(print) <Lisp_Cons>: Detect circular list.

1 files changed, 11 insertions, 3 deletions

diff --git a/src/print.c b/src/print.c
index 2b1e046336d..879b48933c9 100644
--- a/src/print.c
+++ b/src/print.c
@@ -1418,14 +1418,20 @@ print (obj, printcharfun, escapeflag)
           {
             register int i = 0;
             register int print_length = 0;
+            Lisp_Object halftail = obj;
 
             if (INTEGERP (Vprint_length))
               print_length = XINT (Vprint_length);
-            /* Could recognize circularities in cdrs here,
-               but that would make printing of long lists quadratic.
-               It's not worth doing.  */
             while (CONSP (obj))
               {
+                /* Detect circular list.  */
+                if (i != 0 && EQ (obj, halftail))
+                  {
+                    sprintf (buf, " . #%d", i / 2);
+                    strout (buf, -1, -1, printcharfun, 0);
+                    obj = Qnil;
+                    break;
+                  }
                 if (i++)
                   PRINTCHAR (' ');
                 if (print_length && i > print_length)
@@ -1435,6 +1441,8 @@ print (obj, printcharfun, escapeflag)
                   }
                 print (XCAR (obj), printcharfun, escapeflag);
                 obj = XCDR (obj);
+                if (!(i & 1))
+                  halftail = XCDR (halftail);
               }
           }
           if (!NILP (obj))