A file can specify local variable values; Emacs uses these to create buffer-local bindings for those variables in the buffer visiting that file. See Local Variables in Files in The GNU Emacs Manual, for basic information about file-local variables. This section describes the functions and variables that affect how file-local variables are processed.
If a file-local variable could specify an arbitrary function or Lisp expression that would be called later, visiting a file could take over your Emacs. Emacs protects against this by automatically setting only those file-local variables whose specified values are known to be safe. Other file-local variables are set only if the user agrees.
For additional safety,
read-circle is temporarily bound to
nil when Emacs reads file-local variables (see Input Functions). This prevents the Lisp reader from recognizing circular and shared Lisp structures (see Circular Objects).
user option enable-local-variables
This variable controls whether to process file-local variables. The possible values are:
t (the default)
Set the safe variables, and query (once) about any unsafe variables.
Set only the safe variables and do not query.
Set all the variables and do not query.
Don’t set any variables.
Query (once) about all the variables.
This is a list of regular expressions. If a file has a name matching an element of this list, then it is not scanned for any form of file-local variable. For examples of why you might want to use this, see Auto Major Mode.
function hack-local-variables \&optional handle-mode
This function parses, and binds or evaluates as appropriate, any local variables specified by the contents of the current buffer. The variable
enable-local-variables has its effect here. However, this function does not look for the ‘
mode:’ local variable in the ‘
set-auto-mode does that, also taking
enable-local-variables into account (see Auto Major Mode).
This function works by walking the alist stored in
file-local-variables-alist and applying each local variable in turn. It calls
hack-local-variables-hook before and after applying the variables, respectively. It only calls the before-hook if the alist is non-
nil; it always calls the other hook. This function ignores a ‘
mode’ element if it specifies the same major mode as the buffer already has.
If the optional argument
t, then all this function does is return a symbol specifying the major mode, if the ‘
-*-’ line or the local variables list specifies one, and
nil otherwise. It does not set the mode or any other file-local variable. If
handle-mode has any value other than
t, any settings of ‘
mode’ in the ‘
-*-’ line or the local variables list are ignored, and the other settings are applied. If
nil, all the file local variables are set.
This buffer-local variable holds the alist of file-local variable settings. Each element of the alist is of the form
(var . value), where
var is a symbol of the local variable and
value is its value. When Emacs visits a file, it first collects all the file-local variables into this alist, and then the
hack-local-variables function applies them one by one.
Emacs calls this hook immediately before applying file-local variables stored in
Emacs calls this hook immediately after it finishes applying file-local variables stored in
You can specify safe values for a variable with a
safe-local-variable property. The property has to be a function of one argument; any value is safe if the function returns non-
nil given that value. Many commonly-encountered file variables have
safe-local-variable properties; these include
indent-tabs-mode. For boolean-valued variables that are safe, use
booleanp as the property value.
If you want to define
safe-local-variable properties for variables defined in C source code, add the names and the properties of those variables to the list in the “Safe local variables" section of
When defining a user option using
defcustom, you can set its
safe-local-variable property by adding the arguments
:safe function to
defcustom (see Variable Definitions). However, a safety predicate defined using
:safe will only be known once the package that contains the
defcustom is loaded, which is often too late. As an alternative, you can use the autoload cookie (see Autoload) to assign the option its safety predicate, like this:
;;;###autoload (put 'var 'safe-local-variable 'pred)
The safe value definitions specified with
autoload are copied into the package’s autoloads file (
loaddefs.el for most packages bundled with Emacs), and are known to Emacs since the beginning of a session.
user option safe-local-variable-values
This variable provides another way to mark some variable values as safe. It is a list of cons cells
(var . val), where
var is a variable name and
val is a value which is safe for that variable.
When Emacs asks the user whether or not to obey a set of file-local variable specifications, the user can choose to mark them as safe. Doing so adds those variable/value pairs to
safe-local-variable-values, and saves it to the user’s custom file.
function safe-local-variable-p sym val
This function returns non-
nil if it is safe to give
sym the value
val, based on the above criteria.
Some variables are considered risky. If a variable is risky, it is never entered automatically into
safe-local-variable-values; Emacs always queries before setting a risky variable, unless the user explicitly allows a value by customizing
Any variable whose name has a non-
risky-local-variable property is considered risky. When you define a user option using
defcustom, you can set its
risky-local-variable property by adding the arguments
:risky value to
defcustom (see Variable Definitions). In addition, any variable whose name ends in any of ‘
-program’, or ‘
-predicate’ is automatically considered risky. The variables ‘
font-lock-keywords’ followed by a digit, and ‘
font-lock-syntactic-keywords’ are also considered risky.
function risky-local-variable-p sym
This function returns non-
sym is a risky variable, based on the above criteria.
This variable holds a list of variables that should not be given local values by files. Any value specified for one of these variables is completely ignored.
Eval:’ “variable" is also a potential loophole, so Emacs normally asks for confirmation before handling it.
user option enable-local-eval
This variable controls processing of ‘
Eval:’ in ‘
-*-’ lines or local variables lists in files being visited. A value of
t means process them unconditionally;
nil means ignore them; anything else means ask the user what to do for each file. The default value is
user option safe-local-eval-forms
This variable holds a list of expressions that are safe to evaluate when found in the ‘
Eval:’ “variable" in a file local variables list.
If the expression is a function call and the function has a
safe-local-eval-function property, the property value determines whether the expression is safe to evaluate. The property value can be a predicate to call to test the expression, a list of such predicates (it’s safe if any predicate succeeds), or
t (always safe provided the arguments are constant).
Text properties are also potential loopholes, since their values could include functions to call. So Emacs discards all text properties from string values specified for file-local variables.