Output::sendMessage(): lock RtMidiOut::sendMessage() with mutex

Output.cpp

@@ -6,7 +6,10 @@ namespace midi {
 void Output::sendMessage(const Message& message)
 {
 	std::vector<unsigned char> bytes = message.getBytes();
+
+    sendMessage_mutex.lock();
 	RtMidiOut::sendMessage(&bytes);
+    sendMessage_mutex.unlock();
 }
 
 void Output::sendMessages(const MessageList& messageList)

Output.h

@@ -3,11 +3,15 @@
 #include "RtMidi.h"
 #include "Message.h"
 #include "MessageList.h"
+#include <mutex>
 
 namespace midi {
 
 class Output : public RtMidiOut
 {
+private:
+    std::mutex sendMessage_mutex;
+
 public:
 	void sendMessage(const Message& message);
 	void sendMessages(const MessageList& messageList);