浅析SendMessage与PostMessage的区别

SendMessage与PostMessage的区别应该是面试中经常问到的问题，我们可能只会简单用两句话来描述二者的区别。今天就二者的区别做一个深入的分析。

按照国际惯例，msdn上这样描述SendMessage：

Sends the specified message to a window or windows. The SendMessage function calls the window procedure for the specified window and does not return until the window procedure has processed the message.

翻译大致这个意思：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。
函数原型如下：

LRESULT WINAPI SendMessage(  _In_ HWND   hWnd,  _In_ UINT   Msg,  _In_ WPARAM wParam,  _In_ LPARAM lParam);

msdn上PostMessage的如下描述：

Places (posts) a message in the message queue associated with the thread that created the specified window and returns without waiting for the thread to process the message.

翻译大致这个意思：ostMessage是把消息放到消息队列里就返回继续执行下一句。
函数原型如下：

BOOL WINAPI PostMessage(  _In_opt_ HWND   hWnd,  _In_     UINT   Msg,  _In_     WPARAM wParam,  _In_     LPARAM lParam);

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。

#include <windows.h>#include <cstdio>#include <process.h>#define MY_MSG WM_USER+100const int MAX_INFO_SIZE = 20;HANDLE hStartEvent; // thread start event// thread functionunsigned __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;}