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  Craig B. Markwardt, NASA/GSFC Code 662, Greenbelt, MD 20770


  Shift a block of data to a new position in a file (possibly overlapping)

Calling Sequence



  BLKSHIFT moves a block of data forward or backward, to a new
  position in a data file. The old and new positions of the block
  can overlap safely.
  The new position can be specified with either the DELTA parameter,
  which gives the number of bytes to move forward (positive delta) or
  backward (negative delta); or the TO keyword, which give the new
  absolute starting position of the block.
  The block can be moved beyond the current end of file point, in
  which case the intervening gap is filled with zeros (optionally).
  The gap left at the old position of the block is also optionally
  zero-filled. If a set of data up to the end of the file is being
  moved forward (thus making the file smaller) then
  the file is truncated at the new end.using TRUNCATE_LUN.


  UNIT - a logical unit number, opened for reading and writing.
  POS - POS[0] is the position of the block in the file, in bytes,
        before moving. POS[1], if present, is the size of the block
        in bytes. If POS[1] is not given, then the block is from
        POS[0] to the end of the file.
  DELTA - the (optional) offset in bytes between the old and new
          positions, from the start of the block. Positive values
          indicate moving the data forward (toward the end of file),
          and negative values indicate moving the data backward
          (toward the beginning of the file). One of DELTA and TO
          must be specified; DELTA overrides the TO keyword.
          Attempts to move the block beyond the end of the file will
          succeed. A block can never be moved beyond the beginning
          of the file; it will be moved to the beginning instead.

Keyword Parameters

  TO - the absolute file offset in bytes for the new start of the
        block. One of DELTA and TO must be specified; DELTA
        overrides the TO keyword.
  /NOZERO - if set, then newly created gaps will not be explicitly
            zeroed. Note that in same systems (e.g. MacOS) the gaps will
            always be zeroed whether or not /NOZERO is set.
  ERRMSG - If defined and passed, then any error messages will be
            returned to the user in this parameter rather than
            depending on the MESSAGE routine in IDL. If no errors
            are encountered, then a null string is returned.
  BUFFERSIZE - the maximum buffer size for transfers, in bytes.
                Larger values of this keyword impose larger memory
                requirements on the application; smaller values will
                lead to more transfer operations.
                Default: 32768 (bytes)

Modification History

  Written, CM, Apr 2000
  Documented and re-written, CM, 20 Jul 2000
  Renamed from FXSHIFT to BLKSHIFT, CM, 21 Jul 2000
  Documentation, CM, 12 Dec 2002
  Truncate if moving data block forward from the end of file
            using TRUNCATE_LUN W. Landsman Feb. 2005
  Assume since V5.5, remove VMS support W. Landsman Sep 2006
  Assume since V5.6, TRUNCATE_LUN available W. Landsman Sep 2006
  MacOS can point beyond EOF W. Landsman Aug 2009

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