What is the reason for error while returning a structure in this C program?

Question

My program intends to achieve this

(A) Write a C function named larger() that returns the later date of any two dates passed to it. For

Answer 1

As half-promised, a revision of Rüppell's Vulture's answer. This code struggles to avoid the repetition in the other answer — which is already reduced by comparison with the code in the question.

There are multiple changes. Since the type only stores a single date, it is renamed Date. There is a general purpose function read_validate_number() which is used to handle the entry of each component of the date (with a separate prompt). The function returns an error/non-error status and returns the value via a pointer argument. It puts an upper bound on the number of times the user is prompted for a number. It avoids insulting the user but does report the erroneous value. The code uses a while loop rather than a do ... while loop; generally, the latter should be avoided. It would be possible to read to a newline after a failure to read a number instead of simply returning an error.

With this function on hand, writing read_date() becomes trivial. And with read_date() on hand, the main() function simplifies.

I'm still not keen on the interface to the function larger(); in general, I prefer the interface shown in later_date(). However, the code shows the one advantage of the larger() interface; it can identify the larger of the two dates by position in the array, whereas later_date() does that by value.

The functions other than main() are static since they aren't used outside this file; the compiler options I use require either static functions or an extern declaration.

On the whole, I'd prefer a less verbose interface for entering dates. I also prefer the ISO 8601 format for dates since they are unambiguous; however, that would be a user preference item in fully internationalized (and localized) code.

#include 
#include 
#include 

#define NUM 2
typedef struct Date
{
       int month;
       int day;
       int year;
} Date;

enum { MAX_ERRORS = 3 };

static int read_validate_number(const char *tag, int min, int max, int *value)
{
    int errors = 0;
    assert(min <= max);
    while (errors++ < MAX_ERRORS)
    {
        printf("Please enter the %s number (%d-%d): ", tag, min, max);
        if (scanf("%d", value) != 1)
        {
            printf("Failed to read number\n");
            return EOF;
        }
        if (*value >= min && *value <= max)
            return 0;
        printf("The value entered (%d) is outside the range %d-%d\n", *value, min, max);
    }
    printf("Too many errors entering %s\n", tag);
    return EOF;
}

static int read_date(Date *date)
{
    if (read_validate_number("month", 1,   12, &date->month) != 0 ||
        read_validate_number("day",   1,   31, &date->day  ) != 0 ||
        read_validate_number("year",  1, 9999, &date->year ) != 0)
        return EOF;
    return 0;
}

static Date *larger(Date *more)
{
    int days0 = (more[0].month*31)+(more[0].day)+(more[0].year*365);
    int days1 = (more[1].month*31)+(more[1].day)+(more[1].year*365);

    // Resist the temptation to write: return more + (days1 > days0);
    if (days1 > days0)
        return more+1;
    else
        return more+0;
}

static Date later_date(Date d1, Date d2)
{
    int days1 = d1.day + d1.month * 31 + d1.year * 365;
    int days2 = d2.day + d2.month * 31 + d2.year * 365;

    if (days1 > days2)
        return d1;
    else
        return d2;
}

int main(void)
{
    Date user[NUM];

    printf("Enter two dates, and the program will return the larger.\n");
    if (read_date(&user[0]) == 0 && read_date(&user[1]) == 0)
    {
        putchar('\n');
        printf("Date 1: %.4d-%.2d-%.2d\n", user[0].year, user[0].month, user[0].day);
        printf("Date 2: %.4d-%.2d-%.2d\n", user[1].year, user[1].month, user[1].day);
        Date *p_later = larger(user);
        Date  v_later = later_date(user[0], user[1]);
        printf("\nThe later date is the %s (%d/%d/%d)\n",
               (p_later == &user[0]) ? "first" : "second",
               p_later->month, p_later->day, p_later->year);
        printf("Later Date: %.4d-%.2d-%.2d\n", v_later.year, v_later.month, v_later.day);
    }

    return 0;
}

Sample output:

Enter two dates, and the program will return the larger.
Please enter the month number (1-12): 12
Please enter the day number (1-31): 25
Please enter the year number (1-9999): 2013
Please enter the month number (1-12): 1
Please enter the day number (1-31): 1
Please enter the year number (1-9999): 2012

Date 1: 2013-12-25
Date 2: 2012-01-01

The later date is the first (12/25/2013)
Later Date: 2013-12-25

I note that you could reduce the size of the Date structure by using unsigned char (or uint8_t) for the day and month components, and an unsigned short (or uint16_t) for the year component. However, you'd have to modify the read_date() function a bit to do so:

#include 

typedef struct Date
{
       uint8_t  month;
       uint8_t  day;
       uint16_t year;
} Date;

static int read_date(Date *date)
{
    int mm, dd, yyyy;
    if (read_validate_number("month", 1,   12, &mm  ) != 0 ||
        read_validate_number("day",   1,   31, &dd  ) != 0 ||
        read_validate_number("year",  1, 9999, &yyyy) != 0)
        return EOF;
    date->month = mm;
    date->day   = dd;
    date->year  = yyyy;
    return 0;
}

At some point, you might want to prevent someone entering the 31st of February (but remember that there was once a 30th of February — in Sweden, in 1712; or there again, maybe you don't need to remember that).