usr/src/slowsys/h/buf.h

/*
 * Each buffer in the pool is usually doubly linked into 2 lists:
 * the device with which it is currently associated (always)
 * and also on a list of blocks available for allocation
 * for other use (usually).
 * The latter list is kept in last-used order, and the two
 * lists are doubly linked to make it easy to remove
 * a buffer from one list when it was found by
 * looking through the other.
 * A buffer is on the available list, and is liable
 * to be reassigned to another disk block, if and only
 * if it is not marked BUSY.  When a buffer is busy, the
 * available-list pointers can be used for other purposes.
 * Most drivers use the forward ptr as a link in their I/O
 * active queue.
 * A buffer header contains all the information required
 * to perform I/O.
 * Most of the routines which manipulate these things
 * are in bio.c.
 */
struct buf
{
        int     b_flags;                /* see defines below */
        struct  buf *b_forw;            /* headed by d_tab of conf.c */
        struct  buf *b_back;            /*  "  */
        struct  buf *av_forw;           /* position on free list, */
        struct  buf *av_back;           /*     if not BUSY*/
        dev_t   b_dev;                  /* major+minor device name */
        unsigned b_bcount;              /* transfer count */
        union {
            caddr_t b_addr;             /* low order core address */
            int *b_words;               /* words for clearing */
            struct filsys *b_filsys;    /* superblocks */
            struct dinode *b_dino;      /* ilist */
            daddr_t *b_daddr;           /* indirect block */
        } b_un;
        daddr_t b_blkno;                /* block # on device */
        char    b_xmem;                 /* high order core address */
        char    b_error;                /* returned after I/O */
        unsigned int b_resid;           /* words not transferred after error */
};

extern struct buf buf[];                /* The buffer pool itself */
extern struct buf bfreelist;            /* head of available list */

/*
 * These flags are kept in b_flags.
 */
#define B_WRITE 0       /* non-read pseudo-flag */
#define B_READ  01      /* read when I/O occurs */
#define B_DONE  02      /* transaction finished */
#define B_ERROR 04      /* transaction aborted */
#define B_BUSY  010     /* not on av_forw/back list */
#define B_PHYS  020     /* Physical IO potentially using UNIBUS map */
#define B_MAP   040     /* This block has the UNIBUS map allocated */
#define B_WANTED 0100   /* issue wakeup when BUSY goes off */
#define B_AGE   0200    /* delayed write for correct aging */
#define B_ASYNC 0400    /* don't wait for I/O completion */
#define B_DELWRI 01000  /* don't write till block leaves available list */
#define B_TAPE 02000    /* this is a magtape (no bdwrite) */

/*
 * special redeclarations for
 * the head of the queue per
 * device driver.
 */
#define b_actf  av_forw
#define b_actl  av_back
#define b_active b_bcount
#define b_errcnt b_resid