Freeing memory which has been allocated to an array of char pointers (strings). Do I have to free each string or just the “main” pointer?

Question

I have a function that takes a pointer to a char ** and fills it with strings (an array of strings I guess). *list_of_strings* is allocated memory inside the function.

char * *list_of_strings = NULL;

/* list_of_strings malloc'd inside function */
fill_strings_with_stuff(&list_of strings);

use_list_for_something(list_of_strings);

/* Now how do I free it all? */

How would I go about freeing the memory after I've used the strings? If I call

free(list_of_strings);

won't that just free the actual pointers and not the memory each string itself was using? How do I completely free the memory

Just for clarity the function looks something like this:

fill_strings_with_stuff(char *** list)
{
    *list = malloc(AMOUNT);

    for (i = 0; i < SOMETHING; i++) {
        *(list + i) = malloc(LINE_LEN);
        *(list + i) = some_string_from_somewhere
    }

    /* ... */

}

Answer 1

won't that just free the actual pointers and not the memory each string itself was using?

Yes, indeed.

How do I completely free the memory

By looping through the array and freeing each string one by one before freeing up the array itself. Eg

for (i = 0; i < SOMETHING; i++) {
    free(list[i]);
}
free(list);

Answer 2

Basically, there's a rule of thumb to allocating and freeing: You need to call as many free() as you called malloc(). It's as simple as that. In every other case you got yourself a memory leak.

Answer 3

Yes, you have to free() every block you obtained from malloc() . You do it by traversing the array of pointers and caling free() on each element and only then freeing the array itself.

Only you know that there's a tree-like structure that could be freed recursively, that knowledge is not anywhere in the C runtime heap, so the heap manager has no idea about that and your program has to free everything itself.

Answer 4

You need to iterate over list and call free() on every array member. Then free the array.