Arduino AVR SPI master/slave

Master.ino

#include <SPI.h>
 
void setup () {
   Serial.begin(115200); //set baud rate to 115200 for usart
   digitalWrite(SS, HIGH); // disable Slave Select
   SPI.begin ();
   SPI.setClockDivider(SPI_CLOCK_DIV8);//divide the clock by 8
}
 
void loop () {
   char c;
   digitalWrite(SS, LOW); // enable Slave Select
   // send test string
   for (const char * p = "Hello, world!\r" ; c = *p; p++) 
   {
      SPI.transfer (c);
      Serial.print(c);
   }
   digitalWrite(SS, HIGH); // disable Slave Select
   delay(2000);
}

Slave.ino

#include <SPI.h>
char buff [50];
volatile byte indx;
volatile boolean process;
 
void setup () {
   Serial.begin (115200);
   pinMode(MISO, OUTPUT); // have to send on master in so it set as output
   SPCR |= _BV(SPE); // turn on SPI in slave mode
   indx = 0; // buffer empty
   process = false;
   SPI.attachInterrupt(); // turn on interrupt
}
 
ISR (SPI_STC_vect) // SPI interrupt routine 
{ 
   byte c = SPDR; // read byte from SPI Data Register
   if (indx < sizeof buff) {
      buff [indx++] = c; // save data in the next index in the array buff
      if (c == '\r') //check for the end of the word
      process = true;
   }
}
 
void loop () {
   if (process) {
      process = false; //reset the process
      Serial.println (buff); //print the array on serial monitor
      indx= 0; //reset button to zero
   }
}