Service Procedures

STREAMS allows a service procedure to be contained in each QUEUE, in addition to the put procedure, to address the above cases and for either purposes. A service procedure is not required in a QUEUE and is associated with deferred processing.

If a QUEUE has both a put and service procedure, message processing will generally be divided between the procedures. The put procedure is always called first, from a preceding QUEUE. After the put procedure completes its part of the message processing, it arranges for the service procedure to be called by passing the message to the putq utility. The putq utility does two things: it places the message on the message queue of the QUEUE, and it links the QUEUE to the end of the STREAMS scheduling queue. When the putq utility returns to the put procedure, the procedure typically exits. Some time later, the service procedure will be automatically called by the STREAMS scheduler.

The STREAMS scheduler is separate and distinct from the system process scheduler. It is concerned only with QUEUEs linked on the STREAMS scheduling queue. The scheduler calls the service procedure of the scheduled QUEUE one at a time, in a FIFO manner.

Having both a put and service procedure in a QUEUE enables STREAMS to provide the rapid response and the queuing required in systems with many users. The put procedure allows rapid response to certain data and events, such as software echoing of input characters. Put procedures effectively have higher priority than any scheduled service procedures. When called from the preceding STREAMS component, a put procedure starts before the scheduled service procedures of any QUEUE are started.

The service procedure implies message queuing. Queuing results in deferred processing of the service procedure, following all other QUEUEs currently on the scheduling queue. For example, terminal output, input erase, and kill processing would typically be performed in a service procedure because this type of processing does not have to be as timely as echoing. Using a service procedure also allows processing time to be more evenly spread among multiple streams. As with the put procedure, there will generally be a separate service procedure for each QUEUE in a module. The flow-control mechanism uses the service procedures.