Thanks a lot Pierrick, now I have a set of working makefiles:

# this is Makefile.a

SOURCES = a.ml
RESULT = a
all: byte-code-library
-include OCamlMakefile

# this is Makefile.b

SOURCES = b.ml
RESULT = b
all: byte-code-library
-include OCamlMakefile

# this is Makefile.test

SOURCES = test.ml
LIBS = str a b
RESULT = test
all: byte-code
-include OCamlMakefile


Le 06/03/2015 11:44, Pierrick Couderc a écrit :
> Hi,
>
> I'm not an expert at all with OCamlMakefile, but I guess that comes from the fact the compiler embeds Str in both libraries "a.cma" and "b.cma". Doing it by hand:
>
> $ ocamlc -a -o a.cma a.ml 
> $ ocamlc -a -o b.cma b.ml
> $ ocamlc -o a.out str.cma a.cma b.cma test.ml
>
> You don't need to give "str" as Lib when generating your libraries, only when producing your executable :-) If you give str.cma when generating your libraries:
>
> $ ocamlc -a -o a.cma str.cma a.ml
> $ ocamlc -a -o b.cma str.cma b.ml
> $ ocamlc -o a.out a.cma b.cma test.ml
>   File "b.cma(Str)", line 1:
>   Warning 31: files b.cma(Str) and a.cma(Str) both define a module named Str
>
>
> Pierrick
>
> 2015-03-06 11:16 GMT+01:00 Vu Ngoc San <san.vu-ngoc@laposte.net>:
>
>     Hello list
>
>     I have a basic compilation question:
>     I want to write two independent libraries, say A an B, and both use a third library, eg. Str
>
>     When I write a program which uses A and B, I get
>     File "b.cma(Str)", line 1:
>     Warning 31: files b.cma(Str) and a.cma(Str) both define a module named Str
>
>     How to get rid of this warning ? Am I doing something wrong ?
>
>     Thanks
>     San
>
>     ----
>
>     Here is a minimal example:
>
>     (* This is a.ml *)
>
>     let version = "A";;
>     let _ = print_endline (Str.string_before "Loading the A library -- foo" 22);;
>
>     (* This is b.ml *)
>
>     let version = "B";;
>     let _ =  print_endline (Str.string_before ("Loading the " ^ version ^" library -- bar") 23);;
>
>     # this is Makefile.a
>
>     SOURCES = a.ml
>     LIBS = str
>     RESULT = a
>     all: byte-code-library
>     -include OCamlMakefile
>
>     # this is Makefile.b
>
>     SOURCES = b.ml
>     LIBS = str
>     RESULT = b
>     all: byte-code-library
>     -include OCamlMakefile
>
>     (* this is test.ml *)
>
>     let _ = print_endline (Str.string_before ("Hello " ^ A.version ^ " and " ^ B.version ^"---")  13);;
>
>     # this is Makefile.test
>
>     SOURCES = test.ml
>     LIBS = a b
>     RESULT = test
>     all: byte-code
>     -include OCamlMakefile
>
>
>
>     --
>     Caml-list mailing list.  Subscription management and archives:
>     https://sympa.inria.fr/sympa/arc/caml-list
>     Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
>     Bug reports: http://caml.inria.fr/bin/caml-bugs
>
>
>
>
> --
> Pierrick COUDERC
>
> PhD Student at OCamlPro / ENSTA Paristech Université Paris Saclay