Accessing allocated memory without a segmentation fault - c

Question

I am trying to make a char matrix for a tic tak toe game. I allocated memory for it but I can't access it. It shows me a segmentation fault and I can't seem to figure out why.
#include <stdio.h>
#include <stdlib.h>
//function to make a char matrix
void make_board (char** board)
{
board = malloc(sizeof(char*)*3);
for(int i=0;i<3;i++)
{
board[i] = malloc(sizeof(char)*3);
}
for(int i=0;i<3;i++)
{
for(int j=0;j<3;j++)
{
board[i][j]='x';
}
}
}
//main function
int main ()
{
char ** board;
make_board(board);
/*when i try to access it it show me a segmentation fault*/
printf("%c\n",**board);
}

Answer 1

The reason your code is throwing a segmentation fault is related to the way in which you are passing a pointer to an array of chars into your function. In make_board, by reassigning the variable board with malloc you are telling the computer, "Make board point to an array of characters." This sounds like it would work, but when you do this you are only re-assigning the variable board within make_board. In main, the variable board remains untouched since variables are copied into functions in c, rather than passed in with the ability to edit. The other problem with your code is that you never initialize the variable board before you pass it into make_board, so you are passing in 0x0, or as interpreted by the computer, nothing.
To help you understand, this is a play by play of what the computer does in this program (keep in mind, this assumes that you add a line that allocates board in main):
Step 1: (Entering main) The user wants a char** named board. I will allocate it so that it points to the address 0x1234123412341234.
Step 2: Ok, now I should pass board into make_board. (keep in mind, it passes 0x1234123412341234 and nothing else).
Step 3: (Entering make_board) Ok, the user wants to allocate board to 3 char* types, so I will assign board to 0x1234567812345678.
This is where things have gone wrong. The rest of your code works, but when you enter main again, board is still 0x1234123412341234 rather than the value of 0x1234567812345678. This way, when you go to print **board, it has never been assigned and points to nothing, throwing a segmentation fault.
Here is my code that fixes this by passing in a char***, board_ptr, which allows make_board to create a char** pointed to by its parameter without changing it.
#include <stdio.h>
#include <stdlib.h>
//function to make a char matrix
void make_board(char*** board_ptr)
{
*board_ptr = calloc(3, sizeof(char*));
char **board = *board_ptr;
for (int i = 0; i < 3; i++) {
board[i] = calloc(3, sizeof(char));
}
for (int i = 0; i < 3; i++) {
for (int j = 0; j < 3; j++) {
board[i][j]='x';
}
}
}
//main function
int main(int argc, char *argv[])
{
char ***board_ptr = calloc(1, sizeof(char**));
char **board;
make_board(board_ptr);
board = *board_ptr;
printf("%c\n", **board);/* Now it works! */
}

Answer 2

You change board variable inside make_board, but you pass it by value, so it is not changed in the main function. You try to printf the board variable, which was initialized with garbage, thus you received segmentation fault. You should return pointer from make_board: char **board = make_board() . Or pass board variable by pointer:make_board(&board)