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symbol-value  symbol ⇒  value
(setf (         symbol-value symbol) new-value)
symbol—a symbol that must have a value.
value, new-value—an object.
Accesses the symbol’s value cell.
 (setf (symbol-value 'a) 1) ⇒  1
 (symbol-value 'a) ⇒  1
 ;; SYMBOL-VALUE cannot see lexical variables.
 (let ((a 2)) (symbol-value 'a)) ⇒  1
 (let ((a 2)) (setq a 3) (symbol-value 'a)) ⇒  1
 ;; SYMBOL-VALUE can see dynamic variables.
 (let ((a 2)) 
   (declare (special a)) 
   (symbol-value 'a)) ⇒  2
 (let ((a 2)) 
   (declare (special a)) 
   (setq a 3)
   (symbol-value 'a)) ⇒  3
 (let ((a 2))
   (setf (symbol-value 'a) 3)
   a) ⇒  2
 a ⇒  3
 (symbol-value 'a) ⇒  3
 (let ((a 4))
   (declare (special a))
   (let ((b (symbol-value 'a)))
     (setf (symbol-value 'a) 5)
     (values a b))) ⇒  5, 4
 a ⇒  3
 (symbol-value :any-keyword) ⇒  :ANY-KEYWORD
 (symbol-value 'nil) ⇒  NIL
 (symbol-value '()) ⇒  NIL
 ;; The precision of this next one is implementation-dependent.
 (symbol-value 'pi) ⇒  3.141592653589793d0  
makunbound, set, setq
Should signal an error of type type-error if symbol is not a symbol.
Should signal unbound-variable if symbol is unbound and an attempt is made to read its value. (No such error is signaled on an attempt to write its value.)
boundp , makunbound , set , setq
symbol-value can be used to get the value of a constant variable. symbol-value cannot access the value of a lexical variable.