Returning an array from C to .Net: What am I doing wrong?

Question

This is a follow on from this question.

I am trying to return an array of floating point numbers from C, to .Net. I will include some F# code, as well as C# code, so that people of either language can answer.

Unmanaged C code:

extern "C"
{
    __declspec(dllexport) void DisplayHelloFromDLL(c_float* P_x, c_float* x)
    {
        //printf("Hello from DLL !\n");
        //cout << "You gave me ... an int: " << i << endl;

        // Load problem data
        //c_float P_x[4] = { 4., 1., 1., 2., }; //covariance matrix
        c_int   P_nnz = 4; //number of non-zero elements in covar
        c_int   P_i[4] = { 0, 1, 0, 1, }; //row indices?
        c_int   P_p[3] = { 0, 2, 4}; //?
        c_float q[2] = { 1., 1., }; //linear terms
        c_float A_x[4] = { 1., 1., 1., 1., }; //constraint coefficients matrix
        c_int   A_nnz = 4; //number of non zero elements in constraints matrix
        c_int   A_i[4] = { 0, 1, 0, 2, }; //row indices?
        c_int   A_p[3] = { 0, 2, 4}; //?
        c_float l[3] = { 1., 0., 0., }; //lower bounds
        c_float u[3] = { 1., 0.7, 0.7, }; //upper bounds
        c_int n = 2; //number of variables (x)
        c_int m = 3; //number of constraints


        // Problem settings
        OSQPSettings *settings = (OSQPSettings *)c_malloc(sizeof(OSQPSettings));

        // Structures
        OSQPWorkspace *work; // Workspace
        OSQPData *data;      // OSQPData

        // Populate data
        data = (OSQPData *)c_malloc(sizeof(OSQPData));
        data->n = n;
        data->m = m;
        data->P = csc_matrix(data->n, data->n, P_nnz, P_x, P_i, P_p);
        data->q = q;
        data->A = csc_matrix(data->m, data->n, A_nnz, A_x, A_i, A_p);
        data->l = l;
        data->u = u;


        // Define Solver settings as default
        osqp_set_default_settings(settings);

        // Setup workspace
        work = osqp_setup(data, settings);

        // Solve Problem
        osqp_solve(work);

        //return the value
        OSQPSolution* sol = work->solution;
        x = sol->x;

        // Clean workspace
        osqp_cleanup(work);
        c_free(data->A);
        c_free(data->P);
        c_free(data);
        c_free(settings);
    }
}

So all I have done is declared a parameter 'x', and I am setting it after the results are calculated.

F# code

open System.Runtime.InteropServices

module ExternalFunctions =
    [<DllImport("TestLibCpp.dll")>]
    extern void DisplayHelloFromDLL(float[] i, [<In>][<Out>] float[] x)

[<EntryPoint>]
let main argv = 
    let P_x = [|4.; 1.; 1.; 2.|]
    let mutable xResult:float[] = [|0.;0.|]
    ExternalFunctions.DisplayHelloFromDLL(P_x, xResult);

    printfn "This is x:%A" xResult
    0 // return an integer exit code

C# code

class Program
    {
        [DllImport("TestLibCpp.dll")]
        public static extern void DisplayHelloFromDLL(double[] i, [In, Out] double[] x);
        static void Main(string[] args)
        {
            Console.WriteLine("This is C# program");
            double[] P_x = new double[] {4.0, 1.0, 1.0, 2.0};
            double[] x = new double[] { 0.0, 0.0};
            DisplayHelloFromDLL(P_x, x);

            Console.WriteLine("Finished");
        }
    }

In both the F#, and C# cases, the value of x is unchanged. I have tried other variations such as

open System.Runtime.InteropServices

module ExternalFunctions =
    [<DllImport("TestLibCpp.dll")>]
    extern void DisplayHelloFromDLL(float[] i, [<In>][<Out>] float[]& x)

[<EntryPoint>]
let main argv = 
    let P_x = [|4.; 1.; 1.; 2.|]
    let mutable xResult:float[] = [|0.0; 0.0|]
    ExternalFunctions.DisplayHelloFromDLL(P_x, &xResult);

    printfn "This is x:%A" xResult

But I suspect the problem is the C code, not the .Net code. I think the problem is that work->solution->x is a pointer, not an array. I am guessing that I need to convert it from a pointer to an array, but can't work out how to do that. And again, not totally sure this is even the problem to begin with.

Answer 1

Thanks to PetSerAl for pointing (pun intended) me in the right direction.

All I needed to do was change part of the C code to

        //return the value
        OSQPSolution* sol = work->solution;
        for (int i = 0; i != n; ++i)
            x[i] = sol->x[i];