Linking C++ to Matlab via mex: passing arguments

Question

I need to call C++ code from Matlab. I know I have to write a mex wrapper, but I can't figure out how to pass the arguments correctly.

The C++ function to be called is:

foo(int* n1,int* n2,int* n3,double* x,double* y,int* n4) 

x is matrix on which computations are done, y is an output vector. I am having trouble passing the integer arguments which correspond to the dimensions of the matrix.

I wrote the following wrapper:

void mexFunction(
     int          nlhs,
     mxArray      *plhs[],
     int          nrhs,
     const mxArray *prhs[]
     )
{
double  *x, *y;
int *n3,*n4;
mwSize nrow,ncols;

  x = mxGetPr(prhs[0]);
  n3 = mxGetPr(prhs[1]);
  n4 = mxGetPr(prhs[2]);
  nrow = mxGetM(prhs[0]);
  ncols = mxGetN(prhs[0]);

  /* Create a matrix for the return argument */ 
  plhs[0] = mxCreateDoubleMatrix(nrow, 1, mxREAL);
  y = mxGetPr(plhs[0]);


  foo(*nrow,*ncols,*n3, x,y,*n4);
  return;
  }

However, when I try to compile, the following errors arise:

• warning C4267: '=' : conversion from 'size_t' to 'mwSize', possible loss of data (n3-n4 line)
• warning C4267: '=' : conversion from 'size_t' to 'mwSize', possible loss of data (nrow,ncol line)
• error C2100: illegal indirection (foo line)

I can't seem to figure out how to make this work. Any help would be appreciated.

Answer 1

There are some mistakes, use it like this, you need to cast it to mwSize to avoid warnings:

nrow = (mwSize)mxGetM(prhs[0]);
ncols = (mwSize)mxGetN(prhs[0]);

Anad other change is that pointers are passed using '&' and you don't need to give *n4

foo(&nrow,&ncols,n3, x,y,n4);

Answer 2

warning C4267: '=' : conversion from 'size_t' to 'mwSize', possible loss of data (n3-n4 line)
warning C4267: '=' : conversion from 'size_t' to 'mwSize', possible loss of data (nrow,ncol line)

Here your target variables are smaller than the values returned by the function. Obviously, MATLAB changed the return types of these functions to size_t so you should match your variables as well.

At least, this was my first thought. But I am wrong here: as you are passing (resp. should pass) the addresses of the variables, you should natch them to thetype needed by the function.

---

error C2100: illegal indirection (foo line)

This is clear: what do you want to achieve with *nrow and *ncols? They are no pointers, so using * is pointless.