mex compile error caused by different position of mexPrintf()

I have got this code from website, saved it as hello.c and set up mex using MSVC 2010 compiler. It gives me compile errors. Such as illegal use of mxArray and undefined variables error.

#include <matrix.h>
#include <math.h>
#include <mex.h>
void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]) {    
    mexPrintf("hello, world!\n");
    //declare variables
    mxArray *a_in_m;
    mxArray *b_in_m;
    mxArray *c_out_m;
    mxArray *d_out_m;
    const mwSize *dims;
    double *a, *b, *c, *d;
    int dimx, dimy, numdims;    

      //associate inputs
      a_in_m = mxDuplicateArray(prhs[0]);
      b_in_m = mxDuplicateArray(prhs[1]);

      //figure out dimensions
      dims = mxGetDimensions(prhs[0]);
      numdims = mxGetNumberOfDimensions(prhs[0]);
      dimy = (int)dims[0]; dimx = (int)dims[1];

      //associate outputs
      c_out_m = plhs[0] = mxCreateDoubleMatrix(dimy, dimx, mxREAL);
      d_out_m = plhs[1] = mxCreateDoubleMatrix(dimy, dimx, mxREAL);
  }

However, when I put the mexPrintf sentence in after the declaration part. The errors are gone.

#include <matrix.h>
#include <math.h>
#include <mex.h>

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]) {
    //declare variables
    mxArray *a_in_m;
    mxArray *b_in_m;
    mxArray *c_out_m;
    mxArray *d_out_m;
    const mwSize *dims;
    double *a, *b, *c, *d;
    int dimx, dimy, numdims;

      mexPrintf("hello, world!\n");

      //associate inputs
      a_in_m = mxDuplicateArray(prhs[0]);
      b_in_m = mxDuplicateArray(prhs[1]);

      //figure out dimensions
      dims = mxGetDimensions(prhs[0]);
      numdims = mxGetNumberOfDimensions(prhs[0]);
      dimy = (int)dims[0]; dimx = (int)dims[1];

      //associate outputs
      c_out_m = plhs[0] = mxCreateDoubleMatrix(dimy, dimx, mxREAL);
      d_out_m = plhs[1] = mxCreateDoubleMatrix(dimy, dimx, mxREAL);
  }

Could anyone explain why this happens? Thanks!

Answer 1

[Ryan Livingston]

In C (C89 and older I believe) variables must be declared at the beginning of a scope before any other statements are made in the code. So having that function call before declaring variables makes the compiler unhappy.

You could rename the file to .cpp and that should pacify it or just leave the declarations first.