[gdal-dev] concerning VSIInstallSubFileHandler

Dan Greve grevedan at hotmail.com
Mon Feb 9 15:42:47 EST 2009


Exactly the answer I was looking for, I was hoping the MemFileHandler wasn't used directly by the VSISubFileHandler.  You are constantly adding features right before I need them.  Keep up the great work!

-- dan

> Date: Mon, 9 Feb 2009 15:21:04 -0500
> From: warmerdam at pobox.com
> To: grevedan at hotmail.com
> CC: gdal-dev at lists.osgeo.org
> Subject: Re: [gdal-dev] concerning VSIInstallSubFileHandler
> 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?
> Dan,
> 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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