allocb(9F)

NAME

allocb − allocate a message block

SYNOPSIS

#include <sys/stream.h>
mblk_t ∗allocb(int size, uint pri);

ARGUMENTS

size The number of bytes in the message block.

pri Priority of the request (no longer used).

INTERFACE LEVEL

Architecture independent level 1 (DDI/DKI).

DESCRIPTION

allocb() 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.

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

scale=100
define t181 |
[ box invis ht 48 wid 63 with .sw at 0,0
"b_cont (0)" at 0,42 ljust
"b_rptr" at 0,30 ljust
"b_wptr" at 0,18 ljust
"b_datap" at 0,6 ljust
] |
define t176 |
[ box invis ht 30 wid 96 with .sw at 0,0
"message block" at 48,23
"(mblk_t)" at 48,7
] |
define t190 |
[ box invis ht 30 wid 72 with .sw at 0,0
"data block" at 36,23
"(dblk_t)" at 36,7
] |
define t186 |
[ box invis ht 54 wid 115 with .sw at 0,0
"db_base" at 0,48 ljust
"db_lim" at 0,34 ljust
"db_type (M_DATA)" at 0,20 ljust
] |
box invis ht 116 wid 442 with .sw at 0,0
box ht 72 wid 98 with .nw at 0,84
line -> from 76,36 to 152,36
line from 74,62 to 106,62
line from 106,62 to 106,48
line from 106,48 to 74,48
line from 106,56 to 136,56
line from 136,56 to 136,116
t181 with .nw at 12,77
t176 with .nw at 0,-1
t190 with .nw at 192,-1
"data buffer" at 404,-8
line from 238,74 to 312,74
line -> from 312,74 to 360,84
line from 238,62 to 314,62
line -> from 314,62 to 360,12
box ht 72 wid 152 with .nw at 152,84
box ht 72 wid 82 with .nw at 360,84
line from 360,76 to 440,76 dotted
line from 360,20 to 440,20 dotted
line from 138,116 to 402,116
line -> from 400,116 to 400,84
t186 with .nw at 164,76

RETURN VALUES

A pointer to the allocated message block of type M_DATA (defined in sys/stream.h) on success.

A NULL pointer on failure.

CONTEXT

allocb() can be called from user or interrupt context.

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, 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.

Sun Microsystems — Last change: 11 Apr 1991

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