Using shared memory with structs containing arrays

Question

I'm doing an university project and I was asked to create a kind of game between processes: they have to capture some flags placed over a shared matrix, similar to a chessboard. I represented the board using a struct and cells as a matrix but, after allocating the shared memory segment, I cannot attach it correctly because when I try to access to matrix cells I get "segmentation fault", in fact I see that matrix pointer is zero. So, how do I attach the matrix after the whole board is allocated?

Here's the code of the functions I wrote to allocate/attach the board:

int allocate_board(int width, int height){
    size_t size;
    int shm_id;
    key_t shm_key;
    char current_path[PATH_MAX];
    getcwd(current_path, sizeof(current_path));
    shm_key = ftok(current_path, 1);
    size = sizeof(board) + ( sizeof(cell) * height * width );
    shm_id = shmget(shm_key, size, IPC_CREAT | 0660);
    if ( shm_id < 0 ){
        printf("Cannot allocate the game board, aborting.\n");
        exit(1);
    }
    return shm_id;
}

board* get_board(int shm_id){
    void* shm_ptr;
    board* game_board;
    shm_ptr = shmat(shm_id, NULL, 0);
    if ( (int)shm_ptr == -1 ){
        printf("Cannot attach the shared memory segment, aborting.\n");
        printf("Reported error: %s.\n", strerror(errno));
        exit(2);
    }
    game_board = (board *)shm_ptr;
    return game_board;
}

Here's the structs definition:

typedef struct {
    entity_type_t entity_type;
    owner_id_t owner_id;
    float flag_score;
} cell;

typedef struct {
    int width;
    int height;
    sem_t* semaphore;
    cell** cells;
} board;

Here's how I try to access to board's cells:

game_board->width = some_width;
game_board->height = some_height;
game_board->cells[0][0].flag_score = some_score;

You do **not** have arrays. You have **pointers** which makes this **completely different thing**. You cannot have sensible pointers in shared memory, period. Each process has its own memory layout so the pointers of one would not make sense in another process. — Antti Haapala -- Слава Україні, Jan 11 '20 at 23:12
Perhaps you want to define your board so that it has a *flexible array member* at the end. — Antti Haapala -- Слава Україні, Jan 11 '20 at 23:14
https://stackoverflow.com/a/41471343/918959 <= like so. Unfortunately you cannot use this with 2-d indexing but you can use `cells[x + y * game_board->width]` then. — Antti Haapala -- Слава Україні, Jan 11 '20 at 23:16
Note that the `sem_t *` presents a similar problem. You should be storing the `sem_t` object itself in shared memory, or else using a named semaphore that is separate from your `board` structure. — John Bollinger, Jan 11 '20 at 23:55
Also, you are using SysV shared memory together with POSIX semaphores. There's nothing inherently wrong with that, but it's stylistically troubling. I would suggest using POSIX shared memory instead. — John Bollinger, Jan 11 '20 at 23:59

Using shared memory with structs containing arrays

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