在 c 中具有动态 memory 数组元素的结构

Question

I am trying to create a struct. One of the elements of the struct is an array that should be able to grow if needed.

I do this:

int  COLS=2, ROWS=20;
long int (*array)[COLS] = malloc(sizeof(int[ROWS][COLS]));

struct test{
    long int (*arr)[COLS];
};

struct test *s_test = malloc(sizeof(s_test));
s_test->arr = array;

for (int i=0; i<ROWS; i++){
    array[i][0]=i;
    array[i][1]=i+20;

    printf("0:%ld\t1:%ld\n",s_test->arr[i][0], s_test->arr[i][1]);
}

but the compiler says:

test.c:10:14: error: fields must have a constant size: 'variable length array in structure' extension will never be supported
                long int (*arr)[COLS];

This works ok:

int COLS=2, ROWS=20;
long int (*array)[COLS] = malloc(sizeof(int[ROWS][COLS]));

long int (*arr)[COLS];

//struct test{
//  long int (*arr)[COLS];
//};

struct test *s_test = malloc(sizeof(s_test));

//s_test->arr = array;

arr = array;

for (int i=0; i<ROWS; i++){
    array[i][0]=i;
    array[i][1]=i+20;

    printf("0:%ld\t1:%ld\n", arr[i][0], arr[i][1]); //s_test->arr[i][0], s_test->arr[i][1]);
}

BTW, I, mistakenly forgot to delete the declaration of a struct test (which is not defined now) and the compiler didn't complain...). I will also appreciate a very simple (if possible) explanation of why.

1 - I clearly don't know how to solve this problem.
2 - Does the struct HAS to be a pointer?
3 - Can't the element arr be a pointer to theh 2darray?

Thank you a lot!

Answer 1

Disclaimer: I think long int (*arr)[COLS] (flexible array to pointer to long ) is a mistake; but all you need to do to fix it is adjust the type.

That doesn't look quite right. if COLS really is dynamic we need to say:

struct test{
    long int (*arr)[];
};

but this is invalid due to the toy example being too small. The stretchy element has to be the last one, but it can't be the only one either. Needs to look like this:

struct test{
    size_t nelem; /* I suppose you could have another way of knowing how many */
    long int (*arr)[];
};

and you allocate it with a call that looks like this:

    struct test *ptr = malloc(sizeof(struct test) + sizeof (long int *) * COLS);

If COLS is really constant, the way to get to compile is to change the declaration of COLS to be a macro, or an enum like so: enum { COLS = 2; }; enum { COLS = 2; };