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-File: lispref.info, Node: How Programs Do Loading, Next: Autoload, Up: Loading
-
-How Programs Do Loading
-=======================
-
- XEmacs Lisp has several interfaces for loading. For example,
-`autoload' creates a placeholder object for a function in a file;
-trying to call the autoloading function loads the file to get the
-function's real definition (*note Autoload::). `require' loads a file
-if it isn't already loaded (*note Named Features::). Ultimately, all
-these facilities call the `load' function to do the work.
-
- - Function: load filename &optional missing-ok nomessage nosuffix
- This function finds and opens a file of Lisp code, evaluates all
- the forms in it, and closes the file.
-
- To find the file, `load' first looks for a file named
- `FILENAME.elc', that is, for a file whose name is FILENAME with
- `.elc' appended. If such a file exists, it is loaded. If there
- is no file by that name, then `load' looks for a file named
- `FILENAME.el'. If that file exists, it is loaded. Finally, if
- neither of those names is found, `load' looks for a file named
- FILENAME with nothing appended, and loads it if it exists. (The
- `load' function is not clever about looking at FILENAME. In the
- perverse case of a file named `foo.el.el', evaluation of `(load
- "foo.el")' will indeed find it.)
-
- If the optional argument NOSUFFIX is non-`nil', then the suffixes
- `.elc' and `.el' are not tried. In this case, you must specify
- the precise file name you want.
-
- If FILENAME is a relative file name, such as `foo' or
- `baz/foo.bar', `load' searches for the file using the variable
- `load-path'. It appends FILENAME to each of the directories
- listed in `load-path', and loads the first file it finds whose name
- matches. The current default directory is tried only if it is
- specified in `load-path', where `nil' stands for the default
- directory. `load' tries all three possible suffixes in the first
- directory in `load-path', then all three suffixes in the second
- directory, and so on.
-
- If you get a warning that `foo.elc' is older than `foo.el', it
- means you should consider recompiling `foo.el'. *Note Byte
- Compilation::.
-
- Messages like `Loading foo...' and `Loading foo...done' appear in
- the echo area during loading unless NOMESSAGE is non-`nil'.
-
- Any unhandled errors while loading a file terminate loading. If
- the load was done for the sake of `autoload', any function
- definitions made during the loading are undone.
-
- If `load' can't find the file to load, then normally it signals the
- error `file-error' (with `Cannot open load file FILENAME'). But
- if MISSING-OK is non-`nil', then `load' just returns `nil'.
-
- You can use the variable `load-read-function' to specify a function
- for `load' to use instead of `read' for reading expressions. See
- below.
-
- `load' returns `t' if the file loads successfully.
-
- - User Option: load-path
- The value of this variable is a list of directories to search when
- loading files with `load'. Each element is a string (which must be
- a directory name) or `nil' (which stands for the current working
- directory). The value of `load-path' is initialized from the
- environment variable `EMACSLOADPATH', if that exists; otherwise its
- default value is specified in `emacs/src/paths.h' when XEmacs is
- built.
-
- The syntax of `EMACSLOADPATH' is the same as used for `PATH'; `:'
- (or `;', according to the operating system) separates directory
- names, and `.' is used for the current default directory. Here is
- an example of how to set your `EMACSLOADPATH' variable from a
- `csh' `.login' file:
-
- setenv EMACSLOADPATH .:/user/bil/emacs:/usr/lib/emacs/lisp
-
- Here is how to set it using `sh':
-
- export EMACSLOADPATH
- EMACSLOADPATH=.:/user/bil/emacs:/usr/local/lib/emacs/lisp
-
- Here is an example of code you can place in a `.emacs' file to add
- several directories to the front of your default `load-path':
-
- (setq load-path
- (append (list nil "/user/bil/emacs"
- "/usr/local/lisplib"
- "~/emacs")
- load-path))
-
- In this example, the path searches the current working directory
- first, followed then by the `/user/bil/emacs' directory, the
- `/usr/local/lisplib' directory, and the `~/emacs' directory, which
- are then followed by the standard directories for Lisp code.
