Windows Sockets 1.1 introduced the async select mechanism in order to provide network event indications in a manner that did not involve either polling or blocking. The WSPAsyncSelect function is used to register an interest in one or more network events as listed above, and supply a window handle to be used for notification. When a nominated network event occurs, a client-specified Windows message is sent to the indicated window. The service provider must use the WPUPostMessage function to accomplish this.
In Win32 environments, this may not be the most efficient notification mechanism, and is inconvenient for daemons and services which don't want to open any windows. The event object signaling mechanism described below is introduced to solve this problem.