Matrix allocation

Question

I am trying to write a function that should have as result a matrix. The function is showed below:

double **jacobian(double *Xn, double *Clvi, double *c, double *Vinf,
                  int *Np, double *mu, double *dx) {
    /*Calculating the Fn values*/

    int position = *Np / 2;
    int k;
    double *y;
    double **J;

    double *Gama = malloc(position * sizeof(double)); 
    double *Dalpha = malloc(position * sizeof(double));

    memcpy(Gama, Xn, position * sizeof(Gama));
    memcpy(Dalpha, Xn + position, position * sizeof(Gama)); 

    for (k = 1; k <= (position - 2); k++) {
        y[k] = Dalpha[k] - Clvi[k] / (2 * pi) + Gama[k] / (pi * *Vinf * *c) -
               *mu * (Dalpha[k - 1] - 2 * Dalpha[k] + Dalpha[k + 1]); // n values listed as F1, F2,...Fn  - rows
    }

    int i;
    int j;

    //Gamma values
    for (i = 1; i <= (position - 2); i++) {
        for (j = 1; j < position - 2; j++) {
            if (i == j) {
                J[i][j] = ((Dalpha[j] - Clvi[j] / (2 * pi) + 
                            (Gama[j] + *dx) / (pi * *Vinf * *c) -
                             *mu * (Dalpha[j - 1] - 2 * Dalpha[j] + Dalpha[j + 1])) - y[i]) / (*dx);
            } else {
                J[i][j] = 0;
            }
        }
    }
    for (j = position; j < *Np; j++) {  
        if (abs(i - j) > 3) {
            J[i][j] = 0;
        } else {
            J[i][j] = (((Dalpha[j] + *dx) -
                        Clvi[j] / (2 * pi) + Gama[j] / (pi * *Vinf * *c) - 
                        *mu * (Dalpha[j - 1] - 2 * (Dalpha[j] + *dx) + 
                               Dalpha[j + 1])) - y[i]) / (*dx);

        }
    }
}

return J
};

However, every time I try to run it, I receive the Segmentation error message. I guess it is because I am allocating the matrices elements wrong. Can someone help me?

Running script.

double main() {
    double Xn[12] = {1.0,2.0,3.0,4.0,5.0,6.0,0.1,0.2,0.3,0.4,0.5,0.6};
    double Clvi[6] = {0.2,0.3,0.35,0.4,0.5,0.6};
    double alpha[6] = {1.0,2.0,3.0,4.0,5.0,6.0};
    double c = 1.0;
    double Vinf = 300.0;
    double mu = 0.2;
    int Np1 = 12;
    double dx = 0.1;

    **jacobian(Xn, Clvi, &c, &Vinf, &Np1, &mu, &dx);
}

Answer 1

The code does not compile, but it would have undefined behavior anyway, since neither J nor y are ever allocated, storing values to y[k] or J[i][j] are major problems.