Index: head/sys/sys/_bitset.h
===================================================================
--- head/sys/sys/_bitset.h
+++ head/sys/sys/_bitset.h
@@ -47,4 +47,12 @@
         long    __bits[__bitset_words((_s))];				\
 }
 
+/*
+ * Helper to declare a bitset without it's size being a constant.
+ *
+ * Sadly we cannot declare a bitset struct with '__bits[]', because it's
+ * the only member of the struct and the compiler complains.
+ */
+#define BITSET_DEFINE_VAR(t)	BITSET_DEFINE(t, 1)
+
 #endif /* !_SYS__BITSET_H_ */
Index: head/sys/sys/bitset.h
===================================================================
--- head/sys/sys/bitset.h
+++ head/sys/sys/bitset.h
@@ -199,4 +199,10 @@
 #define	BITSET_FSET(n)							\
 	[ 0 ... ((n) - 1) ] = (-1L)
 
+/*
+ * Dynamically allocate a bitset.
+ */
+#define BITSET_ALLOC(_s, mt, mf)					\
+	malloc(__bitset_words(_s) * sizeof(long), mt, (mf))
+
 #endif /* !_SYS_BITSET_H_ */