PostMessage function

Posts a message to the message queue of the specified thread and returns without waiting for the thread to process the message.

Syntax

BOOL PostMessageA(
    HWND   hWnd,
    UINT   Msg,
    WPARAM wParam,
    LPARAM lParam
);

Parameters

Parameter	Description
`hWnd`	Handle to the window whose thread will receive the message. If this parameter is `NULL`, `PostMessage` posts the message to the message queue of the calling thread.
`Msg`	Message identifier. For example, `WM_KEYDOWN` or a custom message defined with `RegisterWindowMessage`.
`wParam`	Additional message-specific information. The content of this parameter depends on the value of `Msg`.
`lParam`	Additional message-specific information. The content of this parameter depends on the value of `Msg`.

Return value

If the function succeeds, the return value is nonzero.
If the function fails, the return value is zero. To get extended error information, call GetLastError.

Remarks

The message is placed in the message queue associated with the thread that created the specified window and then the thread is awakened.
Unlike SendMessage, PostMessage does not wait for the message to be processed.
Do not use PostMessage to send messages that require a response.
For cross-process communication, ensure the receiving thread has a message pump.

Example

The following example posts a WM_CLOSE message to a window identified by its handle.

#include <windows.h>

int main()
{
    HWND hWnd = FindWindowA(NULL, "Untitled - Notepad");
    if (hWnd)
    {
        PostMessageA(hWnd, WM_CLOSE, 0, 0);
    }
    return 0;
}