AVR Arduino serial C

#define F_CPU 16000000UL
#define BAUD 9600


#include <stdio.h>
#include <stdint.h>
#include <avr/io.h>
#include <util/delay.h>
#include <util/setbaud.h>


void uart_init(void) {
    UBRR0H = UBRRH_VALUE;
    UBRR0L = UBRRL_VALUE;

#if USE_2X
    UCSR0A |= _BV(U2X0);
#else
    UCSR0A &= ~(_BV(U2X0));
#endif

    UCSR0C = _BV(UCSZ01) | _BV(UCSZ00); /* 8-bit data */
    UCSR0B = _BV(RXEN0) | _BV(TXEN0);   /* Enable RX and TX */
}

void uart_putchar(char c) {
    //loop_until_bit_is_set(UCSR0A, UDRE0); /* Wait until data register empty. */
    while(!(UCSR0A & (1<<UDRE0)));
    UDR0 = c;
}

char uart_getchar(void) {
    //loop_until_bit_is_set(UCSR0A, RXC0); /* Wait until data exists. */
    while(!(UCSR0A & (1<<RXC0)));
    return UDR0;
}

void uart_putstring(char* string)
{
    char *p = string;
    while(*(p++))
    {
        uart_putchar(*p);
    }
}

char String[] = "Hello world!!\n";

static FILE uart_output = FDEV_SETUP_STREAM((int (*)(char, struct __file *)) uart_putchar, NULL, _FDEV_SETUP_WRITE);
static FILE uart_input  = FDEV_SETUP_STREAM(NULL, (int (*)(struct __file *)) uart_getchar, _FDEV_SETUP_READ);

int main(int main, char *argv[])
{
    uart_init();
    stdout = &uart_output;
    stdin  = &uart_input;

    char input;
    while (1)
    {
        puts("Hello world!");
        input = getchar();
        printf("You wrote %c\n", input);

    }
    return 0;
}