浅谈Sendmessage 和Postmessage

SendMessage函数功能描述:将指定的消息发送到一个或多个窗口。此函数为指定的窗口调用窗口过程，直到窗口过程处理完消息后才返回。

Postmessage函数则是将消息放入消息队列里，并立即返回。

PostThreadMessage函数： BOOL PostThreadMessage( DWORD idThread,
                                                           UINT Msg,
                                                           WPARAM wParam,
                                                            LPARAM lParam
                                                    );

     它可以用于线程之间的异步通讯，因为它不用等待调用者返回，

.常见问题
1）使用SendMessage来实现剪切、复制和粘贴

SendMessage(hwnd, WM_COPY, 0, 0);
SendMessage(hwnd, WM_CUT, 0, 0);
SendMessage(hwnd, WM_PASTE, 0, 0);

2）SendMessage与PostMessage的区别
PostMessage将消息放入消息队列后马上返回，而SendMessage直到窗口过程处理完消息后才返回

3）SendMessage发送WM_COPYDATA消息在进程间传送数据
WM_COPYDATA消息主要目的是允许在进程间传递少量只读数据。SDK文档推荐用户使用SendMessage()函数，接收方在数据复制完成前不返回，这样发送方就不可能删除和修改数据。
例如：

std:string strData = "VC知识库 VCKBASE.COM";
COPYDATASTRUCT cds;
cds.dwData = 0;
cds.cbData = strData.Length();
cds.lpData = strData.c_str();
::SendMessage(hwnd, WM_COPYDATA, NULL, (LPARAM)&cds);

3、PostThreadMessage有时会失败，报1444错误(Invalid thread identifier. )，其实这不一定是线程不存在的原因，也有可能是线程不存在消息队列(message queue)造成的。事实上，并不是每个thread都有message queue,那如何让thread具有呢？ 答案是，至少调用message相关的function一次，比如GetMessage,PeekMessage。

4、如果是postthreadmessage动态分配的memory给另外一个thread,要注意内存的正确释放。并且PostThreadMessage不能够post WM_COPYDATE之类的同步消息，否则会报错！

5、最好不要使用PostThreadMessage post message给一个窗口,使用PostMessage替代。

example：

#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());
        return 1;
    }
    
    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());
        return 1;
    }
    //start thread
    hThread = (HANDLE)_beginthreadex( NULL, 0, &fun, NULL, 0, &nThreadID );
    if(hThread == 0)
    {
        printf("start thread failed,errno:%d\n",::GetLastError());
        CloseHandle(hStartEvent);
        return 1;
    }
    //wait thread start event to avoid PostThreadMessage return errno:1444
    ::WaitForSingleObject(hStartEvent,INFINITE);
    CloseHandle(hStartEvent);
    int count = 0;
    while(true)
    {
        char* pInfo = new char[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;
}

转载自：http://blog.163.com/aq_csl_csl_csl/blog/static/44860738200781091435175/