Tribblix: manual page: allocb.9f

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

NAME

allocb - allocate a message block

SYNOPSIS

#include <sys/stream.h>

mblk_t *allocb(size_t size, uint_t pri);

INTERFACE LEVEL

Architecture independent level 1 (DDI/DKI).

DESCRIPTION

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:

b_cont
set to NULL

b_rptr
points to the beginning of the data buffer

b_wptr
points to the beginning of the data buffer

b_datap
points to the dblk_t structure

The fields in the dblk_t structure are initialized as follows:

db_base
points to the first byte of the data buffer

db_lim
points to the last byte + 1 of the buffer

db_type
set to M_DATA

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

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

PARAMETERS

size
The number of bytes in the message block.

pri
Priority of the request (no longer used).

RETURN VALUES

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

CONTEXT

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

EXAMPLES

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 successful.

1 send_error(q,err)
2 queue_t *q;
3 unsigned char err;
4 {
5 mblk_t *bp;
6
7 if ((bp = allocb(1, BPRI_HI)) == NULL) /* allocate msg. block */
8 return(0);
9
10 bp->b_datap->db_type = M_ERROR; /* set msg type to M_ERROR */
11 *bp->b_wptr++ = err; /* increment write pointer */
12
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 }

NOTES

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

January 16, 2006 ALLOCB(9F)