Tribblix: manual page: mkiocb.9f

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

NAME

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

SYNOPSIS

#include <sys/stream.h>

mblk_t *mkiocb(uint_t command);

INTERFACE LEVEL

illumos DDI specific (illumos DDI).

PARAMETERS

command
ioctl command for the ioc_cmd field.

DESCRIPTION

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:

b_wptr
Set to b_rptr + sizeof(struct iocblk).

b_cont
Set to NULL.

b_datap->db_type
Set to M_IOCTL.

The fields in the iocblk structure are initialized as follows:

ioc_cmd
Set to the command value passed in.

ioc_id
Set to a unique identifier.

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

ioc_count
Set to 0.

ioc_rval
Set to 0.

ioc_error
Set to 0.

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

RETURN VALUES

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

On failure, it returns a null pointer.

CONTEXT

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

EXAMPLES

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;
4
5 if ((mp = mkiocb(TEST_CMD)) == NULL)
6 return (0);
7
8 /* save off ioctl ID value */
9 testinfo->xx_iocid = ((struct iocblk *)mp->b_rptr)->ioc_id;
10
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 ...
4
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;
5
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 }

WARNINGS

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.

May 21, 2022 MKIOCB(9F)