C 中结构内的动态二维数组

Question

At first I'd like to say that despite experience with other programming lanugages, I'm new to C and I'm trying my best to understand all of its concepts. I know that dynamic allocation and especially 2d arrays inside structures aren't things that C is good at, but regardless of that I would like to understand how it functions.

First of all the reason why I'm trying to use a dynamic 2d array instead of two 1d arrays is because I'd like to benefit from the foo[x][y] syntax. Also array foo[x][y] should change it's size according to what's passed to function that allocates memory for the enitre struct.

My problem is that I cannot seem to correctly allocate enough memory for array inside a Bus struct. I've been looking around web but I cannot seem to find a working example of such allocation and adapt it to my example. What I've tried so far was: declaring flexible array memeber inside struct and allocating it with 'make_structure' function, to later access it with macro.

Current example has the "pointer to a pointer" way of declaring dynamic 2d array. The program crashes when it's ran. Functions:

• driver function for the example:

struct Bus *head;
struct Bus *current;
struct Bus *new;
struct Bus *make_structure(int r, int s);

void test() {
    int i, NUM;
    srand(time(NULL));

    head = make_structure(ROW_SIZE, SEAT_AMOUNT);
    current=head;
    random_fill_structure(current);

    current->next=NULL;

    current=head;
    show_structure();
}

• function for allocating the struct

struct Bus *make_structure(int r, int s) {
    int i;
    struct Bus* a;

    a->seats = (int**)calloc(r,sizeof(int*));
    for (i=0;i<r; i++) {
        a->seats[i] = (int*)calloc(s,sizeof(int));
    }
    if(a==NULL)
    {
        puts("Error");
        exit(1);
    }

    return(a);
}

• the struct itself:

struct Bus {
    char name[20];
    int **seats;
    struct Bus *next;
};

The other two functions present in driver function random_fill_structure and show_structure don't do anything with the seats** array so they shouldn't be the case of programme crashing.

Edit 1: I've allocated memory for the struct in make_structure function with this line of a=(struct Bus *)malloc(sizeof(struct Bus)); and the programme doesn't crash anymore. Though whenever I'd try to access it inside show_structure like so:

        for (i = 0; i < ROW_SIZE; i++) {
            for (j = 0; j < SEAT_AMOUNT; j++) {
                printf("[%d]", a->seats[i][j]);
            }
        }

The app will crash as it tries to print the first value of 2d array.

Edit 2: Function used to print the structure:

void show_structure() {
    int i, j;
    struct Bus *ptr = head;

    while(ptr != NULL) {
        printf("%s slots: ",ptr->name);
        printf("\n");
        ptr = ptr->next;
        for (i = 0; i < ROW_SIZE; i++) {
            for (j = 0; j < SEAT_AMOUNT; j++) {
                printf("[%d]", ptr->seats[i][j]);
            }
        }
    }
}

Answer 1

You shoul allocate memory for 'a' before accessing 'a->seats'.

struct Bus *a = malloc(sizeof *a);