I do have graph typedef as a global.
The original problem was, I am not allowed to define
graph_t myGraph;
without all the arguments like this:

graph_t myGraph(array, edge_array + num_arcs, weights, num_nodes);

Since I don't know the arguments before fun_A, I need something like a
"resize" operator, so that i can define graph_t outside the function, but
declare it later inside the function.