Segmentation fault (core dumped) [Conway's game of life] - c

Question

I'm working on a C implementation for Conway's game of life, I have been asked to use the following header:
#ifndef game_of_life_h
#define game_of_life_h
#include <stdio.h>
#include <stdlib.h>
// a structure containing a square board for the game and its size
typedef struct gol{
int **board;
size_t size;
} gol;
// dynamically creates a struct gol of size 20 and returns a pointer to it
gol* create_default_gol();
// creates dynamically a struct gol of a specified size and returns a pointer to it.
gol* create_gol(size_t size);
// destroy gol structures
void destroy_gol(gol* g);
// the board of 'g' is set to 'b'. You do not need to check if 'b' has a proper size and values
void set_pattern(gol* g, int** b);
// using rules of the game of life, the function sets next pattern to the g->board
void next_pattern(gol* g);
/* returns sum of all the neighbours of the cell g->board[i][j]. The function is an auxiliary
function and should be used in the following function. */
int neighbour_sum(gol* g, int i, int j);
// prints the current pattern of the g-board on the screen
void print(gol* g);
#endif
I have added the comments to help out with an explanation of what each bit is.
gol.board is a 2-level integer array, containing x and y coordinates, ie board[x][y], each coordinate can either be a 1 (alive) or 0 (dead).
This was all a bit of background information, I'm trying to write my first function create_default_gol() that will return a pointer to a gol instance, with a 20x20 board.
I then attempt to go through each coordinate through the 20x20 board and set it to 0, I am getting a Segmentation fault (core dumped) when running this program.
The below code is my c file containing the core code, and the main() function:
#include "game_of_life.h"
int main()
{
// Create a 20x20 game
gol* g_temp = create_default_gol();
int x,y;
for (x = 0; x < 20; x++)
{
for (y = 0; y < 20; y++)
{
g_temp->board[x][y] = 0;
}
}
free(g_temp);
}
// return a pointer to a 20x20 game of life
gol* create_default_gol()
{
gol* g_rtn = malloc(sizeof(*g_rtn) + (sizeof(int) * 20 * 20));
return g_rtn;
}
This is the first feature I'd like to implement, being able to generate a 20x20 board with 0's (dead) state for every coordinate.
Please feel free to criticise my code, I'm looking to determine why I'm getting the segmentation fault, and if I'm allocating memory properly in the create_default_gol() function.
Thanks!

Answer 1

The type int **board; means that board must contain an array of pointers, each of which points to the start of each row. Your existing allocation omits this, and just allocates *g_rtn plus the ints in the board.
The canonical way to allocate your board, supposing that you must stick to the type int **board;, is:
gol* g_rtn = malloc(sizeof *g_rtn);
g_rtn->size = size;
g_rtn->board = malloc(size * sizeof *g_rtn->board);
for (int i = 0; i < size; ++i)
g_rtn->board[i] = malloc(size * sizeof **g_rtn->board);
This code involves a lot of small malloc chunks. You could condense the board rows and columns into a single allocation, but then you also need to set up pointers to the start of each row, because board must be an array of pointers to int.
Another issue with this approach is alignment. It's guaranteed that a malloc result is aligned for any type; however it is possible that int has stricter alignment requirements than int *. My following code assumes that it doesn't; if you want to be portable then you could add in some compile-time checks (or run it and see if it aborts!).
The amount of memory required is the sum of the last two mallocs:
g_rtn->board = malloc( size * size * sizeof **g_rtn->board
+ size * sizeof *g_rtn->board );
Then the first row will start after the end of the row-pointers (a cast is necessary because we are converting int ** to int *, and using void * means we don't have to repeat the word int):
g_rtn->board[0] = (void *) (g_rtn->board + size);
And the other rows each have size ints in them:
for (int i = 1; i < size; ++i)
g_rtn->board[i] = g_rtn->board[i-1] + size;
Note that this is a whole lot more complicated than just using a 1-D array and doing arithmetic for the offsets, but it was stipulated that you must have two levels of indirection to access the board.
Also this is more complicated than the "canonical" version. In this version we are trading code complexity for the benefit of having a reduced number of mallocs. If your program typically only allocates one board, or a small number of boards, then perhaps this trade-off is not worth it and the canonical version would give you fewer headaches.
Finally - it would be possible to allocate both *g_rtn and the board in the single malloc, as you attempted to do in your question. However my advice (based on experience) is that it is simpler to keep the board separate. It makes your code clearer, and your object easier to use and make changes to, if the board is a separate allocation to the game object.

