在C中使用malloc初始化结构

Question

So I have what I think is a noob question. Sorry for that in advance, and for the grammar as english is not my primary language.

So I have to make a game of checkers. I have some struct, defined by

struct game {
int **board;
int xsize, ysize;

struct move *moves;
int cur_player;
};

struct coord {
int x, y;
};

struct move_seq {
struct move_seq *next;
struct coord c_old;
struct coord c_new; 
int piece_value;
struct coord piece_taken;
int old_orig;
};

struct move {
struct move *next;
struct move_seq *seq;
};

And I have to initialize a struct game wih the fonction

struct game *new_game(int xsize, int ysize)

So, here's my problem. I call, for now, new_game always with 10 and 10 values for xsize an ysize. Then I initialize the board game, which I want to assign later.

int black = 1;
int white = 5;
int **board;
int i;
int j;
int k;
for(i=0;i<xsize;i++)
{
    for(j=0;j<ysize;j++)
    {
        if(i<(xsize/2) && j<(ysize/2) && (i+j)%2!=0)
        {
            board[i][j] = black;
        }
        else if(i>(xsize/2) && j>(ysize/2) && (i+j)%2!=0)
        {
            board[i][j] = white;
        }
        else board[i][j] = 0;
    }
}
struct game *new = malloc (sizeof(struct game *));
if (new == NULL) return NULL;

So, my problem is after that. I just have Segmentation Fault whatever I do with my struct new.

I tried to do assign new->xsize = xsize and the same with ysize. I do a malloc for the board and the struct move, like I learned to do, but I kept getting this error of Segmentation Fault.

So here's my real question: How to assign and initialize correctly a struct ? Do I Have to make a malloc for each of the member of struct game ? (I tried that too but without any success...)

I don't necessarily want just the answer, I'd prefer to really understand what I have to do in this case and in general, to make less mistakes in the future.

Thanks in advance for your help.

Have a good day.

Answer 1

It happens because you only allocated space for a pointer to your struct . What you need to do, is allocate it for the entire size of it:

struct game *new = malloc (sizeof(struct game));

Edit: Don't be mislead by the return value of malloc, as it returns a pointer to the allocated space, that's why it should be struct game *new as it is.

Answer 2

In addition to Michael's bug fix, I also wonder about board: it is not allocated, yet you write to it. I think it should be like:

struct game *new_game(int xsize, int ysize)
{
    int black = 1;
    int white = 5;
    int i;
    int j;

    struct game *new = malloc (sizeof(struct game));
    if (new == NULL) return NULL;
    game->xsize= xsize;
    game->ysize= ysize;
    game->board= malloc(xsize*ysize*sizeof(int));

    for(i=0;i<xsize;i++)
    {
        for(j=0;j<ysize;j++)
        {
            if(i<(xsize/2) && j<(ysize/2) && (i+j)%2!=0)
            {
                game->board[i*xsize+j] = black;
            }
            else if(i>(xsize/2) && j>(ysize/2) && (i+j)%2!=0)
            {
                game->board[i*xsize+j] = white;
            }
            else game->board[i*xsize+j] = 0;
        }
    }
    return (game);
}

Note also the array indexing: the compiler doesn't know the dynamic rowsize so you have to do that yourself.