From: Michael Edgar <adgar@...>
Date: 2011-07-09T14:16:12+09:00
Subject: Re: FFI with a C++ Library

On Jul 9, 2011, at 12:40 AM, Mike Bethany wrote:
> This may be more of a C++ question but here goes:
> 
> I'm trying to use Ruby FFI with a CPP library but when I compile my CPP
> library and look at the method names there's always a __Z5, __Z4, or __Z3 in
> front of the method name. I see this when I run nm against the dynlib file.
> 
> Oddly when I compile a C file it works fine but I can use a C file because
> I'm using some CPP libraries in my library.

This is due to what's called "name mangling" - it's how C++
differentiates different functions with the same name based on
namespace, overloads, etc. You can learn more on
Wikipedia [0].

To resolve this, you need to specify the functions being
called with C linkage. Wrap the necessary function as
follows in your .cpp/.cc file:

extern "C" {
  void foo(...) {
  }
}

This will give foo the external name "foo" instead of how
it would be defined with C++ linkage.

If you have C headers and see them being used from
C++ and wish to keep the unmangled names, you can
add this to the top of your C header:

 #ifdef __cplusplus
 extern "C" {
 #endif

and this at the bottom:

 #ifdef __cplusplus
 }
 #endif


[0] http://en.wikipedia.org/wiki/Name_mangling

Michael Edgar
adgar@carboni.ca
http://carboni.ca/