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


allocb - allocate a message block


#include <sys/stream.h>

mblk_t *allocb(size_t size, uint_t pri);


Architecture independent level 1 (DDI/DKI).


The allocb() function tries to allocate a STREAMS message block. Buffer
allocation fails only when the system is out of memory. If no buffer is
available, the bufcall(9F) function can help a module recover from an
allocation failure.

A STREAMS message block is composed of three structures. The first
structure is a message block (mblk_t). See msgb(9S). The mblk_t structure
points to a data block structure (dblk_t). See datab(9S). Together these
two structures describe the message type (if applicable) and the size and
location of the third structure, the data buffer. The data buffer
contains the data for this message block. The allocated data buffer is at
least double-word aligned, so it can hold any C data structure.

The fields in the mblk_t structure are initialized as follows:

set to NULL

points to the beginning of the data buffer

points to the beginning of the data buffer

points to the dblk_t structure

The fields in the dblk_t structure are initialized as follows:

points to the first byte of the data buffer

points to the last byte + 1 of the buffer

set to M_DATA

The following figure identifies the data structure members that are
affected when a message block is allocated.

Printed copy or shows a figure that identifies the data
structure members that are affected when a message block is allocated


The number of bytes in the message block.

Priority of the request (no longer used).


Upon success, allocb() returns a pointer to the allocated message block
of type M_DATA. On failure, allocb() returns a NULL pointer.


The allocb() function can be called from user, interrupt, or kernel


Example 1 allocb() Code Sample

Given a pointer to a queue (q) and an error number (err), the
send_error() routine sends an M_ERROR type message to the stream head.

If a message cannot be allocated, NULL is returned, indicating an
allocation failure (line 8). Otherwise, the message type is set to
M_ERROR (line 10). Line 11 increments the write pointer (bp->b_wptr) by
the size (one byte) of the data in the message.

A message must be sent up the read side of the stream to arrive at the
stream head. To determine whether q points to a read queue or to a write
queue, the q->q_flag member is tested to see if QREADR is set (line 13).
If it is not set, q points to a write queue, and in line 14 the RD(9F)
function is used to find the corresponding read queue. In line 15, the
putnext(9F) function is used to send the message upstream, returning 1 if

1 send_error(q,err)
2 queue_t *q;
3 unsigned char err;
4 {
5 mblk_t *bp;
7 if ((bp = allocb(1, BPRI_HI)) == NULL) /* allocate msg. block */
8 return(0);
10 bp->b_datap->db_type = M_ERROR; /* set msg type to M_ERROR */
11 *bp->b_wptr++ = err; /* increment write pointer */
13 if (!(q->q_flag & QREADR)) /* if not read queue */
14 q = RD(q); /* get read queue */
15 putnext(q,bp); /* send message upstream */
16 return(1);
17 }


RD(9F), bufcall(9F), esballoc(9F), esbbcall(9F), putnext(9F), testb(9F),
datab(9S), msgb(9S)

Writing Device Drivers

STREAMS Programming Guide


The pri argument is no longer used, but is retained for compatibility
with existing drivers.

illumos January 16, 2006 ALLOCB(9F)