sorry, let me be very explicit then:

0)
I believed that the following
creates the c object containing:
(a) main ocaml implementation of foo (b) export API (c) C binding to API (d) ocaml startup code

$ ocamlopt -output-obj -o foo_caml.o foo.cmxa foo_stub.c foo_export.ml

now I create a lib

$ ar rs libfoo.a foo_caml.o

and happy ever after. Nope.
No matter how I link it with a main c test, I get undefined symbols for startup code

$ gcc -o foo_test foo_test.c -L. -lfoo -lasmrun

In fact, the following variants don't work:

1)
$ ocamlopt -output-obj -o foo_caml.o foo.cmxa foo_stub.c foo_export.ml
$ ar -rs libfoo.a /sw/lib/ocaml/libasmrun.a foo_caml.o
$ gcc -o foo_test foo_test.c -L. -lfoo

2)
$ ocamlopt -output-obj -o foo_caml.o foo.cmxa foo_stub.c foo_export.ml
$ cp /sw/lib/ocaml/libasmrun.a libfoo.a
$ ar -rs libfoo.a foo_caml.o
$ gcc -o foo_test foo_test.c -L. -lfoo

so it seems I have to compile foo_stub.o separately...

but even if I do:

3)
$ ocamlopt -output-obj -o foo_caml.o foo.cmxa foo_stub.c foo_export.ml
$ ocamlopt foo_stub.c
$ ar -rs libfoo.a foo_caml.o foo_stub.o
$ gcc -o foo_test foo_test.c -L. -lfoo -L/sw/lib/ocaml -lasmrun

4)
$ ocamlopt -output-obj -o foo_caml.o foo.cmxa foo_stub.c foo_export.ml
$ ocamlopt foo_stub.c
$ ar -rs libfoo.a /sw/lib/ocaml/libasmrun.a foo_caml.o
$ gcc -o foo_test foo_test.c -L. -lfoo

both fail, the only thing that works is when
(a) I compile stub and caml objects separately AND 
(b) use copy to bootstrap the runtime

$ ocamlopt -output-obj -o foo_caml.o foo.cmxa foo_stub.c foo_export.ml
$ ocamlopt foo_stub.c
$ cp /sw/lib/ocaml/libasmrun.a libfoo.a
$ ar -rs libfoo.a foo_caml.o foo_stub.o
$ gcc -o foo_test foo_test.c -L. -lfoo

In fact ocamlopt does pretty enigmatic things in the background and ar is an entire mystery with this
copy thing.
If anyone can make me understand the above, I'd be forever grateful.

thanks
Viktor