openMP具有大量循环的分段错误

Question

您好我正在使用openMP进行经典“monte carlo pi approximation”问题的简单C ++练习。我的代码是这样的：

int main () {
    Ran ran(time( NULL ) );
    int N = 2000000;       //run the loop for N times
    int a[N];
    double sum=0;
 // for loop execution
    #pragma omp for private(a[i]) reduction(+:sum)
    for( int i = 0; i < N; i = i + 1 ) {
            double xx=ran.doub();
            double yy=ran.doub();
            double rr=r(xx, yy);
            double cc=count(rr);
            a[i]=cc;
            sum+=a[i];
            //cout << "element of a: " << a[i] << endl;
            //cout << "x: " << xx << endl;
            //cout << "sum: " << sum << endl;
    }
    cout << "sum: " << sum << endl;
    cout << "pi?: " << 4*sum/N << endl;
    return sum/N;
}

它运行良好，N = 2000000，有20个核心。但是当我使N更大时，如2.5e6，它开始出现分段错误。