[gdal-dev] concerning VSIInstallSubFileHandler

Frank Warmerdam warmerdam at pobox.com
Mon Feb 9 15:21:04 EST 2009

Dan Greve wrote:
> Greetings,
> I am currently accessing datasets which are embedded inside a single 
> file.  Each dataset is a given file offset and length inside of the 
> larger file. 
> I saw in the latest doxy-docs here 
> <http://www.gdal.org/ogr/cpl__vsi_8h.html> that there is a 
> VSIInstallSubFileHandler for doing just that, and look forward to 
> testing it out.  My concern with it, as well as the normal 
> VSIMemFileHandler, is in accessing large files.  I have 2 GB images, 
> embedded within files that are up to 100 GB in size.  Does the 
> VSIMemFileHandler do any sort of paging so that the entire source image 
> is not read into memory?


The VSIMemFileHandler treats a memory buffer as a file.  There is no support
in it for paging from a file so it is only suitable when you are prepared
to load the object into RAM.

The VSISubFileHandler on the other hand just manages access to a file through
regular file access handle, but keeping track of an offset to the sub file.
So it does not use appreciably more memory than a normal file access.

I believe the subfilehandler would be the suitable tool for you.  I believe it
is only available in GDAL trunk, not 1.6.x.

Best regards,
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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