linking fd to pipe identity via socket option

src/pipe.hpp

@@ -27,6 +27,7 @@
 #include "stdint.hpp"
 #include "array.hpp"
 #include "blob.hpp"
+#include "fd.hpp"
 
 namespace zmq
 {
@@ -117,6 +118,8 @@ namespace zmq
         // set the high water marks.
         void set_hwms (int inhwm_, int outhwm_);
 
+        // provide a way to link pipe to engine fd. Set on session initialization
+        fd_t assoc_fd; //=retired_fd
     private:
 
         //  Type of the underlying lock-free pipe.