diff --git a/sys/sys/_bitset.h b/sys/sys/_bitset.h
index 86cf539c5abb..2f5301d58a5d 100644
--- a/sys/sys/_bitset.h
+++ b/sys/sys/_bitset.h
@@ -47,4 +47,12 @@ struct t {								\
         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_ */
diff --git a/sys/sys/bitset.h b/sys/sys/bitset.h
index f1c7bf8e0e6d..723c39b0a2cf 100644
--- a/sys/sys/bitset.h
+++ b/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_ */