If the message is from another thread this function determines whether or not the message was initiated by the active thread. The "active thread" is the thread associated with the current active window. (See also the WinIsThreadActive function.)

Typically this function is used by applications to determine how to proceed with errors when the window processing the message is not the active window. For example, if the active window uses the WinSendMsg function to send a request for information to another window, the other window cannot become active until it returns control from the WinSendMsg function. The only methods an inactive window has to inform the user of an error are to create a message box (see WinMessageBox), or to flash a window (see WinFlashWindow).

This function can be used to tell if a function is being called recursively.