Dynamic Array in C - realloc

One long-term problem is that you are not returning the updated array pointer from the addPointerToArray() function:

void addPointerToArray(Vertex v1, Vertex out[], int *size)
{
    int newSize = *size;
    newSize++;

    out = realloc(out, newSize * sizeof(Vertex));
    out[(*size)] = v1;

    //  Update Size
    *size = newSize;
}

When you reallocate space, it can move to a new location, so the return value from realloc() need not be the same as the input pointer. This might work while there is no other memory allocation going on while you add to the array because realloc() will extend an existing allocation while there is room to do so, but it will fail horribly once you start allocating other data while reading the vertices. There are a couple of ways to fix this:

Vertex *addPointerToArray(Vertex v1, Vertex out[], int *size)
{
    int newSize = *size;
    newSize++;

    out = realloc(out, newSize * sizeof(Vertex));
    out[(*size)] = v1;

    //  Update Size
    *size = newSize;
    return out;
}

and invocation:

out = addPointerToArray(vertexToAdd, out, size);

Alternatively, you can pass in a pointer to the array:

void addPointerToArray(Vertex v1, Vertex **out, int *size)
{
    int newSize = *size;
    newSize++;

    *out = realloc(*out, newSize * sizeof(Vertex));
    (*out)[(*size)] = v1;

    //  Update Size
    *size = newSize;
}

and invocation:

out = addPointerToArray(vertexToAdd, &out, size);

Neither of these rewrites addresses the subtle memory leak. The trouble is, if you overwrite the value you pass into realloc() with the return value but realloc() fails, you lose the pointer to the (still) allocated array - leaking memory. When you use realloc(), use an idiom like:

Vertex *new_space = realloc(out, newSize * sizeof(Vertex));
if (new_space != 0)
    out = new_space;
else
    ...deal with error...but out has not been destroyed!...

Note that using realloc() to add one new item at a time leads to (can lead to) quadratic behaviour. You would be better off allocating a big chunk of memory - for example, doubling the space allocated:

int newSize = *size * 2;

If you are worried about over-allocation, at the end of the reading loop, you can use realloc() to shrink the allocated space to the exact size of the array. However, there is then a bit more book-keeping to do; you need to values: the number of vertices allocated to the array, and the number of vertices actually in use.

Finally, for now at least, note that you should really be ruthlessly consistent and use addPointerToArray() to add the first three entries to the array. I'd probably use something similar to this (untested) code:

struct VertexList
{
    size_t    num_alloc;
    size_t    num_inuse;
    Vertex   *list;
};

void initVertexList(VertexList *array)
{
    // C99: *array = (VertexList){ 0, 0, 0 };
    // Verbose C99: *array = (VertexList){ .num_inuse = 0, .num_alloc = 0, .list = 0 };
    array->num_inuse = 0;
    array->num_alloc = 0;
    array->list      = 0;
}

void addPointerToArray(Vertex v1, VertexList *array)
{
    if (array->num_inuse >= array->num_alloc)
    {
        assert(array->num_inuse == array->num_alloc);
        size_t new_size = (array->num_alloc + 2) * 2;
        Vertex *new_list = realloc(array->list, new_size * sizeof(Vertex));
        if (new_list == 0)
            ...deal with out of memory condition...
        array->num_alloc = new_size;
        array->list      = new_list;
    }
    array->list[array->num_inuse++] = v1;
}

This uses the counter-intuitive property of realloc() that it will do a malloc() if the pointer passed in is null. You can instead do a check on array->list == 0 and use malloc() then and realloc() otherwise.

You might notice that this structure simplifies the calling code too; you no longer have to deal with the separate int *size; in the main program (and its memory allocation); the size is effectively bundled into the VertexList structure as num_inuse. The main program might now start:

int main(void)
{
    VertexList array;
    initVertexList(&array);
    addPointerToArray((Vertex){ 1, 1 }, &array);  // C99 compound literal
    addPointerToArray((Vertex){ 2, 2 }, &array);
    addPointerToArray((Vertex){ 3, 3 }, &array);
    addPointerToArray((Vertex){ 9, 9 }, &array);

    for (int i = 0; i < array->num_inuse; i++)
        printf("Vertex %d: (%d, %d)\n", i, array->list[i].x, array->list[i].y, i);

    return 0;
}

(It is coincidental that this sequence will only invoke the memory allocation once because the new size (old_size + 2) * 2 allocates 4 elements to the array the first time. It is easy to exercise the reallocation by adding a new point, or by refining the formula to (old_size + 1) * 2, or ...

If you plan to recover from memory allocation failure (rather than just exiting if it happens), then you should modify addPointerToArray() to return a status (successful, not successful).

Also, the function name should probably be addPointToArray() or addVertexToArray() or even addVertexToList().