void setup() {
   // put your setup code here, to run once:
   Serial.begin(9600);
   String s1 = "Hello World!";
   char buf[30];

   s1.toCharArray(buf,6);
   Serial.println(buf);

   s1.toCharArray(buf,s1.length());
   Serial.println(buf[0]);
}
void loop() {
   // put your main code here, to run repeatedly:
}