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DUPB(9F)							      DUPB(9F)

       dupb - duplicate a message block descriptor

       #include <sys/stream.h>

       mblk_t *dupb(mblk_t *bp);

       Architecture independent level 1 (DDI/DKI).

       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	incre‐
       mented.	 The new mblk_t structure contains the same information as the
       original.  Note that b_rptrand b_wptr are copied from the bp.

       Printed copy or docs.sun.com shows a 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

	      Pointer to the message block to  be  duplicated.	mblk_t	is  an
	      instance of the  msgb(9S) structure.

       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).

       dupb() can be called from user, kernel, or interrupt context.

       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	 qbuf‐
       call(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;
	       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;
	      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   }

       srv(9E), copyb(9F), qbufcall(9F), datab(9S), msgb(9S)

       Writing Device Drivers STREAMS Programming Guide

				 Mar 22, 2002			      DUPB(9F)

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