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+#ifndef _RING_H_INCLUDED_
+#define _RING_H_INCLUDED_
+
+/*++
+/* NAME
+/* ring 3h
+/* SUMMARY
+/* circular list management
+/* SYNOPSIS
+/* #include <ring.h>
+/* DESCRIPTION
+/* .nf
+
+ /*
+ * External interface.
+ */
+typedef struct RING RING;
+
+struct RING {
+ RING *succ; /* successor */
+ RING *pred; /* predecessor */
+};
+
+extern void ring_init(RING *);
+extern void ring_prepend(RING *, RING *);
+extern void ring_append(RING *, RING *);
+extern void ring_detach(RING *);
+
+#define ring_succ(c) ((c)->succ)
+#define ring_pred(c) ((c)->pred)
+
+#define RING_FOREACH(entry, head) \
+ for (entry = ring_succ(head); entry != (head); entry = ring_succ(entry))
+
+ /*
+ * Typically, an application will embed a RING structure into a larger
+ * structure that also contains application-specific members. This approach
+ * gives us the best of both worlds. The application can still use the
+ * generic RING primitives for manipulating RING structures. The macro below
+ * transforms a pointer from RING structure to the structure that contains
+ * it.
+ */
+#define RING_TO_APPL(ring_ptr,app_type,ring_member) \
+ ((app_type *) (((char *) (ring_ptr)) - offsetof(app_type,ring_member)))
+
+/* LICENSE
+/* .ad
+/* .fi
+/* The Secure Mailer license must be distributed with this software.
+/* AUTHOR(S)
+/* Wietse Venema
+/* IBM T.J. Watson Research
+/* P.O. Box 704
+/* Yorktown Heights, NY 10598, USA
+/* LAST MODIFICATION
+/* Tue Jan 28 16:50:20 EST 1997
+/*--*/
+
+#endif