linking error interfacing ocaml for windows with external c library - windows

Question

I am developing a project in ocaml that requires me to interface it with the OGDF external c++ library. It is all up and running on my mac, but now I am trying to create a windows version using Ocaml for Windows (https://fdopen.github.io/opam-repository-mingw/), the MinGW Cygwin port of Ocaml. In this version, I can interface ocaml with c code and it works fine, but as soon as I try to include an external library in that c code I get errors from the linker, which is flexdll (https://github.com/alainfrisch/flexdll) in this case. The linker says it cannot resolve symbols for _Unwind_Resume and __emutls_get_address throughout the library.
Here is a toy example:
My .ml file t.ml:
external print : unit -> unit = "print"
let () =
Printf.printf "platform: %s\n" (Sys.os_type);
print ()
My .cpp file tc.cpp:
#include <stdio.h>
#include "caml/mlvalues.h"
#define CAML_NAME_SPACE
//#include <ogdf/basic/Graph.h>
extern "C" value print(value unused) {
printf("hello from C\n");
return Val_unit;
}
My makefile:
t.exe: t.ml tc.o
ocamlopt -verbose -ccopt -pthread \
-cclib -lstdc++ -w s \
-ccopt -L../cdeg/ogdf/_release \
-cclib -lOGDF \
tc.o t.ml \
-o t.exe
tc.o: tc.cpp
x86_64-w64-mingw32-gcc -c \
-march=x86-64 -mtune=generic -O2 -mms-bitfields -Wall -Wno-unused \
tc.cpp \
-I../cdeg/ogdf -L../cdeg/ogdf/_release -lOGDF \
-I ~/.opam/4.04.0+mingw64c/lib/ocaml \
-lstdc++ -pthread -o tc.o
Like this, it all compiles happily, but if I uncomment the ogdf include line in tc.cpp, I get the following output:
$ make
x86_64-w64-mingw32-gcc -c \
-march=x86-64 -mtune=generic -O2 -mms-bitfields -Wall -Wno-unused \
tc.cpp \
-I../cdeg/ogdf -L../cdeg/ogdf/_release -lOGDF \
-I ~/.opam/4.04.0+mingw64c/lib/ocaml \
-lstdc++ -pthread -o tc.o
ocamlopt -verbose -ccopt -pthread \
-cclib -lstdc++ -w s \
-ccopt -L../cdeg/ogdf/_release \
-cclib -lOGDF \
tc.o t.ml \
-o t.exe
+ x86_64-w64-mingw32-as -o "t.o" "C:\OCaml64\tmp\camlasme5f9bd.s"
+ x86_64-w64-mingw32-as -o "C:\OCaml64\tmp\camlstartupf2b3f1.o" "C:\OCaml64\tmp\camlstartup101e51.s"
+ flexlink -chain mingw64 -stack 33554432 -exe -o "t.exe" "-LC:/OCaml64/home/Nathaniel.Miller/.opam/4.04.0+mingw64c/lib/ocaml" -pthread -L../cdeg/ogdf/_release "C:\OCaml64\tmp\camlstartupf2b3f1.o" "C:/OCaml64/home/Nathaniel.Miller/.opam/4.04.0+mingw64c/lib/ocaml\std_exit.o" "t.o" "C:/OCaml64/home/Nathaniel.Miller/.opam/4.04.0+mingw64c/lib/ocaml\stdlib.a" "-lstdc++" "-lOGDF" "tc.o" "C:/OCaml64/home/Nathaniel.Miller/.opam/4.04.0+mingw64c/lib/ocaml\libasmrun.a" -lws2_32
** Cannot resolve symbols for ../cdeg/ogdf/_release\libOGDF.a(PoolMemoryAllocator.o/
PreprocessorLayout.o/
extended_graph_alg.o/
graph_generators.o/
random_hierarchy.o/
simple_graph_alg.o/
CPlanarEdgeInserter.o/
... [a bunch of other .o files from the library]...
UpwardPlanarModule.o/
UpwardPlanarSubgraphModule.o/
UpwardPlanarSubgraphSimple.o/
VisibilityLayout.o/
):
_Unwind_Resume
__emutls_get_address
** Cannot resolve symbols for ../cdeg/ogdf/_release\libOGDF.a(basic.o):
_Unwind_Resume
File "caml_startup", line 1:
Error: Error during linking
make: *** [makefile:20: t.exe] Error 2
If I don't connect it to ocaml, but instead add a main() function to t.c it compiles just fine under x86_64-w64-mingw32-gcc with the external library included. I've tried including a few other small external libraries and they didn't cause this problem.
My first thought was that maybe the problem had to do with the linked files not all being compiled the same way, but I compiled the library and the .cpp file with the compiler and options given by ocamlopt -configure. And if they weren't all compiled the same way, I wouldn't expect to be able to get tc.cpp to work individually with ocamlopt and with the external library, but I only get errors when I try to use both. So is this an issue with Ocaml for windows, or flexdll, or with my installation of one of these? I'm at a loss for what to try next, and any ideas, suggestions, and/or explanations of what is going on here would be very much appreciated.

Answer 1

I have a partial answer. The issue is coming somehow from flexdll. I switched to using the Cygwin version of ocaml with gcc, and still had the same problem. Then I recompiled ocaml configured with the -no-shared-libs flag, which makes ocamlopt link with gcc instead of flexdll, and now everything compiles.