How does this retarget.c work - c

Question

Can someone explain to me how this retarget.c works?
I am trying to send integers to and from the uart of a microcontroller, i have been successful with using fgets to get a char (16 bits) and returning an integer using the atoi function in the Uart Interupt service routine but I am trying to get an integer using scanf, i am thinking i need to change the retarget file outlined below?
#include <stdio.h>
#include <time.h>
#include <rt_misc.h>
#define AHB_LED_BASE 0x50000000
#define AHB_UART_BASE 0x51000000
#pragma import(__use_no_semihosting)
struct __FILE {
unsigned char * ptr;
};
FILE __stdout = {(unsigned char *) AHB_UART_BASE};
FILE __stdin = {(unsigned char *) AHB_UART_BASE};
int fputc(int ch, FILE *f)
{
return(uart_out(ch));
}
int fgetc(FILE *f)
{
return(uart_in());
}
int ferror(FILE *f)
{
return 0;
}
int uart_out(int ch)
{
int* UARTPtr;
UARTPtr = (int*)AHB_UART_BASE;
*UARTPtr = (int)ch;
return(ch);
}
int uart_in()
{
int ch;
int* UARTPtr;
UARTPtr = (int*)AHB_UART_BASE;
ch = *UARTPtr;
uart_out(ch);
return((int)ch);
}
void _ttywrch(int ch)
{
fputc(ch,&__stdout);
}
void _sys_exit(void) {
while(1);
}
//------------------------------------------------------------------------------
// Cortex-M0
//------------------------------------------------------------------------------
#include <stdio.h>
#include <time.h>
#include <rt_misc.h>
#include <stdlib.h>
#define AHB_LED_BASE 0x50000000
#define AHB_UART_BASE 0x51000000
void UART_ISR(void)
{
int sample;
printf("the value entered is %d\n", sample);
}
//////////////////////////////////////////////////////////////////
// Main Function
//////////////////////////////////////////////////////////////////
int main() {
{
int sample;
scanf ("%d",&sample);
}
}

Answer 1

The scanf function will need ungetc because it must scan ahead in the buffer to see when fields end. E.g., when looking for a number, it needs to pull one character after the number to see where the number ends. When it sees the non-number character, it needs to put it back into the stream so the next call to getc will get it.
Something like this:
struct __FILE
{
unsigned char * ptr;
int unchar; /* place to keep the character put back in the stream */
};
FILE __stdout = {(unsigned char *) AHB_UART_BASE, -1};
FILE __stdin = {(unsigned char *) AHB_UART_BASE, -1};
int fgetc(FILE *f)
{
int c;
if (f->unchar == -1)
{
c = uart_in(); /* just read a character */
}
else
{
c = f->unchar; /* reuse the character put back by ungetc */
f->unchar = -1; /* mark it as used */
}
return c;
}
int fungetc(int c, FILE *f)
{
unsigned char uc = c; /* POSIX says that it is converted first to unsigned char */
f->unchar = (int )uc; /* put back the character */
return (int )uc;
}