dupb(9F) Kernel Functions for Drivers dupb(9F)NAMEdupb - duplicate a message block descriptor
SYNOPSIS
#include <sys/stream.h>
mblk_t *dupb(mblk_t *bp);
INTERFACE LEVEL
Architecture independent level 1 (DDI/DKI).
DESCRIPTIONdupb() 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 incre‐
mented. 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 online man page on docs.sun.com or a print copy
for the diagram.
Figure that shows a new mblk_t structure created, with the original and
new bp both pointing to the dblk_t structure, and db_ref incremented by
one
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 allocate 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).
CONTEXTdupb() can be called from user, kernel, or interrupt context.
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, qbuf‐
call(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) callback
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 ALSOsrv(9E), copyb(9F), qbufcall(9F), datab(9S), msgb(9S)
Writing Device Drivers STREAMS Programming Guide
SunOS 5.10 22 Mar 2002 dupb(9F)
