Passing WORD data type from C file to mex function:

Question

Alexei Halpin 2013-2-28

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Hello, I have a quick question regarding converting between data types in a mex file. I am interfacing to some hardware which calls a WORD pointer in its measuring function, but I'm having trouble passing this quantity back to Matlab as the output appears as a bunch of zeros. Below is a truncated version of the code to highlight the problem I'm having, I've removed the overhead because the code compiles and I'm fairly certain my problems are coming from the mexfunction part:

     #include <windows.h>
  #include "DCamUSB.h"
  #include "DCamStatusCode.h"
  #include "mex.h"
extern void _main();
int mainsubr(long *RetVal, double *data)
{
  WORD*  pDataBuff   = NULL;
  
  pDataBuff = new WORD[ nImageSize * nFrameCount];
  // Acquisition execute, camera acquires data
  
        *data = pDataBuff;
            
      for( nCountH = 0; nCountH < nHeight; nCountH++ )
                  {
        for( nCountW = 0; nCountW < nWidth; nCountW++ )
            {
            printf("%d\n", *data);
            data ++;*              
            }
          }
              
        break;
        }
  
  
  // Process for exit. 
  DcamStop(); // Stop acquisition
  delete [] pDataBuff; // Release acquisition buffer
  // status
  return 0;
}
// ---------------------------- MEX PART --------------------------------
void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
  WORD *Buffer;        // measured spectra
  long mrows2 = 2068, mcols2 = 1;
  plhs[0] = mxCreateDoubleMatrix(mrows2, mcols2, mxREAL);
  Buffer = (WORD *)mxGetData(plhs[1]);
  // call main subroutine
  mainsubr(Status, Buffer);
  return;
}
  end

The issue I suppose is the way I am calling mxGetData, but I'm not certain why it is only returning zeros. The little printf command I have in the for loop in the main subroutine is displaying the values I want in the Matlab shell, but someone I am not able to then pass these values to the array in Matlab. Any tips? I'm assuming this has something to do with recasting the WORD * into a double * somehow but it seems (according to my compiler) that this is not possible, and therefore the conversion has to take place in the mex function I suppose. Thanks for any tips you can provide.

Alexei

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Answer 1

James Tursa 2013-3-1

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I don't see any code where you put the results of your acquisition into data. All I see is you stuffing the pointer into the first location, which btw seems vary suspect since you are converting a pointer to a double. Also, simply putting a (WORD *) in front of the mxGetData call does not change the fact that the underlying storage is doubles. Your mainsubr has the 2nd argument as a (double *) anyway, so what is the point of this? I suspect you want something like this instead (assumes WORD is a 2-byte unsigned data type):

plhs[0] = mxCreateNumericMatrix(mrows2, mcols2, mxUINT16_CLASS, mxREAL);
  :
int mainsubr(long *RetVal, WORD *data)
  :
*data = pDataBuff; // delete this line
  :  
*data = pDataBuff[appropriate index]; // add this inside inner loop

But why are you allocating pDataBuff in the first place? Why can't you just have your data acquision put the results into data directly instead of doing a copy?

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Alexei Halpin 2013-3-1

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Hi James,

Thanks a lot for your suggestion. It immediately allowed us to recover the values of the data into a Matlab array. Regarding the allocation of the pDataBuffer, it was something that was present in the manufacturer's sample code that we have been working from, I guess to account for the fact that the nFrameCount quantity can vary given input parameters from the user. But I'm unsure. One puzzling aspect now is how to read out the pDataBuffer pointer correctly, although this is more of a C++ application specific problem, read my problem :) , but maybe you have some insight on what's immediately going wrong. I don't believe this is on the mex side but perhaps I'm mistaken.

if true
// Data process
for(nCountF = 0; nCountF < nFrameCount; nCountF++)
  {
  for( nCountW = 0; nCountW < nWidth; nCountW++ )
    {
    (*data) = pDataBuff[nCountW];
    data++;
    }
  }      
end

Unfortunately for each increment of nCountF returns the same array as data as the first one. The pDataBuffer array is initialized to [nWidth*nFramecount], so I figure that within the loop the data pointer should keep reading out the pDataBuffer until the last element.

Thanks!

James Tursa 2013-3-1

编辑：James Tursa 2013-3-1

在 MATLAB Online 中打开

You need to run pDataBuff through all the values. E.g., instead of your double loop you could just use a single loop like this:

for( i=0; i<nFrameCount*nWidth; i++ ) {
    data[i] = pDataBuff[i];
}

Alexei Halpin 2013-3-1

Thanks James, turns out we tried that prior, and the reason why it didn't work was that we neglected to initialize a hardware parameter..ha ha. sorry to have troubled you with the follow-up question, but really appreciate that you took the time!

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