Using shared C++ library in Idris

Question

I want to FFI to a third-party C++ library from Idris but I'm getting "undefined symbol". I'm new to C/C++ compilation.

I'm doing this by wrapping the C++ in a pure C layer which I call from Idris. The C++ code is provided as a bunch of .h headers and a single .so shared library. At the moment I only have one C file, but I may have more later.

Here's the C++ library header include/foo.h

#include <some/other/library/header.h>

namespace mynamespace {
  class Foo {
    public:
      Foo();
  }
}

and the C wrapper wrapper.cpp

#include <foo.h>

extern "C" {
    struct cFoo;

    using namespace mynamespace;

    struct cFoo* cFoo_new() {
        return reinterpret_cast<cFoo*>(new Foo());
    }
}

The shared library is lib/libfoo_ext.so and I'm compiling these with

g++ -shared -Iinclude -Llib -lfoo_ext -o libfoo.so wrapper.cpp

That runs without errors. Meanwhile, the Idris code is

module Foo

import System.FFI

export
Foo : Type
Foo = Struct "cFoo" []

%foreign "C:cFoo_new,libfoo"
export
mkFoo : Foo

and I'm calling mkFoo in a test file. When I do this I get

Exception: (while loading libfoo.so) .../build/exec/_tmpchez_app/libfoo.so: undefined symbol: _ZN17mynamespace6FooC1Ev

Answer 1

This command:

g++ -shared -Iinclude -Llib -lfoo_ext -o libfoo.so wrapper.cpp

is incorrect. Assuming libfoo_ext is the 3rd party library which implements mynamespace::Foo::Foo() , the link command should be:

g++ -shared -Iinclude -Llib -o libfoo.so wrapper.cpp -lfoo_ext 

The order of libraries and sources on the link line matters , yours is wrong.