dupb(9F)
NAME
dupb - duplicate a message block descriptor
SYNOPSIS
#include <sys/stream.h>
mblk_t *dupb(mblk_t *bp);
INTERFACE LEVEL
Architecture independent level 1 (DDI/DKI).
DESCRIPTION
dupb() creates a new mblk_t structure (see msgb(9S)) to
reference the message block pointed to by bp.
Unlike copyb(9F), dupb() does not copy the information in
the dblk_t structure (see datab(9S)), but creates a new
mblk_t structure to point to it. The reference count in the
dblk_t structure (db_ref) is incremented. The new mblk_t
structure contains the same information as the original.
Note that b_rptr and b_wptr are copied from the bp.
Please see the the online man page on docs.sun.com or a
print copy for the diagram.
PARAMETERS
bp Pointer to the message block to be duplicated. mblk_t
is an instance of the msgb(9S) structure.
RETURN VALUES
If successful, dupb() returns a pointer to the new message
block. A NULL pointer is returned if dupb() cannot allo-
cate a new message block descriptor or if the db_ref field
of the data block structure (see datab(9S)) has reached a
maximum value (255).
CONTEXT
dupb() can be called from user, kernel, or interrupt con-
text.
EXAMPLES
Example 1: Using dupb()
This srv(9E) (service) routine adds a header to all
M_DATA messages before passing them along. dupb is used
instead of copyb(9F) because the contents of the header
block are not changed.
For each message on the queue, if it is a priority message,
pass it along immediately (lines 10-11). Otherwise, if it is
anything other than an M_DATA message (line 12), and if it
can be sent along (line 13), then do so (line 14).
Otherwise, put the message back on the queue and return
(lines 16-17). For all M_DATA messages, first check to see
if the stream is flow-controlled (line 20). If it is, put
the message back on the queue and return (lines 37-38). If
it is not, the header block is duplicated (line 21).
dupb() can fail either due to lack of resources or because
the message block has already been duplicated 255 times. In
order to handle the latter case, the example calls
copyb(9F) (line 22). If copyb(9F) fails, it is due to
buffer allocation failure. In this case, qbufcall(9F) is
used to initiate a callback (lines 30-31) if one is not
already pending (lines 26-27).
The callback function, xxxcallback(), clears the recorded
qbufcall(9F) callback id and schedules the service procedure
(lines 49-50). Note that the close routine,
xxxclose(), must cancel any outstanding qbufcall(9F) call-
back requests (lines 58-59).
If dupb() or copyb(9F) succeed, link the M_DATA message
to the new message block (line 34) and pass it along (line
35).
1 xxxsrv(q)
2 queue_t *q;
3 {
4 struct xx *xx = (struct xx *)q->q_ptr;
5 mblk_t *mp;
6 mblk_t *bp;
7 extern mblk_t *hdr;
8
9 while ((mp = getq(q)) != NULL) {
10 if (mp->b_datap->db_type >= QPCTL) {
11 putnext(q, mp);
12 } else if (mp->b_datap->db_type != M_DATA) {
13 if (canputnext(q))
14 putnext(q, mp);
15 else {
16 putbq(q, mp);
17 return;
18 }
19 } else { /* M_DATA */
20 if (canputnext(q)) {
21 if ((bp = dupb(hdr)) == NULL)
22 bp = copyb(hdr);
23 if (bp == NULL) {
24 size_t size = msgdsize(mp);
25 putbq(q, mp);
26 if (xx->xx_qbufcall_id) {
27 /* qbufcall pending */
28 return;
29 }
30 xx->xx_qbufcall_id = qbufcall(q, size,
31 BPRI_MED, xxxcallback, (intptr_t)q);
32 return;
33 }
34 linkb(bp, mp);
35 putnext(q, bp);
36 } else {
37 putbq(q, mp);
38 return;
39 }
40 }
41 }
42 }
43 void
44 xxxcallback(q)
45 queue_t *q;
46 {
47 struct xx *xx = (struct xx *)q->q_ptr;
48
49 xx->xx_qbufcall_id = 0;
50 qenable(q);
51 }
52 xxxclose(q, cflag, crp)
53 queue_t *q;
54 int cflag;
55 cred_t *crp;
56 {
57 struct xx *xx = (struct xx *)q->q_ptr;
...
58 if (xx->xx_qbufcall_id)
59 qunbufcall(q, xx->xx_qbufcall_id);
...
60 }
SEE ALSO
srv(9E), copyb(9F), qbufcall(9F), datab(9S), msgb(9S)
Writing Device Drivers STREAMS Programming Guide
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