Question

This is some code I wrote for implementing Sieve of Eratosthenes:

#include <iostream>
#include <vector>
#include <cmath>
#include <cassert>
#include <cstdlib>

int allPrimes (unsigned long n) {
    std::vector<int> track (n, 0);
    int index = 2;
    int m = sqrt(n);
    while(index < n) {
        if (track[index] == 0) {
            std::cout << index << std::endl;
            int mul = 1;
            while ((index <= m) && (n >= (index * ++mul))) {
                track[index * mul] = 1;
            }
        }
        index++;
    }
}

int main() {
    int num;
    std::cin >> num;
    allPrimes(num);
}

Strangely, whenever num is in the series 6, 10, 14, 18, 22, … the code aborts with the folloiwng stack while deallocating memory (runs okay for other n):

raise () from /lib64/libc.so.6
abort () from /lib64/libc.so.6
__libc_message () from /lib64/libc.so.6
malloc_printerr () from /lib64/libc.so.6
_int_free () from /lib64/libc.so.6
__gnu_cxx::new_allocator<int>::deallocate
std::_Vector_base<int, std::allocator<int> >::_M_deallocate
std::_Vector_base<int, std::allocator<int> >::~_Vector_base
std::vector<int, std::allocator<int> >::~vector
allPrimes (n=6) at allprimes.cpp:20
main () at allprimes.cpp:26

But I do not see the bug, or the logic behind these numbers spaced by 4. What is the bug here?

Answer 1

Here:

while(index <= n) {
    if (track[index] == 0) {

you allow index to run up to n, which is out of bounds. You need n-1, or while (index < n). There are other such indexing errors in the code, all of which lead to undefined behaviour.