Use C binding with PPX and dune

Learning
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I come here with a better understanding of my issue but still no way to solve it after reading forums, documentations and try different options (even the minimal command line example).

Context:

I’m using this library for my internship, ocaml_mammut. This is a C++ binding to this library, mammut.
The dune file for the ocaml-mammut is:

(library
 (name mammut)
 (public_name mammut)
 (flags (:standard -w -49 -linkall ))
 (modules mammut)
 (libraries ctypes.stubs mammut.generated threads)
 (c_library_flags -lstdc++)
)

The mammut.generated is here to compile the mammut c++ library and generate the c++ stubs. Its dune is this one (a bit long sorry):

(library
 (name mammut_generated)
 (public_name mammut.generated)
 (wrapped false)
 (modules mammut_generated)
 (libraries ctypes.stubs mammut_stubs_description)
 (foreign_stubs
  (language c)
  (names mammut_stubs)
  (include_dirs ../../vendor/mammut/include))
 (foreign_archives raplcap-msr mammut usb-1.0 smartgauge)
 (c_library_flags -lstdc++)
)

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

(rule
 (targets libraplcap-msr.a dllraplcap-msr%{ext_dll})
 (deps (source_tree ../../vendor))
 (action (progn
  (chdir ../../vendor/raplcap (progn
   (bash
    "cmake -B_build/shared -DBUILD_SHARED_LIBS=ON .")
   (bash
    "cmake --build _build/shared")))
   (bash
    "cp ../../vendor/raplcap/_build/shared/msr/libraplcap-msr.so dllraplcap-msr.so")
  (chdir ../../vendor/raplcap (progn
   (bash
    "cmake -B_build/static .")
   (bash
    "cmake --build _build/static")))
   (bash
    "cp ../../vendor/raplcap/_build/static/msr/libraplcap-msr.a .")
)))

; The vendored libmammut.
(data_only_dirs vendor)

(rule
 (targets libmammut.a dllmammut%{ext_dll} libusb-1.0.a  dllusb-1.0%{ext_dll} libsmartgauge.a dllsmartgauge%{ext_dll})
 (deps (source_tree ../../vendor))
 (action (progn
  (chdir ../../vendor/mammut (progn
   (bash
    "cmake .")
   (bash
    "make")))
  (bash
   "cp ../../vendor/mammut/src/libmammut_static.a libmammut.a")
  (bash
   "cp ../../vendor/mammut/src/libmammut.so dllmammut.so")
  (bash
   "cp ../../vendor/mammut/src/external/libusb-1.0.9/libusb/.libs/libusb-1.0.a .")
  (bash
   "cp ../../vendor/mammut/src/external/libusb-1.0.9/libusb/.libs/libusb-1.0.so dllusb-1.0.so")
  (bash
   "cp ../../vendor/mammut/src/external/odroid-smartpower-linux/libsmartgauge.a .")
  (bash
   "cp ../../vendor/mammut/src/external/odroid-smartpower-linux/libsmartgauge.so dllsmartgauge.so")
  )))

;;;;;;;;;;;;;;;;;;


; Type bindings.
(library
 (name generate_types)
 (public_name mammut.generated_type_descriptions)
 (modules generate_types)
 (libraries ctypes.stubs))

(executable
 (name generate_types_start)
 (modules generate_types_start)
 (libraries ctypes.stubs generate_types))

(rule
 (with-stdout-to mammut_generated.types.c
  (run ./generate_types_start.exe)))

; Based partially on
;   https://github.com/avsm/ocaml-yaml/blob/master/types/stubgen/jbuild#L20
(rule
 (targets mammut_types.exe)
 (deps (:c mammut_generated.types.c))
 (action (bash "\
  %{cc} %{c} \
  -I '%{lib:ctypes:.}' \
  -I %{ocaml_where} \
  -I ../../vendor/mammut/include -o %{targets}")))

(rule
 (with-stdout-to mammut_types.ml
  (run ./mammut_types.exe)))

; Function bindings.
(library
 (name mammut_stubs_description)
 (public_name mammut.generated_function_descriptions)
 (flags (:standard -w -9-16-27))
 (wrapped false)
 (modules mammut_types mammut_stubs_description)
 (libraries ctypes.stubs generate_types))

(executable
 (name generate_c_functions)
 (modules generate_c_functions)
 (libraries ctypes.stubs mammut_stubs_description))

(executable
 (name generate_ml_functions)
 (modules generate_ml_functions)
 (libraries ctypes.stubs mammut_stubs_description))

(rule
 (with-stdout-to mammut_generated.ml
  (run ./generate_ml_functions.exe)))

(rule
 (with-stdout-to mammut_stubs.c
  (run ./generate_c_functions.exe)))

Attempts:

In the current state of ocaml-mammut, when you use it into another project, you have to specify the flags with (flags (-cclib -lraplcap-msr -cclib -lsmartgauge -cclib -lusb-1.0 -cclib -lmammut)) in the project itself. However, according to the manual and this discussion I had with @ivg, if I transform the ocaml-mammut dune in this way:

(library
 (name mammut)
 (public_name mammut)
 (flags (:standard -w -49 -linkall
                    -cclib generated
                    -cclib -lraplcap-msr
                    -cclib -lsmartgauge
                    -cclib -lusb-1.0
                    -cclib -lmammut ))
 (modules mammut)
 (libraries ctypes.stubs mammut.generated threads)
 (c_library_flags -lstdc++)
)

I’m not supposed to have to write the (flags ...) stanza in other projects any more because it should be a standalone library.

Issue:

When I use the previous dune, it compiles, I get the c++ .a libraries at the right place. However, when I reach the link point, it fails with:

ocamlopt src/mammut.cmxs (exit 2)
(cd _build/default && /home/etienne/.opam/default/bin/ocamlopt.opt -w
@1..3@5..28@30..39@43@46..47@49..57@61..62-40 -strict-sequence
-strict-formats -short-paths  -keep-locs -w -49 -linkall -cclib -lstdc++
-cclib -lraplcap-msr
-cclib -lsmartgauge -cclib -lusb-1.0 -cclib 
-lmammut -g -shared -linkall -I src -o src/mammut.cmxs src/mammut.cmxa)
/usr/bin/ld: cannot find -lraplcap-msr
/usr/bin/ld: cannot find -lsmartgauge
/usr/bin/ld: cannot find -lusb-1.0
/usr/bin/ld: cannot find -lraplcap-msr
/usr/bin/ld: cannot find -lsmartgauge
/usr/bin/ld: cannot find -lusb-1.0
collect2: error: ld returned 1 exit status
File "caml_startup", line 1:
Error: Error during linking (exit code 1)

I don’t understand why dune can’t find the .a and add them in the cmxa whereas there are in the _build repository.