Sorry I am still not understanding the functionality you are expecting. If it has to be synchronous, it means that the caller is interested in the result. Howev

Sorry I am still not understanding the functionality you are expecting. If it has to be synchronous, it means that the caller is interested in the result. However, the caller can stop after a specified timeout and continue, which means that getting the result is not absolutely critical to the caller's functionality. So then why is callback not an option? After all, it is the same as setting the timeout to zero and getting interrupted (or polling) at some point in the future.