Customizable variables, also called user options, are
global Lisp variables whose values can be set through the Customize
interface. Unlike other global variables, which are defined with
defvar (see Defining Variables), customizable variables are
defined using the
defcustom macro. In addition to calling
defvar as a subroutine,
defcustom states how the
variable should be displayed in the Customize interface, the values it
is allowed to take, etc.
This macro declares option as a user option (i.e., a customizable variable). You should not quote option.
The argument standard is an expression that specifies the standard value for option. Evaluating the
defcustomform evaluates standard, but does not necessarily bind the option to that value. If option already has a default value, it is left unchanged. If the user has already saved a customization for option, the user's customized value is installed as the default value. Otherwise, the result of evaluating standard is installed as the default value.
defvar, this macro marks
optionas a special variable, meaning that it should always be dynamically bound. If option is already lexically bound, that lexical binding remains in effect until the binding construct exits. See Variable Scoping.
The expression standard can be evaluated at various other times, too—whenever the customization facility needs to know option's standard value. So be sure to use an expression which is harmless to evaluate at any time.
The argument doc specifies the documentation string for the variable.
defcustomdoes not specify any
:group, the last group defined with
defgroupin the same file will be used. This way, most
defcustomdo not need an explicit
When you evaluate a
defcustomform with C-M-x in Emacs Lisp mode (
eval-defun), a special feature of
eval-defunarranges to set the variable unconditionally, without testing whether its value is void. (The same feature applies to
defvar, see Defining Variables.) Using
eval-defunon a defcustom that is already defined calls the
:setfunction (see below), if there is one.
If you put a
defcustomin a pre-loaded Emacs Lisp file (see Building Emacs), the standard value installed at dump time might be incorrect, e.g., because another variable that it depends on has not been assigned the right value yet. In that case, use
custom-reevaluate-setting, described below, to re-evaluate the standard value after Emacs starts up.
In addition to the keywords listed in Common Keywords, this macro accepts the following keywords:
This is meaningful only for certain types, currently including
alist. See the definition of the
individual types for a description of how to use
If you specify this keyword, the variable's documentation string
should describe how to do the same job in hand-written Lisp code.
You have to really understand the workings of Custom to use
:get correctly. It is meant for values that are treated in
Custom as variables but are not actually stored in Lisp variables. It
is almost surely a mistake to specify getfunction for a value
that really is stored in a Lisp variable.
defcustomis evaluated. It should take two arguments, the option name (a symbol) and the value. Here are some predefined functions meant for use in this way:
:setfunction to initialize the variable, but do not reinitialize it if it is already non-void.
custom-initialize-set, but use the function
set-defaultto set the variable, instead of the variable's
:setfunction. This is the usual choice for a variable whose
:setfunction enables or disables a minor mode; with this choice, defining the variable will not call the minor mode function, but customizing the variable will do so.
:setfunction to initialize the variable. If the variable is already non-void, reset it by calling the
:setfunction using the current value (returned by the
:getmethod). This is the default
:setfunction to initialize the variable, if it is already set or has been customized; otherwise, just use
custom-initialize-default, respectively), but catch errors. If an error occurs during initialization, they set the variable to
set-default, and signal no error.
These functions are meant for options defined in pre-loaded files,
where the standard expression may signal an error because some
required variable or function is not yet defined. The value normally
gets updated in startup.el, ignoring the value computed by
defcustom. After startup, if one unsets the value and
defcustom, the standard expression can be
evaluated without error.
risky-local-variableproperty to value (see File Local Variables).
safe-local-variableproperty to function (see File Local Variables).
:set-afterif setting this variable won't work properly unless those other variables already have their intended values.
It is useful to specify the
:require keyword for an option
that “turns on” a certain feature. This causes Emacs to load the
feature, if it is not already loaded, whenever the option is set.
See Common Keywords. Here is an example, from the library
(defcustom save-place nil "Non-nil means automatically save place in each file..." :type 'boolean :require 'saveplace :group 'save-place)
If a customization item has a type such as
alist, which supports
:options, you can add additional
values to the list from outside the
defcustom declaration by
custom-add-frequent-value. For example, if you define a
my-lisp-mode-initialization intended to be called from
emacs-lisp-mode-hook, you might want to add that to the list of
reasonable values for
emacs-lisp-mode-hook, but not by editing
its definition. You can do it thus:
(custom-add-frequent-value 'emacs-lisp-mode-hook 'my-lisp-mode-initialization)
For the customization option symbol, add value to the list of reasonable values.
The precise effect of adding a value depends on the customization type of symbol.
defcustom uses the symbol property
standard-value to record the expression for the standard value,
saved-value to record the value saved by the user with the
customization buffer, and
customized-value to record the value
set by the user with the customization buffer, but not saved.
See Symbol Properties. These properties are lists, the car of
which is an expression that evaluates to the value.
This function re-evaluates the standard value of symbol, which should be a user option declared via
defcustom. If the variable was customized, this function re-evaluates the saved value instead. Then it sets the user option to that value (using the option's
:setproperty if that is defined).
This is useful for customizable options that are defined before their value could be computed correctly. For example, during startup Emacs calls this function for some user options that were defined in pre-loaded Emacs Lisp files, but whose initial values depend on information available only at run-time.