You may announce your intention to use a symbol as a global variable
with a variable definition: a special form, either
In Emacs Lisp, definitions serve three purposes. First, they inform
people who read the code that certain symbols are intended to be
used a certain way (as variables). Second, they inform the Lisp system
of these things, supplying a value and documentation. Third, they
provide information to utilities such as
make-docfile, which create data bases of the functions and
variables in a program.
The difference between
defvar is primarily
a matter of intent, serving to inform human readers of whether the value
should ever change. Emacs Lisp does not restrict the ways in which a
variable can be used based on
declarations. However, it does make a difference for initialization:
defconst unconditionally initializes the variable, while
defvar initializes it only if it is void.
This special form defines symbol as a variable and can also initialize and document it. The definition informs a person reading your code that symbol is used as a variable that might be set or changed. Note that symbol is not evaluated; the symbol to be defined must appear explicitly in the
If symbol is void and value is specified,
defvarevaluates it and sets symbol to the result. But if symbol already has a value (i.e., it is not void), value is not even evaluated, and symbol's value remains unchanged. If value is omitted, the value of symbol is not changed in any case.
If symbol has a buffer-local binding in the current buffer,
defvaroperates on the default value, which is buffer-independent, not the current (buffer-local) binding. It sets the default value if the default value is void. See Buffer-Local Variables.
When you evaluate a top-level
defvarform 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.
If the doc-string argument appears, it specifies the documentation for the variable. (This opportunity to specify documentation is one of the main benefits of defining the variable.) The documentation is stored in the symbol's
variable-documentationproperty. The Emacs help functions (see Documentation) look for this property.
If the documentation string begins with the character ‘*’, Emacs allows users to set it interactively using the
set-variablecommand. However, you should nearly always use
defvarto define such variables, so that users can use M-x customize and related commands to set them. In that case, it is not necessary to begin the documentation string with ‘*’. See Customization.
Here are some examples. This form defines
foobut does not initialize it:(defvar foo) ⇒ foo
This example initializes the value of
23, and gives it a documentation string:(defvar bar 23 "The normal weight of a bar.") ⇒ bar
The following form changes the documentation string for
bar, making it a user option, but does not change the value, since
baralready has a value. (The addition
(1+ nil)would get an error if it were evaluated, but since it is not evaluated, there is no error.)(defvar bar (1+ nil) "*The normal weight of a bar.") ⇒ bar bar ⇒ 23
Here is an equivalent expression for the
defvarspecial form:(defvar symbol value doc-string) == (progn (if (not (boundp 'symbol)) (setq symbol value)) (if 'doc-string (put 'symbol 'variable-documentation 'doc-string)) 'symbol)
defvarform returns symbol, but it is normally used at top level in a file where its value does not matter.
This special form defines symbol as a value and initializes it. It informs a person reading your code that symbol has a standard global value, established here, that should not be changed by the user or by other programs. Note that symbol is not evaluated; the symbol to be defined must appear explicitly in the
defconstalways evaluates value, and sets the value of symbol to the result. If symbol does have a buffer-local binding in the current buffer,
defconstsets the default value, not the buffer-local value. (But you should not be making buffer-local bindings for a symbol that is defined with
An example of the use of
defconstis Emacs' definition of
float-pi—the mathematical constant pi, which ought not to be changed by anyone (attempts by the Indiana State Legislature notwithstanding). As the second form illustrates, however,
defconstis only advisory.(defconst float-pi 3.141592653589793 "The value of Pi.") ⇒ float-pi (setq float-pi 3) ⇒ float-pi float-pi ⇒ 3
This function returns
tif variable is a user option—a variable intended to be set by the user for customization—and
nilotherwise. (Variables other than user options exist for the internal purposes of Lisp programs, and users need not know about them.)
User option variables are distinguished from other variables either though being declared using
defcustom1 or by the first character of their
variable-documentationproperty. If the property exists and is a string, and its first character is ‘*’, then the variable is a user option. Aliases of user options are also user options.
If a user option variable has a
set-variable command uses that value to control reading the
new value for the variable. The property's value is used as if it were
interactive (see Using Interactive). However,
this feature is largely obsoleted by
Warning: If the
forms are used while the variable has a local binding (made with
let, or a function argument), they set the local-binding's
value; the top-level binding is not changed. This is not what you
usually want. To prevent it, use these special forms at top level in
a file, where normally no local binding is in effect, and make sure to
load the file before making a local binding for the variable.
 They may also be declared equivalently in cus-start.el.