allocb(D3)allocb(D3)NAMEallocb - allocate a message block
SYNOPSIS
#include <sys/types.h>
#include <sys/stream.h>
#include <sys/ddi.h>
mblk_t *allocb(int size, uint_t pri);
Arguments
size The number of bytes in the message block.
pri Priority of the request.
DESCRIPTIONallocb tries to allocate a STREAMS message block.
Return Values
If successful, allocb returns a pointer to the allocated message block of
type M_DATA (defined in sys/stream.h). If a block cannot be allocated, a
NULL pointer is returned.
USAGE
Buffer allocation fails only when the system is out of memory. If no
buffer is available, the bufcall(D3) function can help a module recover
from an allocation failure.
The psi argument is no longer used, but is retained for compatibility.
The following figure identifies the data structure members that are
affected when a message block is allocated.
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b_cont (0)
b_rptr
b_wptr
b_datap
message block
(mblk_t)
data block
(dblk_t)
data buffer
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_____________________
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db_base
db_lim
db_type (M_DATA)
Level
Base or Interrupt.
Synchronization Constraints
Does not sleep.
Driver-defined basic locks, read/write locks, and sleep locks may be held
across calls to this function.
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allocb(D3)allocb(D3)
Example
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, 0 is returned, indicating an allocation
failure (line 8). Otherwise, the message type is set to M_ERROR (line
9). Line 10 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 a write
queue, the q->q_flag member is tested to see if QREADR is set (line 12).
If it is not set, q points to a write queue, and on line 13 the RD(D3)
function is used to find the corresponding read queue. In line 14, the
putnext(D3) function is used to send the message upstream. Then
send_error returns 1 indicating success.
1 send_error(q, err)
2 queue_t *q;
3 uchar_t err;
4 {
5 mblk_t *bp;
6 long fl=0;
7 if ((bp = allocb(1, BPRI_HI)) == NULL)
8 return(0);
9 bp->b_datap->db_type = M_ERROR;
10 *bp->b_wptr++ = err;
11 (void) strqget(q, QFLAG, 0, &fl);
12 if (fl & QREADR))
13 q = RD(q);
14 putnext(q, bp);
15 return(1);
16 }
REFERENCESbufcall(D3), esballoc(D3), esbbcall(D3), freeb(D3), msgb(D4)
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