diff --git a/cpu/native/net/wpcap.c b/cpu/native/net/wpcap.c
index 073a615ee..f32ba980f 100644
--- a/cpu/native/net/wpcap.c
+++ b/cpu/native/net/wpcap.c
@@ -192,7 +192,7 @@ uint8_t wfall_send(const uip_lladdr_t *lladdr);
 static uip_ipaddr_t last_sender;
 #endif
 
-static void
+static int
 output(void)
 {
 #if FALLBACK_HAS_ETHERNET_HEADERS&&0
@@ -203,8 +203,8 @@ output(void)
   }
   uip_ipaddr_copy(&last_sender, &UIP_IP_BUF->srcipaddr);
 #endif
-    PRINTF("FUT: %u\n", uip_len);
-	wfall_send(0);
+  PRINTF("FUT: %u\n", uip_len);
+  return wfall_send(0);
 }
 
 const struct uip_fallback_interface rpl_interface = {
diff --git a/examples/cc2530dk/border-router/slip-bridge.c b/examples/cc2530dk/border-router/slip-bridge.c
index 30bd4a429..ff79e2b98 100644
--- a/examples/cc2530dk/border-router/slip-bridge.c
+++ b/examples/cc2530dk/border-router/slip-bridge.c
@@ -83,7 +83,7 @@ init(void)
   slip_set_input_callback(slip_input_callback);
 }
 /*---------------------------------------------------------------------------*/
-static void
+static int
 output(void)
 {
   if(uip_ipaddr_cmp(&last_sender, &UIP_IP_BUF->srcipaddr)) {
@@ -94,6 +94,7 @@ output(void)
     PRINTF("SUT: %u\n", uip_len);
     slip_send();
   }
+  return 0;
 }
 /*---------------------------------------------------------------------------*/
 struct uip_fallback_interface slip_interface = {
diff --git a/examples/ipv6/native-border-router/tun-bridge.c b/examples/ipv6/native-border-router/tun-bridge.c
index 22cb4289b..827f72961 100644
--- a/examples/ipv6/native-border-router/tun-bridge.c
+++ b/examples/ipv6/native-border-router/tun-bridge.c
@@ -222,13 +222,15 @@ tun_init()
 }
 
 /*---------------------------------------------------------------------------*/
-void
+static int
 tun_output(uint8_t *data, int len)
 {
   /* fprintf(stderr, "*** Writing to tun...%d\n", len); */
   if(write(tunfd, data, len) != len) {
     err(1, "serial_to_tun: write");
+    return 1;
   }
+  return 0;
 }
 /*---------------------------------------------------------------------------*/
 int
@@ -246,13 +248,14 @@ init(void)
 {
 }
 /*---------------------------------------------------------------------------*/
-static void
+static int
 output(void)
 {
   PRINTF("SUT: %u\n", uip_len);
   if(uip_len > 0) {
-    tun_output(&uip_buf[UIP_LLH_LEN], uip_len);
+    return tun_output(&uip_buf[UIP_LLH_LEN], uip_len);
   }
+  return 0;
 }
 
 
diff --git a/examples/ipv6/rpl-border-router/slip-bridge.c b/examples/ipv6/rpl-border-router/slip-bridge.c
index 582f626e7..62940d5ef 100644
--- a/examples/ipv6/rpl-border-router/slip-bridge.c
+++ b/examples/ipv6/rpl-border-router/slip-bridge.c
@@ -100,7 +100,7 @@ init(void)
   slip_set_input_callback(slip_input_callback);
 }
 /*---------------------------------------------------------------------------*/
-static void
+static int
 output(void)
 {
   if(uip_ipaddr_cmp(&last_sender, &UIP_IP_BUF->srcipaddr)) {
@@ -115,6 +115,7 @@ output(void)
  //   PRINTF("SUT: %u\n", uip_len);
     slip_send();
   }
+  return 0;
 }
 
 /*---------------------------------------------------------------------------*/
diff --git a/examples/sensinode/border-router/slip-bridge.c b/examples/sensinode/border-router/slip-bridge.c
index edd048a7d..b0ebd066d 100644
--- a/examples/sensinode/border-router/slip-bridge.c
+++ b/examples/sensinode/border-router/slip-bridge.c
@@ -84,7 +84,7 @@ init(void)
   slip_set_input_callback(slip_input_callback);
 }
 /*---------------------------------------------------------------------------*/
-static void
+static int
 output(void)
 {
   if(uip_ipaddr_cmp(&last_sender, &UIP_IP_BUF->srcipaddr)) {
@@ -95,6 +95,7 @@ output(void)
     PRINTF("SUT: %u\n", uip_len);
     slip_send();
   }
+  return 0;
 }
 /*---------------------------------------------------------------------------*/
 const struct uip_fallback_interface slip_interface = {
diff --git a/platform/avr-ravenusb/contiki-raven-main.c b/platform/avr-ravenusb/contiki-raven-main.c
index 6e7f9d28c..e2ca7711e 100644
--- a/platform/avr-ravenusb/contiki-raven-main.c
+++ b/platform/avr-ravenusb/contiki-raven-main.c
@@ -132,7 +132,7 @@ init(void)
 {
 }
 void mac_LowpanToEthernet(void);
-static void
+static int
 output(void)
 {
 //  if(uip_ipaddr_cmp(&last_sender, &UIP_IP_BUF->srcipaddr)) {
@@ -142,6 +142,7 @@ output(void)
     PRINTD("SUT: %u\n", uip_len);
     mac_LowpanToEthernet();  //bounceback trap is done in lowpanToEthernet
 //  }
+    return 0;
 }
 const struct uip_fallback_interface rpl_interface = {
   init, output