Question

I’m trying to learn C and I’ve gotten to a point where I felt like trying something more advanced than small and easy examples of printing text to the console, using pointers in a very simple way and so on.

The problem is that my program crashes on my call to free and I’m sure it is the pointer(s) somehow haunting me…

So, I have created a two-dimensional array like this:

bool InitializeGrid(struct Grid *g, int rows, int columns)
{
    g->rows = rows;
    g->columns = columns;
    g->grid = malloc(g->rows * sizeof(int *));

    if(g->grid == NULL)
    {
        printf("Could not allocate memory for grid rows.\n");
        return false;
    }

    for (int i = 0; i < g->rows; i++)
    {
        g->grid[i] = malloc(g->columns * sizeof(int));

        if(g->grid[i] == NULL)
        {
            printf("Could not allocate memory for grid columns.\n");
            return false;
        }
    }

    / ... /

    return true;
}

The structure Grid looks like this:

struct Grid
{
    int rows;
    int columns;
    int **grid;
};

I then successfully use the grid throughout the program and everything works great. When the program is about to exit, I need/want to free the memory I allocated. This is how I do that:

void CleanupGrid(struct Grid *g)
{
    for(int i = 0; i < g->rows; i++)
        free(g->grid[i]);

    free(g->grid);
}

But on the first free(g->grid[i]) I receive a segmentation fault.

Why does this not work? How can I fix this?

Edit:

stdbool.h has been included in the program for use of bool/true/false.

I feel really stupid for staring blind at the free() statement. When I scale down the program to the absolute minimum, it works great with the free() statements. It is when I access and write to the grid at one point that the free() greets me with a segmentation fault.

Accepting user1083265’s answer.

Answer 1

I compile your code (just alloc/dealloc) and it doesn’t crash.

Possibly, when using allocated grid, you corrupt some memory above allocated fragments. Glibc uses canary values to detect such corruption – so you get segfault when freeing corrupted memory.