Matlab shuts down after executing a mex call

Hi,

I'm calling a C++ function using Mex. The function call works fine. After the file is called and the result is displayed, Matlab shuts down by itself. How do I stop this from happening?

Answer 1

[James Tursa]

plhs[] is an array that MATLAB creates when you call the mex function to hold the return variables. But MATLAB doesn't create these variables for you, it simply creates a pointer array to hold the pointers to the return variables. You the programmer are expected to create these variables. When you enter the mex function, plhs[] values are not yet pointing to any variables. When you do this:

      x = mxGetPr(plhs[0]);

You are asking mxGetPr to get the real data pointer from the variable pointed to by plhs[0], but plhs[0] is an uninitialized pointer, hence the crash. You need to do this:

      plhs[0] = mxCreateDoubleMatrix(1, 1, mxREAL); // create the return variable
      x = mxGetPr(plhs[0]);

And, as Walter points out, even if you do this as you currently have the code you will get a 0 returned value since you don't assign anything to the output yet via the x pointer. E.g., you could change the last line to this:

      *x = test(a,b);

[Jan]

In addition, nrhs is read only. You can check it to see how many values are expected from the caller. But you cannot set it inside the Mex function.

[Vimal]

Thanks James.

That clears it up.

Answer 2

[Walter Roberson]

You probably have some memory corruption in the C++ function, with the effect of the corruption being observed when MATLAB goes to clear the variables that are no longer being used.

[Vimal]

I'm testing mex functionality to see if it will work for something else I need.

I'm just passing 2 variables from matlab to the C++ function which adds them up and prints them.

[Walter Roberson]

Try posting code. James is pretty good at spotting mex problems if he has code to work with.

[Vimal]

#include <iostream>

#include <stdio.h>

#include <conio.h>

#include <math.h>

#include <complex>

using namespace std;

#include "mex.h"

int test(int a, int b)

{

int s;

s=a+b;

cout<<s;

return s;

}

void mexFunction( int nlhs, mxArray *plhs[],

int nrhs, const mxArray *prhs[])

{

int a, b, s;

double *x;

nrhs=2;

a=mxGetScalar(prhs[0]);

b=mxGetScalar(prhs[1]);

x=mxGetPr(plhs[0]);

test(a,b);

}

[Vimal]

I call this by entering the following lines:

mex test.cpp;

a=1;

b=2;

sum=test(a,b)

Answer 3

[Walter Roberson]

What do you do with the value you return from test() at the C level ? You are not somehow putting it in the the output location x (plhs[0]) in your mexFunction routine .

[Vimal]

I'm not doing anything with it.

test() could be a function that returns void.

[Walter Roberson]

True that test() at your C level could return void, but at the MATLAB level you have indicated that you are expecting a return value from the call to the MEX routine, and you never set the return value to anything.