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;
}