C++ Segmentation Fault (SIGSEGV) - c++

Question

I have a problem statement.
Given an even number A ( greater than 2 ), return two prime numbers whose sum will be equal to given number.
The following solution code runs fine for all small and medium inputs but fails for a very large input 16777214. It is a segmentation fault and I tried to read this article on it but I cannot figure out what my program is doing wrong, since it is working for other inputs or what should I fix here. I'm new to C++;
#include <iostream>
#include <vector>
using namespace std;
void form_sieve(int A, int* prime){
for(int c = 0; c < A; c++)
prime[c] = 1;
prime[0] = 0;
prime[1] = 0;
for(int p = 2; p * p <= A; p++){
if(prime[p] == 1){
// update all multiples of p
for(int i = p * p; i <= A; i += p)
prime[i] = 0;
}
}
}
void primesum(int A) {
// find prime numbers less than A
// use sieve method
int prime[A + 1];
form_sieve(A, prime);
vector<int> result;
// for(int c = 0; c <= A; c++)
// cout << prime[c] << "\n";
for(int i = 2; i <= A; i++){
if(prime[i]){
if(i + i == A){
result.push_back(i);
result.push_back(i);
break;
}
else if(prime[A - i]){
result.push_back(i);
result.push_back(A - i);
break;
}
}
}
// cout << result.size();
for(vector<int>::iterator ptr = result.begin(); ptr < result.end(); ptr++){
cout << *ptr << "\n";
}
// return result;
}
// Driver Code
int main()
{
primesum(16777214);
}

Answer 1

You need to allocate new memory.
A user2717954 already suggested using a vector for primes, which is the best way to fix this.
If you wanted to use arrays, you could change int prime[A+1] to int* prime = new int[A+1];. Then delete [] prime; after you're finished using it at the end of the function.
When I do this, the program doesn't segfault. You must allocate dynamic memory and clean up after you're done since the program doesn't know how much memory to allocate since the size A + 1 isn't known at compile-time.
However, vectors will, of course, resize as needed so you won't have to worry about dynamic memory.