emacs, the emacs executable is actually an exact copy of temacs, and the corresponding emacs.pdmp file is installed as well. The variable preloaded-file-list stores a list of the preloaded Lisp files recorded in the dump file or in the dumped Emacs executable. If you port Emacs to a new operating system, and are not able to implement dumping of any kind, then Emacs must load loadup.el each time it starts.

By default the dumped emacs executable records details such as the build time and host name. Use the --disable-build-details option of configure to suppress these details, so that building and installing Emacs twice from the same sources is more likely to result in identical copies of Emacs.

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-portable to-file \&optional track-referrers​

This function dumps the current state of Emacs into a dump file to-file, using the pdump method. Normally, the dump file is called emacs-name.dmp, where emacs-name is the name of the Emacs executable file. The optional argument track-referrers, if non-nil, causes the portable dumper to keep additional information to help track down the provenance of object types that are not yet supported by the pdump method.

Although the portable dumper code can run on many platforms, the dump files that it produces are not portable—they can be loaded only by the Emacs executable that dumped them.

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

function dump-emacs to-file from-file​

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

This function cannot be used in an Emacs that was already dumped. This function is deprecated, and by default Emacs is built without unexec support so this function is not available.

function pdumper-stats​

If the current Emacs session restored its state from a dump file, this function returns information about the dump file and the time it took to restore the Emacs state. The value is an alist ((dumped-with-pdumper . t) (load-time . time) (dump-file-name . file)), where file is the name of the dump file, and time is the time in seconds it took to restore the state from the dump file. If the current session was not restored from a dump file, the value is nil.