Passing pointers to arrays to functions from within another function

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)
    {
      *graph_array = malloc(sizeof **graph_array * vertices);
      if (*graph_array)
      {
        int i;
        for (i = 0; i < vertices; i++)
        {
          (*graph_array)[i] = NULL; // parentheses matter here!
        }
      }
      else
      {
        return -1;
      }

      return 0;
    }

But here's the problem: I don't call GraphIntialize directly from main(). Instead, I first 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 retrieves 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 (triple pointer)' but argument is of type 'struct graph 4ASTERISKS (quadruple pointer)'. This is so confusing. If there is a way I can work this out without needing all these pointers that might be the best answer, but I am trying to create and abstract data type and so I don't want to be creating a graph_array array in my main function.

FreeNickname · Accepted Answer · 2013-12-02 16:43:30Z

I suppose, you don't have to use '&' here:

if ((GraphInitialize(&graph_array, *verts)) == -1)

You want to initialize a double pointer (graph**), but to do that you pass a pointer to it into your functions, so both of them get a triple pointer (graph ***) as an input. The chain of calls looks something like this (this is more of a pseudocode):

void GraphInitialize(struct graph *** graph_array);

void getdata(..., struct graph *** graph_array )
{
    ...
    GraphInitialize(graph_array); //graph_array here is the same triple pointer, that 'getdata' recieved as an input, so there is no need to use '&' operator.
    ...
}

void main() 
{
    graph ** graph_array = ...; // this is a double pointer, obviously
    getdata( ..., &graph_array); //getdata gets a triple pointer as an input, so we get the graph_array address by '&' operator;
}

So the correct form would be

if ((GraphInitialize(graph_array, *verts)) == -1)

Charan Pai · Accepted Answer · 2012-11-28 04:55:21Z

1

use

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

this works i hope..

answered Nov 28, 2012 at 4:55

Charan Pai

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1 Comment

Jonathan Leffler Over a year ago

This explanation is a bit spartan, but the answer is correct. You pass the pointer the graph array in the main function to get_data(), and you can pass that pointer verbatim (without an additional &) to GraphInitialize().

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