Realloc in C for structs containing dynamic array of structs

Question

Issues with reallocing the items list. I am trying to add items into the testList structs items, but i am getting memory address errors when trying to add or print the values for the individual ListItems. Any help would be greatly appreciated.

struct ListItems
{
    int id;
    int name;
};

struct testList
{
    struct ListItems** items;
    int count;
    int size;
};

struct Test
{
    struct testList* list;
};

void printArray(struct testList* list)
{
    for (int i = 0; i < list->count; i++)
    {
        printf("id=%i, name= %i \n", list->items[i]->id, list->items[i]->name);
        fflush(stdout);
    }
    printf("printing accomplished \n");
    fflush(stdout);
}

void growArray(struct testList* list, struct ListItems* item)
{
    int size = list->size;
    list->items[list->count++] = item;
    struct ListItems** user_array = list->items;
    //printf("array count %i, array size %i \n", list->count, size);
    if (list->size == list->count)
    {
        struct ListItems* temp = realloc(*user_array, (size * 2) * sizeof (struct ListItems));
        if (temp == NULL)
        {
            printf("it's all falling apart! \n");
        }
        else
        {
            *user_array = temp;
            list->size = size * 2;
        }
    }
}

/*
 *
 */
int main(int argc, char** argv)
{

    struct Test* test = (struct Test*) malloc(sizeof (struct Test));
    test->list = (struct testList*) malloc(sizeof (struct testList));
    test->list->count = 0;
    test->list->size = 1;
    test->list->items = (struct ListItems**) malloc(sizeof (struct ListItems*));

    for (int i = 0; i < 32; i++)
    {
        struct ListItems* item = (struct ListItems*) malloc(sizeof (struct ListItems));
        item->id = i;
        item->name = i;
        growArray(test->list, item);
    }
    printArray(test->list);
    for (int j = 0; j < sizeof (test->list->items); j++)
    {
        free(test->list->items[j]);
    }
    free(test->list->items);
    free(test->list);
    free(test);
}

Don't cast the return of malloc. And what kind of error do you have ? — Gabson, Dec 04 '13 at 14:30
[Please don't cast the return value of `malloc()` in C](http://stackoverflow.com/a/605858/28169). — unwind, Dec 04 '13 at 14:30
You might want to try some sort of malloc-debugging tool or package. You seem to be freeing memory that has not been allocated, and such errors can be hard to find. — Thomas Padron-McCarthy, Dec 04 '13 at 15:01
the major problem that occurs is during the printArray. Some of the values in the array are incorrect. For example: id=0,name=0, then id=66656, name = 6 — user3066210, Dec 04 '13 at 15:12
A side note: allocated memory contains garbage unless you use `calloc` or `memset` it. — Jite, Dec 04 '13 at 15:15

ensc · Answer 1 · 2013-12-04T15:26:25.523

Your growArray() needs to update list->items. In current code, it will point forever to an 1-element sized area only.

EDIT:

your realloc() allocates for sizeof (struct ListItems)) but pointer holds pointers, not elements.

I would write:

void growArray(struct testList* list, struct ListItems* item)
{
        if (list->size <= list->count) {
              size_t new_size = 2 * list->size;
              struct ListItems** temp = realloc(list->items, new_size * sizeof temp[0]);
              assert(temp);

              list->size = new_size;
              list->items = temp;
        }

        list->items[list->count] = item;
        ++list->count;
}

With this, you do not need the initial list->items = malloc(...) in main() but can assign NULL.

EDIT:

for (int j = 0; j < sizeof (test->list->items); j++)

does not make sense; you probably want to j < test->list->count.

score 0 · Answer 2 · answered Dec 04 '13 at 15:16

The problem starts with the declaration of struct testList. A pointer to an array of items should have just one *:

struct testList
{
    struct ListItems* items;   // Changed from pointer-to-pointer
    int count;
    int size;
};

This will force some other changes in the code.

Realloc in C for structs containing dynamic array of structs

2 Answers2