Task: I need to write an external library in C. A list with the name of the array comes to the object's inlet. How to get a pointer to this array inside an external library in C language?

see d_array.c in the sources as an example: seems like the way to do this is use pd_findbyclass(t_symbol, garray_class)
here's a section from tabread:

if (!(a = (t_garray *)pd_findbyclass(x->x_arrayname, garray_class)))
        pd_error(x, "%s: no such array", x->x_arrayname->s_name);
    else if (!garray_getfloatwords(a, &npoints, &vec))
        pd_error(x, "%s: bad template for tabread", x->x_arrayname->s_name);
    else
    {
        int n = f;
        if (n < 0) n = 0;
        else if (n >= npoints) n = npoints - 1;
        outlet_float(x->x_obj.ob_outlet, (npoints ? vec[n].w_float : 0));
    }