file mapping 2

#include <iostream>
#include <cstdlib>
#include <boost/filesystem/v3/operations.hpp>
#include <boost/interprocess/file_mapping.hpp>
#include <boost/interprocess/mapped_region.hpp>

#include <boost/array.hpp>
#include <fstream>
#include <ctime>

struct random_generator {
    random_generator() { std::srand((unsigned)std::time(NULL)); }

    char operator ()() {
        int res = std::rand() % (26 + 26 + 10);
        switch (res / 26) {
        case 0:
            return res + 'A';
        case 1:
            return res - 26 + 'a';
        case 2:
            return res - 26 * 2 + '0';
        default:
            return 0;
        }
    }
};

int main()
{
    using namespace std;
    using namespace boost;
    const size_t filesize = (16 + 2) * 20000000;
    {
        // 生成一个指定大小的文件
        ofstream fout("mapped_file.txt");
        filesystem3::resize_file("mapped_file.txt", filesize);
    }
    {
        interprocess::file_mapping mfile("mapped_file.txt", interprocess::read_write);
        interprocess::mapped_region region(mfile, interprocess::read_write);
        array<char, 18> * addr = (array<char, 18> *)region.get_address();
        array<char, 18> * const end = addr + 20000000;
        random_generator rg;
        for (; addr != end; ++addr) {
            generate_n(addr->begin(), 16, rg);
            (*addr)[16] = '\r';
            (*addr)[17] = '\n';
        }
    }
}