[Linux-streams] Re: b_flag in msgb structure

[David Grothe]

The mblk_t structure was never designed for users to keep their own information in.  Queue_t structures have a "q_ptr" that is designed for the user (driver) to use to point to some sort of control block structure that belongs to the driver.

For keeping information in messages you have just a few alternatives.

• Set the db_type field to a value of your own.  Just be sure not to send such messages to other drivers; they likely won't understand your message types.
• Put a header in front of the data.  You can make it visible or invisible as you prefer.  You can put it just in front of visible data, just after, at the front of the buffer, at the end, wherever you like.  When the buffer is sent to another module the extra information is presumed lost.

-- Dave

At 02:20 PM 1/16/2003 Thursday, Vinay Channagiri wrote:

Hello Dave,

Here's the msgb structure. The b_flag is quoted as "PRIVATE". Does that mean "The flags defined in b_flag is meant for only STREAMS framework"??? If that is the case, where should STREAMS modules/drivers need to introduce a new flag if required ???

Please respond to me at the earliest. Thanks

/*********************** Excerpts MSG.H ***********************/
typedef
struct  msgb {
  SHARE   
        struct  msgb    *b_next; /* next msg on queue */
        struct  msgb    *b_prev; /* prev msg on queue */
        struct  msgb    *b_cont; /* next blk of msg */
        unsigned char   *b_rptr; /* 1st unread byte */
        unsigned char   *b_wptr; /* 1st unwriten byte */
  EXPORT
        struct datab    *b_datap; /* pointer to data */
        unsigned char   b_band;  /* message priority */
        unsigned char   b_pad1;
  PRIVATE
        unsigned short  b_flag;  /* see below */
        long            b_pad2;
} msgb_t;
/*********************** Excerpts MSG.H ***********************/

Regards,
VINAY
---------------------------


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