struct with a dynamic memory array element in c

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!

You can't grow an array. You can only grow memory that was allocated dynamically with malloc(). long int (*arr)[COLS] declares an array that has COLS rows, and each row is a pointer. You can resize the rows, but not the main array. — Barmar
– Barmar, Commented Dec 18, 2020 at 0:21
1- The mentioned question does not include a variable array within a struct. 2- One of the things that in my view makes this community so helpful is that the answers can be focused on a specific question and not a chapter of a book. 3 - It's not helpful closing the question without providing an answer when the answer is not clearly stated in the mentioned original question. — aerijman
– aerijman, Commented Dec 18, 2020 at 1:08
Just declare long int **arr as the struct member, and do it as in the linked question. — Barmar
– Barmar, Commented Dec 18, 2020 at 1:11

Joshua · Accepted Answer · 2020-12-18 04:22:17Z

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; };

edited Dec 18, 2020 at 4:22

answered Dec 18, 2020 at 4:01

Joshua

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6 Comments

chux Over a year ago

BTW, Could use sizeof *ptr + sizeof ptr->arr[0] * COLS and remove any type matching maintenance issues.

aerijman Over a year ago

@chux-ReinstateMonica. Your addition looks good to me but I get: error: subscript of pointer to incomplete type 'int []'.

aerijman Over a year ago

@Joshua, Thank you very much! 1 - s_truct->arr is now a pointer to array of int (with COL dimensions). Now I have to allocate memory for the array, right? Will I have to allocate memory for each COL separately? My intention is to call the array like:`arr[row][col]. 2 - I use long int because the numbers are genomic locations and unsigned int is not enough. Should I use other type

chux Over a year ago

@aerijman Does sizeof *ptr + sizeof *(ptr->arr) * COLS also error?

aerijman Over a year ago

@chux-ReinstateMonica. I get error: invalid application of 'sizeof' to an incomplete type 'int []'

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