CreateWaitableTimer與SetWaitableTimer詳解

可等待定時器問題:

1. 需要的函數:CreateWaitableTimer,SetWaitableTimer.前者建立一個可等待定時器控制代碼,後者設定等待定時器觸發的條件.我們可以多次調用SetWaitableTimer來重設可等待定時器.重設後之前的定時器將關閉.

2. 主要看兩段代碼:

代碼一:(這段代碼在網上出現的機率是較大的)

[cpp] view plaincopyprint?

1. HANDLE hTimer = CreateWaitableTimer( NULL,FALSE,NULL ); 
2.  
3. LARGE_INTEGER li; 
4.  
5. li.QuadPart = 0; 
6.  
7.  
8. if( !SetWaitableTimer( hTimer,&li,1000,NULL,NULL,FALSE )) 
9.  
10. { 
11.  
12. CloseHandle( hTimer ); 
13.  
14. return; 
15.  
16. } 
17.  
18. while ( 1 ) 
19.  
20. { 
21.  
22. //clock_t c_beg = clock();  
23.  
24. WaitForSingleObject( hTimer,INFINITE ); 
25.  
26. //clock_t end = clock() - c_beg;  
27.  
28. //cout<<"time:"<<end<<endl;  
29.  
30. cout<<"定時器觸發了...\n"; 
31.  
32. } 
33.  
34. CloseHandle(hTimer); 

HANDLE hTimer = CreateWaitableTimer( NULL,FALSE,NULL );LARGE_INTEGER li;li.QuadPart = 0;if( !SetWaitableTimer( hTimer,&li,1000,NULL,NULL,FALSE )){CloseHandle( hTimer );return;}while ( 1 ){//clock_t c_beg = clock();WaitForSingleObject( hTimer,INFINITE );//clock_t end = clock() - c_beg;//cout<<"time:"<<end<<endl;cout<<"定時器觸發了...\n";}CloseHandle(hTimer);

 

 

通過將上面注釋的代碼放開,我們可以發現:

◆ 前兩個定時器出發過程”不穩定”(即它們所耗費的時間並不是1秒.

◆ 就算是穩定了以後,定時器的差距並不是1s而是1000(毫秒)左右,這就奇怪了.

如果說第二個問題是由於執行clock而產生的誤差的話,那麼第一個問題則沒辦法解釋.(其實對於clock的時間幾乎是可以忽略不計的.)

這樣就引入了核心編程裡面作者的代碼:

代碼二:

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1. HANDLE hTimer = CreateWaitableTimer( NULL,FALSE,NULL ); 
2.  
3. LARGE_INTEGER li; 
4.  
5. li.QuadPart = 0; 
6.  
7.  
8. if( !SetWaitableTimer( hTimer,&li,1000,NULL,NULL,FALSE )) 
9.  
10. { 
11.  
12. CloseHandle( hTimer ); 
13.  
14. return; 
15.  
16. } 
17.  
18.  
19. while ( 1 ) 
20.  
21. { 
22.  
23. clock_t c_beg = clock(); 
24.  
25. SleepEx( 1000,TRUE );//這裡並沒有使用WaitForSingleObject,而是使用SleepEx  
26.  
27. clock_t end = clock() - c_beg; 
28.  
29. cout<<"time:"<<end<<endl; 
30.  
31. } 
32.  
33. CloseHandle(hTimer); 

HANDLE hTimer = CreateWaitableTimer( NULL,FALSE,NULL );LARGE_INTEGER li;li.QuadPart = 0;if( !SetWaitableTimer( hTimer,&li,1000,NULL,NULL,FALSE )){CloseHandle( hTimer );return;}while ( 1 ){clock_t c_beg = clock();SleepEx( 1000,TRUE );//這裡並沒有使用WaitForSingleObject,而是使用SleepExclock_t end = clock() - c_beg;cout<<"time:"<<end<<endl;}CloseHandle(hTimer);

 

運行結果發現正常:每次都是1000(毫秒).

從這裡可以間接證明,clock消耗的時間可以忽略不計.

再來看看SleepEx是怎麼定義的.

