docu

doc/zmq_getsockopt.txt

@@ -621,6 +621,23 @@ Option value unit:: N/A
 Default value:: not set
 Applicable socket types:: all, when using TCP transport
 
+ZMQ_IDENTITY_FD: Retrieve FD associated with igven identity
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The 'ZMQ_IDENTITY_FD' option shall retrieve the FD associated with given identity. 
+call _zmq_getsockopt()_ with _option_value_ / _option_len_ pointing to memory 
+holding the identity string. On return the start of _option_value_ buffer will be 
+filled with file descriptor of the pipe with given identity if found. If the identity 
+is not found ENOTSOCK is returned as _zmq_getsockopt()_  result. When the pipe is not 
+using FD as lower transport you might get -1 as FD. NB: _option_value_ must be always 
+big  enough to hold sizeof(fd_t) bytes no matter how small the identity length is.
+
+[horizontal]
+Option value type:: character string/fd_t
+Option value unit:: N/A
+Default value:: not set
+Applicable socket types:: ROUTER
+
 
 RETURN VALUE
 ------------