using openbabel plugins in R

After some more testing I have found that it actually does work if I 
compile without the plugin library but load it with dyn.load. I'm not 
sure why this wasn't working before. It only works though if the plugin 
library is loaded before libobtest2.so (the open babel main lib basically).
     So, to clarify, the following works now:

g++ -shared -o libobtest2.so obtest2.o -fpic -lopenbabel -lR

R>dyn.load("/usr/lib/openbabel/2.2.3/mdlformat.so")
R>dyn.load("libobtest2.so")
R>.Call("test")
   format: 0x7fe114c96d20  #this is the correct result
   NULL


     But now I have a chicken and egg problem. The plugin libraries are 
not stored in a standard directory, but open babel provides a function 
to list their paths. So I need to load the open babel library to fetch 
the plugin paths, then I can load the plugins, but, oops, too late, the 
open babel library is already loaded so loading the plugins now doesn't 
work. I tried using dyn.unload("libobtest2.so") but it didn't work. It 
seems like I'd have to compile a small executable program that uses 
openbabel to fetch the plugin paths, then run it as an external program 
from within R, then load the plugins, then load the open babel lib.
     Does it make any sense that the order in which these are loaded 
affects things? Is there a way to load the plugin lib later and still 
have  it work? If the order does have to be maintained, any better ideas 
how to accomplish this? Thanks.
     Also, here is the dlopen command that openbabel uses:
         dlopen(lib_name.c_str(), RTLD_LAZY | RTLD_GLOBAL)


Kevin

On 25 March 2013 at 12:50, Kevin Horan wrote:
| I posted this in openbabel-devel but didn't get much help, so hopefully
| someone here can help. I don't think its too openbabel specific.
|
| I would like to make use of open babel from within the R language.
| Initially I just need to do some format conversions, but may expand the
| usage to other parts of OpenBabel as well. I am familiar with embedding
| C/C++ code in R, but I'm having some trouble with the plugin mechanism
| of OpenBabel in this case. The  problem is that the formats are not
| available when I run the OpenBabel code from within R. So, for example,
| if I search for the SDF format like so:
|       OBFormat *format = conv.FindFormat("SDF");

[...]

|      The way it works normally in OpenBabel is that each plugin is its
| own shared library and then they get loaded at run time with the dlopen
| function (on linux at least). I have verified that this code is still
| being executed when called from within R, but it doesn't work for some
| reason.

I would try to start from the smallest possible working examples.  R itself
uses dlopen (see eg $RHOME/modules/ for the files it loads), and so does
OpenBabel. Maybe some wires get crossed. You may well have to dig down with
the debugger and see what assumptions / environment variables / ... are valid
or invalid between the working and non-working case.

Dirk