How do I SendMessage() to a window created on another thread?

I have a situation where I want to SendMessage to a window that was created on another thread than the one that is calling SendMessage.

The default behaviour seems to be block forever and not work.

So I changed the call to PostMessage, which didn't block the sending thread, but the message never appears to arrive at the intended window.

So how do I SendMessage to 开发者_开发问答a window created on a separate thread, or is this impossible?

The PostThreadMessage function posts a message to the message queue of the specified thread. you can specified Identifier of the thread to which the message is to be posted. is that you want?

What is the thread that owns the target window doing? It needs to be pumping messages in order to be able to receive messages that are either sent or posted to it. Typically this is done by having a GetMessage/TranslateMessage/DispatchMessage loop. If the thread is doing something else - such as blocking waiting for an event or mutex or IO to complete, or is busy in some other loop carrying out a calculation, it won't receive messages: SendMessage to the thread will block, and PostMessage will post but not get delivered.

If that target thread needs to both manage events or similar, and it also owns a window, you may need to use MsgWaitForMultipleObjects in that thread's message loop.

You are running into a deadlock. An example, if you SendMessage to another thread, then the windowProc in that thread does SendMessage back to your window, they will lock waiting each other forever.

You need to either fix PostMessage (it does deliver messages, there's just error in your code somewhere), or be very careful about who calls who and when.

To protect against threads that are busy or hung, there is SendMessageTimeout.

Each control you create belongs to the thread that created it, which means that the WndProc for that control will be running in the thread that created the control. You send messages with SendMessage and PostMessage freely to any control.

The issue with CWnd::PostMessage and CWnd::SendMessage is the same. The message is being pushed out, and not being received by anything. SendMessage is blocking by design; it's job is to block the thread until the message is received.

MSDN says this about SendMessage:

"The SendMessage function calls the window procedure for the specified window and does not return until the window procedure has processed the message."

It is possible to send a message to window on another thread using CWnd::PostThreadMessage or winapi PostMessage. When you create the window you can use GetSafeHwnd() to get the handle or you can use a thread ID when the thread is created.