How to use the xprintf function on stm32f429?
Register the functions that send\receive one character to UART. 

Read the last paragraph here. Download the Examples and take a look at the stm32 folder.
http://elm-chan.org/fsw/strf/xprintf.html

Here's what I did using the HAL drivers and STM32Cube generated project.

xdev_out(uart_putc);
xdev_in(uart_getc);

void uart_putc(char c){
  HAL_UART_Transmit(&amp;huart2, (uint8_t*)&amp;c, 1, 0xFFFF);
}
void uart_getc(char c){
  HAL_UART_Receive(&amp;huart2, (uint8_t*)&amp;c, 1, 0xFFFF);
}