Write matrix with Matlab and Read with C?

I have a matrix (100 rows and 5 columns) that want to write it as binary and read with C code in the way that I can read it in C row-by-row (5 numbers a time).

How can this be coded with Matlab and C?

Thanks.

Answer 1

[James Tursa]

Use fopen, fwrite, fread.

EXAMPLE:

m-code: xbinary_create.m

x = 1:5;
fid = fopen('xbinary.bin','w');
fwrite(fid, x, 'double');
fclose(fid);

C-mex code: xbinary_read.c

#include "mex.h"
#include <stdio.h>
void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
  FILE *fp;
  char *fname;
  double d;
  int i, n;

    if( nrhs == 2 && mxIsChar(prhs[0]) && mxIsNumeric(prhs[1]) ) {
        fname = mxArrayToString(prhs[0]);
        fp = fopen(fname,"r");
        mxFree(fname);
        if( fp != NULL ) {
            n = mxGetScalar(prhs[1]);
            for( i=0; i<n; i++ ) {
                fread( &d, sizeof(d), 1, fp );
                mexPrintf("%g\n",d);
            }
            fclose (fp);
        } else {
            mexErrMsgTxt("Unable to open file");
        }
    }
  }

Sample run:

>> mex xbinary_read.c
>> xbinary_create
>> xbinary_read('xbinary.bin',5)
1
2
3
4
5

CAVEAT:

If you are writing and reading a 2D matrix, then be advised that MATLAB stores such data column-wise, whereas C stores such data row-wise. That is, if you have a 2D C array like "double x[3][4]", then it will be stored in memory by rows. Whereas if a variable is size 3x4 in MATLAB, it will be stored in memory by columns.

Bottom line is if you want to read the data in C by rows, you should write it out that way from MATLAB. Fortunately this is easy to do. Simply transpose the matrix first, and then write that transposed matrix out to the file via the above code.

[John]

Can you please have an example in both matlab writting and c readding? Thanks, James!