diff --git a/src/forwarder.cpp b/src/forwarder.cpp
index 07288e20..5aab8f2e 100644
--- a/src/forwarder.cpp
+++ b/src/forwarder.cpp
@@ -33,4 +33,6 @@ int zmq::forwarder (socket_base_t *insocket_, socket_base_t *outsocket_)
         insocket_->recv (&msg, 0);
         outsocket_->send (&msg, 0);
     }
+
+    return 0;
 }
diff --git a/src/queue.cpp b/src/queue.cpp
index 361df78b..165e3606 100644
--- a/src/queue.cpp
+++ b/src/queue.cpp
@@ -94,5 +94,7 @@ int zmq::queue (class socket_base_t *insocket_,
             has_response = false;
         }
     }
+
+    return 0;
 }
 
diff --git a/src/streamer.cpp b/src/streamer.cpp
index de23b373..796771b5 100644
--- a/src/streamer.cpp
+++ b/src/streamer.cpp
@@ -33,4 +33,6 @@ int zmq::streamer (socket_base_t *insocket_, socket_base_t *outsocket_)
         insocket_->recv (&msg, 0);
         outsocket_->send (&msg, 0);
     }
+
+    return 0;
 }