STREAMS-UX Programmer's Guide (February 2007)

Messages

Message Structures

Chapter 3

Message Structures

All STREAMS messages consists of one or more message blocks as shown in Figure 3-1, “A Message and Its

Linkage.” STREAMS message is comprised of two data structures — a message block and a data block.

The message block represented by msgb describes the message, the data block represented by datab describes

the data and contains the pointer to the variable-length data buffer.

Figure 3-1 A Message and Its Linkage

The message block and data block are represented and defined as struct msgb (mblk_t) and struct datab

(dblk_t) respectively in <sys/stream.h>.

The mblk_t structure has the following fields:

struct msgb * b_next; /* next message on queue */

struct msgb * b_prev; /* previous message on queue */

struct msgb * b_cont; /* next message block of message */

unsigned char * b_rptr; /* first unread data byte in buffer */

unsigned char * b_wptr; /* first unwritten data byte */

struct datab * b_datap; /* data block */

unsigned char b_band; /* message priority */

unsigned short b_flag; /* message flags restricted for STREAMS framework use only */

data

block

data

block

(type)

buffer

header

queue

b_next

b_prev

b_cont

b_datap

buffer

mblk_t

data

block

(type)

buffer

mblk_t

Message

b_datap

b_cont