Question

I need to pass by reference sub-arrays which are part of a dynamically allocated 2D array. I have tried the following approach, which doesn’t seem to work. Any ideas if it is possible?

void set1(int *a){
    a = malloc(2*sizeof(int));
    a[0] = 5;
    a[1] = 6;
}

void set2(int *a){
    a = malloc(2*sizeof(int));
    a[0] = 7;
    a[1] = 8;
}

int main(){
    int **x = malloc(2*sizeof(int*));

    set1(x[0]);   
    set2(x[1]);

    return 0;
}

Answer 1

You need to pass the address of your sub-arrays, like in this example:

void set(int ** a)
{
    *a = malloc(2 * sizeof(int));
    (*a)[0] = 4;
    (*a)[1] = 9;
}

int main(void)
{
    int ** x = malloc(2 * sizeof(int*));
    set(&x[0]);
    set(&x[1]);
}

Whenever you have a function that needs to modify something in the caller’s scope, you need a de­re­fe­rence in the function (like our *a) and an address-of in the caller (like &x[0]).

(Your code, by contrast, assigns the pointer to allocated memory to a local variable (namely myArray), which is lost when the function returns. And the memory is lost along with it.)