[gdal-dev] What is readlink used for?

[Allen Kempe]

    Allen Kempe

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On 01/06/2014 01:35 PM, Kurt Schwehr wrote:
> Hi all,
>
> I ran into trouble with readlink as can be seen in 
> http://trac.osgeo.org/gdal/ticket/5340.  I'm working with a system 
> where the run directory is all symbolic links.  So if I have 
> nonsquare.vrt refering to "./small.raw", readlink of nonsquare.vrt 
> will end up being in a totally different directory than the small.raw 
> file.
>
> So... That begs the question of the purpose of dereferencing symbolic 
> links using readlink (e.g. 
> http://trac.osgeo.org/gdal/browser/trunk/gdal/frmts/vrt/vrtdataset.cpp#L687). 
>  Is this done just for performance on filesystems where symbolic links 
> might be a performance hit (e.g. nfs file systems over slow networks 
> and an OS that does not do a good job caching)?  Or is there something 
> else important that I'm missing?
>
> I see 4 places where readlink is used:
>
> ./ogr/ogrsf_frmts/generic/ogrsfdriverregistrar.cpp:  int nBytes = 
> readlink(pszName, szPointerFilename, sizeof(szPointerFilename));
> ./gcore/gdalopeninfo.cpp:        int nBytes = readlink(pszFilename, 
> szPointerFilename, sizeof(szPointerFilename));
> ./port/cpl_getexecpath.cpp:    nResultLen = readlink( osExeLink, 
> pszPathBuf, nMaxLength );
> ./frmts/vrt/vrtdataset.cpp:            int bufferSize = 
> readlink(currentVrtFilename, filenameBuffer, sizeof(filenameBuffer));
>
> ogrsfdriverregistar.cpp looks like a possible issue.
>
> http://trac.osgeo.org/gdal/browser/trunk/gdal/ogr/ogrsf_frmts/generic/ogrsfdriverregistrar.cpp#L250
>
> I see that sort of works...
>
> ogrinfo my_remote_poly.shp
> ERROR 4: Unable to open 
> /vsicurl/http://svn.osgeo.org/gdal/trunk/autotest/ogr/data/poly.shp or 
> /vsicurl/http://svn.osgeo.org/gdal/trunk/autotest/ogr/data/poly.SHP.
> Had to open data source read-only.
> INFO: Open of `my_remote_poly.shp'
>       using driver `ESRI Shapefile' successful.
> INFO: Internal data source name 
> `/vsicurl/http://svn.osgeo.org/gdal/trunk/autotest/ogr/data/poly.shp'
>       different from user name `my_remote_poly.shp'.
> 1: poly (3D Polygon)
>
> I'm thinking that I should just disable readlink entirely. 
>  Alternatively, hhould there be a flag that controls readlink just for 
> working with vrt's or a runtime ConfigSetting that could turn on/off 
> readlink?
>
> Any thoughts would be appreciated!
>
> Thanks,
> -kurt
>
>
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