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buf(9s)


NAME

  buf - General: Describes arbitrary I/O


SYNOPSIS

  _________________________________
  Member Name	Data Type
  _________________________________
  b_flags	int
  b_forw	struct buf *
  b_back	struct buf *
  av_forw	struct buf *
  av_back	struct buf *
  b_bcount	int
  b_error	short
  b_dev		dev_t
  b_un.b_addr	caddr_t
  b_lblkno	daddr_t
  b_blkno	daddr_t
  b_resid	int
  b_iodone	void (*b_iodone) ()
  b_proc	struct proc *
  _________________________________


MEMBERS

  b_flags
      Specifies binary status flags. These flags indicate how a request is to
      be handled and the current status of the request. The following flags
      are applicable to kernel modules that are device drivers: B_READ,
      B_DONE, B_ERROR, B_BUSY, and B_PHYS. See the DESCRIPTION section for
      more information on these flags.

  b_forw
      Specifies a hash chain. Only the entity (driver, buffer cache) that
      owns the buf structure can use or reference this member. A driver
      receiving a buf structure from the buffer cache through the strategy
      routine must not use this member.

  b_back
      Specifies a hash chain. Only the entity (driver, buffer cache) that
      owns the buf structure can use or reference this member. A driver
      receiving a buf structure from the buffer cache through the strategy
      routine must not use this member.

  av_forw
      Specifies the position on the free list if the b_flags member is not
      set to B_BUSY.

  av_back
      Specifies the position on the free list if the b_flags member is not
      set to B_BUSY.

  b_bcount
      Specifies the size of the requested transfer (in bytes).

  b_error
      Specifies that an error occurred on this data transfer. This member is
      set to an error code if the b_flags member bit was set.

  b_dev
      Specifies the special device to which the transfer is directed.

  b_un.b_addr
      Specifies the address at which to pull or push the data.

  b_lblkno
      Specifies the logical block number.

  b_blkno
      Specifies the block number on the partition of a disk or on the file
      system.

  b_resid
      Specifies (in bytes) the data not transferred because of some error.

  b_iodone
      Specifies the routine called by iodone.  The device driver calls iodone
      at the completion of an I/O operation.

  b_proc
      Specifies a pointer to the proc structure that represents the process
      performing the I/O.


DESCRIPTION

  The buf data structure describes arbitrary I/O, but is usually associated
  with block I/O and physio.  A systemwide pool of buf data structures exists
  for block I/O; however, many kernel modules that are device drivers also
  include locally defined buf data structures.

  Kernel modules can use the following flags with the b_flags member:

  B_READ  This flag is set if the operation is read and cleared if the
	  operation is write.

  B_DONE  This flag is cleared when a request is passed to a driver strategy
	  routine. The writer must call iodone to mark a buffer as completed.

  B_ERROR This flag specifies that an error occurred on this data transfer.
	  Kernel modules set this flag if an error occurs.

  B_BUSY  This flag indicates that the buffer is in use.

  B_PHYS  This flag indicates that the associated data is in user address
	  space.


NOTES

  The  operating system does not define a B_CALL flag.	The iodone routine
  checks the b_iodone member to determine if you specified a completion
  routine. If so, iodone clears b_iodone and then calls the specified
  completion routine. If you want to reuse this buf data structure, you must
  reset the b_iodone member to a completion routine. In fact, it is good
  programming practice to reset all of the referenced members of a buf data
  structure that you plan to reuse.


FILES

  <sys/buf.h>

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