Why is my pointer being initialized to 0x0?

Question

I've been going through the K&R 2nd edition book and I've been stuck trying to get one of the examples to run properly.

The code taken straight from chapter 5 in the book (with some random variables I added in main):

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}
};

/* day_of_year: set day of year from 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;
}

/* month_day: set month, day from day of year */
void 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;
    *pday = yearday;
}

int main(){
    int x = 5, y = 10;
    int *q, *p;
    month_day(x, y, q, p);
}

This program was giving me segmentation faults, so I tried running it in gdb.

I put a breakpoint in the month_day function, and something immediately caught my attention.

gdb shows that pmonth is initialized to some address, but pday is initialized to 0x0. This can't be right and I know that, but what is causing this in the first place?

This is the month_day function with its parameters in gdb:

month_day (year=5, yearday=10, pmonth=0x7fffffffe150, pday=0x0) at e5-8.c:19

And as expected, when attempting to print *pmonth, it works as it should, but printing *pday does not because its initialized to 0x0.

gdb output:

19      leap = year%4 == 0 && year%100 != 0 || year%400 == 0;
(gdb) print pday
$1 = (int *) 0x0
(gdb) print *pday
Cannot access memory at address 0x0
(gdb) print *pmonth
$2 = 1

This is most likely an easy fix but I'd like to understand what's going on too!

You are dereferencing uninitialized pointers. This is already *undefined behavior*. Why are you passing uninitialized pointers to the function? — UnholySheep, Jan 19 '22 at 18:19
Given `int *q, *p;`, what do you expect the value of `p` and `q` to be, and more importantly, **what do you expect them to point at**? — Andrew Henle, Jan 19 '22 at 18:20
`p` is not initialized, so it doesn't point to a memory location you can write to. So `*pday = yearday` causes a segfault. — William Pursell, Jan 19 '22 at 18:20
@klutt Yes, it's quite an old book. The only thing I added was the part that was wrong (everything in main) — Hoswoo, Jan 19 '22 at 18:34
@Hoswoo Nvm. I missed *"(with some random variables I added in main):"* — klutt, Jan 19 '22 at 18:39
It may be an old book, but it's not careless or full of uninitialized variables! — Steve Summit, Jan 19 '22 at 18:45

score 1 · Accepted Answer · answered Jan 19 '22 at 18:21

1

You need to pass valid addresses. Perhaps you just want:

int main(void){
    int x = 5, y = 10;
    int q, p;
    month_day(x, y, &q, &p);
}

answered Jan 19 '22 at 18:21

William Pursell

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Ugh, I feel so silly... thank you very much – Hoswoo Jan 19 '22 at 18:23

Why is my pointer being initialized to 0x0?

1 Answers1