PRO FXHMODIFY, FILENAME, NAME, VALUE, COMMENT, BEFORE=BEFORE,	$
		AFTER=AFTER, FORMAT=FORMAT, EXTENSION=EXTENSION, ERRMSG=ERRMSG
;+
; NAME: 
;	FXHMODIFY
; Purpose     : 
;	Modify a FITS header in a file on disk.
; Explanation : 
;	Opens a FITS file, and adds or modifies a parameter in the FITS header.
;	Can be used for either the main header, or for an extension header. 
;	The modification is performed directly on the disk file.
; Use         : 
;	FXHMODIFY, FILENAME, NAME, VALUE, COMMENT
; Inputs      : 
;	FILENAME = String containing the name of the file to be read.
;
;	NAME	= Name of parameter.  If NAME is already in the header the
;		  value and possibly comment fields are modified. Otherwise a
;		  new record is added to the header.  If NAME is equal to
;		  either "COMMENT" or "HISTORY" then the value will be added to
;		  the record without replacement.  In this case the comment
;		  parameter is ignored.
;
;	VALUE	= Value for parameter.  The value expression must be of the
;		  correct type, e.g. integer, floating or string.  String
;		  values of 'T'	or 'F' are considered logical values.
;
; Opt. Inputs : 
;	COMMENT	= String field.  The '/' is added by this routine.  Added
;		  starting in position 31.  If not supplied, or set equal to ''
;		  (the null string), then any previous comment field in the
;		  header for that keyword is retained (when found).
; Outputs     : 
;	None.
; Opt. Outputs: 
;	None.
; Keywords    : 
;	EXTENSION = Either the number of the FITS extension, starting with the
;		    first extension after the primary data unit being one; or a
;		    character string containing the value of EXTNAME to search
;		    for.  If not passed, then the primary FITS header is
;		    modified.           
;
;	BEFORE	= Keyword string name.  The parameter will be placed before the
;		  location of this keyword.  For example, if BEFORE='HISTORY'
;		  then the parameter will be placed before the first history
;		  location.  This applies only when adding a new keyword;
;		  keywords already in the header are kept in the same position.
;
;	AFTER	= Same as BEFORE, but the parameter will be placed after the
;		  location of this keyword.  This keyword takes precedence over
;		  BEFORE.
;
;       FORMAT	= Specifies FORTRAN-like format for parameter, e.g. "F7.3".  A
;		  scalar string should be used.  For complex numbers the format
;		  should be defined so that it can be applied separately to the
;		  real and imaginary parts.
;	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.  In order to
;		  use this feature, ERRMSG must be defined first, e.g.
;
;			ERRMSG = ''
;			FXHMODIFY, ERRMSG=ERRMSG, ...
;			IF ERRMSG NE '' THEN ...
;
; Calls       : 
;	FXHREAD, FXPAR, FXADDPAR
; Common      : 
;	None.
; Restrictions: 
;	Adding records to a FITS header is not allowed if it would increase the
;	number of 2880 byte records needed to store the header.  Modifying
;	existing records is always allowed.
;
;	This routine can not be used to modify any of the keywords that control
;	the structure of the FITS file, e.g. BITPIX, NAXIS, PCOUNT, etc.  Doing
;	so could corrupt the readability of the FITS file.
;
; Side effects: 
;	None.
; Category    : 
;	Data Handling, I/O, FITS, Generic.
; Prev. Hist. : 
;	None.
; Written     : 
;	William Thompson, GSFC, 3 March 1994.
; Modified    : 
;	Version 1, William Thompson, GSFC, 3 March 1994.
;	Version 2, William Thompson, GSFC, 31 May 1994
;		Added ERRMSG keyword.
;       Version 3, William Thompson, GSFC, 23 June 1994
;               Modified so that ERRMSG is not touched if not defined.
; Version     :
;       Version 3, 23 June 1994
;	Converted to IDL V5.0   W. Landsman   September 1997
;-
;
	ON_ERROR, 2
;
;  Check the number of parameters.
;
	IF N_PARAMS() LT 3 THEN BEGIN
		MESSAGE = $	;Need at least 3 parameters
		    'Syntax:  FXHMODIFY, FILENAME, NAME, VALUE [, COMMENT ]'
		IF N_ELEMENTS(ERRMSG) NE 0 THEN BEGIN
			ERRMSG = MESSAGE
			RETURN
		END ELSE MESSAGE, MESSAGE
	ENDIF
;
;  If passed, check the type of the EXTENSION parameter.
;
	IF N_ELEMENTS(EXTENSION) GT 1 THEN BEGIN
		MESSAGE = 'EXTENSION must be a scalar'
		IF N_ELEMENTS(ERRMSG) NE 0 THEN BEGIN
			ERRMSG = MESSAGE
			RETURN
		END ELSE MESSAGE, MESSAGE
	END ELSE IF N_ELEMENTS(EXTENSION) EQ 1 THEN BEGIN
		SZ = SIZE(EXTENSION)
		ETYPE = SZ[SZ[0]+1]
		IF ETYPE EQ 8 THEN BEGIN
			MESSAGE = 'EXTENSION must not be a structure'
			IF N_ELEMENTS(ERRMSG) NE 0 THEN BEGIN
				ERRMSG = MESSAGE
				RETURN
			END ELSE MESSAGE, MESSAGE
		ENDIF
;
;  If EXTENSION is of type string, then search for the proper extension by
;  name.  Otherwise, search by number.
