Using C library, function with function pointer as a parameter.

I want to make use of a C library in MATLAB but one of the main functions that the library requires has as a parameter a pointer function.

The library that I am using is proprietary so I can not share too much info, I'll try to explain it with a simpler library.

The reason I need the function pointer is because the library does not know how to find the square root of a number, so I need to make a function in MATLAB for finding the square root of a number, then pass a pointer to that function to the library using a function. Since other functions in the library need to know how to do the square root of a number it needs this function to be able to use the rest of the library.

Sorry about the wierd example. I hope this is clear enough. And I just wanted to know if this was possible.

Answer 1

[James Tursa]

I'm assuming your real function is not actually sqrt(), else you would just call a C math library function for this. But my example will use sqrt() per your request.

You will probably need to write a C-function wrapper for your MATLAB function, then pass a pointer to that function. E.g., something like this:

/* mex_example.c
* Bare bones MATLAB function wrapper example.
* Would need extra argument and return checking for production code.
*/

#include "mex.h"

/* This would be the signature from the header for your library */
/* Takes a function pointer as input and returns a double */
double library_function( double (*mfunction)(double) );

/* This is the C-wrapper function that your library function will call */
double myfunction(double x)
{
    double y;
    mxArray *rhs[1];
    mxArray *lhs[1];
    rhs[0] = mxCreateDoubleScalar(x);
    if( mexCallMATLAB( 1, lhs, 1, rhs, "matlab_sqrt") ) /* callback to MATLAB */
    {
        mexErrMsgTxt("The MATLAB function callback failed!");
    }
    y = mxGetScalar(lhs[0]);
    mxDestroyArray(rhs[0]);
    mxDestroyArray(lhs[0]);
    return y;
}
    
    /* The MATLAB gateway function */
    void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
    {
        double y;
        y = library_function( myfunction );
        plhs[0] = mxCreateDoubleScalar( y );
    }
    
    /* This is the function in your library that takes a function pointer */
    double library_function( double (*mfunction)(double) )
    {
        double x = 2.0; /* Example data created here for simplicity */
        double y = mfunction( x );
        return y;
    }

The MATLAB matlab_sqrt.m file used for the callback is simply this:

    function y = matlab_sqrt(x)
    y = sqrt(x);
    end

Compiling and running this:

>> mex mex_example.c

Building with 'MinGW64 Compiler (C)'.

MEX completed successfully.

>> mex_example

ans =

1.4142

[Axel]

Thank you!