BUFCALL(9F) Kernel Functions for Drivers BUFCALL(9F)


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).


Number of bytes required for the buffer.

Priority of the allocb(9F) allocation request (not used).

Function or driver routine to be called when a buffer becomes

Argument to the function to be called when a buffer becomes


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


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


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

1 struct hdr {
2 unsigned int h_size;
3 int h_version;
4 };
6 void xxxsrv(q)
7 queue_t *q;
8 {
9 mblk_t *bp;
10 mblk_t *mp;
11 struct hdr *hp;
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

January 16, 2006 BUFCALL(9F)