Running into an identifiable Segmentation Fault - c

Question

Whenever I run my code I get a message: Segmentation fault: 11
My question is why this message is popping up. I have done some research prior to this and have tried to fix it but I still have the same message pop up. Please review my code!
To give some context to my code: The goal of this program is to implement the Connect Four game between a computer and a human player. Functions are to be implemented in the file connect4_functions.c. Forward declarations of these functions are in the file connect4_functions.h. connect4.c contains the main () functions.
The code (connect4_functions.c) is below:
#include "connect4_functions.h"
int print_welcome(void) // PROBLEM: returnerer alltid 1
{
printf ("*** Welcome to the Connect Four game!!! ***\n");
printf ("Would you like to make the first move [y/n]: ");
if (getchar() == 'N' || getchar() == 'n') return 2;
else return 1;
while (getchar()!= '\n');
}
void display_board(int board[][BOARD_SIZE_VERT])
{
int i, j;
for (i = BOARD_SIZE_VERT-1; i >= 0; i--)
{
for (j = 0; j < BOARD_SIZE_HORIZ; j++) printf ("+---");
printf ("+\n");
for (j = 0; j < BOARD_SIZE_HORIZ; j++)
{
switch (board[j][i])
{
case 0: printf ("| "); break;
case 1: printf ("| O "); break;
case 2: printf ("| X "); break;
}
}
printf ("|\n");
} // end for
for (j = 0; j < BOARD_SIZE_HORIZ; j++) printf ("+---");
printf ("+\n");
for (j = 1; j <= BOARD_SIZE_HORIZ; j++) printf (" %d ", j);
printf ("\n");
} //end function display_board
int random_move(int board[][BOARD_SIZE_VERT], int computer_num)
{
int m = (rand() % BOARD_SIZE_HORIZ) + 1;
if (!is_column_full(board,m))
{
update_board(board,m,computer_num);
return m;
}
else return random_move(board,computer_num);
}
int player_move(int board[][BOARD_SIZE_VERT], int player_num)
{
int m;
printf ("Please enter your move: ");
scanf ("%d", &m);
while (getchar() != '\n');
if ( 0 > m || m > BOARD_SIZE_HORIZ)
{
printf ("Not a valid move. Enter a column number!\n");
return player_move(board, player_num);
}
if (is_column_full(board, m))
{
printf ("This column is full. Try again!\n");
return player_move(board, player_num);
}
update_board(board,m,player_num);
return m;
}
bool check_win_or_tie(int board[][BOARD_SIZE_VERT], int last_move)
{
int m, count = 0;
if (check_winner(board, last_move))
{
printf("Player %c won!\n", ( check_winner(board,last_move) == 1 ? '1' : '2' ) );
return true;
}
for (m = 0; m < BOARD_SIZE_HORIZ; m++) if ( is_column_full(board, m) ) count++;
if (count == BOARD_SIZE_HORIZ)
{
printf ("Tie game!\n");
return true;
}
else return false;
}
bool is_column_full(int board[][BOARD_SIZE_VERT], int m)
{
return (board[m-1][BOARD_SIZE_VERT-1]);
}
void update_board(int board[][BOARD_SIZE_VERT], int m, int player_num)
{
int i;
for ( i = 0; i < BOARD_SIZE_VERT ; i++)
{
if (!board[m-1][i])
{
board[m-1][i] = player_num;
return;
}
}
}
int check_winner(int board[][BOARD_SIZE_VERT], int last_move)
{
int i, row, count;
// Find row
for (row = 0; row < BOARD_SIZE_VERT; row++)
{
if (board[last_move-1][row]) count++;
}
row = count;
printf ("row = %d\n", row);
// Vertical
for (i = 0; i < BOARD_SIZE_VERT; i++)
{
if (board[last_move-1][i] == board[last_move-1][row]) count++;
else count = 0;
if (count == 4) return board[last_move-1][row];
else return 0;
}
count = 0; // reset
// Horizontal
for (i = 0; i < BOARD_SIZE_HORIZ; i++)
{
if (board[i][row] == board[last_move-1][row]) count++;
else count = 0;
if (count == 4) return board[last_move-1][row];
else return 0;
}
count = 0; // reset
return 0;
}
The code for connect4_functions.h (cannot be changed) is below:
#ifndef CONNECT4_FUNCTIONS
#define CONNECT4_FUNCTIONS
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <stdbool.h>
#define BOARD_SIZE_HORIZ 7
#define BOARD_SIZE_VERT 6
int print_welcome(void);
void display_board(int board[][BOARD_SIZE_VERT]);
int random_move(int board[][BOARD_SIZE_VERT], int computer_num);
int player_move(int board[][BOARD_SIZE_VERT], int player_num);
bool check_win_or_tie(int board[][BOARD_SIZE_VERT], int last_move);
bool is_column_full(int board[][BOARD_SIZE_VERT], int m);
void update_board(int board[][BOARD_SIZE_VERT], int m, int player_num);
int check_winner(int board[][BOARD_SIZE_VERT], int last_move);
int best_move(int board[][BOARD_SIZE_VERT], int computer_num);
#endif
The code for connect4.c (also cannot be changed) is below:
#include "connect4_functions.h"
int main()
{
int board[BOARD_SIZE_HORIZ][BOARD_SIZE_VERT] = { {0} };
int player_num, computer_num;
int last_move;
/* Ask Alice if she wants to go first */
player_num = print_welcome();
if (player_num == 1) computer_num = 2;
else computer_num = 1;
/* If Alice wants to go first, let her make a move */
if (player_num == 1)
{
display_board(board);
last_move = player_move(board,player_num);
display_board(board);
}
/* The main loop */
while (1)
{
/* Make a computer move, then display the board */
last_move = random_move(board,computer_num);
printf("Computer moved in column: %d\n", last_move);
display_board(board);
/* Check whether the computer has won */
if (check_win_or_tie(board,last_move)) return 0;
/* Let Alice make a move, then display the board */
last_move = player_move(board,player_num);
display_board(board);
/* Check whether Alice has won */
if (check_win_or_tie(board,last_move)) return 0;
} /* end of while (1) */
} /* end of main() */
Also, if you would like to see the original pdf it is linked below:
Link to pdf
The output is:
*** Welcome to the Connect Four game!!! ***
Would you like to make the first move [y/n]: y
+---+---+---+---+---+---+---+
| | | | | | | |
+---+---+---+---+---+---+---+
| | | | | | | |
+---+---+---+---+---+---+---+
| | | | | | | |
+---+---+---+---+---+---+---+
| | | | | | | |
+---+---+---+---+---+---+---+
| | | | | | | |
+---+---+---+---+---+---+---+
| | | | | | | |
+---+---+---+---+---+---+---+
1 2 3 4 5 6 7
Please enter your move: 4
+---+---+---+---+---+---+---+
| | | | | | | |
+---+---+---+---+---+---+---+
| | | | | | | |
+---+---+---+---+---+---+---+
| | | | | | | |
+---+---+---+---+---+---+---+
| | | | | | | |
+---+---+---+---+---+---+---+
| | | | | | | |
+---+---+---+---+---+---+---+
| | | | O | | | |
+---+---+---+---+---+---+---+
1 2 3 4 5 6 7
Computer moved in column: 1
+---+---+---+---+---+---+---+
| | | | | | | |
+---+---+---+---+---+---+---+
| | | | | | | |
+---+---+---+---+---+---+---+
| | | | | | | |
+---+---+---+---+---+---+---+
| | | | | | | |
+---+---+---+---+---+---+---+
| | | | | | | |
+---+---+---+---+---+---+---+
| X | | | O | | | |
+---+---+---+---+---+---+---+
1 2 3 4 5 6 7
row = -355898527
Segmentation fault: 11
(If you have any questions about the context or purpose please don't hesitate to ask!)

