Brute force algorithm causes undefined behaviour

I have to create a program for an assignment that solves a sudoku puzzle. User needs to enter the name of a binary file (NOT a true binary file, it just has a .bin extension, it can be opened with notepad, notepad++ etc. as well) that contains numbers. Those numbers represent coordinates on the puzzle as well as the number contained in those coordinates e.g 432 means 4th row 3rd column contains number 2. After filling out the puzzle i need to solve it and print it on screen. After executing the program it crashed, so I decided to use MSVC 2017 debugger which is among the best according to some developers to find and fix the bug. Here is my code:

Sudoku.c

#include <stdio.h>
#include <stdlib.h>
#include "stdafx.h"
#include "sudokulib.h"

#define MALLOC_ERROR 0xFF
#define FILE_NOT_FOUND 0xFFF
#define ROWS 9
#define COLUMNS 9

int main(int argc, char ** argv)
{
    char **matrix;
    int i, args;
    int row, column, num;
    FILE * fp;
    char * filename;
    char * importedData;
    matrix = (char **)malloc(ROWS * sizeof(char *));
    if (!matrix)
        exit(MALLOC_ERROR);
    for (i = 0; i<ROWS; ++i)
    {
        matrix[i] = (char *)malloc(COLUMNS * sizeof(char));
        if (!matrix[i])
            exit(MALLOC_ERROR);
    }
    initSudoku(matrix);
    printf ("Give me the name of data file: ");
    filename = (char *)malloc(100 * sizeof(char));
    if (!filename)
        exit(MALLOC_ERROR);
    scanf("%99s", filename);
    fp = fopen(filename, "rb");
    if (!fp)
    {
        printf ("File not found\n");
        exit(FILE_NOT_FOUND);
    }
    importedData = (char *)malloc(sizeof(char)*ROWS*COLUMNS * 3);
    if (!importedData)
        exit (MALLOC_ERROR);
    args = fread(importedData, 1, 243, fp);
    i = 0;
    while (importedData[i] != ' ' && importedData[i + 1] != ' ' && importedData[i + 2] != ' ' && importedData[i] >= '1' && importedData[i + 1] >= '1' && importedData[i + 2] >= '1' && importedData[i] <= '9' && importedData[i + 1] <= '9' && importedData[i + 2] <= '9' && i < 243)
    {
        row = importedData[i] - '0' - 1; /* Convert from ascii code to number */
        column = importedData[i + 1] - '0' - 1;
        num = importedData[i + 2] - '0';
        matrix[row][column] = num;
        i = i + 3;
    }
    printf("Sudoku after importing data:\n\n");
    printSudoku(matrix);
    system("pause");
    if (solvePuzzle(matrix))
    {
        printSudoku(matrix);
    }
    else
        printf ("Puzzle has no solution\n");
    fclose(fp);
    free(filename);
    for (i = 0; i<9; ++i)
    {
        free(matrix[i]);
    }
    free(matrix);
    return 0;
}

Sudokulib.h

#pragma once
#include <stdlib.h>
#include <stdio.h>

/* Function Prototypes Begin Here */

void printSudoku(char **);
void initSudoku(char **);
int checkRow(char **, int, int);
int checkCol(char **, int, int);
int check3x3(char **, int, int, int);
int checkIfEmpty(char **, int*, int*);
int solvePuzzle (char **);

/* Function Prototypes End Here */

void printSudoku(char ** Mat)
{
    int i, j;
    for (i = 0; i<9; ++i)
    {
        printf ("-------------------\n");
        printf("|");
        for (j = 0; j<9; ++j)
        {
            printf("%d|", Mat[i][j]);
        }
        printf("\n");
    }
    printf ("-------------------\n");
}

void initSudoku(char ** Mat)
{
    int i, j;
    for (i = 0; i<9; ++i)
        for (j = 0; j<9; ++j)
            Mat[i][j] = 0;
}


int checkRow (char ** Mat, int row, int num) // if row is free returns 1 else returns 0
{
    int col;
    for (col = 0; col < 9; col++)
    {
        if (Mat[row][col] == num)
        {
            return 0;
        }
    }
    return 1;
}

