How to return a struct of double* in dymola from an external C code

Question

This question is similar to this one : Is it possible to return multiple values from an external file to Dymola? but somehow, it could not help me.

I have the following C structure:

typedef struct ModelicaExport
{
    double* foo;
    double* bar;
} ModelicaExport;

and the following test function in C.

void testExport(double a, double b, void * modelicaExport)
{

    ModelicaExport* output = modelicaExport;

    double* foo = output->foo;
    double* bar = output->bar; 

    foo[0] = a;
    foo[1] = b;

    bar[0] = a - b;
    bar[1] = a + b;
}

The structure and function behaves normally in C, no issues here. I can export the function as a static library using Visual Studio Build Tools 2019. No issues here either.

The problem arises when I call this function is Dymola. I recreated the "structure" in Dymola as a record ( as indicated here https://mbe.modelica.university/behavior/functions/external/ )

record recordTest
  Real foo[3];
  Real bar[3];
end recordTest;

Then I created the function is dymola to call my external C function:

function testExportC "test calculation through external C code"
  extends Modelica.Icons.Function;
    input Real a;
    input Real b;

    output recordTest cc;

    external "C"  testExport(a, b, cc)
    annotation(Include="#include \"myheader.h\"",Library="mylib");
end testExportC;

I am confident the annotation with the header and library is correct since I tried it on even simpler function and everything works well. However testExportC does not work. It compiles fine, I get a message from dymola saying compilation is successful but then it "stopped without error indication" according to Dymola. What could I be missing ? I am using Dymola 2023x with Visual Studio Build Tools 2019 also.

The page you link to indicates that an array of type `T[d1]` is mapped to a pair `T' *` and `size_t`, where `T'` is the type mapped to by `T`. Presumably the first member of the pair is a pointer to the first element of the array, and the second member is the number of elements. So a record containing `Real foo[3]; Real bar[3];` does not correspond to a structure with `double *foo; double *bar;`. It needs `double *foo; size_t fooSize; double *bar; size_t barSize;`. — Eric Postpischil, Apr 01 '23 at 22:51
@EricPostpischil I tried to modify the C struct as ```struct ModelicaExport { double* foo; size_t fooSize; double* bar; size_t BarSize; }``` as you suggested but to no avail. I get the same error. If I follow the link, shouldn't it be actually ```double **foo, size_t fooSize, etc.``` ? I tried like that also, but it did not work either. — MrBellamy, Apr 02 '23 at 20:10

score 1 · Accepted Answer · answered Apr 03 '23 at 07:00

The external C interface in Modelica is intended for returning "values", and only scalars in records can be handled - specifically arrays as in modelicaExport are not supported, https://specification.modelica.org/master/functions.html#records

This is a recent change and the old link you use hasn't been updated.

The reason for forbidding arrays is that in practice arrays are often variable-length and that would create a mess. In particular in C the following structs:

typedef struct ModelicaExport
{
    double* foo;
    double* bar;
} ModelicaExport;

struct ModelicaExport2 {
  double foo[3];
  double bar[3];
}

are completely different, and not compatible.

You will either have to re-think the original issue (it might be a X-Y problem), or write a C-wrapper for testExport.

In "12.9.1.2 Arrays", it says arrays are mapped like in my original link for "argument type mapping" which is my case since ModelicaExport is passed as an argument and NOT a return type. Am I getting this right ? — MrBellamy, Apr 03 '23 at 19:13
No, 12.9.1.2 is for array arguments, e.g., passing cc.foo directly to the C-function. — Hans Olsson, Apr 04 '23 at 05:58

How to return a struct of double* in dymola from an external C code

1 Answers1