Segmentation fault - Two functions don't run simultaneously

Question

I have the following code for my program

int main(void)
{
    int i,size;
    float vel_x[size],vel_y[size],vel_x0[size],vel_y0[size],rho[size],rho0[size];
    float *ux, *vy, *ux0, *vy0;
    float *r, *r0;
    struct fdparam fdparam_1;

    printf("Enter the number of grid points: \t");
    scanf("%d", &fdparam_1.N);
    printf("Enter the maximum number of iterations: \t");
    scanf("%d", &fdparam_1.MAXIT);
    printf("Enter the value for time domain: \t");
    scanf("%f", &fdparam_1.t_domain);
    printf("Enter the time step and density of the fluid: \t \t");
    scanf("%f\t%f", &fdparam_1.Dt, &fdparam_1.dens);
    printf("Enter the diffusion coefficient and viscosity: \t \t");
    scanf("%f\t%f",&fdparam_1.diff, &fdparam_1.mu);
    printf("The parameters are N=%d, MAXIT=%d, t_domain=%f, Dt=%f, diff=%e, mu=%e, dens=%f \n",fdparam_1.N, fdparam_1.MAXIT, fdparam_1.t_domain, fdparam_1.Dt, fdparam_1.diff, fdparam_1.mu, fdparam_1.dens);

    size=(fdparam_1.N+2)*(fdparam_1.N+2);
    printf("The size is %d \n",size );

    r  = (float*) calloc (size,sizeof(float));
    r0 = (float*) calloc (size,sizeof(float));
    ux = (float*) calloc (size,sizeof(float));
    vy = (float*) calloc (size,sizeof(float));
    ux0 = (float*)calloc (size,sizeof(float));
    vy0 = (float*)calloc (size,sizeof(float));

    var_init(fdparam_1.N,r0,ux0,vy0,fdparam_1.dens);

//      t=0;    
//      Solver functions
//      while (t<fdparam_1.t_domain){
      velocity_solve(fdparam_1.N,ux,vy,ux0,vy0,fdparam_1.Dt,fdparam_1.mu,fdparam_1.MAXIT);    //calculates ux and vy to be used in the density solver
    density_solve(fdparam_1.N,r,r0,ux,vy,fdparam_1.Dt,fdparam_1.diff,fdparam_1.MAXIT);      //uses ux and vy calculated from Navier Stokes in the velocity solver to calculate density r
//      t+=fdparam_1.Dt
//      }
}

    //velocity solver function

    void velocity_solve(int n, float *u, float *v, float *u0, float *v0, float dt, float m, int MAXITER)
{
    int i,j;

    add_source(n,u,u0,dt);          add_source(n,v,v0,dt);
    swap(u0,u);     swap(v0,v);
    diffusion(n,u,u0,dt,m,MAXITER);         diffusion(n,v,v0,dt,m,MAXITER);
    projection(n,u,v,u0,v0,MAXITER);
    swap(u0,u);     swap(v0,v);
    advection(n,u,u0,u0,v0,dt);     advection(n,v,v0,u0,v0,dt);
    projection(n,u,v,u0,v0,MAXITER);

    printf("Printing velocities now \n");
    for (i=0; i<=n+1; i++){
            for (j=0;j<=n+1;j++){
                    printf("%f \t %f \n",u[ix(i,j)],v[ix(i,j)]);
            }
    }
}

// density solver function

void density_solve(int n, float *x, float *x0,  float *u, float *v, float dt, float diff, int MAXITER)
{
    int i,j;

    add_source(n,x,x0,dt);
    swap(x0,x);
    diffusion(n,x,x0,dt,diff,MAXITER);
    swap(x0,x);
    advection(n,x,x0,u,v,dt);

    printf("Printing density now \n");
    for (i=0;i<=n+1;i++){
            for (j=0; j<=n+1;j++){
                    printf("%f \t",x[ix(i,j)]);
            }
    }
    printf("\n");

}

The main problem that I am facing while executing this code is that I get a segmentation fault when I try to run both the functions velocity_solve and density_solve together. The two functions are executed properly when they are run individually i.e. when I comment velocity_solve, density_solve runs fine and vice-versa.

I am pretty sure there is nothing wrong with the functions being used in the velocity_solve and density_solve routines because neither of the two routines would give individual outputs. I suspect there is something going wrong when the two functions are supposed to interact, i.e the output of velocity_solve needs to be used in density_solve.

I tried initializing the size variable but then it doesn't make a difference at all. But then I realized that I do not use the arrays that are defined so I commented them out, instead I use the pointers for which I have already allocated memory, then why does it still give a segmentation fault when I try to call both the functions together in main function?

Can someone suggest what is going wrong in the code?

Answer 1

You don't initialize size, therefore its value will be indeterminate and you will have undefined behavior when you use it to create your arrays.

Undefined behavior makes your whole program ill-formed.

Answer 2

In your code, size is a local variable of type auto storage. Unless initialized explicitly, it will have garbage or indeterminate value, making use of which like

float vel_x[size];

invokes undefined behaviour.

Related : From C11 standard, chapter 6.2.4, "Storage durations of objects",

Paragraph 5:

An object whose identifier is declared with no linkage and without the storage-class specifier static has automatic storage duration, ...

and Paragraph 6:

...The initial value of the object is indeterminate. ...

Also, from the same document, chapter §6.7.9, "Initialization", paragraph 10

If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate.

and finally, from appendix J.2, "Undefined behavior"

The value of an object with automatic storage duration is used while it is indeterminate