DWORD WINAPI SleepEx(

    DWORD dwMillesconds,

    BOOL bAlertable);

這裡主要說明第二個參數的用途:線程是否將自己置為可提醒狀態.(在可提醒I/O中,當I/O完成時,系統將他們添加到線程APC隊列中-回呼函數不會立即被調用.如果我們想讓其被調用需要將這個參數設定為TRUE).

本例並沒有牽涉到這類問題,所以無論FALSE或者TRUE都無所謂.

如果我們只想要定義一個一次性定時器.可以參考如下代碼:

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1. HANDLE hTimer = NULL; 
2.  
3. LARGE_INTEGER liDueTime; 
4.  
5.   liDueTime.QuadPart = -100000000;//10s  
6.  
7. hTimer = CreateWaitableTimer(NULL, TRUE,NULL); 
8.  
9.     if ( hTimer == NULL ) return; 
10.  
11.     if (!SetWaitableTimer(hTimer, &liDueTime, 1000, NULL, NULL, 0)) 
12.  
13.     {  
14.  
15. CloseHandle(hTimer); 
16.  
17.         return; 
18.  
19. } 
20.  
21. clock_t c_beg = clock(); 
22.  
23. if (WaitForSingleObject(hTimer, INFINITE) != WAIT_OBJECT_0) 
24.  
25. { 
26.  
27. CloseHandle(hTimer); 
28.  
29. return ; 
30.  
31. } 
32.  
33. else 
34.  
35. { 
36.  
37. //操作  
38.  
39. } 
40.  
41. CloseHandle(hTimer); 

HANDLE hTimer = NULL;LARGE_INTEGER liDueTime;  liDueTime.QuadPart = -100000000;//10shTimer = CreateWaitableTimer(NULL, TRUE,NULL);    if ( hTimer == NULL ) return;    if (!SetWaitableTimer(hTimer, &liDueTime, 1000, NULL, NULL, 0))    { CloseHandle(hTimer);        return;}clock_t c_beg = clock();if (WaitForSingleObject(hTimer, INFINITE) != WAIT_OBJECT_0){CloseHandle(hTimer);return ;}else{//操作}CloseHandle(hTimer);

最後需要說明的一點是APC(非同步呼叫過程)加入定時器中:

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1. VOID APIENTRY TimerAPCRoutine( PVOID pvArgToCompletionRoutine,DWORD dwTimerLowValue,DWORD dwTimerHighValue) 
2.  
3. { 
4.  
5. cout<<"high:"<<dwTimerHighValue<<"      low"<<dwTimerLowValue<<endl; 
6.  
7. } 
8.  
9. void main() 
10.  
11. { 
12.  
13. HANDLE hTimer = CreateWaitableTimer( NULL,FALSE,NULL ); 
14.  
15. LARGE_INTEGER li; 
16.  
17. li.QuadPart = 0; 
18.  
19.  
20. if( !SetWaitableTimer( hTimer,&li,1000,TimerAPCRoutine,NULL,FALSE )) 
21.  
22. { 
23.  
24. CloseHandle( hTimer ); 
25.  
26. return; 
27.  
28. } 
29.  
30.  
31. while ( 1 ) 
32.  
33. { 
34.  
35. SleepEx( 1000,TRUE ); 
36.  
37. } 
38.  
39. CloseHandle( hTimer ); 
40.  
41. getchar(); 
42.  
43. } 

VOID APIENTRY TimerAPCRoutine( PVOID pvArgToCompletionRoutine,DWORD dwTimerLowValue,DWORD dwTimerHighValue){cout<<"high:"<<dwTimerHighValue<<"      low"<<dwTimerLowValue<<endl;}void main(){HANDLE hTimer = CreateWaitableTimer( NULL,FALSE,NULL );LARGE_INTEGER li;li.QuadPart = 0;if( !SetWaitableTimer( hTimer,&li,1000,TimerAPCRoutine,NULL,FALSE )){CloseHandle( hTimer );return;}while ( 1 ){SleepEx( 1000,TRUE );}CloseHandle( hTimer );getchar();}