windows 使用 Semaphore 进行同步

使用Semaphore进行线程同步

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

#define MAX_SEM_COUNT 2
#define THREADCOUNT 3

HANDLE ghSemaphore;

DWORD WINAPI ThreadProc(LPVOID);

int main(void)
{
	HANDLE aThread[THREADCOUNT];
	DWORD ThreadID;
	int i;

	// Create a semaphore with initial and max counts of MAX_SEM_COUNT

	ghSemaphore = CreateSemaphore(
		NULL,           // default security attributes
		MAX_SEM_COUNT,  // initial count   //初始化资源为2
		MAX_SEM_COUNT,  // maximum count   //初始化资源最大数为2
		NULL);          // unnamed semaphore

	if (ghSemaphore == NULL)
	{
		printf("CreateSemaphore error: %d\n", GetLastError());
		return 1;
	}

	// Create worker threads

	for (i = 0; i < THREADCOUNT; i++)
	{
		aThread[i] = CreateThread(
			NULL,       // default security attributes
			0,          // default stack size
			(LPTHREAD_START_ROUTINE)ThreadProc,
			NULL,       // no thread function arguments
			0,          // default creation flags
			&ThreadID); // receive thread identifier

		if (aThread[i] == NULL)
		{
			printf("CreateThread error: %d\n", GetLastError());
			return 1;
		}
	}

	// Wait for all threads to terminate

	WaitForMultipleObjects(THREADCOUNT, aThread, TRUE, INFINITE);

	// Close thread and semaphore handles

	for (i = 0; i < THREADCOUNT; i++)
		CloseHandle(aThread[i]);

	CloseHandle(ghSemaphore);

	return 0;
}

DWORD WINAPI ThreadProc(LPVOID lpParam)
{

	// lpParam not used in this example
	UNREFERENCED_PARAMETER(lpParam);

	DWORD dwWaitResult;
	BOOL bContinue = TRUE;


	dwWaitResult = WaitForSingleObject(   //当线程使用WaitForSingleObject从ghSemaphore中获取到信号时，资源计数减少1，当ghSemaphore没有资源时，并且参数为INFINITE WaitForSingleObject会卡住
		ghSemaphore,   // handle to semaphore
		INFINITE);           // zero-second time-out interval


	printf("Thread %d: wait succeeded\n", GetCurrentThreadId());
	bContinue = FALSE;

	// Simulate thread spending time on task
	Sleep(5000);

	// Release the semaphore when task is finished

	if (!ReleaseSemaphore( // 该API使ghSemaphore的内部资源计数加1
		ghSemaphore,  // handle to semaphore
		1,            // increase count by one
		NULL))       // not interested in previous count
	{
		printf("ReleaseSemaphore error: %d\n", GetLastError());
	}


	// The semaphore was nonsignaled, so a time-out occurred.
	return TRUE;
}

上面的代码我启动了三个进程，为信号分配了两个资源，那么在程序开始时，有两个线程先通过WaitForSingleObject获取到信号，此时信号的资源计数减到0，执行printf,然后等5s，有一个线程调用了ReleaseSemaphore，使资源计数加1，最后一个线程也执行printf,等待5s，最后所有线程结束，程序退出