Matlab Coder - Create DLL

As an example here is a function, that I want to compiler through Matlab coder in a DLL:

function [C] = Test(A,B) %#codegen

C=A+B;

end

Compiling this function and loading the DLL with "loadlibrary" I can see the required calling convention using libfunctionsview as expected: Return Type Name Arguments double Test (double, double)

Instead, if I have a function with two outputs:

function [C,D] = Test(A,B) %#codegen

C=A+B; D=A-B;

end

The calling convention is the following:

Return Type Name Arguments [doublePtr, doublePtr] Test (double, double, doublePtr, doublePtr)

Why the two additional arguments "... doublePtr, doublePtr)"? I need to avoid this and have a calling convention like this: Return Type Name Arguments [double, double] Test (double, double)

Answer 1

[Ryan Livingston]

When arguments are passed by pointer, loadlibrary creates extra output arguments for them. The third bullet at:

http://www.mathworks.com/help/matlab/matlab_external/passing-arguments-to-shared-library-functions.html#f48607

describes this more.

MATLAB Coder will generate a C function with a signature like:

void Test(double A, double B, double *C, double *D);

where the arguments C and D hold the return values after the call. C does not support returning multiple arguments like MATLAB, so multiple return values must be accomplished by using output reference arguments.

If you are going to use the code in MATLAB, generating a MEX function would make calling the generated code simpler. The generated MEX would be invoked as any other MATLAB function.

You could also generate a MEX function that calls your generated DLL using coder.ceval. There is an example in the documentation that shows how to do this.

[Christian]

Thanks for your answer. Actually, the DLL is needed, as it should be possible to use it in various environments, not only Matlab. Hence, also output reference arguments do not help here, right?

[Ryan Livingston]

I'm not sure what you mean by "output reference arguments do not help here". Could you explain a little more?

Passing outputs via reference arguments is a typical C idiom for returning multiple values. So environments that support calling C code most likely support such arguments.

[Christian]

I changed the output to be a struct: function [result] = Test(A,B,name) %#codegen

C=A+B; D=A-B; result.C=C; result.D=D; name=name;

end

This works now. However, I need to pass to the function a character array, see above. Doing this the calling convention becomes: [int8Ptr, struct_TPtr] Test (double, double, int8Ptr, struct_TPtr)

What is the integer pointers int8Ptr? How do I pass the character array?

[Ryan Livingston]

There is a section in the documentation:

http://www.mathworks.com/help/matlab/matlab_external/working-with-pointers.html#bqgamoj-2

that discusses this in a bit more detail. The main takeaways are to convert the MATLAB string to int8 (I'm not sure how one handles non-ASCII with loadlibrary) and NULL-terminate the string.