Answer 2

create_default_gol() misses to initialise board, so applying the [] operator to it (in main() ) the program accesses "invaid" memory and with ethis provokes undefined behaviour.
Although enough memory is allocated, the code still needs to make board point to the memory by doing
gol->board = ((char*) gol) + sizeof(*gol);
Update
As pointed out by Matt McNabb's comment board points to an array of pointers to int, so initialisation is more complicate:
gol * g_rtn = malloc(sizeof(*g_rtn) + 20 * sizeof(*gol->board));
g_rtn->board = ((char*) gol) + sizeof(*gol);
for (size_t i = 0; i<20; ++i)
{
g_rtn->board[i] = malloc(20 * sizeof(*g_rtn->board[i])
}
Also the code misses to set gol's member size. From what you tell us it is not clear whether it shall hold the nuber of bytes, rows/columns or fields.
Also^2 coding "magic numbers" like 20 is bad habit.
Also^3 create_default_gol does not specify any parameters, which explictily allows any numberm and not none as you might perhaps have expected.
All in all I'd code create_default_gol() like this:
gol * create_default_gol(const size_t rows, const size_t columns)
{
size_t size_rows = rows * sizeof(*g_rtn->board));
size_t size_column = columns * sizeof(**g_rtn->board));
gol * g_rtn = malloc(sizeof(*g_rtn) + size_rows);
g_rtn->board = ((char*) gol) + sizeof(*gol);
if (NULL ! = g_rtn)
{
for (size_t i = 0; i<columns; ++i)
{
g_rtn->board[i] = malloc(size_columns); /* TODO: Add error checking here. */
}
g_rtn->size = size_rows * size_columns; /* Or what ever this attribute is meant for. */
}
return g_rtn;
}

Answer 3

gol* create_default_gol()
{
int **a,i;
a = (int**)malloc(20 * sizeof(int *));
for (i = 0; i < 20; i++)
a[i] = (int*)malloc(20 * sizeof(int));
gol* g_rtn = (gol*)malloc(sizeof(*g_rtn));
g_rtn->board = a;
return g_rtn;
}
int main()
{
// Create a 20x20 game
gol* g_temp = create_default_gol();
int x,y;
for (x = 0; x < 20; x++)
{
for (y = 0; y < 20; y++)
{
g_temp->board[x][y] = 10;
}
}
for(x=0;x<20;x++)
free(g_temp->board[x]);
free(g_temp->board);
free(g_temp);
}

Answer 4

main (void)
{
gol* gameOfLife;
gameOfLife = create_default_gol();
free(gameOfLife);
}
gol* create_default_gol()
{
int size = 20;
gol* g_rtn = malloc(sizeof *g_rtn);
g_rtn = malloc(sizeof g_rtn);
g_rtn->size = size;
g_rtn->board = malloc(size * sizeof *g_rtn->board);
int i, b;
for (i = 0; i < size; ++i){
g_rtn->board[i] = malloc(sizeof (int) * size);
for(b=0;b<size;b++){
g_rtn->board[i][b] = 0;
}
}
return g_rtn;
}
Alternatively, since you also need to add a create_gol(size_t new_size) of custom size, you could also write it as the following.
main (void)
{
gol* gameOfLife;
gameOfLife = create_default_gol();
free(gameOfLife);
}
gol* create_default_gol()
{
size_t size = 20;
return create_gol(size);
}
gol* create_gol(size_t new_size)
{
gol* g_rtn = malloc(sizeof *g_rtn);
g_rtn = malloc(sizeof g_rtn);
g_rtn->size = new_size;
g_rtn->board = malloc(size * sizeof *g_rtn->board);
int i, b;
for (i = 0; i < size; ++i){
g_rtn->board[i] = malloc(sizeof (int) * size);
for(b=0;b<size;b++){
g_rtn->board[i][b] = 0;
}
}
return g_rtn;
}
Doing this just minimizes the amount of code needed.