AVR Use printf() with UART

struct FILE

struct __file {
    char *        buf;   /* buffer pointer */
    unsigned char unget; /* ungetc() buffer */
    uint8_t       flags; /* flags, see below */
#define __SRD     0x0001 /* OK to read */
#define __SWR     0x0002 /* OK to write */
#define __SSTR    0x0004 /* this is an sprintf/snprintf string */
#define __SPGM    0x0008 /* fmt string is in progmem */
#define __SERR    0x0010 /* found error */
#define __SEOF    0x0020 /* found EOF */
#define __SUNGET  0x040  /* ungetc() happened */
#define __SMALLOC 0x80   /* handle is malloc()ed */
#if 0
/* possible future extensions, will require uint16_t flags */
#define __SRW  0x0100 /* open for reading & writing */
#define __SLBF 0x0200 /* line buffered */
#define __SNBF 0x0400 /* unbuffered */
#define __SMBF 0x0800 /* buf is from malloc */
#endif
    int size; /* size of buffer */
    int len;  /* characters read or written so far */
    int (*put)(char,
               struct __file *); /* function to write one char to device */
    int (*get)(struct __file *); /* function to read one char from device */
    void *udata;                 /* User defined and accessible data. */
};

typedef struct __file FILE;

#define FDEV_SETUP_STREAM(p, g, f) \
    { .put = p, .get = g, .flags = f, .udata = 0, }

Set stdout

#include "uart0.h"

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

static int uart_putchar(char c, FILE *stream) {
    if(c == '\n') { uart0_write('\r'); }
    uart0_write(c);
    return 0;
}

static FILE _stdout = FDEV_SETUP_STREAM(uart_putchar, NULL, _FDEV_SETUP_WRITE);

int main() {
    uart0_init(115200);
    stdout = &_stdout;

    printf("Start setup.\n");

    while(1) {

    }
}