casting void** to 2D array of int - C

c, casting, multidimensional-array, void-pointers

Solution

The compiler is right, an array of arrays (or a pointer to an array) is not the same as a pointer to a pointer. Just think about how they would be laid out in memory:

A matrix of size MxN in the form of an array of arrays:

+--------------+--------------+-----+----------------+--------------+-----+------------------+
| matrix[0][0] | matrix[0][1] | ... | matrix[0][N-1] | matrix[1][0] | ... | matrix[M-1][N-1] |
+--------------+--------------+-----+----------------+--------------+-----+------------------+

A and the same "matrix" in the form of pointer to pointer:

+--------+     +-----------+-----------+-----------+-----+
| matrix | --> | matrix[0] | matrix[1] | matrix[2] | ... |
+--------+     +-----------+-----------+-----------+-----+
                 |           |           |
                 |           |           V
                 |           |           +--------------+--------------+-----+
                 |           |           | matrix[2][0] | matrix[2][1] | ... |
                 |           |           +--------------+--------------+-----+
                 |           |
                 |           V
                 |           +--------------+--------------+-----+
                 |           | matrix[1][0] | matrix[1][1] | ... |
                 |           +--------------+--------------+-----+
                 |
                 V
                 +--------------+--------------+-----+
                 | matrix[0][0] | matrix[0][1] | ... |
                 +--------------+--------------+-----+

It doesn't matter if you allocate the correct size, the two variables simply are incompatible which is what your compiler is telling you.

Problem

I am trying to cast a `void**` pointer to an `int**` 2D array in C Here is the code that I am trying to work with (with all the extraneous bits removed): ``` /*assume that I have a data structure called graph with some *element "void** graph" in it and some element "int order" */ void initialise_graph_data(graph_t *graph) { void **graph_data = NULL; int (*matrix)[graph->order]; size_t size = (graph->order * graph->order) * sizeof(int); graph_data = safe_malloc(size); /*safe malloc works fine*/ matrix = (int(*)[graph->order])graph_data; graph->graph = graph_data; } ``` When I compile that, it works fine, but gives me a warning that variable `matrix` is set but not used. I don't really want to have to use the interim matrix variable because the function is just supposed to initialise the array, not put anything in it; but if I try to cast `graph_data` directly to an `int**` when I am assiging it to `graph->graph` like so: ``` graph->graph = (int(*)[graph->order])graph_data; ``` it gives me an assignment from incompatible pointer type warning. Am I just not casting it properly? Does anyone have any suggestions as to how I can make it work without the interim "matrix" variable? Or if not, what I can do with that variable so that it doesnt give me the warning that it is set but not used? Thanks

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