使用PostThreadMessage傳遞消息

 PostThreadMessage的原型是這樣的
BOOL PostThreadMessage( DWORD idThread,
    UINT Msg,
    WPARAM wParam,
    LPARAM lParam
);

PostThreadMessage可以用於線程之間的異步通訊，因爲它不用等待調用者返回，
這也許是線程通訊中最簡單的一種方法了。

但是要注意以下問題
1 .PostThreadMessage有時會失敗，報1444錯誤(Invalid thread identifier. )
其實這不一定是線程不存在的原因，也有可能是線程不存在消息隊列(message queue)造成的。
事實上，並不是每個thread都有message queue,那如何讓thread具有呢？
答案是，至少調用message相關的function一次，比如GetMessage,PeekMessage。

2.如果是post動態分配的memory給另外一個thread,要注意內存的正確釋放。

3.PostThreadMessage不能夠post WM_COPYDATE之類的同步消息，否則會報錯

4.最好不要使用PostThreadMessage post message給一個窗口,使用PostMessage替代。

下面是我寫的一個比較嚴整的例子，僅供參考。

#include<windows.h>
#include <cstdio>
#include <process.h>

#define MY_MSG WM_USER+100
const int MAX_INFO_SIZE = 20;

HANDLE hStartEvent; // thread start event

// thread function
unsigned __stdcall fun(void*param)
{
    printf("thread fun start\n");

    MSG msg;
    PeekMessage(&msg, NULL, WM_USER, WM_USER, PM_NOREMOVE);

    if(!SetEvent(hStartEvent))//set thread start event
    {
        printf("set start event failed,errno:%d\n",::GetLastError());
        return1;
    }
    
    while(true)
    {
        if(GetMessage(&msg,0,0,0))//get msg from message queue
        {
            switch(msg.message)
            {
            case MY_MSG:
                char* pInfo = (char*)msg.wParam;
                printf("recv %s\n",pInfo);
                delete[] pInfo;
                break;
            }
        }
    };
    return 0;
}

int main()
{
    HANDLE hThread;
    unsigned nThreadID;

    hStartEvent = ::CreateEvent(0,FALSE,FALSE,0);//create thread start event
    if(hStartEvent== 0)
    {
        printf("create start event failed,errno:%d\n",::GetLastError());
        return1;
    }

    //start thread
    hThread = (HANDLE)_beginthreadex( NULL,0, &fun, NULL,0, &nThreadID );
    if(hThread== 0)
    {
        printf("start thread failed,errno:%d\n",::GetLastError());
        CloseHandle(hStartEvent);
        return1;
    }

    //wait thread start event to avoid PostThreadMessage return errno:1444
    ::WaitForSingleObject(hStartEvent,INFINITE);
    CloseHandle(hStartEvent);

    int count= 0;
    while(true)
    {
        char* pInfo= newchar[MAX_INFO_SIZE];//create dynamic msg
        sprintf(pInfo,"msg_%d",++count);
        if(!PostThreadMessage(nThreadID,MY_MSG,(WPARAM)pInfo,0))//post thread msg
        {
            printf("post message failed,errno:%d\n",::GetLastError());
            delete[] pInfo;
        }
        ::Sleep(1000);
    }

    CloseHandle(hThread);
    return 0;
}