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+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;
}