UNIX 6th Ed. - /usr/sys/buf.h

   1: /*
   2:  * Each buffer in the pool is usually doubly linked into 2 lists:
   3:  * the device with which it is currently associated (always)
   4:  * and also on a list of blocks available for allocation
   5:  * for other use (usually).
   6:  * The latter list is kept in last-used order, and the two
   7:  * lists are doubly linked to make it easy to remove
   8:  * a buffer from one list when it was found by
   9:  * looking through the other.
  10:  * A buffer is on the available list, and is liable
  11:  * to be reassigned to another disk block, if and only
  12:  * if it is not marked BUSY.  When a buffer is busy, the
  13:  * available-list pointers can be used for other purposes.
  14:  * Most drivers use the forward ptr as a link in their I/O
  15:  * active queue.
  16:  * A buffer header contains all the information required
  17:  * to perform I/O.
  18:  * Most of the routines which manipulate these things
  19:  * are in bio.c.
  20:  */
  21: struct buf
  22: {
  23:     int b_flags;        /* see defines below */
  24:     struct  buf *b_forw;        /* headed by devtab of b_dev */
  25:     struct  buf *b_back;        /*  "  */
  26:     struct  buf *av_forw;       /* position on free list, */
  27:     struct  buf *av_back;       /*     if not BUSY*/
  28:     int b_dev;          /* major+minor device name */
  29:     int b_wcount;       /* transfer count (usu. words) */
  30:     char    *b_addr;        /* low order core address */
  31:     char    *b_xmem;        /* high order core address */
  32:     char    *b_blkno;       /* block # on device */
  33:     char    b_error;        /* returned after I/O */
  34:     char    *b_resid;       /* words not transferred after error */
  35: } buf[NBUF];
  36: 
  37: /*
  38:  * Each block device has a devtab, which contains private state stuff
  39:  * and 2 list heads: the b_forw/b_back list, which is doubly linked
  40:  * and has all the buffers currently associated with that major
  41:  * device; and the d_actf/d_actl list, which is private to the
  42:  * device but in fact is always used for the head and tail
  43:  * of the I/O queue for the device.
  44:  * Various routines in bio.c look at b_forw/b_back
  45:  * (notice they are the same as in the buf structure)
  46:  * but the rest is private to each device driver.
  47:  */
  48: struct devtab
  49: {
  50:     char    d_active;       /* busy flag */
  51:     char    d_errcnt;       /* error count (for recovery) */
  52:     struct  buf *b_forw;        /* first buffer for this dev */
  53:     struct  buf *b_back;        /* last buffer for this dev */
  54:     struct  buf *d_actf;        /* head of I/O queue */
  55:     struct  buf *d_actl;        /* tail of I/O queue */
  56: };
  57: 
  58: /*
  59:  * This is the head of the queue of available
  60:  * buffers-- all unused except for the 2 list heads.
  61:  */
  62: struct  buf bfreelist;
  63: 
  64: /*
  65:  * These flags are kept in b_flags.
  66:  */
  67: #define B_WRITE 0   /* non-read pseudo-flag */
  68: #define B_READ  01  /* read when I/O occurs */
  69: #define B_DONE  02  /* transaction finished */
  70: #define B_ERROR 04  /* transaction aborted */
  71: #define B_BUSY  010 /* not on av_forw/back list */
  72: #define B_PHYS  020 /* Physical IO potentially using UNIBUS map */
  73: #define B_MAP   040 /* This block has the UNIBUS map allocated */
  74: #define B_WANTED 0100   /* issue wakeup when BUSY goes off */
  75: #define B_RELOC 0200    /* no longer used */
  76: #define B_ASYNC 0400    /* don't wait for I/O completion */
  77: #define B_DELWRI 01000  /* don't write till block leaves available list */

Defined variables

Defined struct's

Defined macros

Usage of this include