Octave crashes when running simple .oct file, what is wrong in my code?

Question

I discovered the issue while writing a longer and more complex program (FEM Solver), but to troubleshoot it I started to remove code until I had this simple example that crashes in the same way.

The following .oct file crashes in about 1 in 4 times, but it is random and sometimes it can run for 50 times without crashing. When it crashes it closes Octave without warnings, hence I have not been able to debug it. (The larger code I have with many more variables crashes about 80 % of the time).

I have tried this in both Octave 5.1.0 and 5.1.1 (unofficial) for Windows 10 x64, and on at least one other computer. It also doesn't matter if i run Octave in CLI or GUI mode.

I'm hoping I have made some simple mistake... but I can't seem to find it.

.m script to check when it crashes:

for i = 1:50
  a = test7(5)
end 

test7.cc file that is compiled to .oct:

#include <octave/oct.h>

DEFUN_DLD (test7, args, nargout,
           "Test code that crashes...")
{
  if (args.length () != 1) {
    print_usage ();    
    return octave_value();
  }

  const int N = args(0).int_value();        
  const int savesTot = 500000;
  int iter = 0; 

//Declaration method 1 (also crashes)
  //OCTAVE_LOCAL_BUFFER (double, Q, N); 
//Declaration method 2 (also crashes)
  //double *Q = new double(N);
//Declaration method 3 (crashes)
  NDArray Q_o (dim_vector (N,1));
  double *Q = Q_o.fortran_vec();

  //Q_o(0) = 1.1;   // Calling NDArray class (also crashes)
  Q[0] = 1.1;       // Calling pointer to raw data directly (faster)

  while (iter < savesTot) {

    for (int i = 0; i<N; i++) {    
      if (i == 0) {        
        //Q_o(i+1)      = 1.0;  
          Q[i+1]        = 1.0;        
      } else if ( i == (N-1) ) { 
        //Q_o(i+1)      = 2.0;
          Q[i+1]        = 2.0;  
      } else {        
        //Q_o(i+1)      = 3.0;  
          Q[i+1]        = 3.0;         
      }
    }  

    iter++;
    OCTAVE_QUIT;        // Check if user has pressed Ctrl+C
  }

  octave_value_list retval;    // Define return list (can contain many variables)
  retval(0) = iter; 

  if (! error_state) { return retval; }

  return octave_value_list ();
}

test7.cc is compiled to test7.oct inside Octave 5.1.1 or 5.1.0 on Windows 10 x64 with the following command:

mkoctfile test7.cc

Answer 1

Your problem is that you write out of bounds:

    for (int i = 0; i<N; i++) {    
       //...
       Q[i+1]        = 3.0;
       //...
    }

You might want to loop i=1; i<N; i++ and index Q[i], or loop i=0; i<N-1; i++.