The PostThreadMessage function posts a message to the message queue of the specified thread. It returns without waiting for the thread to process the message.

Syntax

BOOL PostThreadMessage(      
    DWORD idThread,
    UINT Msg,
    WPARAM wParam,
    LPARAM lParam
);

Parameters

idThread

[in] Identifier of the thread to which the message is to be posted.

The function fails if the specified thread does not have a message queue. The system creates a thread's message queue when the thread makes its first call to one of the User or GDI functions. For more information, see the Remarks section.

Microsoft Windows Vista and later.Message posting is subject to User Interface Privilege Isolation (UIPI). The thread of a process can post messages only to posted-message queues of threads in processes of lesser or equal integrity level.

Windows 2000/XP: This thread must either belong to the same desktop as the calling thread or to a process with the same locally unique identifier (LUID). Otherwise, the function fails and returns ERROR_INVALID_THREAD_ID.

Windows Server 2003 SP1: This thread must have the SE_TCB_NAME privilege to post a message to a thread that belongs to a process with the same locally unique identifier (LUID) but is in a different desktop. Otherwise, the function fails and returns ERROR_INVALID_THREAD_ID.

Msg

[in] Specifies the type of message to be posted.

wParam

[in] Specifies additional message-specific information.

lParam

[in] Specifies additional message-specific information.

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. GetLastError returns ERROR_INVALID_THREAD_ID if idThread is not a valid thread identifier, or if the thread specified by idThread does not have a message queue.

Remarks

Microsoft Windows Vista and later. When a message is blocked by UIPI the last error, retrieved with GetLastError, is set to 5 (access denied).

The thread to which the message is posted must have created a message queue, or else the call to PostThreadMessage fails. Use the following method to handle this situation.

• Create an event object, then create the thread. Use the WaitForSingleObject function to wait for the event to be set to the signaled state before calling PostThreadMessage. In the thread to which the message will be posted, call PeekMessage as shown here to force the system to create the message queue.
  PeekMessage(&msg, NULL, WM_USER, WM_USER, PM_NOREMOVE)

  Set the event, to indicate that the thread is ready to receive posted messages.

The thread to which the message is posted retrieves the message by calling the GetMessage or PeekMessage function. The hwnd member of the returned MSG structure is NULL.

Messages sent by PostThreadMessage are not associated with a window. As a general rule, messages that are not associated with a window cannot be dispatched by the DispatchMessage function. Therefore, if the recipient thread is in a modal loop (as used by MessageBox or DialogBox), the messages will be lost. To intercept thread messages while in a modal loop, use a thread-specific hook.

The system only does marshalling for system messages (those in the range 0 to (WM_USER-1)). To send other messages (those >= WM_USER) to another process, you must do custom marshalling.

Windows 2000/XP: There is a limit of 10,000 posted messages per message queue. This limit should be sufficiently large. If your application exceeds the limit, it should be redesigned to avoid consuming so many system resources. To adjust this limit, modify the following registry key.

HKEY_LOCAL_MACHINE
     SOFTWARE
          Microsoft
               Windows NT
                    CurrentVersion
                         Windows
                              USERPostMessageLimit

The minimum acceptable value is 4000.

Windows 95/98/Me: PostThreadMessageW is supported by the Microsoft Layer for Unicode (MSLU). To use this, you must add certain files to your application, as outlined in Microsoft Layer for Unicode on Windows 95/98/Me Systems.

Function Information

Minimum DLL Version	user32.dll
Header	Declared in Winuser.h, include Windows.h
Import library	User32.lib
Minimum operating systems	Windows 95, Windows NT 3.1
Unicode	Implemented as ANSI and Unicode versions.

See Also

Messages and Message Queues Overview, GetCurrentThreadId, GetMessage, GetWindowThreadProcessId, MSG, PeekMessage, PostMessage, Sleep, WaitForSingleObject