Answer 1

Not having actually run the code, I believe the issue may be that count is not initialized in check_winner:
int check_winner(int board[][BOARD_SIZE_VERT], int last_move)
{
int i, row, count;
// Find row
for (row = 0; row < BOARD_SIZE_VERT; row++)
{
if (board[last_move-1][row]) count++;
}
row = count;
// WHAT IF IT EXITS THE for () WITHOUT board[last_move-1][row] being non-zero?
// HINT: count can be anything!
printf ("row = %d\n", row);
You then use row as an access into an array, and boom! You've accessed memory outside your permissions.

Answer 2

In the check_winner function you don't initialise the variable count. I get an unitialised variable runtime error, yet if I continue ahead a couple of lines later you then set:
row = count;
Which in my case is setting row to -858993459;
A couple of lines down in that same function you write:
if (board[last_move - 1][i] == board[last_move - 1][row]) count++;
// You use "row", an initialised variable as the index
// for the second subscript operator of board.
And that's where I get:
Access violation reading location 0x0000005663E82BB4. occurred
I think there should have been a warning shown about this unitialised variable. And honestly, I don't know if that's all that's wrong with it, but it looks like a pretty good game.

Answer 3

First of all, to be able to solve your problem, you should make sure everything that might be printed before the crash, has been printed. In this goal, you should add a fflush(stdout); instruction after each of your printing instructions
secondly, if this doesn't make clear at what place you program crashes, you should add supplementary tracing instruction inside the piece of code where it happens.
And then, when you have identified which instruction causes de crash, you should print it's variables to check if their values are what you expect.
Alternatively, you can use the assert instruction to make sure the variable values are all in the expected ranges.