typedef int (*callback)();

void test( callback v )
{
  int i = v();
}

require 'ffi'

module MYFFI
  extend FFI::Library
  ffi_lib './libtest.so'

  callback :callback, [:int], :int

  attach_function :test, [:callback], :void
end

def callback( v )
  return "string"
end

MYFFI.test( method( :callback ) )

[testors@macpro:~]$ gcc -c -fpic test.c
[testors@macpro:~]$ gcc -shared -o libtest.so test.o
[testors@macpro:~]$ jruby test.rb
(nothing happened)
[testors@macpro:~]$ 

[testors@macpro:~]$ ruby test.rb
test.rb:16:in `test': no implicit conversion of String into Integer (TypeError)
    from test.rb:16:in `<main>'
[testors@macpro:~]$ rbx test.rb
An exception occurred running test.rb:

can't convert String into Integer (TypeError)

Backtrace:

                 Object#__script__ at test.rb:16
  Rubinius::CodeLoader#load_script at kernel/delta/code_loader.rb:66
  Rubinius::CodeLoader.load_script at kernel/delta/code_loader.rb:152
           Rubinius::Loader#script at kernel/loader.rb:656
             Rubinius::Loader#main at kernel/loader.rb:810

[testors@macpro:~/tmp]$ ruby -v
ruby 2.2.2p95 (2015-04-13 revision 50295) [x86_64-darwin14]
[testors@macpro:~/tmp]$ rbx -v
rubinius 2.5.5.c10 (2.1.0 a0f2fa58 2015-05-17 3.3 JI) [x86_64-darwin14.1.0]
[testors@macpro:~/tmp]$ jruby -v
jruby 9.0.0.0.pre2 (2.2.2) 2015-04-28 2755ae0 Java HotSpot(TM) 64-Bit Server VM 25.5-b02 on 1.8.0_05-b13 +jit [darwin-x86_64]