-
- The command line options `-l' or `-load' specify a Lisp library to
- load as part of Emacs startup. Since this file might be in the
- current directory, Emacs 18 temporarily adds the current directory
- to the front of `load-path' so the file can be found there. Newer
- Emacs versions also find such files in the current directory, but
- without altering `load-path'.
-
- Dumping Emacs uses a special value of `load-path'. If the value of
- `load-path' at the end of dumping is unchanged (that is, still the
- same special value), the dumped Emacs switches to the ordinary
- `load-path' value when it starts up, as described above. But if
- `load-path' has any other value at the end of dumping, that value
- is used for execution of the dumped Emacs also.
-
- Therefore, if you want to change `load-path' temporarily for
- loading a few libraries in `site-init.el' or `site-load.el', you
- should bind `load-path' locally with `let' around the calls to
- `load'.
-
- - Function: locate-file filename path-list &optional suffixes mode
- This function searches for a file in the same way that `load' does,
- and returns the file found (if any). (In fact, `load' uses this
- function to search through `load-path'.) It searches for FILENAME
- through PATH-LIST, expanded by one of the optional SUFFIXES
- (string of suffixes separated by `:'s), checking for access MODE
- (0|1|2|4 = exists|executable|writeable|readable), default readable.
-
- `locate-file' keeps hash tables of the directories it searches
- through, in order to speed things up. It tries valiantly to not
- get confused in the face of a changing and unpredictable
- environment, but can occasionally get tripped up. In this case,
- you will have to call `locate-file-clear-hashing' to get it back
- on track. See that function for details.
-
- - Function: locate-file-clear-hashing path
- This function clears the hash records for the specified list of
- directories. `locate-file' uses a hashing scheme to speed lookup,
- and will correctly track the following environmental changes:
-
- * changes of any sort to the list of directories to be searched.
-
- * addition and deletion of non-shadowing files (see below) from
- the directories in the list.
-
- * byte-compilation of a .el file into a .elc file.
-
- `locate-file' will primarily get confused if you add a file that
- shadows (i.e. has the same name as) another file further down in
- the directory list. In this case, you must call
- `locate-file-clear-hashing'.
-
- - Variable: load-in-progress
- This variable is non-`nil' if Emacs is in the process of loading a
- file, and it is `nil' otherwise.
-
- - Variable: load-read-function
- This variable specifies an alternate expression-reading function
- for `load' and `eval-region' to use instead of `read'. The
- function should accept one argument, just as `read' does.
-
- Normally, the variable's value is `nil', which means those
- functions should use `read'.
-
- - User Option: load-warn-when-source-newer
- This variable specifies whether `load' should check whether the
- source is newer than the binary. If this variable is true, then
- when a `.elc' file is being loaded and the corresponding `.el' is
- newer, a warning message will be printed. The default is `nil',
- but it is bound to `t' during the initial loadup.
-
- - User Option: load-warn-when-source-only
- This variable specifies whether `load' should warn when loading a
- `.el' file instead of an `.elc'. If this variable is true, then
- when `load' is called with a filename without an extension, and
- the `.elc' version doesn't exist but the `.el' version does, then
- a message will be printed. If an explicit extension is passed to
- `load', no warning will be printed. The default is `nil', but it
- is bound to `t' during the initial loadup.
-
- - User Option: load-ignore-elc-files
- This variable specifies whether `load' should ignore `.elc' files
- when a suffix is not given. This is normally used only to
- bootstrap the `.elc' files when building XEmacs, when you use the
- command `make all-elc'. (This forces the `.el' versions to be
- loaded in the process of compiling those same files, so that
- existing out-of-date `.elc' files do not make it mess things up.)
-
- To learn how `load' is used to build XEmacs, see *Note Building
-XEmacs::.
-