msgb(D4)

NAME
     msgb - STREAMS message block structure

SYNOPSIS
     #include <sys/types.h>
     #include <sys/stream.h>
     #include <sys/ddi.h>

DESCRIPTION
     A STREAMS message is made up of one or more message blocks, referenced by
     a pointer to a msgb structure.  When a message is on a queue, all fields
     are read-only to drivers and modules.

USAGE
   Structure Definitions
     The msgb structure is defined as type mblk_t and contains the following
     members:

     struct msgb   *b_next;   /* next message on queue */
     struct msgb   *b_prev;   /* previous message on queue */
     struct msgb   *b_cont;   /* next block in message */
     uchar_t       *b_rptr;   /* 1st unread data byte of buffer */
     uchar_t       *b_wptr;   /* 1st unwritten data byte of buffer */
     struct datab  *b_datap;  /* pointer to data block */
     uchar_t       b_band;    /* message priority  */
     ushort_t      b_flag;    /* used by stream head  */
     The b_next and b_prev pointers are used to link messages together on a
     queue(D4).  These fields can be used by drivers and modules to create
     linked lists of messages.

     The b_cont pointer links message blocks together when a message is
     composed of more than one block.  Drivers and modules can use this field
     to create complex messages from single message blocks.

     The b_rptr and b_wptr pointers describe the valid data region in the
     associated data buffer.  The b_rptr field points to the first unread byte
     in the buffer and the b_wptr field points to the next byte to be written
     in the buffer.

     The b_datap field points to the data block [see datab(D4)] associated
     with the message block.  This field should never be changed by modules or
     drivers.

     The b_band field contains the priority band associated with the message.
     Normal priority messages and high priority messages have b_band set to
     zero.  High priority messages are high priority by virtue of their
     message type.  This field can be used to alter the queuing priority of
     the message.  The higher the priority band, the closer to the head of the
     queue the message is placed.
     The b_flag field contains a bitmask of flags that can be set to alter the
     way the stream head will process the message.  Valid flags are:

     MSGMARK       The last byte in the message is ``marked.''  This condition
                   is testable from user level via the I_ATMARK ioctl(2).

REFERENCES
     allocb(D3), datab(D4), esballoc(D3), freeb(D3), free_rtn(D4),
     messages(D5)