A Simple Sudoku Solver in C

Here is a simple Sudoku solver in C. It uses backtracking technique to find all possible solutions.

#include <stdio.h>

/* Display the result. */
void display(const int grid[][9])
{
    int i,j;
    for (i=0; i<9; i++) {
        for (j=0; j<9; j++) {
            printf("%d ", grid[i][j]);
        }
        printf("\n");
    }
}

/* Verify the grid is in valid state */
int verify(const int grid[][9], int c, int x, int y)
{
    int i, j;
    int xs, ys;
    for (i=0; i<9; i++) {
        if (grid[i][y] == c || grid[x][i] == c)
            return 0;
    }
    xs = x/3;
    ys = y/3;
    for (i = xs*3; i<(xs+1)*3; i++) {
        for (j = ys*3; j< (ys+1)*3; j++) {
            if (grid[i][j] == c)
                return 0;
        }
    }
    return 1;
}

/* The core of the solver. */
void solve(int grid[][9], int x, int y)
{
    if (grid[x][y] != 0) {
        if (x==8 && y==8) {
            display(grid);
        } else {
            solve(grid, (x+1)%9, (x==8) ? (y+1) : y);
        }
    } else {
        int c;
        for (c=1; c <= 9; c++) {
            if (verify(grid, c, x, y)) {
                grid[x][y] = c;
                solve(grid, x, y);
            }
        }
        grid[x][y] = 0;
    }
}

int main()
{
    int c[9][9];
    int i,j;
    for (i=0; i<9; i++) {
        for (j=0; j<9; j++) {
            scanf("%d ", &(c[i][j]));
        }
    }
    solve(c, 0, 0);
    return 0;
}

Compile the program:
cl sudoku.c

Run the program:
sudoku < in.txt

Where in.txt the input file. Here is a sample input file:
(I found it here.)

0 0 1 0 0 5 3 0 0
0 5 0 4 9 0 0 0 0
0 0 0 1 0 2 0 6 4
0 0 0 0 0 0 7 5 0
6 0 0 0 0 0 0 0 1
0 3 5 0 0 0 0 0 0
4 6 0 9 0 3 0 0 0
0 0 0 0 2 4 0 9 0
0 0 3 6 0 0 1 0 0