多線程同步之Event（主要用來線程間的等待通知）
一 Event
在所有的內(nèi)核對象中，事件內(nèi)核對象是個(gè)最基本的對象。它們包含一個(gè)使用計(jì)數(shù)（與所有內(nèi)核對象一樣），一個(gè)用于指明該事件是個(gè)自動重置的事件還是一個(gè)人工重置的事件的布爾值，另一個(gè)用于指明該事件處于已通知狀態(tài)還是未通知狀態(tài)的布爾值。
事件能夠通知一個(gè)操作已經(jīng)完成。有兩種不同類型的事件對象。一種是人工重置的事件，另一種是自動重置的事件。當(dāng)人工重置的事件得到通知時(shí)，等待該事件的所有線程均變?yōu)榭烧{(diào)度線程。當(dāng)一個(gè)自動重置的事件得到通知時(shí)，等待該事件的線程中只有一個(gè)線程變?yōu)榭烧{(diào)度線程。
當(dāng)一個(gè)線程執(zhí)行初始化操作，然后通知另一個(gè)線程執(zhí)行剩余的操作時(shí)，事件使用得最多。事件初始化為未通知狀態(tài)，然后，當(dāng)該線程完成它的初始化操作后，它就將事件設(shè)置為已通知狀態(tài)。這時(shí)，一直在等待該事件的另一個(gè)線程發(fā)現(xiàn)該事件已經(jīng)得到通知，因此它就變成可調(diào)度線程。
Microsoft為自動重置的事件定義了應(yīng)該成功等待的副作用規(guī)則，即當(dāng)線程成功地等待到該對象時(shí)，自動重置的事件就會自動重置到未通知狀態(tài)。這就是自動重置的事件如何獲得它們的名字的方法。通常沒有必要為自動重置的事件調(diào)用ResetEvent函數(shù)，因?yàn)橄到y(tǒng)會自動對事件進(jìn)行重置。但是，Microsoft沒有為人工重置的事件定義成功等待的副作用，所以需要調(diào)用ResetEvent()。
二 Event API
| Event function |
Description |
| CreateEvent |
Creates or opens a named or unnamed event object. |
| CreateEventEx |
Creates or opens a named or unnamed event object and returns a handle to the object. |
| OpenEvent |
Opens an existing named event object. |
| PulseEvent |
Sets the specified event object to the signaled state and then resets it to the nonsignaled state after releasing the appropriate number of waiting threads. |
| ResetEvent |
Sets the specified event object to the nonsignaled state. |
| SetEvent |
Sets the specified event object to the signaled state. |
三 代碼實(shí)例
1）使用手動的Event：當(dāng)文件讀入內(nèi)存的時(shí)候，WordCount， SpellCheck，GrammarCheck可以同時(shí)進(jìn)行，這里使用Event，當(dāng)文件一讀入內(nèi)存就通知WordCount，SpellCheck和GrammarCheck線程開始執(zhí)行。
#include <windows.h>
#include <process.h>
#include <stdio.h>
// a global handle to event.
HANDLE g_hEvent;
void OpenFileAndReadContentsIntoMemory();
unsigned __stdcall WordCount(void *pvParam);
unsigned __stdcall SpellCheck(void *pvParam);
unsigned __stdcall GrammarCheck(void *pvParam);
int main()
{
//Create the manual-reset, nonsignaled event.
g_hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
//Spawn 3 new threads.
HANDLE hThread[3];
unsigned dwThreadID[3];
hThread[0] = (HANDLE)_beginthreadex(NULL, 0, WordCount, NULL, 0, &dwThreadID[0]);
hThread[1] = (HANDLE)_beginthreadex(NULL, 0, SpellCheck, NULL, 0, &dwThreadID[1]);
hThread[2] = (HANDLE)_beginthreadex(NULL, 0, GrammarCheck, NULL, 0, &dwThreadID[2]);
OpenFileAndReadContentsIntoMemory();
//Allow all 3 threads to access the memory.
SetEvent(g_hEvent);
printf("main thread exit\n");
return 1;
}
void OpenFileAndReadContentsIntoMemory()
{
printf("Open File and Read contents into memory\n");
}
unsigned __stdcall WordCount(void *pvParam)
{
//Wait until the file's data is in memory.
WaitForSingleObject(g_hEvent, INFINITE);
//Access the memory block.
printf("0:word count\n");
return(0);
}
unsigned __stdcall SpellCheck(void *pvParam)
{
//Wait until the file's data is in memory.
WaitForSingleObject(g_hEvent, INFINITE);
//Access the memory block.
printf("1:Spell check\n");
return(0);
}
unsigned __stdcall GrammarCheck(void *pvParam)
{
//Wait until the file's data is in memory.
WaitForSingleObject(g_hEvent, INFINITE);
//Access the memory block.
printf("2:Grammar check\n");
return(0);
}2）修改上面的代碼，使用自動Event，則必須在3個(gè)子線程中增加SetEvent（）。且要想讓3個(gè)線程都執(zhí)行完，必須的增加Waitfor（）函數(shù)。
#include <windows.h>
#include <process.h>
#include <stdio.h>
// a global handle to event.
HANDLE g_hEvent;
void OpenFileAndReadContentsIntoMemory();
unsigned __stdcall WordCount(void *pvParam);
unsigned __stdcall SpellCheck(void *pvParam);
unsigned __stdcall GrammarCheck(void *pvParam);
int main()
{
//Create the AUTO-reset, nonsignaled event.
g_hEvent = CreateEvent(NULL, FALSE, FALSE, NULL);
//Spawn 3 new threads.
HANDLE hThread[3];
unsigned dwThreadID[3];
hThread[0] = (HANDLE)_beginthreadex(NULL, 0, WordCount, NULL, 0, &dwThreadID[0]);
hThread[1] = (HANDLE)_beginthreadex(NULL, 0, SpellCheck, NULL, 0, &dwThreadID[1]);
hThread[2] = (HANDLE)_beginthreadex(NULL, 0, GrammarCheck, NULL, 0, &dwThreadID[2]);
OpenFileAndReadContentsIntoMemory();
//Allow all 3 threads to access the memory.
SetEvent(g_hEvent);
//wait for child threads to exit
DWORD dwCThd = WaitForMultipleObjects (3, //count of objects
hThread, //thread handle
TRUE, //wait for all
INFINITE); //time out interval
if(dwCThd != WAIT_OBJECT_0)
{
printf("error\n");
exit(-1);
}
//close handles
CloseHandle (g_hEvent);
//close child thread handles
for (int i=0; i<3; i++)
CloseHandle (hThread[i]);
printf("main thread exit\n");
return 1;
}
void OpenFileAndReadContentsIntoMemory()
{
printf("Open File and Read contents into memory\n");
}
unsigned __stdcall WordCount(void *pvParam)
{
//Wait until the file's data is in memory.
WaitForSingleObject(g_hEvent, INFINITE);
//Access the memory block.
printf("0:word count\n");
SetEvent(g_hEvent);
return(0);
}
unsigned __stdcall SpellCheck(void *pvParam)
{
//Wait until the file's data is in memory.
WaitForSingleObject(g_hEvent, INFINITE);
//Access the memory block.
printf("1:Spell check\n");
SetEvent(g_hEvent);
return(0);
}
unsigned __stdcall GrammarCheck(void *pvParam)
{
//Wait until the file's data is in memory.
WaitForSingleObject(g_hEvent, INFINITE);
//Access the memory block.
printf("2:Grammar check\n");
SetEvent(g_hEvent);
return(0);
}
四 參考
windows核心編程