diff --git a/c/unpack.h b/c/unpack.h
index ab202a10..46777b9c 100644
--- a/c/unpack.h
+++ b/c/unpack.h
@@ -46,6 +46,7 @@ void msgpack_unpacker_destroy(msgpack_unpacker* mpac);
 msgpack_unpacker* msgpack_unpacker_new(size_t initial_buffer_size);
 void msgpack_unpacker_free(msgpack_unpacker* mpac);
 
+static inline size_t msgpack_unpacker_buffered_size(const msgpack_unpacker* mpac);
 static inline bool   msgpack_unpacker_reserve_buffer(msgpack_unpacker* mpac, size_t size);
 static inline char*  msgpack_unpacker_buffer(msgpack_unpacker* mpac);
 static inline size_t msgpack_unpacker_buffer_capacity(const msgpack_unpacker* mpac);
@@ -77,6 +78,11 @@ bool msgpack_unpacker_flush_zone(msgpack_unpacker* mpac);
 
 bool msgpack_unpacker_expand_buffer(msgpack_unpacker* mpac, size_t size);
 
+size_t msgpack_unpacker_buffered_size(const msgpack_unpacker* mpac)
+{
+	return mpac->used;
+}
+
 bool msgpack_unpacker_reserve_buffer(msgpack_unpacker* mpac, size_t size)
 {
 	if(mpac->free >= size) { return true; }
diff --git a/cpp/unpack.hpp b/cpp/unpack.hpp
index e6d35dab..6e9b6410 100644
--- a/cpp/unpack.hpp
+++ b/cpp/unpack.hpp
@@ -43,6 +43,9 @@ public:
 	~unpacker();
 
 public:
+	/*! 0. check if the buffered size is not exceed the assumption. */
+	size_t buffered_size() const;
+
 	/*! 1. reserve buffer. at least `size' bytes of capacity will be ready */
 	void reserve_buffer(size_t size);
 
@@ -154,6 +157,12 @@ inline unpacker::~unpacker()
 	msgpack_unpacker_destroy(this);
 }
 
+
+inline size_t unpacker::buffered_size() const
+{
+	return msgpack_unpacker_buffered_size(this);
+}
+
 inline void unpacker::reserve_buffer(size_t size)
 {
 	if(!msgpack_unpacker_reserve_buffer(this, size)) {