线程池之工作项，等待项，计时项 （存在疑问？？？）

线程池函数允许我们做：
1.以异步方式调用函数  //工作项
2.每隔一段时间调用一个函数 //计时项
3.在内核对象触发时调用一个函数 //等待项
4.在异步I/O请求完成时调用一个函数 //I/O项

2.每隔一段时间调用一个函数
如果两个：周期的每隔5-7秒执行一段代码，周期的每隔6-8秒执行一段代码。
不用线程池将有两个线程执行，上下文切换不好，如用线程池，系统可能选择6秒间隔只唤醒一个线程执行这两段代码。

3.在内核对象触发时调用一个函数
同上类似，各个线程WaitForSingleObject各自的内核对象，如用线程池用一个线程WaitForMultipleObjects 最多MAXIMUM_WAIT_OBJECTS个内核对象
一旦回调函数被调用过，必须SetThreadpoolWait再注册下，否则 等待项 是不活跃状态不会再调用回调函数

 

#include <Windows.h>// C RunTime Header Files#include <string>PTP_WORK g_pWorkItem = NULL;volatile LONG g_nCurrentTask = 0;void NTAPI TaskHandler(PTP_CALLBACK_INSTANCE Instance, PVOID Context, PTP_WORK Work) {   LONG currentTask = InterlockedIncrement(&g_nCurrentTask);   wprintf(L"[%u] Task #%u is starting.\n", GetCurrentThreadId(), currentTask);   // Simulate a lot of work   Sleep(currentTask * 1000);   wprintf(L"[%u] Task #%u is done.\n", GetCurrentThreadId(), currentTask);   InterlockedDecrement(&g_nCurrentTask);}void Work(void){// Create the work item that will be used by all tasks//(1)create 线程池工作项   g_pWorkItem = CreateThreadpoolWork(TaskHandler, NULL, NULL);   if (g_pWorkItem == NULL) {      wprintf(L"Impossible to create the work item for tasks.\n");      return;   }   wprintf(L"----Start a new batch----\n");   for(int i = 4; i > 0; --i)   {   //(2)submit workSubmitThreadpoolWork(g_pWorkItem);   }   wprintf(L"4 tasks are submitted.\n");   //Waits for outstanding work callbacks to complete and optionally cancels pending callbacks that have not yet started to execute.   //(3)wait for work over   WaitForThreadpoolWorkCallbacks(g_pWorkItem,FALSE);   // Don't forget to delete the work item   //(4)delete work item   CloseThreadpoolWork(g_pWorkItem);}PTP_TIMER g_timer = NULL;volatile LONG g_nSecLeft = 10;VOID CALLBACK TimeoutCallback(PTP_CALLBACK_INSTANCE pInst, PVOID pvContext, PTP_TIMER pTimer){ LONG current = InterlockedDecrement(&g_nSecLeft); if(current == 0) { SetThreadpoolTimer(g_timer,NULL,0,0); //pftDueTime为NULL时，将停止调用定时器函数 wprintf(L"time out----\n"); } else {wprintf(L"[%u] call timer callback function #%u is left.\n", GetCurrentThreadId(), current); }}void Timer(void){g_timer = CreateThreadpoolTimer(TimeoutCallback,NULL,NULL);// Start the timer in one second to trigger every 1 second   ULARGE_INTEGER ulRelativeStartTime;   ulRelativeStartTime.QuadPart = (LONGLONG) -1;  // start now   FILETIME ftRelativeStartTime;   ftRelativeStartTime.dwHighDateTime = ulRelativeStartTime.HighPart;   ftRelativeStartTime.dwLowDateTime  = ulRelativeStartTime.LowPart;   SetThreadpoolTimer(      g_timer,       &ftRelativeStartTime,       1000, // Triggers every 1000 milliseconds      INFINITE//0      );   //WaitForThreadpoolTimerCallbacks(g_timer,FALSE); //Why this will fail？   getchar();   CloseThreadpoolTimer(g_timer);}VOID NTAPI pttwaitcallback(    _Inout_     PTP_CALLBACK_INSTANCE Instance,    _Inout_opt_ PVOID                 Context,    _Inout_     PTP_WAIT              Wait,    _In_        TP_WAIT_RESULT        WaitResult    ){wprintf(L"waitResult is WAIT_OBJECT_0 + %d.\n",WaitResult);}void WaitKernelObject(){PTP_WAIT pwait = CreateThreadpoolWait(pttwaitcallback,NULL,NULL);HANDLE hEvent1 = CreateEventW(NULL,FALSE,FALSE,L"event1");HANDLE hEvent2 = CreateEventW(NULL,FALSE,FALSE,L"event2");SetThreadpoolWait(pwait,hEvent1,NULL);SetEvent(hEvent1);//here will make pwait 不活跃状态， 此状态不会再去调回调函数。getchar();SetThreadpoolWait(pwait,hEvent2,NULL);//不活跃状态后需要重新设置SetEvent(hEvent2);//WaitForThreadpoolWaitCallbacks(pwait,FALSE);getchar();CloseThreadpoolWait(pwait);}int wmain() {//Work();Timer();//WaitKernelObject();wprintf(L"it is over\n");getchar();   return(0);}