C - Array containing Linked Lists

Question

I think this question got answered in many ways now, but I am still confused and I am doing strange things with the pointers, so I would really appreciate your help. I want to create an array (unknown size) which contains linked lists. I think the main problems are my use of pointers in this context. Please take a look:

int x_format;
int y_format;
typedef struct{
    int id;
    struct item_node *next;
    union{
        struct{
            int freq
        } Sound;
        struct{
            float reduce;
        } Obstacle;
    } data;
}item_node;
item_node *buffer;
item_node **room;
item_node **room_new;

void createRoom(int x, int y)
{
    buffer = malloc(sizeof(item_node)*x*y);
    room = malloc(sizeof(item_node *)*y);
    for(int i = 0; i<y; i++)
    room[i] = &buffer[i*x];
    for(int j = 0; j<x; ++j)
    {
        for(int k = 0; k<y; ++k)
        {
             room[j][k].next = NULL;
        }
    }
}
item_node createItem (int x, int y, int id)
{
    item_node selected = room[x][y];
    //Error
    if(selected == NULL)
        selected = malloc(sizeof(item_node));
    else{
        while(selected->next != NULL)
            selected = selected->next;

        selected->next = malloc(sizeof(item_node));
        selected = selected->next;
    }

    selected->id = id; 
    selected->next = NULL;
    return selected;
}
int main (int argc, char *argv[])
{
    x_format = 100;
    y_format = 100;

    createRoom(x_format,y_format);
    item_node itemtest = createItem(1,1,0);
    free(room);
    free(buffer);
}

So the problem that occurs is:

error: invalid operands to binary expression ('item_node' and 'void *') if(selected == NULL)

I understand the error but I don't now how to fix that. Sorry if this question is way to trivial and I appreciate any help! Thank you.

Answer 1

On topic, the problem is right here:

item_node createItem (int x, int y, int id){
   item_node selected = room[x][y];
   //Error
   if(selected == NULL)
      selected = malloc(sizeof(item_node));

In your case item_node selected is a variable, not a pointer. You declared selected to be a variable of item_node, which is a struct. No pointers here. Then you are comparing a variable of a certain type with NULL (which is defined as (void*) 0 in plain C).

Off topic: The entire program seems flawed at first sight, though. Please revise the algorithm and the data structures, it seems to me you're blending pointers and array a little to freely.

Answer 2

I'm pretty sure that your item_node data type is not a pointer, but a structure. However in the highlighted line you compare an exemplar of such structure to void pointer (and do other stuff later on), which is a mistake. Apparently, the following was intended:

1) item_node* createItem (int x, int y, int id)

2) item_node* selected = room[x][y];