Creating 2D struct [closed] - c

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Closed 10 years ago.
I have aproblem with this code, it compiles ok. But it crashes imidiately after memory could not be written error.
Debugger says the problem is in the line *grid = (grid_t**)malloc(sizeof(grid_t)*GRID_HEIGHT); , i must be missing something obvious.
I'm trying to create a pointer to a 2D struct.
#define GRID_WIDTH 12
#define GRID_HEIGHT 22
typedef struct
{
int piece;
int edge;
}grid_t;
grid_t*** grid;
*grid = (grid_t**)malloc(sizeof(grid_t)*GRID_HEIGHT);
for(int i = 0 ; i < GRID_HEIGHT ; i++)
{
*grid[i] = (grid_t*)malloc(sizeof(grid_t)*GRID_WIDTH);
}

Answer 1

You dereference an unallocated pointer:
grid_t*** grid;
*grid = (grid_t**)malloc(sizeof(grid_t)*GRID_HEIGHT);
grid is not allocated when you do *grid, so it's undefined behavior.
If you want to dynamically allocate two-dimensional structs, you first need to allocate enough memory for pointers (grid_t*) in the first level:
grid_t** grid;
grid = malloc(sizeof(*grid) * GRID_HEIGHT);
Then you can allocate each element with a loop:
for(int i = 0 ; i < GRID_HEIGHT ; i++)
{
grid[i] = malloc(sizeof(**grid) * GRID_WIDTH);
// ...then you can do grid[i]->piece = 42; etc..
}
Now, from what I can see, you probably don't even need dynamic allocation. If you don't need malloc, don't use it, just use good ol' arrays instead:
grid_t grid[GRID_HEIGHT][GRID_WIDTH];