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FXGSEEK

FXGSEEK

Name


  FXGSEEK

Author


  Craig B. Markwardt, NASA/GSFC Code 662, Greenbelt, MD 20770
  craigm@lheamail.gsfc.nasa.gov
  UPDATED VERSIONs can be found on my WEB PAGE:
      http://cow.physics.wisc.edu/~craigm/idl/idl.html

Purpose


  Perform a seek operation on a generic resource.

Major Topics


  File I/O, Pipes, URLs, FITS

Calling Sequence


  FXGSEEK, UNIT, POSITION ;; Sets the current file position
  FXGSEEK, -UNIT, POSITION ;; Queries the current file position

Description



  FXGSEEK performs a seek on the selected resource. Depending on
  the sign of UNIT, the current file position is either queried or
  set, in much the same manner as the built-in IDL procedure
  POINT_LUN.
  If the resource is a stream, the seek operation does not
  necessarily force a read until FXGREAD is called (i.e., reads are
  "lazy").
  You must use the specialized 'FXG' style functions to read, write
  and seek on file units opened with FXGOPEN:
    FXGOPEN - open resource
    FXGCLOSE - close resource
    FXGREAD - read from resource
    FXGWRITE - write to resource
    FXGSEEK - seek on resource (i.e., perform POINT_LUN)
    FXGFILTERED - determine if resource is a normal file.

Inputs



  UNIT - the unit number to operate on. The unit must have been
          previously opened by FXGOPEN. The operation of FXGSEEK
          depends on the sign of UNIT. If UNIT is positive, then the
          current file position of file UNIT is set to POSITION. If
          UNIT is negative, then the current file position of file
          |UNIT| is placed in the variable POSITION.
  POSITION - Depending on the sign of UNIT, the behavior is
              different. When UNIT is positive, POSITION is an input
              variable containing the new file position. When UNIT
              is negative, POSITION is an output variable to contain
              the file's current file position.

Modification History


  Written, 1999, CM
  Documented, 02 Oct 1999, CM
  Changed copyright notice, 21 Sep 2000, CM
  $Id: fxgseek.pro,v 1.3 2007/09/01 23:03:23 craigm Exp $



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