*** glibc detected *** and dynamic allocation issues - C++

Question

I have two issues that I believe are related. I am trying to create a dynamic array of variables and then access different specific members of the array. The code segment that is causing problems is:

        double *F_inertia, *F_drag, *F_KN;
        i = 0;
        F_inertia = new double[i];
        F_drag = new double[i];
        F_KN = new double[i];
        t = 0; 
        x = 0;

        for (z = 0; z >= -d; z = z-8) {
          F_drag[i] = ((0.5 * rho * Cd * Diam * ux * fabs(ux)));
          F_inertia[i] = (rho * Cm * Vol * ax); 
          F_KN[i] = (F_drag[i] + F_inertia[i])/1000;
          cout << i << "\t" << F_KN[i]<< endl;
          i++;
        }
               cout << F_KN[1] << endl;

        delete[] F_inertia;
        delete[] F_drag;
        delete[] F_KN;     

The line cout << i << "\t" << F_KN[i]<< endl; is outputting the values correctly, and all values are correctly being written to a file (earlier in the code). However, when I use the line cout << F_KN[9] << endl; as a test to see if that position holds the correct value, it prints out a 0, which is not the same value as the i = 1tst position in the for loop. Note: the values for F_KN[5]-F_KN[8] do print correctly! Just not any of the lower positions! I'm very confused by this!!

Secondly, after the code compiles and runs, this comes up, after giving me the correct values (except for the first problem):

*** glibc detected *** ./monopile_deflection: free(): invalid next size (fast): 0x0000000001365260 ***
monopile_deflection: malloc.c:2451: sYSMALLOc: Assertion `(old_top == (((mbinptr) (((char *) &((av)->bins[((1) - 1) * 2])) - __builtin_offsetof (struct malloc_chunk, fd)))) && old_size == 0) || ((unsigned long) (old_size) >= (unsigned long)((((__builtin_offsetof (struct malloc_chunk, fd_nextsize))+((2 * (sizeof(size_t))) - 1)) & ~((2 * (sizeof(size_t))) - 1))) && ((old_top)->size & 0x1) && ((unsigned long)old_end & pagemask) == 0)' failed.
Aborted (core dumped)

From reading I know this has to do with incorrectly allocating an array or trying to access an array value that doesn't exist, but I can't see where the problem is?

Thank you in advance for any help or advice!

Answer 1

i = 0;
F_inertia = new double[i];
F_drag = new double[i];
F_KN = new double[i];

The amount of memory allocated by these lines does not magically change when you increment i later in the program.

So, you allocated three zero-sized arrays then started writing to its non-existent elements.
You corrupted plenty of memory there.

Either:

• determine an appropriate value for i in a separate loop that you run before your allocation, or
• use a std::vector<double> which can automatically grow for you.