Dynamically allocate struct of char array and int array

Question

I am trying to use malloc to allocate memory for a struct containing a char array and an int array. I will be populating these two arrays with information from a file whose length I will not know in advance.

typedef struct bulk_data{
    int *l_bulk_pos_arr;
    char (*l_bulk_anc_arr)[10001];
}bulk_data;

I am still learning memory allocation, but what I imagine here is that since the size of each char array element is fixed, I shouldn't have to loop through that array to allocate memory. At this point I now know the number of array elements I need (n_rows). I have tried the following (obviously not all at the same time):

struct bulk_data *bulk_counts;
    bulk_counts = malloc(sizeof(bulk_data)); // 1st attempt
    bulk_counts = (bulk_data *)malloc(sizeof(bulk_data)); // 2nd
    bulk_counts = malloc(sizeof(bulk_data) * n_rows); // 3rd
    bulk_counts = (bulk_data *)malloc(sizeof(bulk_data) * n_rows); // 4th

No errors at compile time, but it appears that the above listed attempts aren't allocating the space properly:

(gdb) p bulk_counts->l_bulk_anc_arr
$1 = (char (*)[10001]) 0x0
(gdb) p bulk_counts->l_bulk_anc_arr[0]
Cannot access memory at address 0x0

(gdb) p bulk_data->l_bulk_pos_arr
$2 = (int *) 0x0
(gdb) p bulk_data->l_bulk_pos_arr[0]
Cannot access memory at address 0x0

I would like to know how I can allocate memory for this stated case, but also in the case when you don't know the number of chars in each char array element.

Incidentally, if you're really using C and you have a decent compiler, you don't need to cast the result of your mallocs — Iskar Jarak
– Iskar Jarak, Commented Nov 3, 2014 at 3:16

R Sahu · Accepted Answer · 2014-11-03 03:09:10Z

4

l_bulk_anc_arr is a pointer to an array of 10001 chars. It is not an array.

You still have to allocate memory for it.

struct bulk_data *bulk_counts;
bulk_counts = malloc(sizeof(bulk_data));
bulk_counts->l_bulk_pos_arr = malloc( /*some size*/ );
bulk_counts->l_bulk_anc_arr = malloc(10001);

Now, you can use:

(*bulk_counts->l_bulk_anc_arr)[0] = 'a';
(*bulk_counts->l_bulk_anc_arr)[1000] = '\0';

or

bulk_counts->l_bulk_anc_arr[0][0] = 'a';
bulk_counts->l_bulk_anc_arr[0][1000] = '\0';

answered Nov 3, 2014 at 3:09

R Sahu

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