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Copyright 1989 AT&T
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bufcall - call a function when a buffer becomes available

#include <sys/types.h>
#include <sys/stream.h>



bufcall_id_t bufcall(size_t size, uint_t pri, void *funcvoid *arg,
 void *arg);

Architecture independent level 1 (DDI/DKI).

size pri func arg

The bufcall() function serves as a timeout(9F) call of indeterminate length. When a buffer allocation request fails, bufcall() can be used to schedule the routine func, to be called with the argument arg when a buffer becomes available. func may call allocb() or it may do something else.

If successful, bufcall() returns a bufcall ID that can be used in a call to unbufcall() to cancel the request. If the bufcall() scheduling fails, func is never called and 0 is returned.

The bufcall() function can be called from user, interrupt, or kernel context.

Example 1 Calling a function when a buffer becomes available:

The purpose of this srv(9E) service routine is to add a header to all M_DATA messages. Service routines must process all messages on their queues before returning, or arrange to be rescheduled

While there are messages to be processed (line 13), check to see if it is a high priority message or a normal priority message that can be sent on (line 14). Normal priority message that cannot be sent are put back on the message queue (line 34). If the message was a high priority one, or if it was normal priority and canputnext(9F) succeeded, then send all but M_DATA messages to the next module with putnext(9F) (line 16).

For M_DATA messages, try to allocate a buffer large enough to hold the header (line 18). If no such buffer is available, the service routine must be rescheduled for a time when a buffer is available. The original message is put back on the queue (line 20) and bufcall (line 21) is used to attempt the rescheduling. It will succeed if the rescheduling succeeds, indicating that qenable will be called subsequently with the argument q once a buffer of the specified size (sizeof (struct hdr)) becomes available. If it does, qenable(9F) will put q on the list of queues to have their service routines called. If bufcall() fails, timeout(9F) (line 22) is used to try again in about a half second.

If the buffer allocation was successful, initialize the header (lines 25-28), make the message type M_PROTO (line 29), link the M_DATA message to it (line 30), and pass it on (line 31).

Note that this example ignores the bookkeeping needed to handle bufcall() and timeout(9F) cancellation for ones that are still outstanding at close time.

 1 struct hdr {
 2 unsigned int h_size;
 3 int h_version;
 4 };
 5
 6 void xxxsrv(q)
 7 queue_t *q;
 8 {
 9 mblk_t *bp;
10 mblk_t *mp;
11 struct hdr *hp;
12
13 while ((mp = getq(q)) != NULL) { /* get next message */
14 if (mp->b_datap->db_type >= QPCTL || /* if high priority */
 canputnext(q)) { /* normal & can be passed */
15 if (mp->b_datap->db_type != M_DATA)
16 putnext(q, mp); /* send all but M_DATA */
17 else {
18 bp = allocb(sizeof(struct hdr), BPRI_LO);
19 if (bp == NULL) { /* if unsuccessful */
20 putbq(q, mp); /* put it back */
21 if (!bufcall(sizeof(struct hdr), BPRI_LO,
 qenable, q)) /* try to reschedule */
22 timeout(qenable, q, drv_usectohz(500000));
23 return (0);
24 }
25 hp = (struct hdr *)bp->b_wptr;
26 hp->h_size = msgdsize(mp); /* initialize header */
27 hp->h_version = 1;
28 bp->b_wptr += sizeof(struct hdr);
29 bp->b_datap->db_type = M_PROTO;/* make M_PROTO */
30 bp->b_cont = mp; /* link it */
31 putnext(q, bp); /* pass it on */
32 }
33 } else { /* normal priority, canputnext failed */
34 putbq(q, mp); /* put back on the message queue */
35 return (0);
36 }
37 }
 return (0);
38 }

srv (9E), allocb (9F), canputnext (9F), esballoc (9F), esbbcall (9F), putnext (9F), qenable (9F), testb (9F), timeout (9F), unbufcall (9F)

Writing Device Drivers

STREAMS Programming Guide

Even when func is called by bufcall(), allocb(9F) can fail if another module or driver had allocated the memory before func was able to call allocb(9F).