Events and Interrupts

The normal execution of a Prolog program may be interrupted by Events and Interrupts:

Events

• they may occur asynchronously (posted by the environment) or synchronously (raised by the program itself).
• they are handled synchronously by a handler goal that is inserted into the resolvent.
• the handler can interact with interrupted execution via global references.
• the handler can cause the interrupted execution to fail or to abort.
• the handler can cause waking of delayed goals.

Errors

Errors are a special case of events. They are raised by built-in predicates (e.g. when the arguments are of the wrong type) and usually pass the culprit goal to the error handler.

Interrupts

Interrupts usually originate from the operating system, e.g. on a Unix host, signals are mapped to ECLiPSe interrupts.

• they occur asynchronously, but may be mapped into a sychronous event.
• in Unix, the handler can be executed asynchronously in a separate ECLiPSe engine. This means that
• the handler cannot interact with interrupted execution, except via global variables, files and the like.
• failure of the handler is ignored.
• the development system catches and handles many operating system signals as interrupts, user abort by typing ^C, data arriving at sockets, memory protection faults, etc.