[Distutils] SWIG runtime libs/Python distutils combo on Mac

[Bill Spotz]

This problem concerns using distutils to compile a set of python 
modules that are generated by SWIG, so I am cross-posting to both SWIG 
and distutils mailing lists.

I use SWIG to wrap several packages that are intended to be both 
independent and interoperable.  So I am creating separate python 
extensions that link to the swig runtime libraries.

Previously, I linked to installed system runtime libraries, but this is 
not the preferred approach, and indeed, swig 1.3.22 does not support 
it.  Instead, I am supposed to generate my own local runtime library, 
and link to it.  Fine; this would seem to be more portable.

The SWIG documentation says to do the following (for Linux), with the 
caveat that compiling dynamic libraries is different on every system:

------------------
     % swig -runtime -python swigrun.i
     % gcc -fpic swigrun_wrap.c -o swigrun_wrap.o
     % gcc -shared swigrun_wrap.o -o libswigrunpy.so

Now, you compile all of the normal SWIG modules using the -noruntime 
option:

     % swig -noruntime -c++ -python A.i
     % swig -noruntime -c++ -python B.i

Finally, when linking the dynamically loadable modules, you need to 
link again the special runtime library above. For example (Linux) :

     % g++ -shared A_wrap.o -L. -lswigrunpy -o _A.so
     % g++ -shared B_wrap.o -L. -lswigrunpy -o _B.so
------------------

To get around the portability problem, I have decided to use python's 
distutils module to handle the compilation issues.  In my setup.py 
script, I define

   SwigRun = Extension("PyTrilinos.libswigrun", ["src/swigrun_wrap.c"])

and this works, as build/lib.blahblahblah/PyTrilinos/libswigrun.so gets 
created properly.  When I define real extensions in setup.py, they are 
created with keyword arguments

     library_dirs    = [..., "build/lib.blahblahblah/PyTrilinos", ...],
     libraries       = [..., "swigrun", ...],

which gives me the link command arguments I want: 
-Lbuild/lib.blahblahblah/PyTrilinons -lswigrun.  However, on my Mac, 
when the link command is invoked, I get

     ld: can't locate file for: -lswigrun

I strongly suspect that the Mac's ld command does not search for 
libswigrun.so when -lswigrun is given as a command line argument (the 
old system libraries have .la and .dylib extensions).  But I cannot 
figure out how to get setup.py to compile to a different suffix.

Any suggestions would be appreciated.

** Bill Spotz                                              **
** Sandia National Laboratories  Voice: (505)845-0170      **
** P.O. Box 5800                 Fax:   (505)284-5451      **
** Albuquerque, NM 87185-0316    Email: wfspotz at sandia.gov **