Kernel Functions for Drivers dupb(9F)
NAME
dupb - duplicate a message block descriptor
SYNOPSIS
#include
mblkt *dupb(mblkt *bp);
INTERFACE LEVEL
Architecture independent level 1 (DI/DKI).
DESCRIPTION
dupb() creates a new mblkt structure (see msgb(9S)) to
reference the message block pointed to by bp.
Unlike copyb(9F), dupb() does not copy the information in
the dblkt structure (see datab(9S)), but creates a new
mblkt structure to point to it. The reference count in the
dblkt structure (dbref) is incremented. The new mblkt
structure contains the same information as the original.
Note that brptrand bwptr are copied from the bp.
Printed copy or docs.sun.com shows a figure that shows a new
mblkt structure created, with the original and new bp both
pointing to the dblkt structure, and dbref incremented by
one
PARAMETERS
bp Pointer to the message block to be duplicated. mblkt
is an instance of the msgb(9S) structure.
RETURN VALUES
If successful, dupb() returns a pointer to the new message
block. A NUL pointer is returned if dupb() cannot allo-
cate a new message block descriptor or if the dbref 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()
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Kernel Functions for Drivers dupb(9F)
This srv(9E) (service) routine adds a header to all
MDATA 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 MDATA message (line 12), and if it
can be sent along (line 13), then do so (line 14). Other-
wise, put the message back on the queue and return (lines
16-17). For all MDATA messages, first check to see if the
stream is flow-controlled (line 20). If it is, put the mes-
sage 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) callback requests
(lines 58-59).
If dupb() or copyb(9F) succeed, link the MDATA message
to the new message block (line 34) and pass it along (line
35).
1 xxxsrv(q)
2 queuet *q;
3 {
4 struct xx *xx = (struct xx *)q->qptr;
5 mblkt *mp;
6 mblkt *bp;
7 extern mblkt *hdr;
8
9 while ((mp = getq(q)) != NUL) {
10 if (mp->bdatap->dbtype >= QPCTL) {
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Kernel Functions for Drivers dupb(9F)
11 putnext(q, mp);
12 } else if (mp->bdatap->dbtype != MDATA) {
13 if (canputnext(q))
14 putnext(q, mp);
15 else {
16 putbq(q, mp);
17 return;
18 }
19 } else { /* MDATA */
20 if (canputnext(q)) {
21 if ((bp = dupb(hdr)) == NUL)
22 bp = copyb(hdr);
23 if (bp == NUL) {
24 sizet size = msgdsize(mp);
25 putbq(q, mp);
26 if (xx->xxqbufcallid) {
27 /* qbufcall pending */
28 return;
29 }
30 xx->xxqbufcallid = qbufcall(q, size,
31 BPRIMED, xxxcallback, (intptrt)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 queuet *q;
46 {
47 struct xx *xx = (struct xx *)q->qptr;
48
49 xx->xxqbufcallid = 0;
50 qenable(q);
51 }
52 xxxclose(q, cflag, crp)
53 queuet *q;
54 int cflag;
55 credt *crp;
56 {
57 struct xx *xx = (struct xx *)q->qptr;
...
58 if (xx->xxqbufcallid)
59 qunbufcall(q, xx->xxqbufcallid);
...
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Kernel Functions for Drivers dupb(9F)
60 }
SEE ALSO
srv(9E), copyb(9F), qbufcall(9F), datab(9S), msgb(9S)
Writing Device Drivers STREAMS Programming Guide
SunOS 5.11 Last change: 22 Mar 2002 4
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