Re: 1.8 file format

[Quincey Koziol <koziol@xxxxxxxxxxxx>]

Hi John,

On Jul 24, 2007, at 6:50 PM, John Caron wrote:

> >     Hmm, looks like the docs are out of date and/or wrong for  
> > region references.  Because region references are variable in  
> > length, the actual reference information is stored in the global  
> > heap and a (fixed length) heap ID is stored in the dataset's raw  
> > data - that part is right, although too briefly described.
>
> the data storage layout messaage seems to imply the HeapID is 12  
> bytes long. Usually they are 16 bytes long.
>
>    type = Layout(8); contiguous storageSize = (2,12) dataAddress=13352
>
> I dont see enough detail in h5dump:
>
>         DATASET "VIIRS-NCC-EDR_Gran_0" {
>            DATATYPE  H5T_REFERENCE
>            DATASPACE  SIMPLE { ( 2 ) / ( 2 ) }
>
> is there another option that would verify that?
>
> in any case is 12 bytes correct? if so, what does a 12-byte HeapId  
> look like?

	Yes, 12 bytes is correct.  The format of a heap ID is: an 8 byte  
file address of the global heap to use, followed by a 4 byte heap  
"index" without that global heap.  You can see how this is decoded  
and used by the library in the H5R_dereference routine in src/H5R.c.

	Quincey


> The format of the data in the
> > heap is wrong however.  The information actually stored in the  
> > heap is as follows:
> >     - The offset of the object header of the object (ie. dataset)  
> > pointed to (yes, an object ID)
> >     - A serialized form of a dataspace _selection_ of elements (in  
> > the dataset pointed to).  I don't have a formal description of  
> > this information now, but it's encoded in the H5S_<foo>_serialize 
> > () routines in src/H5S<foo>.c, where foo = {all, hyper, point, none}.
> >     There is _no_ datatype information stored for these sort of  
> > selections currently.
> >     I'll file a bug report in our bugzilla to make certain this  
> > description gets fixed in the file format spec.
> >     Quincey
>
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