DragonFly On-Line Manual Pages

BUFCACHE(9)           DragonFly Kernel Developer's Manual          BUFCACHE(9)

NAME
     bufinit, bread, bwrite -- Buffer Cache Functions

SYNOPSIS
     #include <sys/param.h>
     #include <sys/systm.h>
     #include <sys/buf.h>
     #include <sys/buf2.h>

     int
     bread(struct vnode *vp, off_t loffset, int size, struct buf **bpp);

     int
     bwrite(struct buf *bp);

DESCRIPTION
     The buffer cache functions are at the heart of all storage file systems;
     they are used for reading from and writing to the underlying storage.
     The bread() and bwrite() functions observe most activity in the kernel
     from file systems, but other functions such as breadn() are also used.

     At boot time, the bufinit() function is invoked to initialize various
     accounting code.  It also initializes nbuf number of buffers and inserts
     them into the empty queue BQUEUE_EMPTY.  The variable nbuf is a global
     variable in the kernel that is tunable at boot time using the loader(8).

FUNCTIONS
     bread(*vp, loffset, size, **bpp)
              Retrieve a buffer with specified data.  An internal function,
              getblk() is called to check whether the data is available in
              cache or if it should be read from the vp.  If the data is
              available in cache, the B_CACHE flag will be set otherwise size
              bytes will be read starting at block number loffset from the
              block special device vnode vp.

              In case when the buffer is not in cache or not cacheable this
              function will put the calling process or thread to sleep, using
              bp as the wait channel and `biord' as the wait message.

              On successful return, the b_data field of bp will point to valid
              data address and b_count will contain the number of bytes read.

     bwrite(*bp)
              Write a buffer back to the device pointed to by b_dev field.
              Until the write operation is complete, the calling thread or
              process will be put to sleep by the kernel using bp as the wait
              channel and `biowr' as the wait message.

              Before calling this function, the following fields are the least
              to be set:

              b_data   This field should be set to a valid data buffer to be
                       written by bwrite().

              b_bcount
                       Size of buffer to be written, analogous to the size
                       argument of bread().

              b_blkno  Logical block number at which the buffer should be
                       written.

              b_dev    This can be set by using the vn_todev() function on the
                       device vnode.

              b_vp     This should be set to the vnode of the device to which
                       the buffer will be written.

              This function will put the calling process or thread to sleep if
              the data cannot be written when operating synchronously, using
              bp as the wait channel and `biowr' as the wait message.  On
              successful return the b_resid field of bp will be set to the
              value zero, thus indicating a successful write.

CODE REFERENCES
     The file system code, located under sys/vfs directory are the main source
     of reference.

SEE ALSO
     buf(9), VFS(9)

AUTHORS
     This manual page was written by Hiten Pandya <hmp@freebsd.org>.

DragonFly 5.5                  December 27, 2018                 DragonFly 5.5