作者：ani_di 
版权所有，转载务必保留此链接 http://blog.csdn.net/ani_di

网络授时服务是一些网络上的时间服务器提供的时间，一般用于本地时钟同步。 授时服务有很多种，一般我们选择RFC-868。这个协议的工作流程是：（S代表Server，C代表Client）

1. S: 检测端口37
2. U: 连接到端口37
3. S: 以32位二进制数发送时间
4. U: 接收时间
5. U: 关闭连接
6. S: 关闭连接

协议非常简单，用TCP连接上后，服务器直接把时间发送回来。发送的是从1900年1月1日午夜到现在的秒数。

使用luasocket

实现的方案有很多种，Lua不一定是最简单的，选择只是出于个人兴趣。直接上代码吧

-----------------------------------------------------------------------------
-- Network Time Protocal
-- Author: ani_di
-----------------------------------------------------------------------------
package.cpath = package.cpath .. ';D:\\tools\\Lua\\5.1\\clibs\\?.dll;?.dll'
local socket = require "socket.core"

server_ip = {
        -- "129.6.15.29",
        "132.163.4.101",
        "132.163.4.102",
        "132.163.4.103",
        "128.138.140.44",
        "192.43.244.18",
        "131.107.1.10",
        "66.243.43.21",
        "216.200.93.8",
        "208.184.49.9",
        "207.126.98.204",
        "207.200.81.113",
        "205.188.185.33"}

function nstol(str)
    assert(str and #str == 4)
    local t = {str:byte(1,-1)}
    local n = 0
    for k = 1, #t do
        n= n*256 + t[k]
    end
    return n
end

-- get time from a ip address, use tcp protocl
function gettime(ip)
    print('connect ', ip)
    local tcp = socket.tcp()
    tcp:settimeout(10)
    tcp:connect(ip, 37)
    success, time = pcall(nstol, tcp:receive(4))
    tcp:close()
    return success and time or nil
end

function nettime()
    for _, ip in pairs(server_ip) do
        time = gettime(ip)
        if time then 
            return time
        end
    end
end

代码原理不细说，非常简单。唯一值得一提的是socket库包含。最开始用的这句 require "socket"

在解释器中表现很好，但在用C中调用会找不到相应的module。错误提示

    no field package.preload['socket']
    no file '.\socket.lua'
    no file 'F:\Projects\Lua\nettime\lua\socket.lua'
    no file 'F:\Projects\Lua\nettime\lua\socket\init.lua'
    no file 'F:\Projects\Lua\nettime\socket.lua'
    no file 'F:\Projects\Lua\nettime\socket\init.lua'
    no file 'D:\tools\Lua\5.1\lua\socket.luac'
    no file '.\socket.dll'
    no file '.\socket51.dll'
    no file 'F:\Projects\Lua\nettime\socket.dll'
    no file 'F:\Projects\Lua\nettime\socket51.dll'
    no file 'F:\Projects\Lua\nettime\clibs\socket.dll'
    no file 'F:\Projects\Lua\nettime\clibs\socket51.dll'
    no file 'F:\Projects\Lua\nettime\loadall.dll'
    no file 'F:\Projects\Lua\nettime\clibs\loadall.dll'.

网上也有好多类似的提问，大抵是没仔细看作者的Guide。显著的有这么一句

The other two environment variables instruct the compatibility module to look for dynamic libraries and extension modules in the appropriate directories and with the appropriate filename extensions.>

LUAPATH=/?.lua;?.lua LUACPATH=/?.dll;?.dll

至于"socket.core"，windows默认安装位于“\socket\core.dll”。

C宿主调用

#include <stdio.h>
#include <string.h>
#include <lua.h>
#include <lauxlib.h>
#include <lualib.h>
#include <time.h>
#include <Windows.h>

int load(lua_State* L, const char* func, unsigned int* utc) {
   lua_getglobal(L, func);
   if (lua_pcall(L, 0, 1, 0)) {
        printf("Error Msg pcall %s.\n", lua_tostring(L, -1));
        return -1;
   }

   if (!lua_isnumber(L,-1)) {
       printf("time should be a number\n" );
       return -2;
   }

   *utc = lua_tonumber(L,-1);
   lua_pop(L, -1);
   return 0;
}

void TimetToFileTime( time_t t, LPFILETIME pft )
{
    LONGLONG ll = Int32x32To64(t, 10000000) + 116444736000000000;
    pft->dwLowDateTime = (DWORD) ll;
    pft->dwHighDateTime = ll >>32;
}

int main()
{
   lua_State* L = luaL_newstate();
   unsigned int utc = 0;
   luaL_openlibs(L);
  if (luaL_loadfile(L, "nettime.lua") || lua_pcall(L, 0, 0, 0)) {
       printf("Error Msg load %s.\n", lua_tostring(L, -1));
       return -1;
   }
   do {
    if(load(L,"nettime", &utc) == 0) {
        time_t tt = utc - 2208988800L;
        SYSTEMTIME st;
        FILETIME ft;
        TimetToFileTime(tt, &ft);
        if (FileTimeToSystemTime(&ft, &st))
        {
            printf("Today is: %d-%d-%d\n", st.wYear, st.wMonth, st.wDay);
            SetSystemTime(&st);
        }
        break;
    } else {
        puts("No network!");
        Sleep(10000);
    }
   } while (1);
   lua_close(L);
   return 0;
}