Unable to read memory. Can't find what's wrong

Question

I've got a piece of code, it's purpose is to draw a background image on one of the game levels. For this purpose I create this structure.

typedef struct crate_t {
            int x = 0;
            int y = 0;
            int h = 0;
            int w = 0;
            int type = BACKGROUND;
        }crate;

Then in the main function I create a 2D array

crate **Crates = (crate**)malloc(sizeof(crate)*(SCREEN_WIDTH / GrassBlock->w));
for (int i = 0; i <= SCREEN_HEIGHT/GrassBlock->h; i++) {
    Crates[i] = (crate*)malloc(sizeof(crate)*(SCREEN_HEIGHT / GrassBlock->h));
}

and I pass it to the function counter = DrawLevelBG(screen, GrassBlock, Border, Crates);. The problem is that the function causes error. "Access violation writing location." at Obstacles[i][j].x = x;

int DrawLevelBG(SDL_Surface *screen, SDL_Surface *sprite, SDL_Surface *border, crate **Obstacles) {
        int x = 0;
        int y = 0;
        int i = 0;
        int j = 0;
        bool condition = 0;
        while (y < SCREEN_HEIGHT + sprite->h) {
            DrawSurface(screen, sprite, x + (sprite->w / 2), y + (sprite->h / 2));
            if (x >= SCREEN_WIDTH - sprite->w || x == 0 || y == 0 || y >= SCREEN_HEIGHT - sprite->h) {
                DrawSurface(screen, border, x + (sprite->w / 2), y + (sprite->h / 2));
                Obstacles[i][j].x = x;
                Obstacles[i][j].y = y;
                Obstacles[i][j].h = border->h;
                Obstacles[i][j].w = border->w;
                Obstacles[i][j].type = WALL;
                i++;
                if (x >= SCREEN_WIDTH - sprite->w) {
                    y += sprite->h;
                    x = 0;
                    j++;
                    condition = 1;
                }
            }
            if (!condition) {
                x += sprite->w;
            }
            condition = 0;
        }
        return i;
    }

I know that these ones are caused by pointers not pointing actually to anything but I can't understand what's wrong here. Any help would be greatly appreciated. Thanks.

EDIT

I've changed my memory allocation piece of code so it looks like that now:

crate **Crates = (crate**)malloc(sizeof(crate*)*(SCREEN_WIDTH / GrassBlock->w)*(SCREEN_HEIGHT / GrassBlock->h));
for (int i = 0; i <= SCREEN_WIDTH/GrassBlock->w; i++) {
    Crates[i] = (crate*)malloc(sizeof(crate)*(SCREEN_HEIGHT / GrassBlock->h));
}

According to all your replies guys. Unfortunately this doesnt solve the problem. +Important info, the function DrawLevelBG causes ERROR on the first iteration of loop.

You don't see anything wrong with you using `sizeof(crate)` for *both* allocations? — Some programmer dude, Jan 09 '18 at 14:57
Have you tried using a debugger? That's the first thing to do when you get errors like this. — underscore_d, Jan 09 '18 at 14:59
There's a smell in your for loop for initialization. You allocate `SCREEN_WIDTH / GrassBlock->w` pointers, but to initialize them you iterate `SCREEN_HEIGHT/GrassBlock->h` times... these numbers may not be the same (also you're not allocating pointers, and also this looks like a 2D array, not 3D) — AndyG, Jan 09 '18 at 14:59
As for your problem, this is the perfect time to [learn how to debug your programs](https://ericlippert.com/2014/03/05/how-to-debug-small-programs/). What are the valid indexes into `Obstacles`? What indexes are you using? — Some programmer dude, Jan 09 '18 at 14:59
@underscore_d I've got a problem with a debugger, can't use it because I've got "sdl_windows_main.c not found" I'll fix that ASAP and try to debug it. — Jkee, Jan 09 '18 at 15:06
By the way, the reason you need to cast those `malloc` calls, and the reason you can use inline initialization of the `crate_t` structure members is because you're programming in *C++!* Either stop doing that and program in C properly, or [get a good beginners C++ book (or two)](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) and learn C++ properly. — Some programmer dude, Jan 09 '18 at 15:09
For an array of N elements, what are the valid indexes? Check those loop conditions! — Some programmer dude, Jan 09 '18 at 15:26
I notice you allocate `SCREEN_WIDTH / GrassBlock->w` pointers, but then initialize `1 + SCREEN_HEIGHT/GrassBlock->h` of them. Is this intentional? — Davislor, Jan 09 '18 at 16:20

score 2 · Answer 1 · answered Jan 09 '18 at 15:00

2

In the first allocation you create an array from pointers. So you need to allocate memory for pointers:

crate **Crates = (crate**)malloc(sizeof(crate*)*(SCREEN_WIDTH / GrassBlock->w));

answered Jan 09 '18 at 15:00

Paul Götzinger

145
6

While this is more correct, it should not impact anything considering that `sizeof(create)` should be larger by some margin to `sizeof(crate*)`. – Some programmer dude Jan 09 '18 at 15:08

score 0 · Answer 2 · answered Jan 09 '18 at 15:43

Thanks for all the help guys. The problem was iterators, not only did I make my 2D array SCREEN_HEIGHT wide and SCREEN_WIDTH high which was the opposite of what I wanted but aswell the iteration in DrawLevelBG was wrong as pointed out. I had to swap my "i" and "j" and make some corrections, so thanks alot Some programmer dude for pointing that out. Thanks alot.

Unable to read memory. Can't find what's wrong

EDIT

2 Answers2