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structure length in files [message #84907]

Tue, 18 June 2013 08:20

Helder Marchetto
Messages: 520
Registered: November 2011

Senior Member

Hi,

[Short summary: When IDL writes structures in unformatted binary data files, what size will the resulting file have, that described by n_tags(Example1,/LENGTH) or n_tags(Example1,/DATA_LENGTH)?]

I was just looking at the length/size of a structure. I'm reading data from a file and using structures looking like this:

Example1 = {Field1: 0 ,$ ; Integer, 2 byte
Field2: 0 ,$ ; Integer, 2 byte
Field3: 0 ,$ ; Integer, 2 byte
Field4: 0 ,$ ; Integer, 2 byte
Field5: 0ULL } ; Unsigned Long64, 8 byte

Example2 = {Field1: 0 ,$ ; Integer, 2 byte
Field2: 0 ,$ ; Integer, 2 byte
Field3: 0 ,$ ; Integer, 2 byte
Field4: 0 ,$ ; Integer, 2 byte
Field5: 0ULL ,$ ; Unsigned Long64, 8 byte
Field6: 0 } ; Integer, 2 byte

When I look at it using the help command with /structure, I get:
IDL> help, example1, example2, /struct

** Structure <edbf6a0>, 5 tags, length=16, data length=16, refs=1:
...
** Structure <104d4de0>, 6 tags, length=24, data length=18, refs=1:
...

What is bothering me is that the addition of a field in the second structure results in an increased length (24 bytes) for a data length of 18 bytes. In the first example, the length is the same as the "data length". [the origin of this is probably filling the structure up to multiples of 8 bytes...]
The same results can be obtained using n_tags with the /length and data_length keywords.

So, reading the n_tags documentation, it seems like the Data_length is constant (machine independent) and the length is machine dependent (as in 32-64 bit and other things).

However, my question:
I noticed that when I read structures, the sizes are the expected "data_length" (machine independent). When IDL writes, what size will I have to expect: "length" or "data_length"?

Thanks,
Helder

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[Message index]

		structure length in files By: Helder Marchetto on Tue, 18 June 2013 08:20
		Re: structure length in files By: David Fanning on Tue, 18 June 2013 08:31
		Re: structure length in files By: Bill Nel on Tue, 18 June 2013 09:20
		Re: structure length in files By: Helder Marchetto on Tue, 18 June 2013 11:13
		Re: structure length in files By: David Fanning on Tue, 18 June 2013 11:26
		Re: structure length in files By: lecacheux.alain on Tue, 18 June 2013 12:05
		Re: structure length in files By: David Fanning on Tue, 18 June 2013 12:14

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