| OPENR F77 SWAP_IF_LITTLE_ENDIAN [message #90724] |
Mon, 06 April 2015 11:04  |
MarioIncandenza
Messages: 231
Registered: February 2005
|
Senior Member |
|
|
; open and write a file using F77_UNFORMATTED
IDL> openw,lun,/get_lun,/f77,'/tmp/test_f77'
IDL> writeu,lun,long(2015032518),float(6),long(0)
IDL> free_lun,lun
; read the file back in
IDL> openr,lun,/get_lun,/f77,'/tmp/test_conc'
IDL> qq1=long(0) & qq2=float(0) & qq3=long(0)
IDL> readu,lun,qq1,qq2,qq3
IDL> print,qq1,qq2,qq3
2013052306 6.00000 0
IDL> free_lun,lun
; now try reading it with /SWAP_IF_LITTLE_ENDIAN
IDL> openr,lun,/get_lun,/f77,'/tmp/test_conc',/swap_if_little_end ian
IDL> readu,lun,qq1,qq2,qq3
% READU: Corrupted f77 unformatted file detected. Unit: 100
File: /tmp/test_conc
% Execution halted at: $MAIN$
Has anyone encountered this behavior?
--Edward H.
|
|
|
|
| Re: OPENR F77 SWAP_IF_LITTLE_ENDIAN [message #90725 is a reply to message #90724] |
Mon, 06 April 2015 12:43  |
Lajos Foldy
Messages: 176
Registered: December 2011
|
Senior Member |
|
|
On Monday, April 6, 2015 at 8:04:05 PM UTC+2, Edward Hyer wrote:
> ; open and write a file using F77_UNFORMATTED
> IDL> openw,lun,/get_lun,/f77,'/tmp/test_f77'
> IDL> writeu,lun,long(2015032518),float(6),long(0)
> IDL> free_lun,lun
> ; read the file back in
> IDL> openr,lun,/get_lun,/f77,'/tmp/test_conc'
> IDL> qq1=long(0) & qq2=float(0) & qq3=long(0)
> IDL> readu,lun,qq1,qq2,qq3
> IDL> print,qq1,qq2,qq3
> 2013052306 6.00000 0
> IDL> free_lun,lun
> ; now try reading it with /SWAP_IF_LITTLE_ENDIAN
> IDL> openr,lun,/get_lun,/f77,'/tmp/test_conc',/swap_if_little_end ian
> IDL> readu,lun,qq1,qq2,qq3
> % READU: Corrupted f77 unformatted file detected. Unit: 100
> File: /tmp/test_conc
> % Execution halted at: $MAIN$
>
>
> Has anyone encountered this behavior?
>
> --Edward H.
F77 unformatted files contain record length data. Byte swapping them will result in corrupted records.
regards,
Lajos
|
|
|
|
| Re: OPENR F77 SWAP_IF_LITTLE_ENDIAN [message #90726 is a reply to message #90724] |
Mon, 06 April 2015 12:45 |
Paul Van Delst[1]
Messages: 1157
Registered: April 2002
|
Senior Member |
|
|
Hello,
On 04/06/15 14:04, Edward Hyer wrote:
> ; open and write a file using F77_UNFORMATTED
> IDL> openw,lun,/get_lun,/f77,'/tmp/test_f77'
> IDL> writeu,lun,long(2015032518),float(6),long(0)
> IDL> free_lun,lun
> ; read the file back in
> IDL> openr,lun,/get_lun,/f77,'/tmp/test_conc'
> IDL> qq1=long(0) & qq2=float(0) & qq3=long(0)
> IDL> readu,lun,qq1,qq2,qq3
> IDL> print,qq1,qq2,qq3
> 2013052306 6.00000 0
> IDL> free_lun,lun
> ; now try reading it with /SWAP_IF_LITTLE_ENDIAN
> IDL> openr,lun,/get_lun,/f77,'/tmp/test_conc',/swap_if_little_end ian
> IDL> readu,lun,qq1,qq2,qq3
> % READU: Corrupted f77 unformatted file detected. Unit: 100
> File: /tmp/test_conc
> % Execution halted at: $MAIN$
>
>
> Has anyone encountered this behavior?
Yes. At least, when I read in the same file I write! (you have
'/tmp/test_f77' and '/tmp/test_conc')
On my (little-endian) machine:
IDL> print, !version
{ x86_64 linux unix linux 8.3 Nov 15 2013 64 64}
IDL> openw,lun,/get_lun,/f77,'tmp.test_f77'
IDL> writeu,lun,long(2015032518),float(6),long(0)
IDL> free_lun,lun
IDL> openr,lun,/get_lun,/f77,'tmp.test_f77'
IDL> qq1=long(0) & qq2=float(0) & qq3=long(0)
IDL> readu,lun,qq1,qq2,qq3
IDL> print,qq1,qq2,qq3
2015032518 6.00000 0
IDL> free_lun,lun
IDL> openr,lun,/get_lun,/f77,'tmp.test_f77',/swap_if_little_endia n
IDL> readu,lun,qq1,qq2,qq3
% READU: Corrupted f77 unformatted file detected. Unit: 100, File:
tmp.test_f77
% Execution halted at: $MAIN$
This is what should happen on a little endian machine.
