C++ passing an array pointer as a function argument

You’re over-complicating it – it just needs to be:

void generateArray(int *a, int si)
{
    for (int j = 0; j < si; j++)
        a[j] = rand() % 9;
}

int main()
{
    const int size=5;
    int a[size];

    generateArray(a, size);

    return 0;
}

When you pass an array as a parameter to a function it decays to a pointer to the first element of the array. So there is normally never a need to pass a pointer to an array.