| File pointer problem on PC-IDL [message #14602] |
Thu, 11 March 1999 00:00  |
landsman
Messages: 93
Registered: August 1991
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Member |
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I am trying to debug a program (a "FITS" reader) that is failing on PC-IDL
Unfortunately, I don't have access to PC-IDL myself so I apologizing for not
doing as much debugging as I would have liked before posting my problem.
Basically, then internal file position is getting screwed up. I open a
binary file, readu 2880 bytes, and find myself located 4095 bytes into the
file. Below is the simple script (as done by a remote user with PC-IDL).
IDL> print, !version
{ x86 Win32 Windows 5.2 Oct 30 1998}
IDL> openr,lun,'C:\burst_image_1.fit',/block,error=error,/get_lun
IDL> if eof(lun) then message, 'Reached end of file'
IDL> buf = replicate(32b,80,36) ;Total of 2880 bytes
IDL> readu,lun,buf
IDL> help,/str,fstat(lun)
** Structure FSTAT, 12 tags, length=36:
UNIT LONG 100
NAME STRING 'C:\ROTSE\GRB 990123\burst_image_1.fit'
OPEN BYTE 1
ISATTY BYTE 0
ISAGUI BYTE 0
INTERACTIVE BYTE 0
READ BYTE 1
WRITE BYTE 0
TRANSFER_COUNT LONG 2880
CUR_PTR LONG 4095
SIZE LONG 25920
REC_LEN LONG 0
How did the current pointer get at 4095 bytes??? Calling EOF() is not
supposed to change the file position. The situation seems reminiscent of
a VMS mode where one always reads in multiple of 512 bytes. (To avoid this
problem is why the VMS-only /BLOCK keyword is added to the OPENR statement
above.) But I wasn't aware of such fixed block I/O on a PC.
Anyone have any ideas? (The file burst_image_1.fit is available at
ftp://idlastro.gsfc.nasa.gov/landsman/fits)
thanks, --Wayne Landsman landsman@mpb.gsfc.nasa.gov
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| Re: File pointer problem on PC-IDL [message #14632 is a reply to message #14602] |
Thu, 18 March 1999 00:00  |
menakkis
Messages: 37
Registered: June 1998
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Member |
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landsman@stars.gsfc.nasa.gov (Wayne Landsman) wrote:
<...>
> Basically, then internal file position is getting screwed up. I open a
> binary file, readu 2880 bytes, and find myself located 4095 bytes into the
> file.
<...>
Wayne, don't ask me why, but ask your remote user to include the switch
/NOAUTOMODE in the OPEN call.
Oh well, it's hard to turn down any opportunity to pick on RSI for this
little Windows feature of theirs. :-) It has bothered me since the day it
sidled onto the scene, announced - as it were - somewhere in the bowels of
the readme file. Basically IDL's I/O on Windows has a "text mode" and a
"binary mode". One of these (the "binary" mode, would you believe?) fools
around with <CR><LF> pairs. For some reason your EOF check is triggering IDL
to switch to "binary" mode or something. (I haven't checked, but it seems
unlikely to me that your file would have enough <CR><LF> pairs to fool with
to account for the discrepancy - it looks more like a bug.) Another strange
thing is that (in this case) if you use the /BINARY switch in the OPEN call
it will also fix the problem. (I don't even want to speculate on this one.)
Peter Mason
-----------== Posted via Deja News, The Discussion Network ==----------
http://www.dejanews.com/ Search, Read, Discuss, or Start Your Own
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| Re: File Pointer [message #41870 is a reply to message #14602] |
Mon, 06 December 2004 14:50 |
Paul Van Delst[1]
Messages: 1157
Registered: April 2002
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Senior Member |
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Pravesh wrote:
> Hi:
> HOw do i implement a feature, that on clicking of a button will
> automatically display the next file in the directory. the files are
> images.
Some sort of widget code that, e.g., after using findfile to get a list of the requested
files (*.png or *.gif or whatever), displays the first one in the list. The button widget
then just goes through the list with its event handler displaying the file.
paulv
--
Paul van Delst
CIMSS @ NOAA/NCEP/EMC
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| Re: File Pointer [message #41872 is a reply to message #14602] |
Mon, 06 December 2004 13:28 |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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Pravesh writes:
> HOw do i implement a feature, that on clicking of a button will
> automatically display the next file in the directory. the files are
> images.
Well, you just go to the next file in the list of files
you are keeping, read the file, and display the image.
Then you probably update your file position pointer because
you always want to know where you are in your list.
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.dfanning.com/
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