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

mkiocb - allocates a STREAMS ioctl block for M_IOCTL messages in the kernel.

#include <sys/stream.h>
mblk_t *mkiocb(uint_t command);

illumos DDI specific (illumos DDI).

ioctl command for the ioc_cmd field.

STREAMS modules or drivers might need to issue an ioctl to a lower module or driver. The mkiocb() function tries to allocate (using allocb(9F)) a STREAMS M_IOCTL message block (iocblk(9S)). Buffer allocation fails only when the system is out of memory. If no buffer is available, the qbufcall(9F) function can help a module recover from an allocation failure.

The mkiocb function returns a mblk_t structure which is large enough to hold any of the ioctl messages (iocblk(9S), copyreq(9S) or copyresp(9S)), and has the following special properties:


Set to b_rptr + sizeof(struct iocblk).


Set to NULL..


Set to M_IOCTL.

The fields in the iocblk structure are initialized as follows:


Set to the command value passed in.


Set to a unique identifier.


Set to point to a credential structure encoding the maximum system privilege and which does not need to be freed in any fashion.


Set to 0.


Set to 0.


Set to 0.


Set to IOC_NATIVE to reflect that this is native to the running kernel.

Upon success, the mkiocb() function returns a pointer to the allocated mblk_t of type M_IOCTL.

On failure, it returns a null pointer.

The mkiocb() function can be called from user, interrupt, or kernel context.

Example 1 M_IOCTL Allocation

The first example shows an M_IOCTL allocation with the ioctl command TEST_CMD. If the iocblk(9S) cannot be allocated, NULL is returned, indicating an allocation failure (line 5). In line 11, the putnext(9F) function is used to send the message downstream.

1  test_function(queue_t *q, test_info_t *testinfo)
 2  {
 3   mblk_t *mp;
 5   if ((mp = mkiocb(TEST_CMD)) == NULL)
 6       return (0);
 8       /* save off ioctl ID value */
 9       testinfo->xx_iocid = ((struct iocblk *)mp->b_rptr)->ioc_id;
11       putnext(q, mp);       /* send message downstream */
12       return (1);
13  }

Example 2 The ioctl ID Value

During the read service routine, the ioctl ID value for M_IOCACK or M_IOCNAK should equal the ioctl that was previously sent by this module before processing.

 1  test_lrsrv(queue_t *q)
 2  {
 3      ...
 5      switch (DB_TYPE(mp)) {
 6      case M_IOCACK:
 7      case M_IOCNAK:
 8          /* Does this match the ioctl that this module sent */
 9          ioc = (struct iocblk*)mp->b_rptr;
10          if (ioc->ioc_id == testinfo->xx_iocid) {
11              /* matches, so process the message */
12              ...
13              freemsg(mp);
14          }
15          break;
16      }
17      ...
18  }

Example 3 An iocblk Allocation Which Fails

The next example shows an iocblk allocation which fails. Since the open routine is in user context, the caller may block using qbufcall(9F) until memory is available.

1  test_open(queue_t *q, dev_t devp, int oflag, int sflag,
                           cred_t *credp)
 2  {
 3       while ((mp = mkiocb(TEST_IOCTL)) == NULL) {
 4            int id;
 6            id = qbufcall(q, sizeof (union ioctypes), BPRI_HI,
 7                dummy_callback, 0);
 8            /* Handle interrupts */
 9            if (!qwait_sig(q)) {
10                qunbufcall(q, id);
11                return (EINTR);
12            }
13       }
14       putnext(q, mp);
15  }

allocb(9F), putnext(9F), qbufcall(9F), qwait_sig(9F), copyreq(9S), copyresp(9S), iocblk(9S)

Writing Device Drivers

STREAMS Programming Guide

It is the module's responsibility to remember the ID value of the M_IOCTL that was allocated. This will ensure proper cleanup and ID matching when the M_IOCACK or M_IOCNAK is received.
January 16, 2006 OmniOS