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### 11.11 File Local Variables

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.

  `:safe`

    Set only the safe variables and do not query.

  `:all`

    Set all the variables and do not query.

  `nil`

    Don’t set any variables.

  anything else

    Query (once) about all the variables.

Variable: inhibit-local-variables-regexps

  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 mode-only

  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 ‘`-*-`’ line. `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 `before-hack-local-variables-hook` and `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 mode-only is non-`nil`, 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 nor any other file-local variable.

Variable: file-local-variables-alist

  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.

Variable: before-hack-local-variables-hook

  Emacs calls this hook immediately before applying file-local variables stored in `file-local-variables-alist`.

Variable: hack-local-variables-hook

  Emacs calls this hook immediately after it finishes applying file-local variables stored in `file-local-variables-alist`.

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 `fill-column`, `fill-prefix`, and `indent-tabs-mode`. For boolean-valued variables that are safe, use `booleanp` as the property value.

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).

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 `safe-local-variable-values` directly.

Any variable whose name has a non-`nil` `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 ‘`-command`’, ‘`-frame-alist`’, ‘`-function`’, ‘`-functions`’, ‘`-hook`’, ‘`-hooks`’, ‘`-form`’, ‘`-forms`’, ‘`-map`’, ‘`-map-alist`’, ‘`-mode-alist`’, ‘`-program`’, or ‘`-predicate`’ is automatically considered risky. The variables ‘`font-lock-keywords`’, ‘`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-`nil` if sym is a risky variable, based on the above criteria.

Variable: ignored-local-variables

  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.

The ‘`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 `maybe`.

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.

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