[gdal-dev] Can't use two datasets read-only from same files?

[Even Rouault]

Nik,

what you try to do is perfectly valid and should work. I've just tried it 
myself, but I can't reproduce any error. If you are working on Linux, could 
you run your program with valgrind and look if it reports some errors ?
The only explanation I can imagine for now is that the file system on which 
your shapefile is located doesn't support a file to be opened more than once at 
a time, but this would be extremely surprising !

Best regards,

Even

> Hi GDAL devs,
> 
> I have a need to open the same set of Shapefiles as two independent
> datasets, BOTH of which are READ ONLY.  However, when I do this, and
> attempt to get the number of layers in the second data set, I get a whole
> lot of errors of the form:
> 
> 	ERROR 4: Failed to open file <shapefile>.shp.
> 	It may be corrupt or read-only file accessed in update mode.
> 
> ...for most (but not all) of the layers, and of course the dataset is
> incomplete and not useful.
> 
> I've crafted the following very simple example of this problem:
> 
> 		 OGRDataSourceH firstDS = OGROpen(path, FALSE, NULL);
> 		 OGRDataSourceH secondDS = OGROpen(path, FALSE, NULL);
> 		 OGR_DS_GetLayerCount(firstDS);   //  THIS LINE WORKS FINE
> 		 OGR_DS_GetLayerCount(secondDS);  //  THIS LINE PRODUCES MANY ERRORS
> 
> If I OGR_DS_Destroy the first data set before running the
> OGR_DS_GetLayerCount on the second data set it works fine.
> 
> Is this expected?
> 
> I really need to have both datasets readable and useable simultaneously as
> they are both in completely independent parts of the application.  The
> first data set should be long-lived and needs to be persistent before,
> during and after the entire short life-cycle of the second data set.
> 
> The error messages imply that it is either corrupt or accessed in update
> mode.  But clearly it is not corrupt (at the file level) because it works
> fine in the first data set (and in the second if the first data set is
> destroyed first).  It is also being explicitly opened in READ-ONLY mode in
> both cases and therefore should not be in update mode.
> 
> I'm using gdal-1.10.1.
> 
> Any advice would be very much appreciated.
> 
> Cheers,
> Nik.
> 
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