C K&R: printf pointer = nil?

Question

int *month, *day;

printf("%p,%p\n", month, day);

Executing the codesample above returns: 0x7fffeda86190,(nil)

Also i get a segmentation fault whenever i assign something to the *day pointer as demonstrated in the code below coming from K&R chapter 5.7:

What is causing this behavior and where can i learn more about this kind of behavior?

static char daytab[2][13] = {
    {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
    {0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31} 
};

int main()
{
    int *month, *day;

    printf("%p,%p\n", month, day);

    month_day(2009, 78, month, day);

    printf("%d,%d\n", *month, *day);

}


int day_of_year(int year, int month, int day)
{
    int i, leap;

    leap = year%4 == 0 && year%100 != 0 || year%400 == 0;
    for (i = 1; i < month; i++)
        day+= daytab[leap][i];
    return day; /*day of the year*/
}
int month_day(int year, int yearday, int *pmonth, int *pday)
{
    int i, leap;

    leap = year%4 == 0 && year%100 != 0 || year%400 == 0;
    for (i = 1; yearday > daytab[leap][i]; i++)
        yearday -= daytab[leap][i];
    *pmonth = i;
    printf("%d,%d\n", *pmonth, yearday);
    *pday = yearday;
}

Answer 1

Your month and day pointer point to nowhere (or some random location). Let's look at the day pointer.

*day = value;

When you're trying to write the memory pointed by day, your OS will figure out writing to this address (0?) makes no sense and kills your program.

Solution 1: Use malloc to dynamically allocate some memory and point day there:

day = (int*)malloc(sizeof(int));
*day = 10; //this is now legal

You could also make the pointers point memory inside some array etc.

Solution 2: Pass pointers to 'regular' variables to the month_day function:

int mymonth, myday;
month_day(2009, 78, &mymonth, &myday);
// now mymonth and myday should have expected values

Hope this helps.