Add mailbox struct to mailbox.h

mailbox.h

@@ -1,8 +1,11 @@
 #ifndef MAILBOX_H
 #define MAILBOX_H
 
+#include <semaphore.h>
+
 struct msg {
 	// index of thread that sent the message
+	// if iFrom is -1, then the message is considered invalid.
 	int iFrom;
 	// value of message
 	int value;
@@ -12,4 +15,13 @@ struct msg {
 	int tot;
 };
 
+struct mailbox {
+	// the current message in the mailbox
+	struct msg stored_message;
+	// represents whether or not a message can be sent
+	sem_t send_sem;
+	// represents whether or not a message can be receieved
+	sem_t recv_sem;
+};
+
 #endif