As for ldd blib/arch/auto/TwinTree/TwinTree.so it says

        libc.so.6 => /lib/libc.so.6 (0xb7f24000)
        /lib/ld-linux.so.2 (0x0071d000)
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All my compile switches are in the Makefile...

Of course they are :D ok, I can compile libtwintree with mingw(strawberryperl/win32), but it exports no symbols. So I end up with

TwinTree.o:TwinTree.c:(.text+0x29e): undefined reference to `return_on
+e'
TwinTree.o:TwinTree.c:(.text+0x3de): undefined reference to `return_ze
+ro'
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Your ldd output shows it isn't looking for libtwintree , so whatever is going on, its all c to me :)

All of this seems normal to me. Purpose of h2xs is for generating starting point, you have to do the c programming :)

It's just that the XSUB code is supposed to make the actual call to the library, yes? As long as this is missing, the library could implement the meaning of life, universe and everything and the perl script would never know.

And the thing is, even if I write the XSUB code manually, say for instance like this

int
return_one()
  CODE:
    RETVAL = return_one();
  OUTPUT:
    RETVAL

int
return_zero()
  CODE:
    RETVAL = return_one();
  OUTPUT:
    RETVAL
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..it still doesn't work:

PERL_DL_NONLAZY=1 /usr/bin/perl "-MExtUtils::Command::MM" "-e" "test_h
+arness(0, 'blib/lib', 'blib/arch')" t/*.t
t/TwinTree....NOK 1                                                   
+       
#   Failed test 'use TwinTree;'
#   at t/TwinTree.t line 9.
#     Tried to use 'TwinTree'.
#     Error:  Can't load '/home/floyd/twintree/TwinTree/blib/arch/auto
+/TwinTree/TwinTree.so' for module TwinTree: /home/floyd/twintree/Twin
+Tree/blib/arch/auto/TwinTree/TwinTree.so: undefined symbol: return_on
+e at /usr/lib/perl5/5.8.8/i386-linux-thread-multi/DynaLoader.pm line 
+230.
#  at (eval 4) line 2
# Compilation failed in require at (eval 4) line 2.
# BEGIN failed--compilation aborted at (eval 4) line 2.
# Looks like you failed 1 test of 1.
t/TwinTree....dubious                                                 
+       
        Test returned status 1 (wstat 256, 0x100)
DIED. FAILED test 1
        Failed 1/1 tests, 0.00% okay
Failed Test  Stat Wstat Total Fail  Failed  List of Failed
----------------------------------------------------------------------
+---------
t/TwinTree.t    1   256     1    1 100.00%  1
Failed 1/1 test scripts, 0.00% okay. 1/1 subtests failed, 0.00% okay.
make: *** [test_dynamic] Error 1
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...the pieces just don't fit. There's probably just a tiny command-line switch or something missing somewhere, but I still don't have the faintest clue where to look. Logically I would expect h2xs to discover this and point it out, instead of generating a lot of code that just doesn't compile.
-- Time flies when you don't know what you're doing