Question

I am storing my information in an array of pointers to structs. In other words, each element of the array is a pointer to a linked list.

I don’t know how long the array should be, so instead of initializing the array in my main() function, I instead intialize the double pointer

struct graph** graph_array;

Then once I obtain the length of the array, I try to initialize each element of graph_array using a function GraphInitialize:

int GraphInitialize(struct graph* *graph_array,int vertices)
{ 
  struct graph* graph_array2[vertices+1];
  graph_array = graph_array2;
  int i;

  for (i=0;i<vertices+1;i++)
  {
    graph_array[i] = NULL;
  }

  return 0;
}

But for some reason this is not returning the updated graph_array to main(). Basically, this function is updating graph_array locally, and no change is being made. As a result, any time I try to access an element of graph_array it seg faults because it is not initialized. What am I doing wrong?

Edit: Following the convo with Tom Ahh I should add something else that makes this more confusing.

I don’t call GraphIntialize directly from main(). Instead, I call getdata() from main, and pass a pointer to graph_array to getdata as shown below.

    getdata(argc, argv, vertpt, edgept, &graph_array)

    int getdata(int argc, char *argv[], int *verts, int *edges, struct graph* **graph_array)

Then getdata gets the number of vertices from my input file, and uses that to call GraphInitialize:

    if ((GraphInitialize(&graph_array, *verts)) == -1)
    {
      printf("GraphCreate failed");
      return 0;
    }

This results in an error: “expected ‘struct graph 3ASTERISKS’ but argument is of type ‘struct graph 4ASTERISKS’.

Answer 1

When you assign something to graph_array, you simply assign it to its local copy. The changes made to it in the function will not be see-able by the caller. You need to pass it by pointer value to be able to change its value. Change your function prototype to int GraphInitialize(struct graph ***graph_array,int vertices) and when you call it, use GraphInitialize(&graph_array, 42).

Second problem in your code is when you create graph_array2, you declare it to be local to your GraphInitialize() function. Thus, when exiting your function, graph_array2 is destroyed, even if you assigned it to *graph_array. (the star dereferences the pointer to assign it to the value it points to).

change your assignation to *graph_array = malloc(sizeof(*graph_array) * vertices); and you should be fine.