| Re: converting VMS idl files to UNIX pvwave [message #473] |
Thu, 16 July 1992 07:47  |
thompson
Messages: 584
Registered: August 1991
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Senior Member |
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In article <1992Jul16.004709.31221@urz.unibas.ch>, hofer@urz.unibas.ch (Remo
Hofer) writes...
> In article <1992Jul11.034822.19367@bnlux1.bnl.gov>,
> jacobsen@bnlls1.nsls.bnl.gov (Chris Jacobsen) writes:
>> I had the same question. Mark Rivers of University Chicago/Brookhaven
>> Lab set things up using the XDR file format option. That allows
>> me to ship files back and forth between VMS and Unix
>
> Since XDR uses a description tag for every scalar data and not for a whole
> array, arrays written with XDR are often two times as big as without XDR.
> We ended up only wrtiting a stream of bytes to our data files using our own
> portable convertion routines to convert e.g. an array of floats into an array
> of bytes. We are transfering data (floats, longints, ints, bytes) between a VAX
> running VMS (little endian, VMS floats) and a Silicon Graphics Iris
> (big endian, IEEE floats) without any troubles this way.
I thought I might mention that IDL now supports converting floating point
numbers with the BYTEORDER function. The relevant calls would be for
converting from XDR (IEEE) formats to the host format:
BYTEORDER,DATA,/NTOHS ;Short integer
BYTEORDER,DATA,/NTOHL ;Long integer
BYTEORDER,DATA,/XDRTOF ;Floating point
BYTEORDER,DATA,/XDRTOD ;Double precision
BYTEORDER,DATA,/XDRTOF ;Complex
and from host to XDR (IEEE) formats
BYTEORDER,DATA,/HTONS ;Short integer
BYTEORDER,DATA,/HTONL ;Long integer
BYTEORDER,DATA,/FTOXDR ;Floating point
BYTEORDER,DATA,/DTOXDR ;Double precision
BYTEORDER,DATA,/FTOXDR ;Complex
That way you avoid the description tags. These calls are very fast.
> The bigger problem is the record formats on VMS.
> I'm not able to read a big file in stream format with pv~wave routines.
> (An error message saying that some buffer is too small is shown.)
> Converting the file to stream_lf format helps about this.
> Transfering a file written with pv~wave on the VAX in variable lenght record
> format to the Iris using ftp results in a mixture between actual data and
> record info on the unix side. And these are just two examples of troubles with
> VMS record orientated files.
If you use fixed length or stream records, rather than variable length, then it
is possible to access particular bytes in a file using
OPEN,/BLOCK Opens the file for block access (on VAX)
POINT_LUN Select starting byte to read
READU Read data
WRITEU Write data
This is more portable than ASSOC, or any of the other more record-associated
I/O calls. That way, on the VAX, then the actual file attributes are ignored.
The file has to be written so that the bytes follow one another without
interruption. Therefore, it won't work with variable length record files which
have extra embedded bytes describing the records, which is different between
VAXs and other computers.
I use the above commands to read and write FITS files, which can be transported
without change between different computers, in particular, VAX/VMS computers
and Sun workstations.
Bill Thompson
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