Now, if you're on a big-endian machine (e.g. Power chips?) then this
would be strange behaviour. But, last time I checked/cared, IDL wasn't
being developed anymore for those platforms (e.g. IBM AIX RISC type of
thing).
cheers,
paulv
|
|
|
|
| Re: OPENR F77 SWAP_IF_LITTLE_ENDIAN [message #90727 is a reply to message #90725] |
Mon, 06 April 2015 13:00 |
Paul Van Delst[1]
Messages: 1157
Registered: April 2002
|
Senior Member |
|
|
On 04/06/15 15:43, fawltylanguage@gmail.com wrote:
> On Monday, April 6, 2015 at 8:04:05 PM UTC+2, Edward Hyer wrote:
>> ; open and write a file using F77_UNFORMATTED
>> IDL> openw,lun,/get_lun,/f77,'/tmp/test_f77'
>> IDL> writeu,lun,long(2015032518),float(6),long(0)
>> IDL> free_lun,lun
>> ; read the file back in
>> IDL> openr,lun,/get_lun,/f77,'/tmp/test_conc'
>> IDL> qq1=long(0) & qq2=float(0) & qq3=long(0)
>> IDL> readu,lun,qq1,qq2,qq3
>> IDL> print,qq1,qq2,qq3
>> 2013052306 6.00000 0
>> IDL> free_lun,lun
>> ; now try reading it with /SWAP_IF_LITTLE_ENDIAN
>> IDL> openr,lun,/get_lun,/f77,'/tmp/test_conc',/swap_if_little_end ian
>> IDL> readu,lun,qq1,qq2,qq3
>> % READU: Corrupted f77 unformatted file detected. Unit: 100
>> File: /tmp/test_conc
>> % Execution halted at: $MAIN$
>>
>>
>> Has anyone encountered this behavior?
>>
>> --Edward H.
>
> F77 unformatted files contain record length data. Byte swapping them
> will result in corrupted records.
Oh, it didn't even occur to me that the OP didn't know about the
(ubiquitous but non-standard) format of Fortran unformatted sequential
files.
For the OP (apolgoies if you do know this):
Fortran unformatted sequential files have the following structure:
[next record length]
[record]
[previous record length]
repeated for each record. That is, each record is bookended with a
record marker indicating the length of the record. Having the record
length at the end of the record as well is to allow code to BACKSPACE
records like one would have had to do on magnetic tapes back in old day.
At any rate, when you byte swap the data via the
"/swap_if_little_endian" keyword, you are also byteswapping the record
markers. And that leads to potentially screwed up reads.
Doing an octal dump of the original *little-endian* file:
$ od -t d4 tmp.test_f77
0000000 12 2015032518 1086324736 0
0000020 12
0000024
Note the "12" at the beginning and end. The record is 12 bytes long.
Now lets do the same, but for a purposefully written big-endian file:
IDL> openw,lun,/get_lun,/f77,'be.tmp.test_f77',/swap_endian
IDL> writeu,lun,long(2015032518),float(6),long(0)
IDL> free_lun,lun
$ od -t d4 be.tmp.test_f77
0000000 201326592 -957080968 49216 0
0000020 201326592
0000024
Because of the byteswapping, this is the equivalent of the little-endian
file when the "/swap_if_little_endian" keyword is used. When this file
is read IDL thinks the record should be 201326592 bytes long. Which is
isn't.
Thus, error.
cheers,
paulv
|
|
|
|
| Re: OPENR F77 SWAP_IF_LITTLE_ENDIAN [message #90728 is a reply to message #90726] |
Mon, 06 April 2015 17:27 |
MarioIncandenza
Messages: 231
Registered: February 2005
|
Senior Member |
|
|
On Monday, April 6, 2015 at 12:45:49 PM UTC-7, Paul van Delst wrote:
> This is what should happen on a little endian machine.
>
Paul,
I had somehow gotten the notion that the 'endianness' was a property of the *file*, rather than the *machine reading or writing the file*. So what I needed was for the /SWAP_IF_LITTLE_ENDIAN used on the OPENR to also be used on the OPENW (basically forcing big-endian behavior [I can't be the only person coding against a legacy system built on SGI hardward-- right?]).
Thanks for the help, and the explanation.
--Edward H.
|
|
|
|
comp.lang.idl-pvwave archive
Members
Search
Help
Login
Home
