MATLAB: Simple mex function help

cc programming ;MATLABmexmex compilermexfunction

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You have defined x and y in mexFunction to be pointers to double, not double. But your xtimes10 routine is expecting double, not pointer to double. Hence the error. Also, unrelated, you create plhs[0] twice which is unnecessary. One way to fix things is to have the xtimes10 routine work with pointers, and dereference them inside the routine. E.g.,
#include "mex.h"                // Matlab mex header file
// C function multiply arrays x and y (of size) to give z
void xtimes10(double *x, double *y)
{
    *y = *x * 10;
}
void mexFunction( int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
    double *x,*y;
      x = mxGetPr(prhs[0]);
      plhs[0] = mxCreateDoubleMatrix( 1 ,  1, mxREAL);
      y = mxGetPr(plhs[0]);
      xtimes10(x,y);
      mexPrintf("result y= %g\n", y);
}
Also you will notice that I fixed your mexPrintf call. It was named incorrectly, and you were using the wrong format for a double (%d is for integer types), and you did not have a newline.
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