A debug application uses WinQuerySendMsg to process messages sent to the program it is debugging. The call returns whether or not the thread of the program is currently processing the message. If it is, the thread is waiting for the scheduler to give it another time slice.

Applications that have sent messages to the process of the program being debugged normally have to wait until the program processes those messages. This means that, if the program is currently waiting at a breakpoint, these applications also have to wait. A debug application uses the WinQuerySendMsg function to find out which sent messages are currently waiting to be processed. It then uses WinReplyMsg to release the sending applications.