diff --git a/hardware/arduino/cores/arduino/Arduino.h b/hardware/arduino/cores/arduino/Arduino.h
index 425dfcb8d5a..8673ab8abff 100755
--- a/hardware/arduino/cores/arduino/Arduino.h
+++ b/hardware/arduino/cores/arduino/Arduino.h
@@ -115,6 +115,8 @@ typedef uint8_t byte;
 void init(void);
 void initVariant(void);
 
+int atexit(void (*func)()) __attribute__((weak));
+
 void pinMode(uint8_t, uint8_t);
 void digitalWrite(uint8_t, uint8_t);
 int digitalRead(uint8_t);
diff --git a/hardware/arduino/cores/arduino/main.cpp b/hardware/arduino/cores/arduino/main.cpp
index 091c365fcef..a60980da57a 100644
--- a/hardware/arduino/cores/arduino/main.cpp
+++ b/hardware/arduino/cores/arduino/main.cpp
@@ -19,6 +19,9 @@
 
 #include <Arduino.h>
 
+//Declared weak in Arduino.h to allow user redefinitions.
+int atexit(void (*func)()) { return 0; }
+
 // Weak empty variant initialization function.
 // May be redefined by variant files.
 void initVariant() __attribute__((weak));