无法理解为什么发生分段错误

Question

I have written the following code

#include <stdio.h>
#include <stdlib.h>
#define MAX 300003;

int **a, **cost, **prev_x, **prev_y, **b;
int N, M;

int mincost(int n, int m)
{
    //printf("For %d %d\n", n, m);

    printf("prev_x[6][8] = %d\n", prev_x[6][8]);
    printf("prev_y[6][8] = %d\n", prev_y[6][8]);

    printf("cost[%d][%d] %d\n", n, m, cost[n][m]);
    return cost[n][m];
}


int main()
{
    int T;
    scanf("%d", &T);
    while(T--)
    {
        scanf("%d %d", &N, &M);

        a  = (int **)calloc(N, sizeof(int));
        b  = (int **)calloc(N, sizeof(int));

        cost = (int  **)calloc(N, sizeof(int));
        prev_x = (int  **)calloc(N, sizeof(int));
        prev_y = (int  **)calloc(N, sizeof(int));

        for(int i = 0; i < N; i++)
        {
            a[i] = (int *)calloc(M, sizeof(int));
            b[i] = (int *)calloc(M, sizeof(int));

            cost[i] = (int *)calloc(M, sizeof(int));
            prev_x[i] = (int *)calloc(M, sizeof(int));
            prev_y[i] = (int *)calloc(M, sizeof(int));
        }

        printf("%d %d\n", N, M);
        printf("prev_y[6][8] = %d\n", prev_y[6][8]);
        printf("prev_x[6][8] = %d\n", prev_x[6][8]);


        char *str = (char *)calloc(M+1, sizeof(char));
        for(int i = 0; i < N; i++)
        {
            scanf("%s", str);
            for(int j = 0; str[j]; j++)
            {
                if(str[j] == '1')
                    a[i][j] = 1;
                cost[i][j] = -1;
            }
        }
        cost[0][0] = 0;

        mincost(N-1, M-1);

    }
}

For the input

1
7 9
010101110
110110111
010011111
100100000
000010100
011011000
000100101

it gives segmentation fault on line 13. Can some explain why I can't access prev_x[6][8] in mincost()

Answer 1

To use calloc you pass the number of elements as the first argument and the size of the element as the second.

So to create the arrays it should be

a  = (int **)calloc(N, sizeof(int*));

And to create the elements your code is right:

a[i]  = (int *)calloc(M, sizeof(int));

Incidentally, your code would work in a typical 32-bit machine, as int and int* have the same size, but will fail in a 64-bit machine, where sizeof(int*) is 8 and sizeof(int) is 4.