[Caml-list] Using OCaml from C

[Tom Ridge <tom.j.ridge+caml@googlemail.com>]

Dear List,

I want to use my ocaml code from a C program.

I have read the section "Interfacing C with OCaml" in the manual:

http://caml.inria.fr/pub/docs/manual-ocaml/manual033.html

The example in section 19.8 works fine. The compilation commands are
(in a Makefile; OCAMLPATH points to /home/tr61/.opam/4.00.1):

  ocamlc -custom -output-obj -o modcaml.o mod.ml
  ocamlc -c modwrap.c
  cp $(OCAMLPATH)/lib/ocaml/libcamlrun.a mod.a
  ar r mod.a modcaml.o modwrap.o
  cc -o prog main.c mod.a -lcurses -lm -ldl

(I had to add -lm and -ldl to get the example to work).

If my code makes use of the OCaml Unix library, I do:

cc -o prog main.c mod.a -lcurses -lm -ldl -L$(OCAMLPATH)/lib/ocaml -lunix

And if my code makes use of threads, I do:

cc -o prog main.c mod.a -lcurses -lm -ldl -L$(OCAMLPATH)/lib/ocaml
-lunix -lthreads -lpthread

So far, so good. The real code I want to use makes use of the Core
library. The compilation commands I finally got working are:

  ocamlfind ocamlc -package core -package str -linkpkg -thread -custom
-output-obj -o modcaml.o pqueue.ml mod.ml
  ocamlc -c modwrap.c
  cp $(OCAMLPATH)/lib/ocaml/libcamlrun.a mod.a
  ar r mod.a modcaml.o modwrap.o
  cc -pthread -Xlinker --allow-multiple-definition -o prog main.c
mod.a -lcurses -lm -ldl -lpthread  -L$(OCAMLPATH)/lib/ocaml -lunix
-lthreads -lbigarray -lcamlstr -lnums  -L$(OCAMLPATH)/lib/core_kernel
-lcore_kernel_stubs -L$(OCAMLPATH)/lib/core -lcore_stubs
-L$(OCAMLPATH)/lib/bin_prot -lbin_prot_stubs -lrt

For the last "cc" command, tt took quite a long time to figure out all
these flags and options (I do not know very much about linkers etc).
The errors I got were:

--

/home/tr61/.opam/4.00.1/lib/ocaml/libthreads.a(st_stubs_b.o): In
function `caml_thread_initialize':
st_stubs.c:(.text+0xed1): undefined reference to `pthread_atfork

fixed by adding -pthread after cc command

--

/home/tr61/.opam/4.00.1/lib/core/libcore_stubs.a(unix_stubs.o): In
function `unix_clock_gettime':
unix_stubs.c:(.text+0x2776): undefined reference to `clock_gettime'

I had -lrt after -lpthread, but the error was still reported; when I
moved -lrt to end of command line things seemed to work; why?

--

/home/tr61/.opam/4.00.1/lib/core/libcore_stubs.a(unix_stubs.o): In
function `unix_initgroups':
unix_stubs.c:(.text+0x3b90): multiple definition of `unix_initgroups'
/home/tr61/.opam/4.00.1/lib/ocaml/libunix.a(initgroups.o):initgroups.c:(.text+0x0):
first defined here
/home/tr61/.opam/4.00.1/lib/core/libcore_stubs.a(unix_stubs.o): In
function `unix_nice':
unix_stubs.c:(.text+0x46bc): multiple definition of `unix_nice'
/home/tr61/.opam/4.00.1/lib/ocaml/libunix.a(nice.o):nice.c:(.text+0x0):
first defined here

fixed by adding -Xlinker --allow-multiple-definition to cc options

--

My real question is: what command *should* I be using to compile my
example? Or is the above more-or-less expected?

Thanks

Tom