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### 12.9 Accessing Function Cell Contents

The function definition of a symbol is the object stored in the function cell of the symbol. The functions described here access, test, and set the function cell of symbols.

See also the function `indirect-function`. See Definition of indirect-function.

Function: symbol-function symbol

  This returns the object in the function cell of symbol. It does not check that the returned object is a legitimate function.

  If the function cell is void, the return value is `nil`. To distinguish between a function cell that is void and one set to `nil`, use `fboundp` (see below).

      (defun bar (n) (+ n 2))
      (symbol-function 'bar)
           ⇒ (lambda (n) (+ n 2))

      (fset 'baz 'bar)
           ⇒ bar

      (symbol-function 'baz)
           ⇒ bar

If you have never given a symbol any function definition, we say that that symbol’s function cell is void. In other words, the function cell does not have any Lisp object in it. If you try to call the symbol as a function, Emacs signals a `void-function` error.

Note that void is not the same as `nil` or the symbol `void`. The symbols `nil` and `void` are Lisp objects, and can be stored into a function cell just as any other object can be (and they can be valid functions if you define them in turn with `defun`). A void function cell contains no object whatsoever.

You can test the voidness of a symbol’s function definition with `fboundp`. After you have given a symbol a function definition, you can make it void once more using `fmakunbound`.

Function: fboundp symbol

  This function returns `t` if the symbol has an object in its function cell, `nil` otherwise. It does not check that the object is a legitimate function.

Function: fmakunbound symbol

  This function makes symbol’s function cell void, so that a subsequent attempt to access this cell will cause a `void-function` error. It returns symbol. (See also `makunbound`, in Void Variables.)

      (defun foo (x) x)
      (foo 1)
           ⇒1

      (fmakunbound 'foo)
           ⇒ foo

      (foo 1)
      error→ Symbol's function definition is void: foo

Function: fset symbol definition

  This function stores definition in the function cell of symbol. The result is definition. Normally definition should be a function or the name of a function, but this is not checked. The argument symbol is an ordinary evaluated argument.

  The primary use of this function is as a subroutine by constructs that define or alter functions, like `defun` or `advice-add` (see Advising Functions). You can also use it to give a symbol a function definition that is not a function, e.g., a keyboard macro (see Keyboard Macros):

      ;; Define a named keyboard macro.
      (fset 'kill-two-lines "\^u2\^k")
           ⇒ "\^u2\^k"

  It you wish to use `fset` to make an alternate name for a function, consider using `defalias` instead. See Definition of defalias.

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