关于Win32的事件和线程的一个例子

例子来自MSDN。

整个过程应该是这样的：一些线程要去读取一个缓冲区，但是要在对该缓冲区完成一些操作后才能让其读取，而且完成后要通知这些线程。这里的通知当然就是事件了（如果说最底层的原理，不如说是一个无限循环）。在创建这些线程的时候，线程就被赋予一个要求，必须等到对该缓冲区的标识变为可行的时候才能执行，这样的话就完成了对线程读取的控制。对缓冲区的操作线程完成后，就会将标识变为可行，此后，那些线程的无限循环又执行到检查标识时，就会通过检查，这样的表现就是，对缓冲区的操作完成后通知读取缓冲区的操作，虽然实际上不是这样的，但结果和表现是这样的，这就是事件。

 

#include <windows.h>
#include <stdio.h>

#define THREADCOUNT 4 

HANDLE ghWriteEvent; 
HANDLE ghThreads[THREADCOUNT];

DWORD WINAPI ThreadProc(LPVOID);

void CreateEventsAndThreads(void) 
{
    int i; 
    DWORD dwThreadID; 

    // Create a manual-reset event object. The write thread sets this
    // object to the nonsignaled state when it finishes writing to a 
    // shared buffer. 
	// 事件本事就是一个handle，可以通过它获得某个标识
    ghWriteEvent = CreateEvent( 
        NULL,               // default security attributes
        TRUE,               // manual-reset event
        FALSE,              // initial state is nonsignaled
        TEXT("WriteEvent")  // object name
        ); 

    if (ghWriteEvent == NULL) 
    { 
        printf("CreateEvent failed (%d)\n", GetLastError());
        return;
    }

    // Create multiple threads to read from the buffer.

    for(i = 0; i < THREADCOUNT; i++) 
    {
        // TODO: More complex scenarios may require use of a parameter
        //   to the thread procedure, such as an event per thread to  
        //   be used for synchronization.
        ghThreads[i] = CreateThread(
            NULL,              // default security
            0,                 // default stack size
            ThreadProc,        // name of the thread function 
            NULL,              // no thread parameters
            0,                 // default startup flags
            &dwThreadID); 

        if (ghThreads[i] == NULL) 
        {
            printf("CreateThread failed (%d)\n", GetLastError());
            return;
        }
    }
}

void WriteToBuffer(VOID) 
{
    // TODO: Write to the shared buffer.
    
    printf("Main thread writing to the shared buffer...\n");

    // Set ghWriteEvent to signaled
	// 这里就是体现其标识的地方，能够改变它
    if (! SetEvent(ghWriteEvent) ) 
    {
        printf("SetEvent failed (%d)\n", GetLastError());
        return;
    }
}

void CloseEvents()
{
    // Close all event handles (currently, only one global handle).
    
    CloseHandle(ghWriteEvent);
}

void main()
{
    DWORD dwWaitResult;

    // TODO: Create the shared buffer

    // Create events and THREADCOUNT threads to read from the buffer

    CreateEventsAndThreads();

    // At this point, the reader threads have started and are most
    // likely waiting for the global event to be signaled. However, 
    // it is safe to write to the buffer because the event is a 
    // manual-reset event.
    
    WriteToBuffer();

    printf("Main thread waiting for threads to exit...\n");

    // The handle for each thread is signaled when the thread is
    // terminated.
    dwWaitResult = WaitForMultipleObjects(
        THREADCOUNT,   // number of handles in array
        ghThreads,     // array of thread handles
        TRUE,          // wait until all are signaled
        INFINITE);

    switch (dwWaitResult) 
    {
        // All thread objects were signaled
        case WAIT_OBJECT_0: 
            printf("All threads ended, cleaning up for application exit...\n");
            break;

        // An error occurred
        default: 
            printf("WaitForMultipleObjects failed (%d)\n", GetLastError());
            return;
    } 
            
    // Close the events to clean up

    CloseEvents();
}

DWORD WINAPI ThreadProc(LPVOID lpParam) 
{
    DWORD dwWaitResult;

    printf("Thread %d waiting for write event...\n", GetCurrentThreadId());
    // 这里通过标识限制线程的执行
    dwWaitResult = WaitForSingleObject( 
        ghWriteEvent, // event handle
        INFINITE);    // indefinite wait

    switch (dwWaitResult) 
    {
        // Event object was signaled
        case WAIT_OBJECT_0: 
            //
            // TODO: Read from the shared buffer
            //
            printf("Thread %d reading from buffer\n", 
                   GetCurrentThreadId());
            break; 

        // An error occurred
        default: 
            printf("Wait error (%d)\n", GetLastError()); 
            return 0; 
    }

    // Now that we are done reading the buffer, we could use another
    // event to signal that this thread is no longer reading. This
    // example simply uses the thread handle for synchronization (the
    // handle is signaled when the thread terminates.)

    printf("Thread %d exiting\n", GetCurrentThreadId());
    return 1;
}