在写入模式下打开Windows主机文件失败

Question

#include <cstdio>
#include <cstdlib>

void main()
{
    const char * hosts_filename = "C:\\Windows\\System32\\drivers\\etc\\hosts";
    FILE * hosts= fopen(hosts_filename, "a");//also tried r+ and w modes
    if(!hosts)
    {
        perror("failed");//failed: Permission denied
        system("PAUSE");
        exit(1);
    }
    fprintf(hosts, "\ntest\n");
}

但它在r模式下成功打开。

如果有限制this dude如何打开文件进行书写？