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srv(9e)
srv(9E) Driver Entry Points srv(9E)
NAME
srv - service queued messages
SYNOPSIS
#include <sys/types.h>
#include <sys/stream.h>
#include <sys/stropts.h>
#include <sys/ddi.h>
#include <sys/sunddi.h>
int prefix_rsrv(queue_t *q); /* read side */
int prefix_wsrv(queue_t *q); /* write side */
INTERFACE LEVEL
Architecture independent level 1 (DDI/DKI). This entry point is re‐
quired for STREAMS.
ARGUMENTS
q Pointer to the queue(9S) structure.
DESCRIPTION
The optional service srv() routine may be included in a STREAMS module
or driver for many possible reasons, including:
o to provide greater control over the flow of messages in a
stream;
o to make it possible to defer the processing of some messages
to avoid depleting system resources;
o to combine small messages into larger ones, or break large
messages into smaller ones;
o to recover from resource allocation failure. A module's or
driver's put(9E) routine can test for the availability of a
resource, and if it is not available, enqueue the message
for later processing by the srv() routine.
A message is first passed to a module's or driver's put(9E) routine,
which may or may not do some processing. It must then either:
o Pass the message to the next stream component with
putnext(9F).
o If a srv() routine has been included, it may call putq(9F)
to place the message on the queue.
Once a message has been enqueued, the STREAMS scheduler controls the
service routine's invocation. The scheduler calls the service routines
in FIFO order. The scheduler cannot guarantee a maximum delay srv()
routine to be called except that it will happen before any user level
process are run.
Every stream component (stream head, module or driver) has limit values
it uses to implement flow control. Each component should check the tun‐
able high and low water marks to stop and restart the flow of message
processing. Flow control limits apply only between two adjacent compo‐
nents with srv() routines.
STREAMS 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 M_IOCACK). High priority messages are
always placed at the head of the srv() 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 con‐
trolled band is stopped, all lower priority bands are also stopped.
Once the STREAMS scheduler calls a srv() 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
srv() routine should be written depend of the implementation, the di‐
rection of flow (upstream or downstream), and whether it is for a mod‐
ule or a driver.
1. Use getq(9F) to get the next enqueued message.
2. If the message is high priority, process (if appropriate)
and pass to the next stream component with putnext(9F).
3. If it is not a high priority message (and therefore subject
to flow control), attempt to send it to the next stream com‐
ponent with a srv() routine. Use bcanputnext(9F) to deter‐
mine if this can be done.
4. If the message cannot be passed, put it back on the queue
with putbq(9F). If it can be passed, process (if appropri‐
ate) and pass with putnext().
RETURN VALUES
Ignored.
SEE ALSO
put(9E), bcanput(9F), bcanputnext(9F), canput(9F), canputnext(9F),
getq(9F), nulldev(9F), putbq(9F), putnext(9F), putq(9F), qinit(9S),
queue(9S)
Writing Device Drivers in Oracle Solaris 11.4
STREAMS Programming Guide
WARNINGS
Each stream module must specify a read and a write service srv() rou‐
tine. If a service routine is not needed (because the put() routine
processes all messages), a NULL pointer should be placed in module's
qinit(9S) structure. Do not use nulldev(9F) instead of the NULL
pointer. Use of nulldev(9F) for a srv() routine can result in flow con‐
trol errors.
Oracle Solaris 11.4 9 September 2024 srv(9E)