How to fix ' Error: invalid read of size 4' - c

Question

I'm trying to learn C and am completing different challenges in order to learn faster. The code seems logical to me and should progress. However, I'm getting an invalid read of size four on line 29
I've tried to change the data type that gets parsed through, but it didn't seem to work.
#include <unistd.h>
typedef struct s_grid
{
int x;
int y;
} t_grid;
void set_point(t_grid *coord, int i, int j)
{
coord->x = i;
coord->y = j;
}
//check loccation of next zero
int where_is_zero(int **sudoku)
{
int x;
int y;
t_grid coord;
x = 0;
y = 0;
while (x < 9)
{
y = 0;
while (y < 9)
{
if (sudoku[x][y] == 0) //Here I'm getting the error.
{
set_point(&coord, x, y);
return (0);
}
x++;
}
y++;
}
return (1);
}
int solve_sudoku(int **grid)
{
int row;
int col;
int num;
t_grid coord;
if (where_is_zero(grid) == 1)
return (1);
row = coord.x;
col = coord.y;
//where_is_zero(grid);
num = 1;
while (num <= 9)
{
if (check_number(grid, row, col, num) == 1)
{
grid[row][col] = num;
if (solve_sudoku(grid) == 1)
return (1);
grid[row][col] = 0;
}
num++;
}
return (0);
}
void ft_putchar(char c)
{
write(1, &c, 1);
}
void ft_putstr(char *str)
{
while (*str != '\0')
{
ft_putchar(*str);
str++;
}
}
//my main function
int main(int argc, char **argv)
{
int board[9][9] ={ { 0, 9, 0, 0, 0, 0, 8, 5, 3 },
{ 0, 0, 0, 8, 0, 0, 0, 0, 4 },
{ 0, 0, 8, 2, 0, 3, 0, 6, 9 },
{ 5, 7, 4, 0, 0, 2, 0, 0, 0 },
{ 0, 0, 0, 0, 0, 0, 0, 0, 0 },
{ 0, 0, 0, 9, 0, 0, 6, 3, 7 },
{ 9, 4, 0, 1, 0, 8, 5, 0, 0 },
{ 7, 0, 0, 0, 0, 6, 0, 0, 0 },
{ 6, 8, 2, 0, 0, 0, 0, 9, 0 } };
solve_sudoku(board);
return (0);
}
I removed some code just to highlight where the problem is. It should find the next 0 in the table and set the coordinations to my structure.

Answer 1

The problem lies in how you have the loop set up. See my comments below:
while (x < 9)
{
y = 0;
while (y < 9)
{
if (sudoku[x][y] == 0) //Here I'm getting the error.
{
set_point(&coord, x, y);
return (0);
}
x++; // This increments the x index while you're inside the y loop
}
y++; // This increments the y index when you meant to increment the x
}
If you swap both lines, that should work fine and will resolve your out-of-bounds error.
What would also help you write this better is instead of using a while loop, use a for loop. If you haven't covered that topic yet, it's really straightforward:
for(int x = 0; x < 9 ; ++x)
{
for (int y = 0; y < 9; ++y)
{
if (sudoku[x][y] == 0) //Here you won't get the error anymore!
{
set_point(&coord, x, y);
return (0);
}
}
}
What the for loop line means there is this: Start the loop by setting it to 0, at the end of an iteration, increment it. On the next iteration, check if x is still less than 9. If so, proceed with the next iteration.

Answer 2

Your problem is due to mixing types. In main() you declare:
int board[9][9] = { ....
You then pass:
solve_sudoku(board);
board has the type int (*)[9] (a pointer to array of int [9]). See C11 Standard - 6.3.2.1 Other Operands - Lvalues, arrays, and function designators(p3). Your solve_soduku expects a parameter of type int** (pointer to pointer to int).
int solve_sudoku(int **grid) { ...
The type are NOT compatible. (and your compiler should be screaming warnings at you)
board is a pointer to array of int [9], while grid is a pointer to pointer to int. You must make the types compatible. The easiest way is to change solve_sudoku to
int solve_sudoku(int (*grid)[9]) { ...
Otherwise you will need to declare board as int** and allocate and fill in main().