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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html><head>
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<title>liboop: How?</title>
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<link rel="stylesheet" type="text/css" href="style.css">
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</head><body>
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<h2>Overview of liboop.</h2>
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<h4>The basic idea.</h4>
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Liboop is primarily an <em>interface definition</em>.  It defines an interface
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which components may use to request notification when an <em>event</em>
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(activity on a file descriptor, the real-time clock reaches a certain value,
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a particular signal is received) occurs.  The component which owns the event
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loop -- the component whose code is active when the system is idle --
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implements the interface; it is an <em>event source</em>.  Components which
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are interested in events register themselves with the event source; they are
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<em>event sinks</em>.  Event sinks may themselves source other, higher-level
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events, but that is outside liboop's scope.
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<h4>Control flow.</h4>
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During initialization, the event source is created.  At least one event sink
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is also created, and registered with the event source.  Once initialization
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completes, control is transferred to the event source, which (at its core)
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waits for events, usually using a system function like select() or poll().
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When an event occurs, the event source gives a <em>callback</em> to all the
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event sinks which registered interest in that event.
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<p>
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During callbacks, the event sinks react to the event as appropriate (usually
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performing some I/O, or at least modifying internal state).  Event sinks for
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events which are no longer relevant may be unregistered; new event sinks may
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be registered for additional events.  Each event sink, when it finishes,
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returns a value which tells the event source whether to continue processing
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events or whether to terminate.
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<p>
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While the event source must be fully reentrant (registration and deregistration
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may, and indeed usually are, performed within the context of an event), event
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sinks need not be; no event sink will be called while another event sink is
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active.
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<p>
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If no event sink instructs the event source to terminate, the event source
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continues waiting for events.  Otherwise, the event source returns to its
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caller, which usually shuts down the system.
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<h4>The system event source.</h4>
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Liboop comes with a single "reference" implementation of an event source.
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This event source uses select() to dispatch events.  Most programs built
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around liboop will probably use the standard system event source; legacy
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programs with their own event loop, or programs with specialized needs may
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implement their own event source.
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<h4>Adapters.</h4>
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Liboop supports <em>adapters</em> to enable legacy components to use the liboop
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interface.  For example, many widget sets have their own event loop and their
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own mechanism for registering callbacks on timeouts and file descriptor
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activity; liboop uses <em>source adapters</em> that accept registration,
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register corresponding callbacks with the widget set's event loop, and route
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events appropriately.  Such adapters let general-purpose liboop-based
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components work in an application based on that widget set.
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<p>
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Similarly, some components are designed to work in a non-blocking fashion, and
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they might be used with a <em>sink adapter</em> to work with liboop.  An
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asynchronous DNS query package, for example, could work as a liboop sink that
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ultimately generates a higher-level "success" or "failure" callback to the
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invoking routine.
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<h4>Code.</h4>
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Liboop's abstract event source interface is implemented as a structure
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containing C function pointers.  These functions accept a pointer to the
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structure as their first argument; sources are expected to include their
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own data (in whatever format) with the core function pointers.  Callbacks
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are also C function pointers, with "void *" arguments to pass data.
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<p>
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For more about the liboop interface, see the <a href="ref">reference</a>.
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<hr><a href="">liboop home</a></body></html>