gistfile1.c

#include "avr/interrupt.h"

int inPin = 3;
int outPin = 4;

volatile boolean state = false;

void setup() {
  pinMode(inPin, INPUT);
  digitalWrite(inPin, HIGH);
  pinMode(outPin, OUTPUT);
  GIMSK = 0b00100000;    // turns on pin change interrupts
  PCMSK = 0b00001000;    // turn on interrupts on pin PB3
  sei();
}

void loop() {
  
}

ISR(PCINT0_vect) {
  digitalWrite(outPin, digitalRead(inPin));
}