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allocb(9f)
allocb(9F) Kernel Functions 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 struc‐
ture 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 af‐
fected when a message block is allocated.
The printed copy and the Oracle Technology Network version of this man‐
ual page show a figure that identifies the data structure members that
are affected when a message block is allocated..SH 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_er‐
ror() routine sends an M_ERROR type message to the stream head.
If a message cannot be allocated, NULL is returned, indicating an allo‐
cation 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 }
SEE ALSO
RD(9F), bufcall(9F), esballoc(9F), esbbcall(9F), putnext(9F),
testb(9F), datab(9S), msgb(9S)
Writing Device Drivers in Oracle Solaris 11.4
STREAMS Programming Guide
NOTES
The pri argument is no longer used, but is retained for compatibility
with existing drivers.
Oracle Solaris 11.4 02 Nov 2017 allocb(9F)