fprintf.c

#include <stdio.h>

int main()
{
    FILE * fout = fopen("output.txt", "w");
    int i;
    for (i = 0; i < 1e6; ++i)
        fprintf(fout, "%d", 0);
    return 0;
}

fstream.cpp

#include <fstream>

using namespace std;

int main()
{
    fstream fout("output.txt");
    for (int i = 0; i < 1e6; ++i)
        fout << 0;
    return 0;
}