[gdal-dev] Re: [SoC] GDAL WKTRaster weekly report #9

Even Rouault even.rouault at mines-paris.org
Sun Aug 2 08:28:05 EDT 2009

Le Sunday 02 August 2009 13:56:52 Jorge Arévalo, vous avez écrit :
> Hello,
> So, I can create an array of children datasets to store the overviews,
> but I shouldn't add the datasets as metatada under SUBDATASETS domain.
> Is it right?

Right, you shouldn't use SUBDATASETS for reporting overviews.

> When I create an overview (a children dataset), its properties are the
> same properties of the general dataset (his parent), apart from pixel
> size, table name and column_name, of course. The problem appears when
> sharing the PGconn object.
> My code looks like this:
> poDS->overview[i]->hPGconn = poDS->hPGconn
> If each dataset's destructor (from the general one and from the
> overviews) calls PQfinish over the same object, I get a segmentation
> fault after the first time, of course. So, I have 2 options:
> - Detect when a connection object has been freed, and perform only one
> call. How should I do this?
> - Don't share the PGconn object. Should I declare a static PGconn pointer?
> What is the best approach?

A static variable is rarely a good idea. It would prevent the user from having 
multiple WKTRaster datasets opened at the same time.

Perhaps you should add a bBase flag to the WKTRasterDataset object (like it's 
done for the GTiffDataset). You set it to TRUE for the base dataset and to 
FALSE for the overviews. Then you only call PQfinish() if it is TRUE.

(I've no looked at what your code does recently, so the following remark might 
be irrelevant. But if you use transactions and their life cycle runs accross 
several calls of GDAL API, using the same connection object for the base 
dataset and its overviews might not be appropriate, or might require having a 
flag to know that a transaction is opened. --> You want to be able to mix 
RasterIO() calls on the base dataset and overviews)

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