Re: [Caml-list] C libs from Ocaml libs

[Alain Frisch <alain@frisch.fr>]

viktor tron wrote:
> If anyone can make me understand the above, I'd be forever grateful.

Here is the more direct way to achive what you want:

===========================================================
ocamlopt -output-obj -o foo_caml.o foo.cmxa foo_export.ml
ocamlopt -c foo_stub.c
gcc -o foo_test foo_test.c foo_stub.o foo_caml.o \
    $(OCAMLLIB)/libasmrun.a -lm -ldl
===========================================================


Note that "ocamlopt -output-obj" uses the so-called partial linking 
feature (ld -r) to produce a single object file that contains:
- the startup code
- all the native OCaml code (including the code of the libraries, like 
  foo.a and stdlib.a)

But *not* the extra C objects and libraries (-ccopt, *.c/*.o/*.a 
arguments). (Thanks to Rabih for this off-line reminder.) I don't know 
whether this qualifies as a bug or as a feature.

Here is a variant where we pack foo_caml.o and foo_stub.o together in a 
single object using the partial linker manually:

===========================================================
ocamlopt -output-obj -o foo_caml.o foo.cmxa foo_export.ml
ocamlopt -c foo_stub.c
ld -r -o libfoo.o foo_stub.o foo_caml.o	
gcc -c foo_test.c
gcc -o foo_test foo_test.c libfoo.o $(OCAMLLIB)/libasmrun.a -lm -ldl
===========================================================

We can do more and also put the libasmrun.a library in the same object:

===========================================================
ocamlopt -output-obj -o foo_caml.o foo.cmxa foo_export.ml
ocamlopt -c foo_stub.c
ld -r -o libfoo.o foo_stub.o foo_caml.o	$(OCAMLLIB)/libasmrun.a
gcc -c foo_test.c
gcc -o foo_test foo_test.c libfoo.o -ldl -lm
===========================================================

One could try to put foo_stub.o and foo_caml.o together in a library:

===========================================================
ocamlopt -output-obj -o foo_caml.o foo.cmxa foo_export.ml
ocamlopt -c foo_stub.c
ar r libfoo.a foo_stub.o foo_caml.o	
gcc -c foo_test.c
gcc -o foo_test foo_test.c libfoo.a $(OCAMLLIB)/libasmrun.a -lm -ldl
===========================================================

This does not work because the two libraries libfoo.a and libasmrun.a 
depend on symbols defined in the other library. This can be addressed 
with the linker options --start-group/--end-group:

===========================================================
ocamlopt -output-obj -o foo_caml.o foo.cmxa foo_export.ml
ocamlopt -c foo_stub.c
ar r libfoo.a foo_stub.o foo_caml.o	
gcc -c foo_test.c
gcc -o foo_test foo_test.c \
  -Wl,--start-group libfoo.a $(OCAMLLIB)/libasmrun.a -Wl,--end-group \
  -lm -ldl
===========================================================


In the current development version of OCaml, it is also possible to go 
through a shared library:

===========================================================
ocamlopt -output-obj -o foo_caml.so foo.cmxa foo_export.ml foo_stub.c
gcc -c foo_test.c
gcc -o foo_test foo_test.c foo_caml.so -Wl,-R.
===========================================================

When -output-obj is used to produce a shared library, it will put extra 
C objects and options (here, foo_stub.o, libasmrun.a -lm, -ldl) in the 
result.

Feel free to ask more questions (and to write a tutorial on this issues)!


Alain