The best way to return a result - c

Question

I wrote the following function. This function receives the address of a hex value e.g. 0x4571 and calculates with Day, Month and Year from the bit positions of the hex value.
void fat_dir_date(char *dateAr) {
const unsigned int MaskDayOfMonth = 0x1F; //0000000000011111
const unsigned int MaskMonthOfYear = 0x1E0; //0000000111100000
const unsigned int MaskYear = 0xFE00; //1111111000000000
unsigned int DayOfMonth = hex & MaskDayOfMonth; //AND Bit Operation
unsigned int MonthOfYear = hex & MaskMonthOfYear; //AND Bit Operation
MonthOfYear = MonthOfYear >> 5; //Bitshift to right position
unsigned int Year = hex & MaskYear; //AND Bit Operation
Year = Year >> 9; //Bitshift to right position
printf("%d.%d.%d", DayOfMonth, MonthOfYear, 1980+Year);
}
The calculation works fine. I get the right numbers in the integer DayOfMonth,MonthOfYear and Year. But instead of printing them out with printf, I want to return the values to the caller function. In the best way in concatenated in a single value or string.
How is the best way to solve this in C?

Answer 1

You have several options there:
Create a struct that has the three fields, and return it,
Let the caller pass you the struct that you fill in,
Let the caller pass you a string buffer to which you print using sprintf, or
Create a string dynamically, print to it, and return.
The first option is clean and easy to understand. It requires some copying, but it is fine for small structures like the one that you need:
struct DateTime {
int DayOfMonth;
int MonthOfYear;
int Year;
};
struct DateTime fat_dir_date(unsigned int hex) {
struct DateTime res;
res.DayOfMonth = ...
res.MonthOfYear = ...
res.Year = ...
return res;
}

Answer 2

If you need to print the data and you don't need to use the numeric values, I think this:
char * fat_dir_date(char *dateAr,unsigned int hex) {
const unsigned int MaskDayOfMonth = 0x1F; //0000000000011111
const unsigned int MaskMonthOfYear = 0x1E0; //0000000111100000
const unsigned int MaskYear = 0xFE00; //1111111000000000
unsigned int DayOfMonth = hex & MaskDayOfMonth; //AND Bit Operation
unsigned int MonthOfYear = hex & MaskMonthOfYear; //AND Bit Operation
MonthOfYear = MonthOfYear >> 5; //Bitshift to right position
unsigned int Year = hex & MaskYear; //AND Bit Operation
Year = Year >> 9; //Bitshift to right position
sprintf(dateAr,"%02u.%02u.%4u", DayOfMonth,MonthOfYear, 1980+Year);
return dateAr;
}
int main(void)
{
char dateAr[11];
unsigned int hex=0x1010; //Random :)
printf("%s\n" , fat_dir_date(dateAr,hex))
return 0;
}