title: CFFI - Is Python callback for C code possible in ABI mode?
body: <p>I need a provide a callback for C code. Basically, I need this to work:</p>
  
  <p>In C:</p>
  
  <pre><code>typedef int callback_t(int i);
  callback_t *callback = NULL;
  </code></pre>
  
  <p>in Python:</p>
  
  <pre><code>from cffi import FFI
  
  def py_callback(i):
      return 0
  
  ffi_builder = FFI()
  lib = ffi_builder.dlopen('foobar.so')
  lib.callback = py_callback
  </code></pre>
  
  <p>Adding <code>@ffi_builder.def_extern()</code> to <code>py_callback</code> throws an exception: <code>ffi.def_extern() is only available on API-mode FFI objects</code>.</p>
  
  <p>So is it possible in ABI mode at all?</p>