How to convert a C structure (returned by dll) to MATLAB structure?

Hi,

I have a dll that can read A2L files to a C structure that I would then link to manipulate in MATLAB so that I can use information from Simulink to make changes.

Is there a way for MATLAB to read the C structure as a MATLAB structure?

I have a header file that describes the structure and functions.

Thanks

[Rik]

You could have a look on this GitHub page, which contains C code to deal with Matlab structs. I don't know what the 'correct' way would be to deal with the conversion.

Answer 1

[Sameer]

Hi Jack,

I understand that you're looking to integrate a C structure from a DLL into MATLAB and interact with it as a MATLAB structure. To achieve this, you can write custom code using a MATLAB Executable (MEX) file. MEX files serve as a bridge, allowing you to execute C, C++, or Fortran code within MATLAB's environment.

Let's consider a scenario where you have a C structure that represents a point in 3D space, and you have a DLL that provides this structure. The structure in C might look like this:

// C structure for a 3D point
typedef struct 
{
    double x;
    double y;
    double z;
} Point3D;

Assuming you have a function in your DLL that returns this Point3D structure, and you want to convert it into a MATLAB structure. Here's how you can write a MEX function to do this:

Step 1: Include Necessary Headers

#include "mex.h"
// Include the header that defines Point3D and the function to get a point
#include "your_header.h"

Step 2: Define the MEX Function The entry point of a MEX file is the mexFunction. It has a specific signature that MATLAB recognizes:

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]) 
{
    // code
}

Step 3: Get the Point3D Structure from the DLL Inside mexFunction, call the DLL function that returns the Point3D structure:

// Assume getPointFromDLL is a function in your DLL that returns a Point3D
Point3D point = getPointFromDLL();

Step 4: Create MATLAB Structure Define the field names and create the MATLAB structure that corresponds to the Point3D C structure:

// Define field names
const char *fieldNames[] = {"x", "y", "z"};
int numberOfFields = 3; // We have three fields: x, y, z

// Create a 1x1 MATLAB structure with these fields
plhs[0] = mxCreateStructMatrix(1, 1, numberOfFields, fieldNames);

Step 5: Populate the MATLAB Structure Convert the C structure fields into MATLAB data and populate the MATLAB structure:

// Populate the MATLAB structure with data from the Point3D structure
mxSetField(plhs[0], 0, "x", mxCreateDoubleScalar(point.x));
mxSetField(plhs[0], 0, "y", mxCreateDoubleScalar(point.y));
mxSetField(plhs[0], 0, "z", mxCreateDoubleScalar(point.z));

Put it all together to form the complete MEX function:

#include "mex.h"
#include "your_header.h"

// Example C structure for a 3D point
typedef struct 
{
    double x;
    double y;
    double z;
} Point3D;

// Function to interface with the DLL (placeholder)
Point3D getPointFromDLL() 
{
    Point3D point;
    // Assume your DLL populates this structure
    point.x = 1.0;
    point.y = 2.0;
    point.z = 3.0;
    return point;
}

// MEX function to convert Point3D to MATLAB structure
void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]) 
{
    // Get the Point3D structure from the DLL
    Point3D point = getPointFromDLL();
    
    // Define field names
    const char *fieldNames[] = {"x", "y", "z"};
    int numberOfFields = 3;
    
    // Create a 1x1 MATLAB structure with these fields
    plhs[0] = mxCreateStructMatrix(1, 1, numberOfFields, fieldNames);
    
    // Populate the MATLAB structure with data from the Point3D structure
    mxSetField(plhs[0], 0, "x", mxCreateDoubleScalar(point.x));
    mxSetField(plhs[0], 0, "y", mxCreateDoubleScalar(point.y));
    mxSetField(plhs[0], 0, "z", mxCreateDoubleScalar(point.z));
}

Step 6: Compile the MEX File In MATLAB:

mex your_mex_file.c

Step 7: Once compiled, you can call the MEX file from MATLAB like any other function:

pointStruct = your_mex_file;
%This will return a MATLAB structure pointStruct with fields x, y, and z.

For additional information, please refer to the links below:

www.mathworks.com/help/releases/R2019b/matlab/ref/mex.html

www.mathworks.com/help/releases/R2019b/matlab/apiref/mxcreatestructmatrix.html

www.mathworks.com/help/releases/R2019b/matlab/apiref/mxsetfield.html

www.mathworks.com/help/releases/R2019b/matlab/apiref/mxcreatedoublescalar.html