matlab - How to use a struct in a mex file

Posted by rolf on Thursday, 18 December 2014

It took me some time until I figured out how to pass a struct to a mex file and especially how to get out the values.

To get the values from the struct following lines are important:

 /* get the values from the struct */
  const mxArray  *mxTmp; 
  double         *tmp;
 
  mxTmp = mxGetField(prhs[1],0,"par1");
  tmp   = mxGetPr(mxTmp);

Here the full code

Code (expand / collapse)

#include "mex.h"
/* #include "math.h" */
/* #include "matrix.h" */
/* #include <stdlib.h> */

/* The gateway function */
void mexFunction( int nlhs, mxArray *plhs[],
                  int nrhs, const mxArray*prhs[])
{
  /* Check for proper number of arguments */
  if (nrhs !=3) /* TODO: 8 is not correct */
    mexErrMsgTxt("Three input arguments required.");
  else if (nlhs > 1)
    mexErrMsgTxt("Too many output arguments.");

  /* Declare variables */
  double *distMtx;   /* input: matrix containing the distance values */
  const mxArray *entryStruct;
  double *triangulared_dist; /* output: matrix with the triangular function applied to each of the distance values */

  /* Check if prhs[1] is a struct */
  mexPrintf("Is Structure or Not: %i \n",mxIsStruct(prhs[1]));

  /* print out the names of the fields */
  /* prints (null) if field does not exist */
  mexPrintf("1st field name: %s \n", mxGetFieldNameByNumber(prhs[1],0) );
  mexPrintf("2ns field name: %s \n", mxGetFieldNameByNumber(prhs[1],1) );
  mexPrintf("3rd field name: %s \n", mxGetFieldNameByNumber(prhs[1],2) );
  mexPrintf("4th field name: %s \n", mxGetFieldNameByNumber(prhs[1],3) );
  mexPrintf("5th field name: %s \n", mxGetFieldNameByNumber(prhs[1],4) );

  /* get the values from the struct */
  const mxArray  *mxTmp; 
  double         *tmp;
 
  mxTmp = mxGetField(prhs[1],0,"par1");
  tmp   = mxGetPr(mxTmp);

  mexPrintf(" \n ----- printing values of par1 ----- \n");
  int iPar1;
  for (iPar1=0; iPar1<4; iPar1++)
  {
    mexPrintf("par1[%i] = %.2f \n",iPar1,tmp[iPar1]);
  }
  return;
}

To compile the code, put the code in a file, e.g. passStruct2Mex_MEX.c and use following command within matlab

cd /path/to/file
mex passStruct2Mex_MEX.c

To test the code you can use following matlab script

% test the mex function

approxPar.par1 = [1,2,3,4];
approxPar.par2 = [10,20,30,40];
approxPar.par3 = [100,200,300,400];
approxPar.delta = 0.02;

distMtx = rand(375,375,375);
triangulared_distMtx = zeros(375,375,375);
passStruct2Mex_MEX(distMtx,approxPar,triangulared_distMtx);
keyboard

It should print out the following lines

Is Structure or Not: 1
1st field name: par1
2ns field name: par2
3rd field name: par3
4th field name: delta
5th field name: (null)

----- printing values of par1 -----
par1[0] = 1.00
par1[1] = 2.00
par1[2] = 3.00
par1[3] = 4.00

blog_external_links:

another .c mex example

Files:

passStruct2Mex_MEX.c

blog_tags:

matlab

mex

Comments

Sukruth (not verified)

Fri, 08/21/2015 - 10:00

Permalink

Code errors

Hi there are a few execution errors while running the program such as syntax errors and undeclared identifiers.Hope you could give me a elaborate explaination in executing this file

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