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Re: Bug? yea or nay. [message #23727 is a reply to message #23726]

Fri, 16 February 2001 06:35

Paul van Delst
Messages: 364
Registered: March 1997

Senior Member

William Daffer wrote:
>
> Again, aside from the disagreement between N_tags and the actual I/O
> that's going on, I was hoping someone could explain to me *how* a
> structure that starts with the largest type and works downward could
> *possibly* have any padding.

If one of the requirements for efficient data storage and retrieval in memory is to align
data boundaries defined by the largest type in the structure? (I'm postulating... not
stating).

IDL> print,!version
{ mipseb IRIX unix 5.3 Nov 11 1999}
IDL> r={a:0.d, c:0L} & print,n_tags(r,/length)
16
== Everything lined up on 8-byte boundary

IDL> r={c:0L,a:0.d} & print,n_tags(r,/length)
16
== Still on 8-byte boundary...even though 4-byte is first.

IDL> r={a:0L, c:0L, d:0L} & print,n_tags(r,/length)
12
== Whoa... 4-byte boundaries now. How 'bout that (I'm not being sarcastic, I did not
expect this result)

The last result suggests to me that how IDL/the OS (insert hand-waving here) decides to
allocate space in memory is dependent on the structure types.

> Clearly I'm missing something.

I don't think so, apart from the fact that memory allocation for structures seems to be
(sort of) dynamic depending on what is in the structure.

Also, FWIW, I disagree with Nigel Wade that this is "operator error". I think N_TAGS
should return the sum total of the sizes of the structure elements, NOT the amount of
memory said structure occupies. If one was dealing with accessing system memory for OS
purposes, fine. But IDL is a simple tool for playing with data - why should the user care
whether the structure is padded in memory or not? I think your definition of TOTSIZE( r )
is the "correct" one IMO.

paulv

--
Paul van Delst A little learning is a dangerous thing;
CIMSS @ NOAA/NCEP Drink deep, or taste not the Pierian spring;
Ph: (301)763-8000 x7274 There shallow draughts intoxicate the brain,
Fax:(301)763-8545 And drinking largely sobers us again.
pvandelst@ncep.noaa.gov Alexander Pope.

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[Message index]

		Re: Bug? yea or nay. By: Dave Greenwood on Fri, 16 February 2001 08:25
		Re: Bug? yea or nay. By: Craig Markwardt on Fri, 16 February 2001 07:22
		Re: Bug? yea or nay. By: Paul van Delst on Fri, 16 February 2001 06:35
		Re: Bug? yea or nay. By: Paul van Delst on Fri, 16 February 2001 06:16
		Re: Bug? yea or nay. By: Nigel Wade on Fri, 16 February 2001 02:43
		Re: Bug? yea or nay. By: William Daffer on Thu, 15 February 2001 20:21
		Re: Bug? yea or nay. By: Todd Clements on Thu, 15 February 2001 15:57
		Re: Bug? yea or nay. By: Paul van Delst on Thu, 15 February 2001 14:34
		Re: Bug? yea or nay. By: Nigel Wade on Mon, 19 February 2001 04:21

Previous Topic:	Re: Help with reading structure from file
Next Topic:	JHU/APL/S1R IDL Library update.

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