pro mrd_skip, unit, nskip ;+ ; NAME: ; MRD_SKIP ; PURPOSE: ; Skip a number of bytes from the current location in a file or a pipe ; EXPLANATION: ; First tries using POINT_LUN and if this doesn't work, perhaps because ; the unit is a pipe, MRD_SKIP will just read in the requisite number ; of bytes. GZIP files opened with /COMPRESS (in V5.3 or later) are ; also read as a series of bytes, since this is faster than using ; POINT_LUN when not at the beginning of a file. ; CALLING SEQUENCE: ; MRD_SKIP, Unit, Nskip ; ; INPUTS: ; Unit - File unit for the file or pipe in question, integer scalar ; Nskip - Number of bytes to be skipped, positive integer ; NOTES: ; This routine should be used in place of POINT_LUN wherever a pipe ; may be the input unit (see the procedure FXPOSIT for an example). ; Note that it assumes that it can only work with nskip >= 0 so it ; doesn't even try for negative values. ; ; For reading a pipe, MRD_SKIP currently uses a maximum buffer size ; of 8 MB. This chunk value can be increased for improved efficiency ; (or decreased if you really have little memory.) ; REVISION HISTORY: ; Written, Thomas A. McGlynn July 1995 ; Don't even try to skip bytes on a pipe with POINT_LUN, since this ; might reset the current pointer W. Landsman April 1996 ; Increase buffer size, check fstat.compress W. Landsman Jan 2001 ; Only a warning if trying read past EOF W. Landsman Sep 2001 ; Use 64bit longword for skipping in very large files W. Landsman Sep 2003 ;- On_error,2 if nskip le 0 then return ; Since V5.4 we can test for the presence of gzip compressed files using the ; .compress field of fstat(unit). For Unix compress or earlier IDL versions ; we test whether we can read the current position. If we can then it is not ; a pipe and we can go ahead and use POINT_LUN. if !VERSION.RELEASE GE '5.4' then compress = (fstat(unit)).compress $ else compress = 0 if compress EQ 0 then begin on_ioerror, byte_read point_lun, -unit, curr_pos on_ioerror, null if curr_pos NE -1 then point_lun, unit, long64(curr_pos) + nskip return endif ; Otherwise, we have to explictly read the number of bytes to skip ; If the number is very large we don't want to create a array so skip ; in chunks of 8 Megabyte byte_read: chunk = 8000000L buf = bytarr(nskip
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Last modified on 2nd Mar 2009 by Richard Massey.