What kind of external code is it that you are writing? An FMU master algorithm with some of your own solvers that you want to import generated Modelica code into?
I suggest looking into FMIL in that case (to parse the FMI XML data, etc): http://www.jmodelica.org/FMILibrary

Thank You for such a quick reply.
I don't want to write a FMU master algorithm but if it would be benefitial I will consider it. I want to improve my process of model development. Now I'm developing models in OMedit or Octave and then port it to RT C/C++ online simulation. I was wondering if it would be posible to skip the porting phase with the use of OpenModelica and generate code automatically.

Adam

It depends what kind of porting you need done (what is it that needs OpenModelica generates that you want done differently?). Some things are easy to generate.

For example I have a model in OMEdit which I integrate with Runge-Kutta method. It would be great if it was possible to generate files with function f which was integrated by RK numerical routine and a function with initialization of states. I hope that I only need to add some flag to omc.

Adam

That's FMI for co-simulation. Our support for FMI co-simulation is very limited.

Thank You for your help. I will try to simulate my model with the FMI Library.