How to construct array of pointer of structs in C?

Question

I have problem working with dynamic array of pointer structs. It seemed that at the end of the loop array have only 2 pointers inside. I don't know why is that. I new to C and low level stuff. I am asking for help! Could You please explain me why this is happening?

#include <stdlib.h>

struct Coordinates {
    short x;
    short y;
};

const short BOARD_SIZE = 8;
const short MAX_SIZE = 10;

int main() {


    struct Coordinates **possible_moves = malloc(MAX_SIZE * sizeof(struct Coordinates));

    for (short i = 0; i < BOARD_SIZE; ++i) {
        struct Coordinates *current_coordinates = malloc(sizeof(struct Coordinates));
        current_coordinates->x = i;
        current_coordinates->y = i;
        possible_moves[i] = current_coordinates;
    }

    return 0;
}

There is a typo in this declaration struct Coordinates **possible_moves = malloc(MAX_SIZE * sizeof(struct Coordinates));. You mean sizeof(struct Coordinates *) — Vlad from Moscow
– Vlad from Moscow, Commented Oct 22, 2020 at 20:06

MikeCAT · Accepted Answer · 2020-10-22 19:49:08Z

1

The elements of the array to be assigned to possible_moves is pointers, so allocating size should be one of pointers, not one of the structure.

In other words,

    struct Coordinates **possible_moves = malloc(MAX_SIZE * sizeof(struct Coordinates));

should be

    struct Coordinates **possible_moves = malloc(MAX_SIZE * sizeof(struct Coordinates*));

or

    struct Coordinates **possible_moves = malloc(MAX_SIZE * sizeof(*possible_moves));

answered Oct 22, 2020 at 19:49

MikeCAT

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dspr · Accepted Answer · 2020-10-22 19:52:10Z

1

Or you can do like this :

#include <stdlib.h>

struct Coordinates {
    short x;
    short y;
};

const short BOARD_SIZE = 8;
const short MAX_SIZE = 10;

int main() {
    // struct Coordinates * instead of struct Coordinates **
    struct Coordinates *possible_moves = (Coordinates *)malloc(MAX_SIZE * sizeof(struct Coordinates));

    for (short i = 0; i < BOARD_SIZE; ++i) {
        // struct Coordinates instead of struct Coordinates *
        struct Coordinates current_coordinates = {i, i};
        possible_moves[i] = current_coordinates;
    }
    free(possible_moves);
    return 0;
}

answered Oct 22, 2020 at 19:52

dspr

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