UART on Raspberry Pi 4

Question

I open the port and set the UART

uart_fd = open("/dev/serial0", O_RDWR | O_NOCTTY | O_NDELAY);

if (uart_fd  < 0)
{
    printf("Failed to open UART\n");
    return -1;
}

config.c_cflag = UART_SPEED | CS8 | CLOCAL | CREAD;
config.c_iflag = IGNPAR;
config.c_oflag = 0;
config.c_lflag = 0;

if (tcsetattr(uart_fd, TCSANOW, &config) < 0)  
{
    printf("Error setting UART configuration\n");
    return -1;
}

And I send strings to terminal

void UART_WriteString(const char *str)
{
    write (uart_fd, str, strlen(str));

    //wait all chars are sent
    tcdrain(uart_fd);
}

now I get gibberish on terminal no matter what baud rate I choose. What could be a problem?