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24.2 Access to Documentation Strings

Function: documentation-property symbol property &optional verbatim
This function returns the documentation string that is recorded in symbol's property list under property property. It retrieves the text from a file if the value calls for that. If the property value isn't nil, isn't a string, and doesn't refer to text in a file, then it is evaluated to obtain a string.

Finally, documentation-property passes the string through substitute-command-keys to substitute actual key bindings, unless verbatim is non-nil.

(documentation-property 'command-line-processed
     => "Non-nil once command line has been processed"
(symbol-plist 'command-line-processed)
     => (variable-documentation 188902)

Function: documentation function &optional verbatim
This function returns the documentation string of function.

If function is a symbol, this function first looks for the function-documentation property of that symbol; if that has a non-nil value, the documentation comes from that value (if the value is not a string, it is evaluated). If function is not a symbol, or if it has no function-documentation property, then documentation extracts the documentation string from the actual function definition, reading it from a file if called for.

Finally, unless verbatim is non-nil, it calls substitute-command-keys so as to return a value containing the actual (current) key bindings.

The function documentation signals a void-function error if function has no function definition. However, it is OK if the function definition has no documentation string. In that case, documentation returns nil.

Here is an example of using the two functions, documentation and documentation-property, to display the documentation strings for several symbols in a `*Help*' buffer.

(defun describe-symbols (pattern)
  "Describe the Emacs Lisp symbols matching PATTERN.
All symbols that have PATTERN in their name are described
in the `*Help*' buffer."
  (interactive "sDescribe symbols matching: ")
  (let ((describe-func
          (lambda (s)
            ;; Print description of symbol.
            (if (fboundp s)             ; It is a function.
                 (format "%s\t%s\n%s\n\n" s
                   (if (commandp s) 
                       (let ((keys (where-is-internal s)))
                         (if keys
                              "Keys: "
                              (mapconcat 'key-description 
                                         keys " "))
                           "Keys: none"))
                   (or (documentation s) 
                       "not documented"))))
            (if (boundp s)              ; It is a variable.
                 (format "%s\t%s\n%s\n\n" s
                   (if (user-variable-p s) 
                       "Option " "Variable")
                   (or (documentation-property 
                         s 'variable-documentation)
                       "not documented")))))))

    ;; Build a list of symbols that match pattern.
    (mapatoms (function 
               (lambda (sym)
                 (if (string-match pattern (symbol-name sym))
                     (setq sym-list (cons sym sym-list))))))

    ;; Display the data.
    (with-output-to-temp-buffer "*Help*"
      (mapcar describe-func (sort sym-list 'string<))

The describe-symbols function works like apropos, but provides more information.

(describe-symbols "goal")

---------- Buffer: *Help* ----------
goal-column     Option 
*Semipermanent goal column for vertical motion, as set by ...

set-goal-column Keys: C-x C-n
Set the current horizontal position as a goal for C-n and C-p.
Those commands will move to this position in the line moved to
rather than trying to keep the same horizontal position.
With a non-nil argument, clears out the goal column
so that C-n and C-p resume vertical motion.
The goal column is stored in the variable `goal-column'.

temporary-goal-column   Variable
Current goal column for vertical motion.
It is the column where point was
at the start of current run of vertical motion commands.
When the `track-eol' feature is doing its job, the value is 9999.
---------- Buffer: *Help* ----------

The asterisk `*' as the first character of a variable's doc string, as shown above for the goal-column variable, means that it is a user option; see the description of defvar in 11.5 Defining Global Variables.

Function: Snarf-documentation filename
This function is used only during Emacs initialization, just before the runnable Emacs is dumped. It finds the file offsets of the documentation strings stored in the file filename, and records them in the in-core function definitions and variable property lists in place of the actual strings. See section E.1 Building Emacs.

Emacs reads the file filename from the `emacs/etc' directory. When the dumped Emacs is later executed, the same file will be looked for in the directory doc-directory. Usually filename is "DOC-version".

Variable: doc-directory
This variable holds the name of the directory which should contain the file "DOC-version" that contains documentation strings for built-in and preloaded functions and variables.

In most cases, this is the same as data-directory. They may be different when you run Emacs from the directory where you built it, without actually installing it. See data-directory in 24.5 Help Functions.

In older Emacs versions, exec-directory was used for this.

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