If false, the printing process merely proceeds by recursive descent without attempting to detect circularity and sharing.

If true, the printer will endeavor to detect cycles and sharing in the structure to be printed, and to use #n= and #n# syntax to indicate the circularities or shared components.

If true, a user-defined print-object method can print objects to the supplied stream using write, prin1, princ, or format and expect circularities and sharing to be detected and printed using the #n# syntax. If a user-defined print-object method prints to a stream other than the one that was supplied, then circularity detection starts over for that stream.

Note that implementations should not use #n# notation when the Lisp reader would automatically assure sharing without it (e.g., as happens with interned symbols).

 (let ((a (list 1 2 3)))
   (setf (cdddr a) a)
   (let ((*print-circle* t))
     (write a)
     :done))
#1=(1 2 3 . #1#)
 :DONE