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

[Even Rouault]

Le dimanche 23 février 2014 08:40:11, Even Rouault a écrit :
> 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 !

oh wait, is the "path" in your example a directory that contains lots of 
shapefiles ? In that case, you might reach the maximum number of opened files by 
a process. But... since GDAL 1.9, there is a mechanism in the shapefile driver 
to limit the number of layers opened simultaneously to 100 per datasource (so 
200 files for .shp and .dbf). With 2 datasources, this would be 400 files which 
is still below the 1024 limit on Linux.

> 
> 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.
> > 
> > _______________________________________________
> > gdal-dev mailing list
> > gdal-dev at lists.osgeo.org
> > http://lists.osgeo.org/mailman/listinfo/gdal-dev

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