CreateWaitableTimer与SetWaitableTimer详解

可等待定时器问题:

1. 需要的函数:CreateWaitableTimer,SetWaitableTimer.前者创建一个可等待定时器句柄,后者设置等待定时器触发的条件.我们可以多次调用SetWaitableTimer来重置可等待定时器.重置后之前的定时器将关闭.

2. 主要看两段代码:

代码一:(这段代码在网上出现的概率是较大的)

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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<<"定时器触发了...\n"; } 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<<"定时器触发了...\n";

}

CloseHandle(hTimer);

通过将上面注释的代码放开,我们可以发现:

◆ 前两个定时器出发过程”不稳定”(即它们所耗费的时间并不是1秒.

◆ 就算是稳定了以后,定时器的差距并不是1s而是1000(毫秒)左右,这就奇怪了.

如果说第二个问题是由于执行clock而产生的误差的话,那么第一个问题则没办法解释.(其实对于clock的时间几乎是可以忽略不计的.)

这样就引入了核心编程里面作者的代码:

代码二:

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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 );//这里并没有使用WaitForSingleObject,而是使用SleepEx clock_t end = clock() - c_beg; cout<<"time:"<<end<<endl; } 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 );//这里并没有使用WaitForSingleObject,而是使用SleepEx

clock_t end = clock() - c_beg;

cout<<"time:"<<end<<endl;

}

CloseHandle(hTimer);

运行结果发现正常:每次都是1000(毫秒).

从这里可以间接证明,clock消耗的时间可以忽略不计.

再来看看SleepEx是怎么定义的.

DWORD WINAPI SleepEx(

DWORD dwMillesconds,

BOOL bAlertable);

这里主要说明第二个参数的用途:线程是否将自己置为可提醒状态.(在可提醒I/O中,当I/O完成时,系统将他们添加到线程APC队列中-回调函数不会立即被调用.如果我们想让其被调用需要将这个参数设置为TRUE).

本例并没有牵涉到这类问题,所以无论FALSE或者TRUE都无所谓.

如果我们只想要定义一个一次性定时器.可以参考如下代码:

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HANDLE hTimer = NULL; LARGE_INTEGER liDueTime; liDueTime.QuadPart = -100000000;//10s hTimer = 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);

HANDLE hTimer = NULL;

LARGE_INTEGER liDueTime;

liDueTime.QuadPart = -100000000;//10s

hTimer = 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);

最后需要说明的一点是APC(异步调用过程)加入定时器中:

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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(); }

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();

}