int checkCol (char ** Mat, int col, int num) // if column is free returns 1 else returns 0
{
    int row;
    for (row = 0; row < 9; row++)
    {
        if (Mat[row][col] == num)
        {
            return 0;
        }
    }
    return 1;
}

int check3x3 (char ** Mat, int row, int col, int num) // if number doesnt exist in the 3x3 grid returns 1 else returns 0
{
    row = (row / 3) * 3; // set to first row in the grid
    col = (col / 3) * 3; // set to first col in the grid
    int i;
    int j;
    for (i = 0; i < 3; i++) // grid is 3x3
    {
        for (j = 0; j < 3; j++)
        {
            if (Mat[row + i][col + j] == num)
            {
                return 0;
            }
        }
    }
    return 1;
}

int isValid (char ** Mat, int row, int col, int num)
{
    return (checkRow(Mat, row, num) && checkCol(Mat, col, num) && check3x3(Mat, row, col, num));
}

int checkIfPuzzleSolved (char ** Mat, int *row, int *col) // if function finds a box empty (puzzle not solved) returns 0 else returns 1
{
    for (*row = 0; *row < 9; *row++)
    {
        for (*col = 0; *col < 9; *col++)
        {
            printf("ROW: %d COL: %d\n",*row,*col);
            if (Mat[*row][*col] == 0)
            {
                return 0;
            }
        }
    }
    return 1;
}


int solvePuzzle (char ** Mat)
{
    int row;
    int col;
    if (checkIfPuzzleSolved(Mat, &row, &col))
    {
        return 1;
    }
    int num;
    for (num = 1; num <= 9; num++)
    {
        //if (checkRow (Mat,row,num) && checkCol (Mat,col,num) && check3x3 (Mat,row,col,num))
        if (isValid(Mat, row, col, num))
        {
            Mat[row][col] = num;
            if (solvePuzzle(Mat))
                return 1;
            Mat[row][col] = 0;
        }
    }
    return 0;
}

The debugger found a bug at this function:

int checkIfPuzzleSolved (char ** Mat, int *row, int *col) // if function finds a box empty (puzzle not solved) returns 0 else returns 1
{
    for (*row = 0; *row < 9; *row++)
    {
        for (*col = 0; *col < 9; *col++)
        {
            printf("ROW: %d COL: %d\n",*row,*col);
            if (Mat[*row][*col] == 0) /* DEBUGGER ERROR CODE 0xC0000005: Access violation reading location 0xCDCA247C
            {
                return 0;
            }
        }
    }
    return 1;
}

Two things that confused me:

1) I don't understand the reason solvePuzzle gets stuck brute forcing the first box in the puzzle (1st row 1st column). It seems that checkIfPuzzleSolved thinks that the first box is empty (containing 0), even though using printSudoku I can see the algorithm modifying that box toggles its value between 3 and 4 and obviously 0 != 3 and 0 != 4.

2) In checkIfPuzzleSolved, printf prints on screen row and column number and it constantly produces the following result:

ROW: 0 COL: 0
ROW: 0 COL: 0
ROW: 0 COL: -858993460

Also double checked this with the debugger and the values are indeed those mentioned.

My train of thought was the following:

1) Use checkIfEmpty to determine if a box of the puzzle contained 0, that would mean that the puzzle would not be solved yet. Row and col variables are sent into the function by reference, so when function finds an empty box and returns, row and col would save the coordinates of the empty box.

2) In the loop, call checkRow, checkCol and check3x3 to check if a number can be put into the desired box without breaking the sudoku rules. isValid is there for readability purposes.

3) Call solvePuzzle recursively until the puzzle is solved, meanwhile if a number is wrong, reset it to 0.

I have tried everything i could think of to solve this problem, wasting hours reading again and again my code to find a logical error, but everything seems okay. Any ideas?

EDIT: On request of Michael Beer, here is a sample binary file:

data.bin

142156177191216228257289311329364375418422441484534546562579625663682698739743787794824855883896917933951968

*row++; parses as *(row++);, which is equivalent to just row++. You're incrementing the pointer, not the counter. – melpomene

I see. So am I incrementing the pointer by sizeof(int) and not increasing the value that it refers to by 1? If so what is the correct way of writing "increment the value of the address you are pointing to by 1" regarding the syntax?

(*row)++ or ++(*row) or ++*row or *row += 1. – melpomene