Note, in the following, that an "xdr_counted_string" is not part of the XDR
standard, but a definition added on top of the standard.  It consists of an
xdr_short, containing the number of characters in the string, possibly followed
by an xdr_string.  Only if the xdr_short value is nonzero will an xdr_string
follow it.  Thus an empty string looks like this:

	<xdr_short=0>

and a nonempty string looks like this:

	<xdr_short=3><xdr_string>

-----------------------------------------------------------------------

scan file format

FILE HEADER
	xdr_float:	VERSION  (1.3 == 3FA66666)
	xdr_long:	scan number
	xdr_short	data's rank
	xdr_vector(rank, xdr_int) dims;
	xdr_int		isRegular (true=1, false=0)			
	xdr_long:	pointer to the extra pvs


SCAN
	HEADER:
	xdr_short:		this scan's rank
	xdr_long:		number of requested points (NPTS)
	xdr_long:		current point (CPT)
	if the scan rank is > 1
	  xdr_vector(NPTS, xdr_long)	pointer to the lower scans

	INFO:
	xdr_counted_string:	scan name
	xdr_counted_string:	time stamp


	xdr_int:		number of positioners
	xdr_int:		number of detectors
	xdr_int:		number of triggers

	for each positioner
	  xdr_int:		positioner number
	  xdr_counted_string:	positioner name
	  xdr_counted_string:	positioner desc
	  xdr_counted_string:	positioner step mode
	  xdr_counted_string:	positioner unit
	  xdr_counted_string:	readback name
	  xdr_counted_string:	readback description
	  xdr_counted_string:	readback unit

	for each detector
	  xdr_int:		detector number
	  xdr_counted_string:	detector name
	  xdr_counted_string:	detector desc
	  xdr_counted_string:	detector unit

	for each trigger
	  xdr_int:		trigger number
	  xdr_counted_string:	trigger name
	  xdr_float:		trigger command

	DATA:
	for each positioner
	  xdr_vector(NPTS, xdr_double):	readback array

	for each detector
	  xdr_vector(NPTS, xdr_float):	detector array

[SCAN]
...
...
...
[SCAN]

EXTRA PVs
	xdr_int:	number of extra pvs

	for each pv
	  xdr_counted_string:	name
	  xdr_counted_string:	desc
	  xdr_int:		type
	  if type != DBR_STRING
	    xdr_long:		count
	    xdr_counted_string:	unit

	  depending on the type:
	    DBR_STRING:
		xdr_counted_string:		value
	    DBR_CTRL_CHAR:
		xdr_vector(count, xdr_char):	value
	    DBR_CTRL_SHORT:
		xdr_vector(count, xdr_short):	value
	    DBR_CTRL_LONG:
		xdr_vector(count, xdr_long):	value
	    DBR_CTRL_FLOAT:
		xdr_vector(count, xdr_float):	value
	    DBR_CTRL_DOUBLE:
		xdr_vector(count, xdr_double):	value
-----------------------------------------------------------------------

A 1D scan looks like this:

	header
	scan1
	extra PV's

A 2D scan looks like this

	header
	scan2
		scan1
		scan1
		...
	extra PV's

A 3D scan looks like this

	header
	scan3
		scan2
			scan1
			scan1
			...
		scan2
			scan1
			scan1
			...
		...
	extra PV's

A counted string looks like this:
	int number of characters
	# if number of characters>0:
		int number of characters
		char[number_of_characters]