Program hangs after modifying 2D array in a function in C

Question

The program is a Tic-Tac-Toe game, and it hangs after taking the X and Y values from the user.

This is the main file:

#include <stdio.h>
#include <string.h>
#include "input.cpp"

int makeMove(char** board, char* moveX, char* moveY, char playerMove) {
    printf("Make your x move: ");
    int answerX = (int)getNum(moveX, 2);

    printf("Make your y move: ");
    int answerY = (int)getNum(moveY, 2);

    answerX--;
    answerY--;

    if ((answerX < 0) || (answerX > 2)) {
        return -1;
    }

    if ((answerY < 0) || (answerY > 2)) {
        return -1;
    }

    board[answerY][answerX] = playerMove;
    return 0;
}

int main()
{
    int turns = 0;
    const int MAX_TURNS = 9;
    char playerOneChar = 'X';
    char playerTwoChar = 'O';
    char currentChar = playerOneChar;

    while (turns <= MAX_TURNS) {
        char board[3][3];
        memset(board, ' ', 9);

        char moveX[2];
        memset(moveX, ' ', 2);

        char moveY[2];
        memset(moveY, ' ', 2);

        int result = makeMove(board, moveX, moveY, currentChar);
        
        if (result == 0) {
            if (currentChar == playerOneChar) {
                currentChar = playerTwoChar;
            }

            else {
                currentChar = playerOneChar;
            }

            turns++;
        }

        else {
            printf("Player move was out of bounds.");
        }
    }
}

This is input.cpp, and it is designed to safely get a number from the user, without allowing for buffer overruns or other problems:

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>

long getNum(char* buffer, int bufferSize)
{
    long answer;
    int success = 0;

    while (!success)
    {
        // This gets the input from the user
        if (!fgets(buffer, bufferSize, stdin))
        {
            //If fgets fails, return with exit code 1.
            return 1;
        }

        char* endptr;

        errno = 0;
        answer = strtol(buffer, &endptr, 10);

        // If an error occurs, the user is told the number is too small or large.
        if (errno == ERANGE)
        {
            printf("Sorry, this number is too small or too large.\n");
            success = 0;
        }

        else if (endptr == buffer)
        {
            success = 0;
        }

        else if (*endptr && *endptr != '\n')
        {
            success = 0;
        }

        else
        {
            success = 1;
        }
    }

    return answer;
}

I attempted to input 2 numbers, but the program hanged afterwards. I attempted to research on the internet, but there was a lack of any explanation for why a function was not able to write to a 2D array properly. I am assuming that it hangs because char** is a pointer to char*, rather than being a pointer to a 2D array of chars.

Answer 1

This function declaration:

int makeMove(char** board, char* moveX, char* moveY, char playerMove) {

is invalid.

You are passing to the function:

int result = makeMove(board, moveX, moveY, currentChar);

the two-dimensional array board declared like

char board[3][3];

Used as an argument expression it is implicitly converted to pointer to its first element of the type char ( * )[3]. But the function parameter has the type char **. There is no implicit conversion between objects of the pointer types char ( * )[3] and char **. So already this function call invokes undefined behavior.

You should declare the function like:

int makeMove(char board[][3], char* moveX, char* moveY, char playerMove) {

Another problem is that call of fgets:

if (!fgets(buffer, bufferSize, stdin))

As the bufferSize is equal to 2 then the new line character '\n' will not be stored in the input buffer (It will be stored only if the user at once will press the Enter key. In this case the array will look like { '\n', '\0' }). As a result a subsequent call of fgets will read an empty string that will contain only the new line character.

Pay attention to that it seems due to the extension of the file:

#include "input.cpp"

you are compiling your program as a C++ program. C and C++ are two different languages. But in any case it is a bad idea to include a module as a header. You need to compile it separately.

Answer 2

The first argument of makeMove() needs to be char board[][3], which is a fixed 2-D array with rows of length 3, which matches what you declared in main(). char** board is a rather different thing, which is a 1-D array of pointers, each pointing to a 1-D array of chars. Either one can be accessed with board[i][j], but how that is implemented for those two declarations is different.