;
		IF ETYPE EQ 7 THEN BEGIN
			S_EXTENSION = STRTRIM(STRUPCASE(EXTENSION),2)
		END ELSE BEGIN
			I_EXTENSION = FIX(EXTENSION)
			IF I_EXTENSION LT 1 THEN BEGIN
				MESSAGE = 'EXTENSION must be greater than zero'
				IF N_ELEMENTS(ERRMSG) NE 0 THEN BEGIN
					ERRMSG = MESSAGE
					RETURN
				END ELSE MESSAGE, MESSAGE
			ENDIF
		ENDELSE
	ENDIF
;
;  Get the UNIT number, and open the file.
;
	OPENU, UNIT, FILENAME, /BLOCK, /GET_LUN
;
;  Read in the primary FITS header.
;
	FXHREAD,UNIT,HEADER,STATUS
	IF STATUS NE 0 THEN BEGIN
		FREE_LUN,UNIT
		MESSAGE = 'Unable to read FITS header'
		IF N_ELEMENTS(ERRMSG) NE 0 THEN BEGIN
			ERRMSG = MESSAGE
			RETURN
		END ELSE MESSAGE, MESSAGE
	ENDIF
	MHEAD0 = 0
	I_EXT = 0
;
;  If the EXTENSION parameter was passed, then look for the requested
;  extension.
;
	IF N_ELEMENTS(EXTENSION) EQ 1 THEN BEGIN
;
;  Make sure that the file does contain extensions.
;
		IF NOT FXPAR(HEADER,'EXTEND') THEN BEGIN
			FREE_LUN, UNIT
			MESSAGE = 'File ' + FILENAME +	$
				' does not contain extensions'
			IF N_ELEMENTS(ERRMSG) NE 0 THEN BEGIN
				ERRMSG = MESSAGE
				RETURN
			END ELSE MESSAGE, MESSAGE
		ENDIF
;
;  Get the number of bytes taken up by the data.
;
NEXT_EXT:
		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)
;
;  Read the next extension header in the file.
;
		NREC = LONG((NBYTES + 2879) / 2880)
		POINT_LUN, -UNIT, POINTLUN		;Current position
		MHEAD0 = POINTLUN + NREC*2880L
		POINT_LUN, UNIT, MHEAD0			;Next FITS extension
		FXHREAD,UNIT,HEADER,STATUS
		IF STATUS NE 0 THEN BEGIN
			FREE_LUN,UNIT
			MESSAGE = 'Requested extension not found'
			IF N_ELEMENTS(ERRMSG) NE 0 THEN BEGIN
				ERRMSG = MESSAGE
				RETURN
			END ELSE MESSAGE, MESSAGE
		ENDIF
		I_EXT = I_EXT + 1
;
;  Check to see if the current extension is the one desired.
;
		IF ETYPE EQ 7 THEN BEGIN
			EXTNAME = STRTRIM(STRUPCASE(FXPAR(HEADER,'EXTNAME')),2)
			IF EXTNAME EQ S_EXTENSION THEN GOTO, DONE
		END ELSE IF I_EXT EQ I_EXTENSION THEN GOTO, DONE
		GOTO, NEXT_EXT
DONE:
	ENDIF
;
;  Add or modify the keyword parameter in the header, keeping track of the
;  initial size of the header array.
;
	IEND = WHERE(STRMID(HEADER,0,8) EQ 'END     ')
	N_INITIAL = 1 + IEND[0]/36
	IF N_PARAMS() EQ 4 THEN BEGIN
		FXADDPAR, HEADER, NAME, VALUE , COMMENT, BEFORE=BEFORE,	$
			AFTER=AFTER, FORMAT=FORMAT
	END ELSE BEGIN
		FXADDPAR, HEADER, NAME, VALUE, BEFORE=BEFORE, AFTER=AFTER, $
			FORMAT=FORMAT
	ENDELSE
;
;  If the length of the header has changed, then print an error message.
;
	IEND = WHERE(STRMID(HEADER,0,8) EQ 'END     ')
	N_FINAL = 1 + IEND[0]/36
	IF N_FINAL NE N_INITIAL THEN BEGIN
		MESSAGE, /CONTINUE, 'Adding parameter would increase ' + $
			'header length, no action taken.'
;
;  Otherwise, rewind to the beginning of the header, and write the new header
;  over the old header.  Convert to byte and force into 80 character lines.
;
	END ELSE BEGIN
		BHDR = REPLICATE(32B, 80, 36*N_INITIAL)
		FOR N = 0,IEND[0] DO BHDR[0,N] = BYTE(STRMID(HEADER[N],0,80))
		POINT_LUN, UNIT, MHEAD0
		WRITEU, UNIT, BHDR
	ENDELSE
;
;  Close the file and return.
;
	FREE_LUN, UNIT
	IF N_ELEMENTS(ERRMSG) NE 0 THEN ERRMSG = ''
	RETURN
	END