Hello,

I am trying to call an external function in C that returns an array (size of 2, double) of the derivative of the system's state (der(enthalpy_fluid) and der(enthalpy_solid)). I had some success in calling an external function in C that returns an array (without involving any "der" function), by using an implicit call, as shown below:

function foo
    input Real raw_input[:];
    input Real params;
    output Real out[outputsize];
    external "C" run_surrogate_multi_output(
            raw_input,
            params,
            out
    )
     
    annotation(IncludeDirectory="modelica://SolarTherm/Resources/Include",
              Include="#include \"tf.c\"",
              Library = "tensorflow");
  end foo;

I followed the same logic (literally using the same function), and calling my function like this in the equation:

equation

     {out[1,1], out[1,2]} = foo(
                X_in[1,1:inputdim],
                params
     );
     der(h_fluid[1]) = out[1,1];
     der(h_solid[1]) = out[1,2];

however, I got an error in compilation, and the message was:

SolarTherm.Systems.Publications.Thermocline.Constant_Charging.Time_Controlled.Part2_MgO_6h_10h_8h_NeuralNetTank_02nls.c:8100:40: warning: implicit declaration of function 'der' is invalid in C99 [-Wimplicit-function-declaration]
    tmp7 = (*real_array_element_addr1(&der(thermocline_Tank.Tank_A.h_p[89,1]), 1, ((modelica_integer) 1))) - ((*real_array_element_addr1(&thermocline_Tank.Tank_A.out[89,2,1], 1, ((modelica_integer) 1))));
                                       ^
SolarTherm.Systems.Publications.Thermocline.Constant_Charging.Time_Controlled.Part2_MgO_6h_10h_8h_NeuralNetTank_02nls.c:8100:72: warning: expression result unused [-Wunused-value]
    tmp7 = (*real_array_element_addr1(&der(thermocline_Tank.Tank_A.h_p[89,1]), 1, ((modelica_integer) 1))) - ((*real_array_element_addr1(&thermocline_Tank.Tank_A.out[89,2,1], 1, ((modelica_integer) 1))));
                                                                       ^~
SolarTherm.Systems.Publications.Thermocline.Constant_Charging.Time_Controlled.Part2_MgO_6h_10h_8h_NeuralNetTank_02nls.c:8100:44: error: use of undeclared identifier 'thermocline_Tank'
    tmp7 = (*real_array_element_addr1(&der(thermocline_Tank.Tank_A.h_p[89,1]), 1, ((modelica_integer) 1))) - ((*real_array_element_addr1(&thermocline_Tank.Tank_A.out[89,2,1], 1, ((modelica_integer) 1))));
fatal error: too many errors emitted, stopping now [-ferror-limit=]


I read online and I suspected that no function name "der" is declared in the code, but how can this happen since "der" function is "native" to Modelica? Does anybody have any idea of how to tackle this problem?

Thanks a lot and have a great day!

Cheers,
Philipe