msgb(4) Kernel Interfaces Manual msgb(4)NAME
msgb - Defines a STREAMS message block
SYNOPSIS
#include <sys/stream.h> struct msgb { struct msgb *b_next; struct msgb *b_prev; struct msgb *b_cont; unsigned char
*b_rptr; unsigned char *b_wptr; struct datab *b_datap; MSG_KERNEL_FIELDS };
PARAMETERS
b_next A pointer to the next message on the queue.
b_prev A pointer to the previous message on the queue.
b_cont A pointer to the next message block in the message.
b_rptr A pointer to the first unread data byte in the buffer.
b_wptr A pointer to the first unwritten data byte in the buffer.
b_datap A pointer to the datab structure (data block) that contains the data for the message.
MSG_KERNEL_FIELDS
Additional fields that are visible within the kernel. The fields included and their contents depend on the kernel configuration.
DESCRIPTION
The msgb structure defines a message block. A message block carries data or information in a stream. A STREAMS message consists of mes-
sage blocks linked through b_cont. Each message block points to a data block descriptor, which in turn points to a data buffer.
The msgb structure is typedefed as mblk_t. The associated data block is stored in a datab structure, which is typedefed as dblk_t.
The datab structure is defined (in sys/stream.h) as:
struct datab {
struct datab * db_freep;
unsigned char * db_base;
unsigned char * db_lim;
unsigned char db_ref;
unsigned char db_type;
unsigned char db_class;
unsigned char db_pad[1]; };
The datab fields are defined as follows:
db_freep Used internally by the STREAMS memory allocator.
db_base The first byte of the buffer.
db_lim The last byte of the buffer, plus one.
db_ref The number of message blocks ( struct msgb) that reference this data block.
db_type The message type.
db_class Used internally.
db_pade Padding.
Messages are typed according to the value in the db_type field in the associated datab structure. Some possible type values are:
M_DATA The message contains ordinary data.
M_PROTO The message contains internal control information and data.
As part of its support for STREAMS, Tru64 UNIX provides the following interfaces for exchanging messages betweens STREAMS modules on the
one hand and sockets and network protocols on the other: mbuf_to_mblk() - Converts an mbuf chain to an mblk chain mblk_to_mbuf() - Converts
an mblk chain to an mbuf chain delim off
msgb(4)
Check Out this Related Man Page
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
allocb() tries to allocate a STREAMS message block. Buffer allocation fails only when the system is out of memory. If no buffer is avail-
able, 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 affected when a message block is allocated.
Please see the online man page on docs.sun.com or a print copy for the diagram.
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
allocb() can be called from user or interrupt 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 }
SEE ALSO RD(9F), bufcall(9F), esballoc(9F), esbbcall(9F), putnext(9F), testb(9F), datab(9S), msgb(9S)
Writing Device Drivers
STREAMS Programming Guide
NOTES
The pri argument is no longer used, but is retained for compatibility with existing drivers.
SunOS 5.10 22 March 2002 allocb(9F)