avr_demo/hello_uart/uart.h

int uart_putchar(char c, FILE *stream);
int uart_getchar(FILE *stream);

void uart_init(void);

/* http://www.ermicro.com/blog/?p=325 */

FILE uart_output = FDEV_SETUP_STREAM(uart_putchar, NULL, _FDEV_SETUP_WRITE);
FILE uart_input = FDEV_SETUP_STREAM(NULL, uart_getchar, _FDEV_SETUP_READ);