SPI Arduino

#include<SPI.h>
char buf[100];
byte c=0,b=0;
volatile byte pos;
volatile boolean processing;
byte i;
void setup(void) {
// put your setup code here, to run once:
Serial.begin(9600);
pinMode(MISO,OUTPUT);
pinMode(MOSI,INPUT);
SPCR |= _BV(SPE);
pos = 0;
processing = false;
SPI.attachInterrupt();
}
ISR(SPI_STC_vect){
c = SPDR;
// SPDR = 0x55;
// if(pos<20){
// buf[pos++] = c;
// if(c == '\n')
// processing=true;
// }
processing = true;
}
void loop(void) {
// put your main code here, to run repeatedly:
if(processing){
// Serial.println("Inside Processing");
Serial.println(c);
processing = false;
SPDR = i;
i = i+1;
}
}