What is vsipreload?

vsipreload is a library included with gdal which can be used to extend UserDocs/ReadInZip functionality to non-VSI-aware programs.

It works by using the dynamic linker's LD_PRELOAD system to override certain libc function calls with GDAL equivalents.

vsipreload currently only works on Linux.

Building vsipreload

1. Build GDAL configured with --with-pic
2. Build the VSIPreload library:

     g++ -Wl,-soname,libgdal_vsipreload.so.1 -Wall -fPIC \
       port/vsipreload.cpp \
       -shared -o libgdal_vsipreload.so.1 -Iport -L. -L.libs \
       -Wl,-Bstatic -lgdal \
       -Wl,-Bdynamic -ldl -lpthread -lcurl
   

Some notes:

1. statically linked -lgdal. This means using LD_PRELOAD loads a fairly minimal symbol set, so conflicting symbols with libraries loaded later aren't a problem. Otherwise you may run into problems with conflicting symbols from the various GDAL dependencies. An example is the ESRI FileGDB API, which exports a lot of libxml symbols, and when preloaded it may cause segfaults in other code that uses libxml symbols.

2. (1) requires libgdal to be configured --with-pic, otherwise the static linking doesn't work.

3. debhelper/lintian seems to dislike libraries without sonames. You might be able to skip this step; YMMV.

Using vsipreload

Just put LD_PRELOAD before your executable, and add the necessary /vsi... path parts to what you're doing:

$ LD_PRELOAD="./libgdal_vsipreload.so.1" ls /vsizip/vsicurl/http://svn.osgeo.org/gdal/trunk/autotest/ogr/data/poly.zip
poly.dbf  poly.PRJ  poly.shp  poly.shx