What happens to a sent message when SendMessageTimeout reaches its timeout?

The Send­Message­Timeout function tries to send a message, but gives up if the timeout elapses. What exactly happens when the timeout elapses?

It depends.

The first case is if the receiving thread never received the message at all. (I.e., if during the period the sender is waiting, the receiving thread never called GetMessage, PeekMessage, or a similar message-retrieval function which dispatches inbound sent messages.) In that case, if the timeout is reached, then the entire operation is canceled; the window manager cleans up everything and makes it look as if the call to SendMessageTimeout never took place. The message is removed from the list of the thread's non-queued messages, and when it finally gets around to calling GetMessage (or whatever), the message will not be delivered.

The second case is if the receiving thread received the message, and the message was delivered to the destination window procedure, but the receiving thread is just slow to process the message and either return from its window procedure or call Reply­Message. In that case, if the timeout is reached, then the sender is released from waiting, but the message is allowed to proceed to completion.

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