What is call out table in unix?

Can anybody tell me what a 'call out table' is in Unix? Maurice J. Bach gives an explanation in his book Desig开发者_如何学编程n of the UNIX Operating System, but I'm having difficulty in understanding the examples, especially the one explaining the reason of negative time-out fields. Why are software interrupts used there?

Thanks!

---

Interrupts stop the current code and start the execution of a high-priority handler; while this handler runs, nothing else can get the CPU. So if you need to do something complex, your interrupt handler would hang the whole system.

The solution: Fill a data structure with all the necessary data and then store this data structure with a pointer to the handler in the call out table. Some service (usually the clock handler) will eventually visit the table and execute the entries one by one in a standard context (i.e. one which doesn't block the process switching).

---

In System V unix, the kernel or device drivers could schedule some function to be run (or "called out") by the kernel at a later time. The kernel clock handler was in charge of making sure such registered call outs were executed. The call out table was the kernel data structure in which such registered "call outs" were stored.

I don't know to what end they were generally used.