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If applicable, add the following below this CDDL HEADER, with the fields enclosed by brackets "[]" replaced with your own identifying information: Portions Copyright [yyyy] [name of copyright owner] .TH srv 9E "12 Nov 1992" "SunOS 5.11" "Driver Entry Points" .SH NAME srv \- service queued messages .SH SYNOPSIS .LP .nf #include #include #include #include #include \fBintprefix\fR\fBrsrv\fR(\fBqueue_t *\fR\fIq\fR/* read side */ .fi .LP .nf \fBintprefix\fR\fBwsrv\fR(\fBqueue_t *\fR\fIq\fR/* write side */ .fi .SH INTERFACE LEVEL .sp .LP Architecture independent level 1 (DDI/DKI). This entry point is required for \fBSTREAMS\fR. .SH ARGUMENTS .sp .ne 2 .mk .na \fB\fIq\fR\fR .ad .RS 5n .rt Pointer to the \fBqueue\fR(9S) structure. .RE .SH DESCRIPTION .sp .LP The optional service \fBsrv()\fR routine may be included in a \fBSTREAMS \fRmodule or driver for many possible reasons, including: .RS +4 .TP .ie t \(bu .el o to provide greater control over the flow of messages in a stream; .RE .RS +4 .TP .ie t \(bu .el o to make it possible to defer the processing of some messages to avoid depleting system resources; .RE .RS +4 .TP .ie t \(bu .el o to combine small messages into larger ones, or break large messages into smaller ones; .RE .RS +4 .TP .ie t \(bu .el o to recover from resource allocation failure. A module's or driver's \fBput\fR(9E) routine can test for the availability of a resource, and if it is not available, enqueue the message for later processing by the \fBsrv()\fR routine. .RE .sp .LP A message is first passed to a module's or driver's \fBput\fR(9E) routine, which may or may not do some processing. It must then either: .RS +4 .TP .ie t \(bu .el o Pass the message to the next stream component with \fBputnext\fR(9F). .RE .RS +4 .TP .ie t \(bu .el o If a \fBsrv()\fR routine has been included, it may call \fBputq\fR(9F) to place the message on the queue. .RE .sp .LP Once a message has been enqueued, the \fBSTREAMS \fRscheduler controls the service routine's invocation. The scheduler calls the service routines in \fBFIFO \fRorder. The scheduler cannot guarantee a maximum delay \fBsrv()\fR routine to be called except that it will happen before any user level process are run. .sp .LP Every stream component (stream head, module or driver) has limit values it uses to implement flow control. Each component should check the tunable high and low water marks to stop and restart the flow of message processing. Flow control limits apply only between two adjacent components with \fBsrv()\fR routines. .sp .LP \fBSTREAMS \fR messages can be defined to have up to 256 different priorities to support requirements for multiple bands of data flow. At a minimum, a stream must distinguish between normal (priority zero) messages and high priority messages (such as \fBM_IOCACK\fR). High priority messages are always placed at the head of the \fBsrv()\fR routine's queue, after any other enqueued high priority messages. Next are messages from all included priority bands, which are enqueued in decreasing order of priority. Each priority band has its own flow control limits. If a flow controlled band is stopped, all lower priority bands are also stopped. .sp .LP Once the \fBSTREAMS \fRscheduler calls a \fBsrv()\fR routine, it must process all messages on its queue. The following steps are general guidelines for processing messages. Keep in mind that many of the details of how a \fBsrv()\fR routine should be written depend of the implementation, the direction of flow (upstream or downstream), and whether it is for a module or a driver. .RS +4 .TP 1. Use \fBgetq\fR(9F) to get the next enqueued message. .RE .RS +4 .TP 2. If the message is high priority, process (if appropriate) and pass to the next stream component with \fBputnext\fR(9F). .RE .RS +4 .TP 3. If it is not a high priority message (and therefore subject to flow control), attempt to send it to the next stream component with a \fBsrv()\fR routine. Use \fBbcanputnext\fR(9F) to determine if this can be done. .RE .RS +4 .TP 4. If the message cannot be passed, put it back on the queue with \fBputbq\fR(9F). If it can be passed, process (if appropriate) and pass with \fBputnext()\fR. .RE .SH RETURN VALUES .sp .LP Ignored. .SH SEE ALSO .sp .LP \fBput\fR(9E), \fBbcanput\fR(9F), \fBbcanputnext\fR(9F), \fBcanput\fR(9F), \fBcanputnext\fR(9F), \fBgetq\fR(9F), \fBnulldev\fR(9F), \fBputbq\fR(9F), \fBputnext\fR(9F), \fBputq\fR(9F), \fBqinit\fR(9S), \fBqueue\fR(9S) .sp .LP \fIWriting Device Drivers\fR .sp .LP \fISTREAMS Programming Guide\fR .SH WARNINGS .sp .LP Each stream module must specify a read and a write service \fBsrv()\fR routine. If a service routine is not needed (because the \fBput()\fR routine processes all messages), a \fBNULL\fR pointer should be placed in module's \fBqinit\fR(9S) structure. Do not use \fBnulldev\fR(9F) instead of the \fBNULL\fR pointer. Use of\fBnulldev\fR(9F) for a \fBsrv()\fR routine can result in flow control errors.