main.c

#include <stdio.h>
#include "serial.h"
#include <avr/pgmspace.h>

int main(void) {

	serial_init(19200);

	int i;
	char buffer[8];
	for(i = 0;; i ^= 1) {
		if (!i) {
			sprintf(buffer, "%s", "Foo");
		} else {
			sprintf(buffer, "%s", "Bar");
		}
		printf_P(PSTR("State: %s\r\n"), buffer);
	}

}

serial.c

#include <stdio.h>
#include <avr/io.h>
#include "serial.h"

void serial_init(uint32_t baudrate) {
    // Set Baudrate
    UBRR0H = (UBRR_ASYNC_NORMAL(baudrate) >> 8);
    UBRR0L = UBRR_ASYNC_NORMAL(baudrate);
    
    // Enable RX and TX
    UCSR0B |= _BV(RXEN0) | _BV(TXEN0);

    // Set 8N1 Framing
    UCSR0C |= _BV(UCSZ00) | _BV(UCSZ01);
    
    fdevopen(serial_write, serial_read);
}

int serial_write(char c, FILE *fp) {
    (void)*fp;
    while ( !(UCSR0A & (1 << UDRE0)) );
    UDR0 = c;
    return 0;
}

int serial_read(FILE *fp) {
    (void)*fp;
    while ( !(UCSR0A & (1 << RXC0)) );
    return UDR0;
}

serial.h

#ifndef SERIAL_H
#define SERIAL_H
#include <stdint.h>
#define UBRR_ASYNC_NORMAL(br) ((F_CPU / (br * 16UL)) - 1)

void serial_init(uint32_t baudrate);   // Initializes Serial
int serial_write(char c, FILE *fp); // Send a character over serial
int serial_read(FILE *fp);  // Read a character from serial buffer (blocking)

#endif