diff --git a/cores/esp8266/Arduino.h b/cores/esp8266/Arduino.h
index c712bf756..ad0e7cb36 100644
--- a/cores/esp8266/Arduino.h
+++ b/cores/esp8266/Arduino.h
@@ -39,6 +39,7 @@ extern "C" {
 #include "twi.h"
 
 void yield(void);
+void optimistic_yield(void);
 
 #define HIGH 0x1
 #define LOW  0x0
diff --git a/cores/esp8266/HardwareSerial.cpp b/cores/esp8266/HardwareSerial.cpp
index 7f1f2cc92..493561a24 100644
--- a/cores/esp8266/HardwareSerial.cpp
+++ b/cores/esp8266/HardwareSerial.cpp
@@ -551,8 +551,6 @@ bool HardwareSerial::isRxEnabled(void) {
     return _uart->rxEnabled;
 }
 
-extern "C" void optimistic_yield();
-
 int HardwareSerial::available(void) {
     int result = 0;
 
diff --git a/cores/esp8266/core_esp8266_main.cpp b/cores/esp8266/core_esp8266_main.cpp
index 53f154bd0..ca7805569 100644
--- a/cores/esp8266/core_esp8266_main.cpp
+++ b/cores/esp8266/core_esp8266_main.cpp
@@ -87,7 +87,7 @@ extern "C" void __yield() {
 }
 extern "C" void yield(void) __attribute__ ((weak, alias("__yield")));
 
-extern "C" void optimistic_yield() {
+extern "C" void optimistic_yield(void) {
     if (!ETS_INTR_WITHINISR() && 
         (system_get_time() - g_micros_at_last_task_yield) > OPTIMISTIC_YIELD_TIME_US)
     {
diff --git a/libraries/ESP8266WiFi/src/WiFiClient.cpp b/libraries/ESP8266WiFi/src/WiFiClient.cpp
index 4327b9041..083463444 100644
--- a/libraries/ESP8266WiFi/src/WiFiClient.cpp
+++ b/libraries/ESP8266WiFi/src/WiFiClient.cpp
@@ -177,8 +177,6 @@ size_t WiFiClient::write(const uint8_t *buf, size_t size)
     return _client->write(reinterpret_cast<const char*>(buf), size);
 }
 
-extern "C" void optimistic_yield();
-
 int WiFiClient::available()
 {
     int result = 0;
diff --git a/libraries/ESP8266WiFi/src/WiFiServer.cpp b/libraries/ESP8266WiFi/src/WiFiServer.cpp
index 8c5378cd5..cf975a330 100644
--- a/libraries/ESP8266WiFi/src/WiFiServer.cpp
+++ b/libraries/ESP8266WiFi/src/WiFiServer.cpp
@@ -89,8 +89,6 @@ bool WiFiServer::hasClient(){
   return false;
 }
 
-extern "C" void optimistic_yield();
-
 WiFiClient WiFiServer::available(byte* status)
 {
     if (_unclaimed)
diff --git a/libraries/ESP8266WiFi/src/WiFiUdp.cpp b/libraries/ESP8266WiFi/src/WiFiUdp.cpp
index aa7e071d7..71f8ff41a 100644
--- a/libraries/ESP8266WiFi/src/WiFiUdp.cpp
+++ b/libraries/ESP8266WiFi/src/WiFiUdp.cpp
@@ -113,8 +113,6 @@ uint8_t WiFiUDP::beginMulticast(IPAddress interfaceAddr, IPAddress multicast, ui
     return 1;
 }
 
-extern "C" void optimistic_yield();
-
 /* return number of bytes available in the current packet,
    will return zero if parsePacket hasn't been called yet */
 int WiFiUDP::available() {
diff --git a/libraries/Wire/Wire.cpp b/libraries/Wire/Wire.cpp
index 39a2526ba..84b4c8c22 100644
--- a/libraries/Wire/Wire.cpp
+++ b/libraries/Wire/Wire.cpp
@@ -160,8 +160,6 @@ size_t TwoWire::write(const uint8_t *data, size_t quantity){
   return quantity;
 }
 
-extern "C" void optimistic_yield();
-
 int TwoWire::available(void){
     int result = rxBufferLength - rxBufferIndex;