| Re: Fortran and byte swapping [message #7674 is a reply to message #7673] |
Fri, 13 December 1996 00:00   |
Simon Williams
Messages: 5
Registered: October 1996
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Junior Member |
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Liam Gumley wrote:
>
> Simon Williams wrote:
>> I have a large dataset which is in Unformatted variable-length
>> records written in FORTRAN on a SUN (big endian). I want to
>> read it on a DEC alpha running OSF (little endian) using IDL.
>> It is not really practical to convert the whole dataset.
>>
>> 1. I Can't use "openr, ..... ,/f77_unformatted" because the
>> record length information stored in the file is byte-swapped
>> so it falls over on the first read.
>>
>> 2. I could drop the /f77_unformatted keyword and handle the
>> record structure myself, but even then I can't see a simple
>> way of doing it - I would need an IDL function which behaves
>> like readu, so that I can pass an arbitrary number of arguments
>> to it or I will have to clog up my routine with a load of
>> calls to byteorder for each parameter I read.
>>
>> Any ideas? The nicest solution would be a "byteswap keyword" to
>> the open routines - like the one provided by dec fortran, but I
>> guess that's a bit much to ask!
>
> If you don't want to convert the datafile, then:
>
> First, open the file without the /F77_UNFORMATTED keyword.
>
> Create a record length index array by reading (READU) the first length
> word, swapping it using SWAP_ENDIAN, store it in the array, skip to the
> next length word (POINT_LUN), swap it, etc. to the end of the file.
>
> You now have a record length index array that you can use to read the
> file. Point back to the beginning of the file, and skip the length
> word, read the number of data words indicated in the length array, and
> then swap the data array using SWAP_ENDIAN.
>
> Seems simple enough. I'd write some code for you but I haven't had my
> coffee yet.
>
> Cheers,
> Liam.
--
Thanks Liam,
What you suggested is more or less what I've come up with - (although
I'd not come across SWAP_ENDIAN - my documentation's out of
date - thanks).
However it doesn't really solve the problem - I've been given
a stack of read code which runs on a SUN and contains
MANY lines like this:
readu, lun, param1, param2 ......paramN
readu, lun, next_param1, next_param2 ....
....
....
where the parameters may be scalars or arrays of any type. If I
can possibly avoid it I'd rather not have to call swap_endian for
each parameter after each readu call - I'd have to do it for
several hundred named parameters. It would be much nicer to
incorporate the swapping into a "readu-like" call, but I can't
see any way of doing it. I'm likely to have to do similar things
with other datasets in the future, so a more general solution would
be welcome.
Simon Williams
------------------------------------------------------------ -------
Simon Williams British Atmospheric Data Centre
s.r.williams@rl.ac.uk Tel : 01235 446314 FAX : 01235 445848
------------------------------------------------------------ -------
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