Conway's Game of Life, Segfault 11

Question

I'm trying to write Conway's game of life in C. This is what I have so far. I'm using pointers to refer to the arrays, which has never caused me problems before, but the function place_cell is causing a segfault.

Here's what I've tried so far: - I tried making the grid with constants, 100 x 100, and 10 x 10. Modifying values inside of those constant grids still gives me a segfault. - I tried using constants for place_cell, still got a segfault.

int** make_grid(int x, int y) {
    int** is = (int**)malloc(sizeof(int*) * y);
    if(! is) {
        fprintf(stderr, "make_grid: malloc failed");
        exit(1);
    }
    int j;
    for(j = 0; j < y; j++) {
        is[j] = (int*)malloc(sizeof(int) * x);
        if(!is[j]) {
            fprintf(stderr, "make_grid: malloc failed");
            exit(1);
        }
    }
    return is;
}

/* takes two integers and places a cell at those coords */
void place_cell(int** is, int sidex, int sidey, int x, int y) {
    if(x >= sidex || y >= sidey) {
        fprintf(stderr, "place_cell: out of grid range\n");
        exit(1);
    }
    is[y][x] = 1;
}

int check_surroundings(int** is, int sidex,
                       int sidey, int x, int y) {
    int y_less = y - 1;
    if(y == 0) {
        y_less = sidey - 1;
    }
    int y_more = y + 1;
    if(y == sidey - 1) {
        y_more = 0;
    }
    int x_less = x - 1;
    if(x == 0) {
        x_less = sidex - 1;
    }
    int x_more = x + 1;
    if(x == sidex - 1) {
        x_more = 0;
    }
    int p = is[y_less][x_less] +
        is[y_less][x] +
        is[y_less][x_more] +
        is[y][x_less] +
        is[y][x_more] +
        is[y_more][x_less] +
        is[y_more][x_less] +
        is[y_more][x_more];
    return p;
}

void change_condition(int** is,
                      int sidex, int sidey, int x, int y) {
    int* state = &is[y][x];
    int surr = check_surroundings(is, sidex, sidey, x, y);
    if(surr > 3) {
        *state = 0;
    } else if(surr == 3 || surr == 2) {
        *state = 1;
    } else {
        *state = 0;
    }
}

void print_grid(int** is, int sidex, int sidey) {
    int i, j;
    for(i = 0; i < sidey; i++) {
        for(j = 0; j < sidex; j++) {
            if(is[i][j] == 1) {
                printf("*");
            } else {
                printf(" ");
            }
        }
        printf("\n");
    }
}

void new_generation(int** is, int sidex, int sidey) {
    int i, j;
    for(i = 0; i < sidey; i++) {
        for(j = 0; j < sidex; j++) {
            change_condition(is, sidex, sidey, j, i);
        }
    }
}

void play(int** is, int sidex, int sidey) {
    int i = 0;
    while(i < 100) {
        new_generation(is, sidex, sidey);
        print_grid(is, sidex, sidey);
        i++;
    }
}

here's my main:

int main(int argc, char* argv[]) {

    int sidex = atoi(argv[0]);
    int sidey = atoi(argv[1]);
    int** is = make_grid(10, 10);
    int i;
    for(i = 2; i < argc; i += 2) {
        place_cell(is, sidex, sidey,
                   atoi(argv[i]), atoi(argv[i + 1]));
    }

    return 0;
}

edit:

int** make_grid(int x, int y) {
    int (*is)[x] = (int*)malloc(sizeof(int) * y * x);
    if(! is) {
        fprintf(stderr, "make_grid: malloc failed");
        exit(1);
    }
    int j;
    for(j = 0; j < y; j++) {
        is[j] = (int*)malloc(sizeof(int) * x);
        if(!is[j]) {
            fprintf(stderr, "make_grid: malloc failed");
            exit(1);
        }
    }
    return is;
}

This isn't right at all but I can't put my finger on why. Can someone explain to me what to change like I'm five? (a five year-old who knows C, I guess)

Answer 1

I just copied your entire code and tried to run the program. The memory access violation (at least for me) is in this line:

int sidex = atoi(argv[0]);
int sidey = atoi(argv[1]);    <-- memory access violation

The reason is (in my case at least) that I just ran the program with no arguments.

Now, even if I did provide the arguments on the command line the indexing is still off. The first argument argv[0] is the name of the executable, not the first argument after the name.

So, a few things to note for your code:

1. It is not guaranteed that there will be arguments. You should always check the argc to make sure you can index into argv
2. Those arguments are not guaranteed to be integer numbers either - you better check for that too, before you use them as your dimensions

Of course with the indexing shift you should adjust for your "array reading" code accordingly as well. But once you fix the indexing this should be an easy one for you

Answer 2

You are not declaring a two-dimensional array with that syntax, so the memory is not aligned the way you think, thus a segmentation fault. Declaring a pointer int** does not make it a 2-D array. (Surely you don't think int *** would get you a data cube ?).

Heap allocate a 2D array (not array of pointers)

One of the comments above gives the other problem, the zero parameter to a C program argv[0] is the name of the program, not the first parameter on the command line, that is argv[1].