Index: share/man/man3/tree.3
===================================================================
--- share/man/man3/tree.3
+++ share/man/man3/tree.3
@@ -30,7 +30,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd January 24, 2015
+.Dd August 12, 2015
 .Dt TREE 3
 .Os
 .Sh NAME
@@ -95,6 +95,7 @@
 .Nm RB_FOREACH_REVERSE_SAFE ,
 .Nm RB_INIT ,
 .Nm RB_INSERT ,
+.Nm RB_COUNT ,
 .Nm RB_REMOVE
 .Nd "implementations of splay and red-black trees"
 .Sh SYNOPSIS
@@ -186,6 +187,8 @@
 .Fn RB_INSERT NAME "RB_HEAD *head" "struct TYPE *elm"
 .Ft "struct TYPE *"
 .Fn RB_REMOVE NAME "RB_HEAD *head" "struct TYPE *elm"
+.Ft void
+.Fn RB_COUNT "VARNAME" "NAME" "RB_HEAD *head" "COUNTER"
 .Sh DESCRIPTION
 These macros define data structures for different types of trees:
 splay trees and red-black trees.
@@ -557,6 +560,14 @@
 The
 .Fn RB_EMPTY
 macro should be used to check whether a red-black tree is empty.
+.Pp
+The
+.Fn RB_COUNT
+macro traverses the tree referenced by
+.Ar head ,
+counting the number of node.
+.Ar COUNTER
+is initialized to 0 before traversing and incremented by one for each nodes.
 .Sh NOTES
 Trying to free a tree in the following way is a common error:
 .Bd -literal -offset indent
Index: sys/sys/tree.h
===================================================================
--- sys/sys/tree.h
+++ sys/sys/tree.h
@@ -798,4 +798,11 @@
 	    ((x) != NULL) && ((y) = name##_RB_PREV(x), (x) != NULL);	\
 	     (x) = (y))
 
+#define RB_COUNT(x, name, head, cnt) do {	\
+	(cnt) = 0;				\
+	RB_FOREACH((x), name, (head)) {		\
+		(cnt)++;			\
+	}					\
+} while (0)
+
 #endif	/* _SYS_TREE_H_ */