/* * 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) */#define	B_PBUSY	04000#define	B_PACK	010000/* * 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