Cannot link an object file

[pdonadeo]

It's probably something I don't get right, but I can't compile a very simple program in which I try to link together a piece of OCaml and a piece of C code. The code is here:

https://gist.github.com/pdonadeo/bbc4a1ea176110ab93dd/

The program compiles but ctypes seems to ignore the linked symbols... or the command line for ocamlopt is wrong.

$ ./foo_ml_linked_a
Fatal error: exception Dl.DL_error("./foo_ml_linked_a: undefined symbol: my_foo_function")

[dbuenzli]

It seems you are trying to statically link things in. I don't know on which platform but this may help:

#41 (comment)

[pdonadeo]

@dbuenzli yes, I have a bunch of object files compiled outside my project, so I'm trying to link them and to make functions available to OCaml.

[yallop]

As @dbuenzli suggests, this is often a matter of finding the right linker flags. The first problem is that the symbols aren't being included in the executable at all:

$ nm foo_ml_linked_a  | grep foo
$ 

On my platform I can force the symbols to be included by passing --whole-archive to the linker. In practice this means bracketing the archive of interest on the command-line as follows:

-cclib -Wl,--whole-archive libfoo_c.a -cclib -Wl,--no-whole-archive

After this change the symbol of interest is included in the executable:

$ nm foo_ml_linked_a  | grep foo
000000000046006f T my_foo_function
$

Unfortunately the program still doesn't work:

$ ./foo_ml_linked_a
Fatal error: exception Dl.DL_error("./foo_ml_linked_a: undefined symbol: my_foo_function")

The problem is that the symbol isn't included in the dynamic symbol table, which is where Foreign.foreign (which uses dlsym) looks for it. The -D flag to nm prints the dynamic symbol table:

$ nm -D foo_ml_linked_a  | grep foo
$ 

On my platform passing -E (or --export-dynamic) to the linker adds the symbols to the dynamic symbol table. Here's the modified command-line fragment:

-cclib -Wl,--whole-archive libfoo_c.a -cclib -Wl,--no-whole-archive -cclib -Wl,-E

After this change the symbol of interest is found in the dynamic symbol table of the executable:

$ nm -D foo_ml_linked_a  | grep foo
00000000004925ef T my_foo_function

Even better, the program actually works:

$ ./foo_ml_linked_a
This is printed by OCaml
This is printed by C

[yallop]

@pdonadeo, did the suggestions above solve your problem?

[pdonadeo]

@yallop sorry for the delay, I just came back from holidays. Yes, your suggestion works, I'm closing this "issue" which is more a documentation issue than a problem with ctypes itself.