5.1.3. struct buf Structure

The struct buf structure is used to interface with the buffer cache. It is defined in /usr/src/sys/sys/buf.h:

/*
 * The buffer header describes an I/O operation in the kernel.
 */
struct buf {
        LIST_ENTRY(buf) b_hash;         /* Hash chain. */
        LIST_ENTRY(buf) b_vnbufs;       /* Buffer's associated vnode. */
        TAILQ_ENTRY(buf) b_freelist;    /* Free list position if not active. */
        struct  buf *b_actf, **b_actb;  /* Device driver queue when active. */
        struct  proc *b_proc;           /* Associated proc; NULL if kernel. */
        volatile long   b_flags;        /* B_* flags. */
        int     b_qindex;               /* buffer queue index */
        int     b_error;                /* Errno value. */
        long    b_bufsize;              /* Allocated buffer size. */
        long    b_bcount;               /* Valid bytes in buffer. */
        long    b_resid;                /* Remaining I/O. */
        dev_t   b_dev;                  /* Device associated with buffer. */
        struct {
                caddr_t b_addr;         /* Memory, superblocks, indirect etc. */
        } b_un;
        void    *b_saveaddr;            /* Original b_addr for physio. */
        daddr_t b_lblkno;               /* Logical block number. */
        daddr_t b_blkno;                /* Underlying physical block number. */
                                        /* Function to call upon completion. */
        void    (*b_iodone) __P((struct buf *));
                                        /* For nested b_iodone's. */
        struct  iodone_chain *b_iodone_chain;
        struct  vnode *b_vp;            /* Device vnode. */
        int     b_pfcent;               /* Center page when swapping cluster. */
        int     b_dirtyoff;             /* Offset in buffer of dirty region. */
        int     b_dirtyend;             /* Offset of end of dirty region. */
        struct  ucred *b_rcred;         /* Read credentials reference. */
        struct  ucred *b_wcred;         /* Write credentials reference. */
        int     b_validoff;             /* Offset in buffer of valid region. */
        int     b_validend;             /* Offset of end of valid region. */
        daddr_t b_pblkno;               /* physical block number */
        caddr_t b_savekva;              /* saved kva for transfer while bouncing
 */
        void    *b_driver1;             /* for private use by the driver */
        void    *b_driver2;             /* for private use by the driver */
        void    *b_spc;
        struct  vm_page *b_pages[(MAXPHYS + PAGE_SIZE - 1)/PAGE_SIZE];
        int             b_npages;
};