C: Memory usage and problems with Array - c

Question

I have two arrays (representing rooms) with items that are traveling through the space. I found an interesting way to allocate the ram here in the forum. Here is what I am doing:
First I create an empty room with some default values.
After that I put some elements into it. There are two different items. An obstacle and an item that travels through the room. I have an iteration that runs for example 100 times and puts all items one coordinate further. The obstacles keep their position.
Every iteration has to do the following:
First it creates a new temporary room (new_room). It copies all obstacles because they stay at the same place(id = 3). Next, every item from the old room(room) gets his new coordinate in the new_room. After that I change the rooms, so room gets new_room. I have some big problems with the memory usage. I want to free the old new_room every time I create a new one with createRoomNew().In this implementation I get a segmentation fault. I think because of the function changeroom().
I am really confused right now because I am new to C.... I hope I pointed out what I mean. Thank you very much!
item_node ***room;
item_node ***room_new;
void createRoom(int x, int y, int z)
{
if (room == NULL) {
item_node *allElements = malloc(x * y * z * sizeof(item_node));
room = malloc(x * sizeof(item_node **));
for(int i = 0; i < x; i++)
{
room[i] = malloc(y * sizeof(item_node *));
for(int j = 0; j < y; j++)
{
room[i][j] = allElements + (i * y * z) + (j * z);
}
}
for (j = 0; j < x_format; j++) {
for (k = 0; k < y_format; k++) {
for (l = 0; l < z_format; l++) {
room[j][k][l].id = 3;
room[j][k][l].next = NULL;
}
}
}
}
}
void createRoomNew(int x, int y, int z)
{
if (room_new != NULL) {
for(int i = 0; i < x; i++)
{
free(room_new[i]);
}
free (room_new);
room_new = NULL;
}
if (room_new == NULL) {
item_node *allElements = malloc(x * y * z * sizeof(item_node));
room_new = malloc(x * sizeof(item_node **));
for(int i = 0; i < x; i++)
{
room_new[i] = malloc(y * sizeof(item_node *));
for(int j = 0; j < y; j++)
{
room_new[i][j] = allElements + (i * y * z) + (j * z);
}
}
}
for (j = 0; j < x_format; j++) {
for (k = 0; k < y_format; k++) {
for (l = 0; l < z_format; l++) {
if ((room[j][k][l].next) != NULL) {
if ((room[j][k][l].next->id) == 1) {
room_new[j][k][l] = room[j][k][l];
} else {
room_new[j][k][l].id = 3;
room_new[j][k][l].next = NULL;
}
}
else {
room_new[j][k][l].id = 3;
room_new[j][k][l].next = NULL;
}
}
}
}
}
void changeRoom(item_node *** newRoom)
{
room = newRoom;
}
Example call:
createRoom(200, 200, 200);
createRoomNew(200, 200, 200);
changeRoom(room_new);
createRoomNew(200, 200, 200);
changeRoom(room_new);

Answer 1

From the code it seems you think that when you do free, e.g. free(room_new) that room_new ends up being set to NULL. That is not the case
free() doesn't set a pointer to NULL (it can't), it is still pointing to wherever it was pointing just that the memory is no longer usable. You need to manually set the pointer to NULL after freeing.