Sudoku solver in c crashes

I am trying to figure out what is wrong with my code which is supposed to solve a 9x9 sudoku which crashes when I run it with the example in the Wikipedia for sudoku.

I have also tried using recursion but am not very familiar with it so it did not work either.

findunassignedlocation returns 1 if there are still 0s in the sudoku and 0 if there are no more 0s.
isSafe returns 1 if all rows, columns and 3x3 grid does not contain the number and 0 otherwise.

If the problem is probably in one of the other functions, please let me know, I will send code. Ignore the depth variable.

Below is my code:

/* Returns an int which indicates whether if the sudoku has any
   more empty entries (which shows as a 0) */
int FindUnassignedLocation(int grid[9][9]) { 
    for (int row = 0; row < 9; row++) 
        for (int col = 0; col < 9; col++) 
            if (grid[row][col] == 0) 
                return 1; 
    return 0; 
}

/* Returns an int which indicates whether an assigned entry 
   in the specified row matches the given number. */
int UsedInRow(int grid[9][9], int row, int num) { 
    for (int col = 0; col < 9; col++) 
        if (grid[row][col] == num) 
            return 1; 
    return 0; 
} 

/* Returns an int which indicates whether an assigned entry 
   in the specified column matches the given number. */
int UsedInCol(int grid[9][9], int col, int num) { 
    for (int row = 0; row < 9; row++) 
        if (grid[row][col] == num) 
            return 1; 
    return 0; 
} 

/* Returns an int which indicates whether an assigned entry 
   within the specified 3x3 box matches the given number. */
int UsedInBox(int grid[9][9], int boxStartRow, int boxStartCol, int num) { 
    for (int row = 0; row < 3; row++) 
        for (int col = 0; col < 3; col++) 
            if (grid[row + boxStartRow][col + boxStartCol] == num) 
                return 1; 
    return 0; 
} 

/* Returns an int which indicates whether it will be legal to assign 
   num to the given row,col location. */
int isSafe(int grid[9][9], int row, int col, int num) { 
    /* Check if 'num' is not already placed in current row, 
       current column and current 3x3 box */
    if (!UsedInRow(grid, row, num) && 
        !UsedInCol(grid, col, num) && 
        !UsedInBox(grid, row - row % 3, col - col % 3, num) && 
        grid[row][col] == 0) {
        return 1;
    } else {
        return 0;
    }
} 

void solve_sudoku(int sudoku[9][9]) {
    int row = 0;
    int col = 0;
    if (FindUnassignedLocation(sudoku) == 0) { //if sudoku is completely filled
        return;
    }
    for (int num = 1; num <= 9; num++) {
        if (isSafe(sudoku, row, col, num) == 1) {
            sudoku[row][col] = num;
            solve_sudoku(sudoku, depth);
            sudoku[row][col] = 0;
        }
    }
}

A simple (but not the most efficient because of symmetries/rotations) way is to try all the possibilities trying to place the number 1 up to 9 on the successive positions (0,0 0,1 .. 0,8 1,0 ... ) recursively, each time a number can be put on 8,8 it is a solution

Few changes are needed from your code, just to remove the useless FindUnassignedLocation and add the recursion

If I replace 9 by SZ to also be able to search for the sizes 3, 6 or 9 and I add the draw of the solutions the code can be :

#include <stdio.h>

// 3, 6 or 9
#define SZ 9

/* Returns an int which indicates whether an assigned entry 
   in the specified row matches the given number. */
int UsedInRow(int grid[][SZ], int row, int num) 
{ 
  for (int col = 0; col < SZ; col++)
    if (grid[row][col] == num) 
      return 1; 

  return 0; 
} 

/* Returns an int which indicates whether an assigned entry 
   in the specified column matches the given number. */
int UsedInCol(int grid[][SZ], int col, int num) 
{ 
  for (int row = 0; row < SZ; row++) 
    if (grid[row][col] == num) 
      return 1; 

  return 0; 
} 

/* Returns an int which indicates whether an assigned entry 
   within the specified 3x3 box matches the given number. */
int UsedInBox(int grid[][SZ], int boxStartRow, int boxStartCol, int num) { 
  for (int row = 0; row < 3; row++) 
    for (int col = 0; col < 3; col++) 
      if (grid[row + boxStartRow][col + boxStartCol] == num) 
        return 1; 

  return 0; 
} 


/* Returns an int which indicates whether it will be legal to assign 
   num to the given row,col location. */
int isSafe(int grid[][SZ], int row, int col, int num) 
{ 
  /* Check if 'num' is not already placed in current row, 
     current column and current 3x3 box */
  return (!UsedInRow(grid, row, num) && 
          !UsedInCol(grid, col, num) && 
          !UsedInBox(grid, row - row % 3, col - col % 3, num));
} 

/* print a solution */
void draw(int sudoku[][SZ])
{
  for (int row = 0; row != SZ; ++row) {
    for (int col = 0; col != SZ; ++col)
      printf("%d", sudoku[row][col]);
    putchar('\n');
  }
  putchar('\n');
}

void solve_sudoku(int sudoku[][SZ], int row, int col)
{
  for (int num = 1; num <= 9; num++) { //loop through numbers 1 to SZ
    if (isSafe(sudoku, row, col, num) == 1) { //the number is safe
      sudoku[row][col] = num;
      if ((col + 1) == SZ) {
        if ((row + 1) == SZ) {
          // done
          draw(sudoku);
        }
        else
          solve_sudoku(sudoku, row + 1, 0);
      }
      else
        solve_sudoku(sudoku, row, col + 1);
      sudoku[row][col] = 0;
    }
  }
}

int main()
{
  int sudoku[SZ][SZ] = {0};

  solve_sudoku(sudoku, 0, 0);
}

first solutions found for SZ 3 (362880 possible solutions) :

First solutions found for SZ 6 :

First solutions found for SZ 9