Added new internal header.
diff --git a/upb/upb.int.h b/upb/upb.int.h
new file mode 100644
index 0000000..b857560
--- /dev/null
+++ b/upb/upb.int.h
@@ -0,0 +1,29 @@
+
+#ifndef UPB_INT_H_
+#define UPB_INT_H_
+
+#include "upb/upb.h"
+
+struct mem_block;
+typedef struct mem_block mem_block;
+
+struct upb_arena {
+ _upb_arena_head head;
+ uint32_t *cleanups;
+
+ /* Allocator to allocate arena blocks. We are responsible for freeing these
+ * when we are destroyed. */
+ upb_alloc *block_alloc;
+ uint32_t last_size;
+
+ /* When multiple arenas are fused together, each arena points to a parent
+ * arena (root points to itself). The root tracks how many live arenas
+ * reference it. */
+ uint32_t refcount; /* Only used when a->parent == a */
+ struct upb_arena *parent;
+
+ /* Linked list of blocks to free/cleanup. */
+ mem_block *freelist, *freelist_tail;
+};
+
+#endif /* UPB_INT_H_ */
