03等待多个线程返回WaitForMultipleObject

二. WaitForMultipleObject 等待单个线程返回

1. 函数原型

DWORD WINAPI WaitForMultipleObjects(
  _In_       DWORD   nCount,
  _In_ const HANDLE  *lpHandles,
  _In_       BOOL    bWaitAll,
  _In_       DWORD   dwMilliseconds
);

2. 参数说明

• 第一个参数 nCount 为等待的内核对象个数，可以是 0 到 MAXIMUM_WAIT_OBJECTS （64）中的一个值。
• 第二个参数 lpHandles 为一个存放被等待的内核对象句柄的数组。
• 第三个参数 bWaitAll 是否等到所有内核对象为已通知状态后才返回，如果为 TRUE，则只有当等待的所有内核对象为已通知状态时函数才返回，如果为 FALSE，则只要一个内核对象为已通知状态，则该函数返回。
• 第四个参数 dwMilliseconds 为等待时间，和 WaitForSingleObject 中等待 dwMilliseconds 参数类似。

3. 返回值

• WAIT_ABANDONED_0 表示所有对象都发出消息，而且其中有一个或对个属于互斥体（一旦拥有他们的进程结束，就会发出信号）
• WAIT_TIMEOUT 对象保持未发出信号的状态，但规定的等待超时时间已经超过
• WAIT_OBJECT_0 所有对象都发出信号
• WAIT_FAILED 表示函数执行失败，会设置错误码，错误码可通过 GetLastError 获取
• WAIT_IO_COMPLETION （仅适用于 WaitForMultipleObjectsEx ）由于一个 I/O 完成操作已做好准备执行，所以造成了函数的返回值
  —— 如果 bWaitAll 为 FALSE，那么返回结果相似，只是可能还会返回相对于 WAIT_ABANDONED_0 或 WAIT_OBJECT_0 的一个正偏移量，指出哪个对象时被抛弃还是发出信号。
  补充: WAIT_OBJECT_0 时微软定义的一个宏，就可以把这个宏当做一个数字。

例如， 返回值 WAIT_OBJECT_0 + 5 意味着列表中的第 5 个对象发出了信号。当 bWaitAll 参数为 FALSE 时可以等待其中之一的信号。

4. 示例

（1）参数 bWaitAll 为 FALSE，只要一个内核对象为已通知状态，则该函数返回。

#include <Windows.h>
#include <stdio.h>

const unsigned int THREAD_NUM = 3;
DWORD WINAPI  ThreadFunc(LPVOID p)
{
	printf("I am a child thread 0 which pid is %d ...\n", GetCurrentThreadId());   //输出子线程pid
	Sleep(500);
	printf("The child thread 0 which pid is %d quit ...\n", GetCurrentThreadId());
	return 0;
}

DWORD WINAPI  ThreadFunc1(LPVOID p)
{
	printf("I am a child thread 1 which pid is %d ...\n", GetCurrentThreadId());   //输出子线程pid
	Sleep(2000);
	printf("The child thread 1 which pid is %d quit ...\n", GetCurrentThreadId());
	return 0;
}

DWORD WINAPI  ThreadFunc2(LPVOID p)
{
	printf("I am a child thread 2 which pid is %d ...\n", GetCurrentThreadId());   //输出子线程pid
	Sleep(4000);
	printf("The child thread 2 which pid is %d quit ...\n", GetCurrentThreadId());
	return 0;
}

int main()
{
	printf("I am the main thread that pid is %d ...\n", GetCurrentThreadId());  //输出主线程pid
	HANDLE hThread[THREAD_NUM];
	hThread[0] = CreateThread(NULL, 0, ThreadFunc, 0, 0, NULL); // 创建线程
	hThread[1] = CreateThread(NULL, 0, ThreadFunc1, 0, 0, NULL); // 创建线程
	hThread[2] = CreateThread(NULL, 0, ThreadFunc2, 0, 0, NULL); // 创建线程

	DWORD resulut = WaitForMultipleObjects(THREAD_NUM, hThread, FALSE, INFINITE); //只要有一个线程返回就结束
	if (WAIT_OBJECT_0 == resulut)
	{
		printf("The child thread 0 finished waiting ...\n");
	}
	else if (WAIT_OBJECT_0 + 1 == resulut)
	{
		printf("The child thread 1 finished waiting ...\n");
	}
	else if (WAIT_OBJECT_0 + 2 == resulut)
	{
		printf("The child thread 2 finished waiting ...\n");
	}
	else if (WAIT_FAILED == resulut)
	{
		printf("The fuction WaitForMultipleObjects error! \n");
	} 
	printf("The main thread which pid is %d quit ...\n", GetCurrentThreadId());
	return 0;
}

输出结果为：

—— 如果将代码中的 “Thread 0” 的 Sleep 时间加长，让 “Thread 1” 先退出，则 会进入 WAIT_OBJECT_0 + 1 == resulut 分支，打印 “The child thread 1 finished waiting ...”，然后退出主线程。这样就可以通过返回值判断是哪个对象发送的信号。

（2）参数 bWaitAll 为 TRUE，等待所有线程返回。

#include <Windows.h>
#include <stdio.h>

const unsigned int THREAD_NUM = 3;
DWORD WINAPI  ThreadFunc(LPVOID p)
{
	printf("I am a child thread 0 which pid is %d ...\n", GetCurrentThreadId());   //输出子线程pid
	Sleep(500);
	printf("The child thread 0 which pid is %d quit ...\n", GetCurrentThreadId());
	return 0;
}

DWORD WINAPI  ThreadFunc1(LPVOID p)
{
	printf("I am a child thread 1 which pid is %d ...\n", GetCurrentThreadId());   //输出子线程pid
	Sleep(2000);
	printf("The child thread 1 which pid is %d quit ...\n", GetCurrentThreadId());
	return 0;
}

DWORD WINAPI  ThreadFunc2(LPVOID p)
{
	printf("I am a child thread 2 which pid is %d ...\n", GetCurrentThreadId());   //输出子线程pid
	Sleep(4000);
	printf("The child thread 2 which pid is %d quit ...\n", GetCurrentThreadId());
	return 0;
}

int main()
{
	printf("I am the main thread that pid is %d ...\n", GetCurrentThreadId());  //输出主线程pid
	HANDLE hThread[THREAD_NUM];
	//创建第一个子线程
	hThread[0] = CreateThread(NULL, 0, ThreadFunc, 0, 0, NULL); // 创建线程
	hThread[1] = CreateThread(NULL, 0, ThreadFunc1, 0, 0, NULL); // 创建线程
	hThread[2] = CreateThread(NULL, 0, ThreadFunc2, 0, 0, NULL); // 创建线程

	int wait_num = 0;
	while (wait_num < THREAD_NUM)
	{
		DWORD resulut = WaitForMultipleObjects(THREAD_NUM, hThread, TRUE, INFINITE); //所有子线程返回才结束
		if (WAIT_OBJECT_0 <= resulut <= WAIT_OBJECT_0 + 2)
		{
			printf("The Return value is %d, One child thread finished waiting ...\n", resulut);
			wait_num++;
		}	
	}

	printf("The main thread which pid is %d quit ...\n", GetCurrentThreadId());
	return 0;
}

输出结果为：