usr/sys/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 devtab of b_dev */
        struct  buf *b_back;            /*  "  */
        struct  buf *av_forw;           /* position on free list, */
        struct  buf *av_back;           /*     if not BUSY*/
        int     b_dev;                  /* major+minor device name */
        int     b_wcount;               /* transfer count (usu. words) */
        char    *b_addr;                /* low order core address */
        char    *b_xmem;                /* high order core address */
        char    *b_blkno;               /* block # on device */
        char    b_error;                /* returned after I/O */
        char    *b_resid;               /* words not transferred after error */
} buf[NBUF];

/*
 * Each block device has a devtab, which contains private state stuff
 * and 2 list heads: the b_forw/b_back list, which is doubly linked
 * and has all the buffers currently associated with that major
 * device; and the d_actf/d_actl list, which is private to the
 * device but in fact is always used for the head and tail
 * of the I/O queue for the device.
 * Various routines in bio.c look at b_forw/b_back
 * (notice they are the same as in the buf structure)
 * but the rest is private to each device driver.
 */
struct devtab
{
        char    d_active;               /* busy flag */
        char    d_errcnt;               /* error count (for recovery) */
        struct  buf *b_forw;            /* first buffer for this dev */
        struct  buf *b_back;            /* last buffer for this dev */
        struct  buf *d_actf;            /* head of I/O queue */
        struct  buf *d_actl;            /* tail of I/O queue */
};

/*
 * This is the head of the queue of available
 * buffers-- all unused except for the 2 list heads.
 */
struct  buf bfreelist;

/*
 * 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_RELOC 0200    /* no longer used */
#define B_ASYNC 0400    /* don't wait for I/O completion */
#define B_DELWRI 01000  /* don't write till block leaves available list */