RFC 27: Improved Supporting Data File Options
Author: Frank Warmerdam
Currently GDAL depends on a variety of supporting data files from the gdal data directory. The largest part of these are coordinate system dictionaries from EPSG and other sources. It also includes S-57 dictionaries, seed DGN and DXF files, and project logos. Uncompressed it currently comes to roughly 1.8MB and it is expected to grow as additional dictionaries are added (for PCI and IAU coordinate systems for instance).
It has also been a frequent problem at run time to find the data files when they are installed in unusual locations.
This RFC aims to overhaul support file handling with two new major features.
The ability to read from compressed data files to reduce the disk footprint of GDAL.
The ability to embed the data files with the GDAL DLL or shared library to remove the “finding” problem.
CPL CSV Access via VSI*L
The large majority of the support data file access is via the CPL CSV API (gdal/port/cpl_csv.cpp). Finding support data files is done via CPLFindFile(). It turns out these functions are still using the old VSI API which does not support special handlers (like /vsizip/), or in at least one case direct fopen() calls. So the first stage of this RFC is to convert these functions to all use the VSI*L API. A patch has been prepared that demonstrates the bulk of the required changes. With this patch it is possible to access files from a GDAL_DATA setting like /vsizip//home/warmerda/gdal/data/gdaldata.zip.
Note that we are explicitly changing the contract about the nature of the FILE* passed to functions like CSVReadParseLine() (real FILE* vs. VSIL style FILE). It is possible, though relatively unlikely that application code, or private driver implementations will be using the SV functions and will need to be changed. This change should be noted in the GDAL 1.8 release notes.
It is also unclear if there will be bad interactions with the cpl_csv implementation embedded in libgeotiff in some situations, such as when using libgeotiff as an external library. Some review will be needed.
Another point to investigate is what the performance impact of doing all the file finding through the VSI*L API will be.