Process returned -1073741819 (0xC0000005) (why though??)

Question

So i was solving my homework, and i did this piece of code which is supposed to find the biggest difference beetween two prime numbers in the interval of [a,b], and i got "Process returned -1073741819 (0xC0000005)"

#include <iostream>
#include <vector>
#include <bitset>

using namespace std;
bitset <10000000>v;
int main()
{
    for (int i = 2; i < 10000000; i++)
    {
        if (v[i] == 0)
        {
            for (int j = i * i; j < 10000000; j += i)
                v[j] = 1;
        }
    }
    int n, a, b, maxi = 0, mini = 0, smax = 0;
    cin >> a >> b;
    int poz = a;
    while (v[poz] == 1)
        poz++;
    int prev = poz;
    poz++;
    while (v[poz] == 1 && poz < b)
        poz++;
    if (poz == b && v[b] == 1)
    {
        cout << -1; return 0;
    }
    int next = poz;
    poz++;
    while (poz <= b)
    {
        if (next - prev > smax)
        {
            smax = next - prev;
            maxi = next;
            mini = prev;
        }
        if (v[poz] == 0)
        {
            prev = next;
            next = poz;
        }
        poz++;
    }
    cout << mini << " " << maxi;
    return 0;
}

i expected 43 with 47

Read [this](https://stackoverflow.com/questions/4629712/simple-array-cause-exception) — Scamtex, Mar 25 '19 at 16:21
i guess the task is about biggest difference between two consecutive prime numbers, otherwise it is just the difference between the first and the last prime — 463035818_is_not_an_ai, Mar 25 '19 at 16:22
@Scamtex This is not a problem, due to 1) `v` not being on the stack; 2) Asker not getting stack overflow, but rather access violation. — Algirdas Preidžius, Mar 25 '19 at 16:22
Try searching for the return value (both decimal and hexadecimal). It should give you some hints. — Some programmer dude, Mar 25 '19 at 16:23
You have to [debug](https://ericlippert.com/2014/03/05/how-to-debug-small-programs/) your program. Primarily, you should use a debugger to catch the crash "in action", and then you will be able to pinpoint where in your code it happens, as well as be able to examine your variables and their values to make sure they are okay. — Some programmer dude, Mar 25 '19 at 16:38

Alan Birtles · Accepted Answer · 2019-03-25T16:45:19.943

3

In your initialisation loop when i is 46349, i*i is 2,148,229,801, this is larger than fits inside a signed 32-bit integer so evaluates to -2,146,737,495. v[j] then causes the crash.

You should either modify your code to use a larger data type or set the limit for i to sqrt(10000000) rather than 10000000.

edited Mar 25 '19 at 16:45

answered Mar 25 '19 at 16:37

Alan Birtles

32,622
4
31
60

No, `i*i` is correct for the sieve. When considering multiples of 5, you don't need to consider 5x2, or 5x3 (they will have been eliminated as multiple of 2 and 3), - you start at 5x5. (Point is though, that similarly you don't need to consider values of `i` above sqrt(10million) ). – Martin Bonner supports Monica Mar 25 '19 at 16:40
ok, was just going by wikipedia which says to start at 2p in step 3 https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes – Alan Birtles Mar 25 '19 at 16:43
Works on my machine. However, I'm using 64-bit int. I think it will fail on machines using 32-bit int. – Eljay Mar 25 '19 at 16:43
@Eljay What compiler are you using that has 64-bit *int* ? (64-bit pointers and longs, sure - but int ??!?) – Martin Bonner supports Monica Mar 25 '19 at 16:45
@MartinBonner • a modified version of clang. I'm also using separate heaps for `malloc`, `new` and `new[]`. – Eljay Mar 25 '19 at 16:48
2

64-bit int -- int64_t, include < cstdint > – 2785528 Mar 25 '19 at 17:37

score 2 · Answer 2 · answered Mar 25 '19 at 16:37

2

I'm guessing that i*i overflows when i is large, leading to a negative value for j and an access violation on v[j]=1;.

answered Mar 25 '19 at 16:37

john

85,011
4
57
81

score 0 · Answer 3 · edited Mar 25 '19 at 16:31

0

You have a potential segmentation fault on line v[j]=1; where j may exceed 10000000.

Please check your boundaries.

edited Mar 25 '19 at 16:31

Aykhan Hagverdili

28,141
6
41
93

answered Mar 25 '19 at 16:29

Byung Kim

21
5

Process returned -1073741819 (0xC0000005) (why though??)

3 Answers3