FUNCTION FXPOSIT2, unit, EXT_NO, oEXT_NO
;+
; NAME:
;	FXPOSIT2
; PURPOSE:
;	Return the unit number of a file positioned to the beginning
;       of a particular extension.
;
; CALLING SEQUENCE:
;	unit=FXPOSIT(FILE, EXT_NO, /READONLY)
;
; INPUT PARAMETERS:
;	FILE	= FITS file name, scalar string
;       EXT_NO  = Extension to be moved to, scalar nonnegative integer
;
; RETURNS:
;       Unit number of file or -1 if an error is detected.
;
; OPTIONAL KEYWORD PARAMETER:
;	READONLY - If this keyword is set and non-zero, then OPENR rather 
;		than OPENU will be used to open the FITS file.
;
; COMMON BLOCKS:
;	None.
; SIDE EFFECTS:
;	Opens and returns the descriptor of a file.
; PROCEDURE:
;	Each FITS header is read in and parsed, and the file pointer is moved
;	to where the next FITS extension header until the desired
;       extension is reached.
; PROCEDURE CALLS:
;	FXPAR(), MRD_HREAD, MRD_SKIP
; MODIFICATION HISTORY:
;	Derived from William Thompson's FXFINDEND routine.
;       Modified by T.McGlynn, 5-October-1994.
;	Modified by T.McGlynn, 25-Feb-1995 to handle compressed
;          files.  Pipes cannot be accessed using FXHREAD so
;          MRD_HREAD was written.
;	W. Landsman 23-Apr-1997    Force the /bin/sh shell when uncompressing 
;	W. Landsman 26-May-1997    Non-unix is not just VMS
;	Converted to IDL V5.0   W. Landsman   September 1997
;-
;
	ON_ERROR,2
;
;  Check the number of parameters.
;
	IF N_PARAMS() LT 2 THEN BEGIN 
	    print,'Syntax:  unit = FXPOSIT(file, EXT_NO)'
	    return,-1
        ENDIF
;
;  Check if this is a compressed file.
;

;	unit = -1
	
	
;	len = strlen(file)
;	if len gt 3 then tail = strmid(file, len-3, 3) else tail = ' '
;	ucmprs = ' '
;	if strmid(tail,1,2) eq '.Z' or strmid(tail,1,2) eq '.z' then  $
;			   ucmprs = 'uncompress -c '
;	if tail eq '.gz'  or tail eq '.GZ' then ucmprs = 'gunzip -c '
		
;  Handle compressed files.
;	if ucmprs ne ' ' then begin
;		if (!version.os ne 'vms') and (!version.os ne 'windows') and $
;                   (!version.os ne 'MacOS') then begin
;			spawn, ucmprs+file, unit=unit,/sh
;		endif else begin
;			print, 'MRDFITS: Only Unix IDL supports piped spawns'
;			print, '         File must be uncompressed manually'
;			return, -1			
;	        endelse
		
;        endif else begin
;
;  Go to the start of the file.
;
;		if keyword_set(readonly) then begin
;			openr, unit, file, /block, ERROR = error
;		endif else begin
;			openu, unit, file, /block, ERROR = error
;		endelse
;		if ERROR NE 0 then begin
;			print,!ERR_STRING
;			return,-1
;		endif
;		if ext_no le 0 then return, -1
;	endelse
	
	for ext = oext_no, ext_no-1 do begin
	       
;
;  Read the next header, and get the number of bytes taken up by the data.
;
		IF EOF(UNIT) THEN BEGIN
			return, -1
		ENDIF

		; Can't use FXHREAD to read from pipe, since it uses
		; POINT_LUN.  So we read this in ourselves using mrd_hread

		mrd_hread, unit, header, status
		
		if status lt 0 then return, -1
			
		; Get parameters that determine size of data
		; region.
		
		BITPIX = FXPAR(HEADER,'BITPIX')
		NAXIS  = FXPAR(HEADER,'NAXIS')
		GCOUNT = FXPAR(HEADER,'GCOUNT') 
		IF GCOUNT EQ 0 THEN GCOUNT = 1
		PCOUNT = FXPAR(HEADER,'PCOUNT')
		
		IF NAXIS GT 0 THEN BEGIN 
			DIMS = FXPAR(HEADER,'NAXIS*')		;Read dimensions
			NDATA = DIMS[0]
			IF NAXIS GT 1 THEN FOR I=2,NAXIS DO NDATA = NDATA*DIMS[I-1]
			
		ENDIF ELSE NDATA = 0
		
		NBYTES = (ABS(BITPIX) / 8) * GCOUNT * (PCOUNT + NDATA)
;
;  Move to the next extension header in the file.
;
		NREC = LONG((NBYTES + 2879) / 2880)
		
		mrd_skip, unit, nrec*2880L
;		print,'Unit = ',unit,nrec,oext_no,ext_no-1

	endfor
	
	return,0
	
	END