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Re: Changing File Size with OPENU? [message #10212 is a reply to message #10203]

Tue, 28 October 1997 00:00

Peter Mason
Messages: 145
Registered: June 1996

Senior Member

On Mon, 27 Oct 1997, Wayne Landsman wrote:
> I always thought that when opening a file with OPENU that the file wouldn't
> change size, unless one wrote past the end of the file. But below I give an
> example of a file changing size, even though I never reached the end of the
> file.
>
> I suspect this might be a IDL (or Solaris 2.6) bug, because if I uncomment
> the ;print,fstat(lun) line below, then the file size stays unchanged.
> Anybody know if the behaviour in the example below is expected?
<example cut>

The size change also happens on Digital Unix 4.0A with IDL 4.0.1 (and earlier
versions).
I wasn't surprised by this behaviour in IDL, having first been totally freaked
out by it in the C world. (I found out about it the hard way.) In C, you
have to do a fseek() or fflush() in between changing from a read to a write
(or vice versa) on the same streamed file unit, otherwise things just go wrong.
(At least C documentation on fopen() usually points this out.)

IDL's normal I/O routines are based on C routines, and I guess that this is
why they have the same problem. (IDL's POINT_LUN is equivalent to a fseek().
Also, I guess that FSTAT() incorporates a seek.)
It would be nice if the IDL docs mentioned this somewhere.

So, anyway, when changing from a read to a write (or vice versa) on the
same file unit, you should always do something like:
POINT_LUN, -lun, junk
or, as you've tried, an FSTAT(),
or possibly FLUSH,lun.

Peter Mason
CSIRO division of Exploration and Mining
P.O Box 136, North Ryde, NSW, 2113, Australia
E-Mail: p.mason@syd.dem.csiro.au Tel: +61 2 9490-8883 Fax: 9490-8960/8921
Web: http://www.syd.dem.csiro.au/research/MMTG/

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[Message index]

		Re: Changing File Size with OPENU? By: Peter Mason on Tue, 28 October 1997 00:00
		Re: Changing File Size with OPENU? By: Jack Saba on Tue, 28 October 1997 00:00
		Re: Changing File Size with OPENU? By: Peter Mason on Tue, 28 October 1997 00:00
		Re: Changing File Size with OPENU? By: Wayne Landsman on Mon, 27 October 1997 00:00
		Re: Changing File Size with OPENU? By: thompson on Mon, 27 October 1997 00:00

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