3 files changed, 8 insertions, 3 deletions
diff --git a/cores/arduino/HardwareSerial.h b/cores/arduino/HardwareSerial.h
index 8a5bf95..17000c2 100644
--- a/cores/arduino/HardwareSerial.h
+++ b/cores/arduino/HardwareSerial.h
@@ -124,7 +124,7 @@ class HardwareSerial : public Stream
     virtual int available(void);
     virtual int peek(void);
     virtual int read(void);
-    int availableForWrite(void);
+    virtual int availableForWrite(void);
     virtual void flush(void);
     virtual size_t write(uint8_t);
     inline size_t write(unsigned long n) { return write((uint8_t)n); }
diff --git a/cores/arduino/Print.h b/cores/arduino/Print.h
index d099a3b..4d8a2f5 100644
--- a/cores/arduino/Print.h
+++ b/cores/arduino/Print.h
@@ -22,6 +22,7 @@
 
 #include <inttypes.h>
 #include <stdio.h> // for size_t
+#include <limits.h> // for INT_MAX
 
 #include "WString.h"
 #include "Printable.h"
@@ -57,7 +58,11 @@ class Print
     size_t write(const char *buffer, size_t size) {
       return write((const uint8_t *)buffer, size);
     }
-    
+
+    // default to zero, meaning "a single write may block"
+    // should be overriden by subclasses with buffering
+    virtual int availableForWrite() { return 0; }
+
     size_t print(const __FlashStringHelper *);
     size_t print(const String &);
     size_t print(const char[]);
diff --git a/cores/arduino/USBAPI.h b/cores/arduino/USBAPI.h
index 358444e..479ced9 100644
--- a/cores/arduino/USBAPI.h
+++ b/cores/arduino/USBAPI.h
@@ -98,7 +98,7 @@ public:
 	virtual int available(void);
 	virtual int peek(void);
 	virtual int read(void);
-	int availableForWrite(void);
+	virtual int availableForWrite(void);
 	virtual void flush(void);
 	virtual size_t write(uint8_t);
 	virtual size_t write(const uint8_t*, size_t);