c: Access violation writing location using dynamic array

Question

I am currently working on a project which uses 2D arrays. I have a function which makes a 2D array of integers, and others that find the shortest path etc, and they all work perfectly.

However, I'm running into issues with another part, where I am getting 'Access Violation errors' when attempting to initialise these arrays:

    // |V|x|V| arrays
int** fwdistance = malloc(vertexCount*vertexCount*(sizeof(int)));
int** fwnext = malloc(vertexCount*vertexCount*(sizeof(int)));

for (int i = 0; i < vertexCount; i++) {
    for (int j = 0; j < vertexCount; j++) {
        fwdistance[i][j] = INT_MAX; // Distances = infinity
        fwnext[i][j] = -1;          // Next nodes are unknown
    }
}

Error occurs when attempting to initialise fwdistance[0][0]. As I mentioned, I have other arrays initialised the same way, and they work perfectly:

// Initialises arrays for Dijkstra's Shortest Path algorithm
int *distance = malloc(vertexCount*(sizeof(int)));
int *previous = malloc(vertexCount*(sizeof(int)));

I honestly have no idea!

Answer 1

You allocate a plain array of int s, but you access it as if it were a jagged array of int pointers. This causes undefined behavior, which ultimately leads to segmentation fault.

There are two solutions to this problem:

• Make fwdistance an int* instead of int** , and access its elements either with fwdistance[i*vertexCount+j] or fwdistance[j*vertexCount+i] , depending on the order you choose, or
• Make fwdistance a pointer to array, ie int (*fwdistance)[vertexCount] = malloc ... . This approach would let you keep matrix indexing, ie fwdistance[i][j] .

Answer 2

if you wanted to create a 2D array you would have to allocate each row (or column ) separately.

 int fwdistance[][] = malloc(vertexCount*sizeof(int*));
 for(i=0;i<vertexCount;++i) {
     fwdistance[i] = malloc(vertexCount*sizeof(int));
 }
 ...
 fwdistance[i][j] = -1;

Alternatively you could emulate a 2d rectangular array by calculating the offset.

 int fwdistance[] = alloc(vertexCount*vertexCount*sizeof(int));
 ...
 fwdistance[i*vertexCount+j] = -1;