Examples: Update serial to also use printf.

Author: microbit-carlos

examples/serial.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright 2020 Micro:bit Educational Foundation and contributors
+ * Copyright 2020-2022 Micro:bit Educational Foundation and contributors
  * SPDX-License-Identifier: Apache-2.0
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
@@ -16,19 +16,29 @@
  */
 #include "mbed.h"
 
-static UnbufferedSerial pc_serial(USBTX, USBRX, 115200);
+UnbufferedSerial pc_serial(USBTX, USBRX, 115200);
 
-void rx_irq() {
+// Wire up printf to serial
+FileHandle *mbed::mbed_override_console(int fd) {
+    return &pc_serial;
+}
+
+void rx_irq_echo() {
     char single_byte;
-    // Echo each byte received
     if (pc_serial.read(&single_byte, 1)) {
         pc_serial.write(&single_byte, 1);
     }
 }
 
 int main(void) {
+    printf("printf test! (MbedOS %d.%d.%d)\n",
+           MBED_MAJOR_VERSION, MBED_MINOR_VERSION, MBED_PATCH_VERSION);
+
+    // Send a string directly (size calculation removes the null terminator)
     char hello[] = "hello world!\n";
-    size_t hello_size = sizeof(hello) / sizeof(hello[0]);
+    size_t hello_size = (sizeof(hello) / sizeof(hello[0])) - 1;
     pc_serial.write(hello, hello_size);
-    pc_serial.attach(&rx_irq, SerialBase::RxIrq);
+
+    // Configure RX IRQ to a function to echos back the data
+    pc_serial.attach(&rx_irq_echo, SerialBase::RxIrq);
 }