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7.2 The SERVE-EVENT Function

The system:serve-event function is the standard way for an application to wait for something to happen. For example, the Lisp system calls system:serve-event when it wants input from X or a terminal stream. The idea behind system:serve-event is that it knows the appropriate action to take when any interesting event happens. If an application calls system:serve-event when it is idle, then any other applications with pending events can run. This allows several applications to run “at the same time” without interference, even though there is only one thread of control. Note that if an application is waiting for input of any kind, then other applications will get events.

Function: system:serve-event &optional timeout

This function waits for an event to happen and then dispatches to the correct handler function. If specified, timeout is the number of seconds to wait before timing out. A time out of zero seconds is legal and causes system:serve-event to poll for any events immediately available for processing. system:serve-event returns t if it serviced at least one event, and nil otherwise. Depending on the application, when system:serve-event returns t, you might want to call it repeatedly with a timeout of zero until it returns nil.

If input is available on any designated file descriptor, then this calls the appropriate handler function supplied by system:add-fd-handler.

Since events for many different applications may arrive simultaneously, an application waiting for a specific event must loop on system:serve-event until the desired event happens. Since programs such as Hemlock call system:serve-event for input, applications usually do not need to call system:serve-event at all; Hemlock allows other application’s handlers to run when it goes into an input wait.

Function: system:serve-all-events &optional timeout

This function is similar to system:serve-event, except it serves all the pending events rather than just one. It returns t if it serviced at least one event, and nil otherwise.


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