diff --git a/locked_queue.hh b/locked_queue.hh
index e96faac..ccf45ea 100644
--- a/locked_queue.hh
+++ b/locked_queue.hh
@@ -15,6 +15,7 @@ namespace utility
         typedef std::recursive_mutex     Mutex;
         typedef std::unique_lock<Mutex>  Lock;
         
+        int    _waiting = 0;
         Mutex  _mtx;
         std::condition_variable_any  _cv;
         std::deque<T> _q;
@@ -83,11 +84,14 @@ namespace utility
         bool try_pop_back(T& out, std::chrono::steady_clock::time_point end_time)
         {
             Lock L(_mtx);
+            ++_waiting;
             if(! _cv.wait_until(L, end_time, [this]{ return !_q.empty(); } ) )
             {
+                --_waiting;
                 return false;
             }
             
+            --_waiting;
             out = std::move(_q.back());
             _q.pop_back();
             return true;
@@ -98,11 +102,14 @@ namespace utility
         bool try_pop_front(T& out, std::chrono::steady_clock::time_point end_time)
         {
             Lock L(_mtx);
+            ++_waiting;
             if(! _cv.wait_until(L, end_time, [this]{ return !_q.empty(); } ) )
             {
+                --_waiting;
                 return false;
             }
             
+            --_waiting;
             out = std::move(_q.front());
             _q.pop_front();
             return true;
@@ -113,11 +120,14 @@ namespace utility
         bool try_pop_front(T& out, std::chrono::seconds duration)
         {
             Lock L(_mtx);
+            ++_waiting;
             if(! _cv.wait_for(L, duration, [this]{ return !_q.empty(); } ) )
             {
+                --_waiting;
                 return false;
             }
             
+            --_waiting;
             out = std::move(_q.front());
             _q.pop_front();
             return true;
@@ -162,6 +172,13 @@ namespace utility
             Lock lg(_mtx);
             return _q.empty();
         }
+
+        // returns the number of threads that are waiting in pop_...() or try_pop_...()
+        int waiting()
+        {
+            Lock L(_mtx);
+            return _waiting;
+        }
     };
-}
 
+} // end of namespace utility